Determining the effectiveness of best management practices ... · Figure 5-6 – Nutrient...

March 2010

Determining the Effectiveness of Best Management Practices to Reduce Nutrient Flows in Urban Drains Managed by the Water Corporation: Part 1 Water Quality and Water Regime in Perth Urban Drains

Olga Barron, Mike Donn, Daniel Pollock and Chris Johnstone March 2010

Water for a Healthy Country Flagship Report series ISSN: 1835-095X

Australia is founding its future on science and innovation. Its national science agency, CSIRO, is a powerhouse of ideas, technologies and skills.

CSIRO initiated the National Research Flagships to address Australia’s major research challenges and opportunities. They apply large scale, long term, multidisciplinary science and aim for widespread adoption of solutions. The Flagship Collaboration Fund supports the best and brightest researchers to address these complex challenges through partnerships between CSIRO, universities, research agencies and industry.

The Water for a Healthy Country Flagship aims to achieve a tenfold increase in the economic, social and environmental benefits from water by 2025. The work contained in this report is a collaboration between CSIRO and the Water Corporation of Western Australia.

For more information about Water for a Healthy Country Flagship or the National Research Flagship Initiative visit www.csiro.au/org/HealthyCountry.html

Citation: Barron O, Donn MJ, Pollock D and Johnstone C 2010. Determining the effectiveness of best management practices to reduce nutrient flows in urban drains managed by the Water Corporation: Part 1 – Water quality and water regime in Perth urban drains. CSIRO: Water for a Healthy Country National Research Flagship. 67 pp.

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Important Disclaimer:

CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.

Cover Photograph:

From CSIRO Description: Mooney St Wetland Biofilter Bayswater main drain catchment April 2009 Photographer: Mike Donn © 2009 CSIRO

Best management practices Part 1: Water quality and water regime in Perth urban drains Page iii

CONTENTS Acknowledgments .................................... .................................................................vi

Glossary ........................................... ..........................................................................vi

Executive Summary.................................. ................................................................vii

1. Introduction....................................... .................................................................1 1.1. Aim and Objectives ................................................................................................... 1 1.2. Water Quality in the Perth Urban Drains .................................................................. 1 1.3. Currently Adopted Water Quality Control Measures (BMPs) ................................... 5

2. Project methodology ................................ .........................................................7 2.1. Selection of the Drain Catchments for More Detailed Investigation ......................... 7 2.2. Collation of the Currently Available Data ................................................................ 10 2.3. Analysis of the Available Data ................................................................................ 11

2.3.1. Spatial analysis....................................................................................................11 2.3.2. Catchment water balance....................................................................................11 2.3.3. Drainage water quality.........................................................................................14 2.3.4. Water quality sampling program (CSIRO snapshot)............................................15 2.3.5. Data limitation......................................................................................................16

2.4. Literature Review on BMPs and Water Quality in Urban Environment .................. 17

3. Nutrients sources, pathways and attenuation process es in the urban environment and Best Management Practices for nutri ent remediation .....18

4. Catchment characteristics and climate.............. ............................................23 4.1. Climate.................................................................................................................... 23 4.2. Land Use................................................................................................................. 23 4.3. Drainage ................................................................................................................. 26 4.4. Water Balance Table .............................................................................................. 28

5. Definition of the relationship between water qualit y and catchment characteristics .................................... .............................................................29 5.1. Nutrient Inputs in the Urban Environment............................................................... 31 5.2. Development Stage: Legacy Nutrients ................................................................... 34 5.3. Hydrogeological Conditions .................................................................................... 35 5.4. In-Stream Processes .............................................................................................. 36 5.5. Seasonal Concentration and Load ......................................................................... 39

6. Identification of the most appropriate BMPs for wat er quality improvement in urban drainage in the Perth metropolitan area...... ........................................45 6.1. Storm Water Runoff ................................................................................................ 46 6.2. Baseflow ................................................................................................................. 47 6.3. Individual Drains ..................................................................................................... 49

7. Conclusions ........................................ .............................................................51

References ......................................... .......................................................................56

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LIST OF FIGURES Figure 1-1 - Mean Total Phosphorus concentrations in urban streams (data cortesy of WA Department of Water) ............................................................................................................ 3 Figure 1-2 - Mean Total Nitrogen concentrations in urban streams (data cortesy of WA Department of Water) ............................................................................................................ 4 Figure 2-1 - Project activities structure................................................................................... 8 Figure 2-2 - Catchments selected for analysis within the project............................................ 9 Figure 2-3 - An example of hydrograph separation (Bayswater main drain 2007) ................ 12 Figure 2-4 - The water quality data separation based on the hydrograph analysis ............... 16 Figure 3-1 – Nutrient source, pathway and receiving water continuum ................................ 19 Figure 3-2 – The nitrogen cycle (Environment Canada 2001) .............................................. 19 Figure 3-3 – The phosphorus cycle (Environment Canada 2001) ........................................ 19 Figure 3-4 - Options for BMPs and their application at catchment areas to control water quality during various hydrological stages............................................................................ 22 Figure 4-1 - Annual rainfall for three meteorological stations (Perth Airport, Jandakot, Perth Metro) from 1940 to 2008 .................................................................................................... 24 Figure 4-2 - Monthly average rainfall and potential evaporation for the three stations from 1975. ................................................................................................................................... 25 Figure 4-3 – Catchment urban development indicated by the increase in roof area over time............................................................................................................................................ 26 Figure 4-4 - Drain density (a) and the area of compensation basins (b) ............................... 27 Figure 4-5 – Studied catchment locations and the groundwater table (vertical scale is exaggerated, red dots indicate location of bores where observation data was available for groundwater table interpretation) ......................................................................................... 27 Figure 5-1 - Sediments observed in Bayswater main drain in the (a) western and eastern branches (upstream [b] and downstream [c] from the outfall from the CSBP site), (d) at the confluence and (e) in the main channel downstream from the gauging station. These show the level of suspended materials contributed by the CSBP site. They are evident along the drain channel up to the confluence with Swan River. ........................................................... 33 Figure 5-2 – Changes in dissolved organic carbon (DOC) with catchment urban development............................................................................................................................................ 34 Figure 5-3 - Baseflow rates and seasonal variation in DOC concentration........................... 36 Figure 5-4 – TP concentrations in leachate from sources (such as septic tanks or soil as shown in Table 2.3 and Table 2.4 in Part 2 of this report), branch drains and main drains, indicating high level of TP attenuation in Mills Street main drain .......................................... 37 Figure 5-5 – Ammonium attenuation in the Mills Street main drain channel ......................... 37 Figure 5-6 – Nutrient attenuation effect of a Mills Street main drain compensation basin (a) nitrate (NO3) and (b) dissolved organic nitrogen plus particulate N (DON+PN) .................... 38 Figure 5-7 – In-stream processes – organic matter build-up in Brown’s Lake wetland (9 April 2009) ................................................................................................................................... 38 Figure 5-8 – Variation in soluble reactive P (SRP) concentration with the ratio of dissolved inorganic nitrogen (DIN) to SRP for three catchments.......................................................... 39 Figure 5-9 – Variation in nutrient concentrations with discharge separated for baseflow and peak flow periods, (a) soluble reactive phosphorus (SRP) and (b) ammonium (NH4)........... 40 Figure 5-10 – Variation in (a) soluble reactive phosphorus (SRP) with total P concentration and (b) ammonium (NH4) with total N for baseflow and peak flow periods. .......................... 41 Figure 5-11– Main sources of nutrient in surface water during baseflow period in Bayswater main drain catchment; septic tanks area; CSBP area; irrigated and fertilised area............... 42 Figure 5-12 - Main sources of nutrients in surface water during storm flow period (peak flow) in the Bayswater main drain catchment: roads and in-stream features (compensation basins and open drains).................................................................................................................. 42

Best management practices Part 1: Water quality and water regime in Perth urban drains Page v

Figure 5-13 – Main sources of nutrient in surface water during baseflow period in the Mills Street main drain catchment: septic tanks area (mainly industrial) and irrigated and fertilised area ..................................................................................................................................... 43 Figure 5-14- Main sources of nutrient in surface water during storm flow period (peak flow) in the Mills Street main drain catchment: roads (and other hard surfaces in the industrial area) and in-stream features (compensation basins and open drains) .......................................... 43 Figure 5-15– Main sources of nutrient in surface water during baseflow period in Bannister main drain catchment: septic tanks area; irrigated and fertilised area .................................. 44 Figure 5-16- Main sources of nutrient in surface water during storm flow period (peak flow) in Bannister main drain catchment: roads and in-stream features (compensation basins and open drains)......................................................................................................................... 44 Figure 6-1 - Storm runoff discharge point (Mills Street and Bayswater main drains) ............ 46 Figure 6-2 – Filtration/biofiltration system, which allows the discharge of road runoff over a grassed are prior to discharge to the drains (Bletchley Park, Southern River)...................... 47 Figure 6-3 – Decision tree for determining the most effective BMPs for the Bayswater main drain catchment ................................................................................................................... 50

LIST OF TABLES Table 1-1 - Nutrient export (TN and TP) from Avon River Catchment and Swan-Canning Catchment, modified from Table 15, Swan River Trust (2009). .............................................. 2 Table 1-2 - Areas of the urban catchments with various mean Total Phosphorus and Total Nitrogen concentrations in urban streams.............................................................................. 5 Table 2-1 - Data gathered during the project and data sources............................................ 10 Table 2-2 - Water quality data** availability.......................................................................... 15 Table 3-1 – Structural BMP nutrient attenuation processes.................................................. 21 Table 4-1 - Rainfall, potential evaporation and winter temperature at three meteorological stations ................................................................................................................................ 23 Table 4-2 Summary of the catchment water balances.......................................................... 28 Table 5-1 – Indicative sources and speciation of baseflow nitrogen export from the Bayswater main drain catchment (baseflow* as 79% annual load in 2007 as 7.6 t-N).......... 30 Table 5-2 – Indicative sources and speciation of baseflow nitrogen export from the Mills Street main drain catchment (baseflow* as 45% annual load in 2007 as 3.8 t N) ................. 30 Table 5-3 – Indicative sources and speciation of baseflow phosphorus export from the Forrestdale main drain catchment (baseflow* as 42% annual load in 2007 as 0.36 t P)....... 31 Table 5-4 - Indicative sources and speciation of baseflow nitrogen export from the Bannister Creek catchment.................................................................................................................. 31

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ACKNOWLEDGMENTS The project was funded by the Water Corporation of Western Australia and delivered through the CSIRO Water for Healthy Country Flagship under the Urban Water Research Theme. The Water Corporation has funded this research as a contribution to the science underpinning

� the definition of appropriate targets for nutrient and contaminant reduction, and

� cost-effective management practices to achieve those targets,

so as to safeguard natural water resources in the Perth urban environment.

The authors are grateful for the Technical Reference group, including representatives from the Water Corporation, Department of Water and Swan River Trust, which were involved in the project progress review and discussion on the project direction.

GLOSSARY BMP: Best Management Practices as land management measures or infrastructure, which allow improvement of water quality or discharge control.

Urban Drains in Perth : initially installed to control the shallow groundwater table in the eastern Perth suburbs and to convey storm runoff

All drains whether ‘main’ or ‘branch’ discussed in this report relate to drains controlled by the Water Corporation of Western Australian as per mapping provided for this study. The Water Corporation drains are integrated with a network of smaller drains controlled by local government authorities, thus issues relating to BMPs are relevant to all drain managers.

Baseflow: drains flow associated with groundwater discharge

Storm runoff: drains flow associated with storm runoff from the urban impervious sources directly connected to drains (such as roads)

Nutrients

C: Organic carbon

DOC: dissolved organic carbon

POC: particulate organic carbon

N: Nitrogen

NH4-N: ammonium-nitrogen

NO3-N: nitrate-nitrogen

DON: dissolved organic N

PN: particulate N

TKN: total Kjeldahl N (nitrogen analysis method which includes both total organic nitrogen and ammonium-N)

P: Phosphorus

SRP: soluble reactive P

DOP: dissolved organic P

PP: particulate P

PO4: phosphate (the predominate form of P measured by SRP)

Best management practices Part 1: Water quality and water regime in Perth urban drains Page vii

EXECUTIVE SUMMARY

The efficiency of the best management practices (BMPs) to reduce nutrient concentrations and loads in Perth’s urban main drains is poorly understood. Results from the other cities cannot be reliably extrapolated because of Perth’s unusually sandy catchments and the major contribution of groundwater to the nutrient loads in the streams.

The research project, funded by the Water Corporation and CSIRO Flagship Water for a Healthy Country, has been established to address the issues associated with the generation and transfer to surface water of nutrients in the selected catchments with the following objectives:

1. Define the existing water quality issues, including nutrient speciation, concentrations, and their relation to drainage hydrological characteristics;

2. Define the relationship between water quality and land use, water management in the catchment, climate and opportunities for structural interventions; and

3. Establish the requirements for BMPs which are most likely to address the issues identified and relevant technology (in-situ treatments) based upon Western Australian, national and international experience.

The study region covered developed areas where urban main drains contribute to drainage discharge to the Swan and Canning rivers. Four drains were selected for a review of available information; these included the Bayswater, Mills Street, Bannister and Forrestdale main drains.

The project brought together and provided analysis of a large number of data sets characterising the individual catchments, including geological, hydrogeological and hydrological information, current land use and some aspects of catchment development history, as well as the water quality data collected over the last two decades by Western Australian water authorities. The emphasis was placed on identification of nutrient speciation in urban drains and their variation both seasonally and over the period of water quality monitoring. The nutrient forms considered included:

• For Phosphorus (P): soluble reactive P (SRP), dissolved organic P (DOP) and particulate P (PP)

• For Nitrogen (N): ammonium-nitrogen (NH4-N), nitrate-nitrogen (NO3-N), dissolved organic N (DON) and particulate N (PN)

• For Organic carbon (C): dissolved organic carbon (DOC) and particulate organic carbon (POC)

Over 120 published papers and reports were reviewed, summarising the current national and international knowledge on in-situ water quality control BMPs and their ability to attenuate various nutrient species and the associated limitations. The results are presented in the Part 2 of this report.

This summary provides the main research results related to the factors influencing water quality in urban drainage, overall BMP efficiency, effectiveness of BMPs in Perth, and the needs for future research.

It was concluded that for any catchment the success of in-situ water quality control measures is greatly dependent on the following:

1. Identification of the sources of nutrients in the catchment, their transfer pathways from the catchment sources to surface water drainage, and the processes which influence in-stream nutrient transformation; this allows a most appropriate intervention both in terms of BMP location and technologies deployed.

Best management practices Part 1: Water quality and water regime in Perth urban drains Page viii

2. Identification of not only total concentrations, but P and N species and their seasonal variation, as well as their links to the relevant sources and pathways: similarly to the previous statement, this provides a guide to the most appropriate treatment technology.

3. Definition of the significance of nutrients load in terms of ecological conditions in the receiving water environment, when risk level needs to be established for various seasons and for P and N species: allows the effort to be directed to the priority seasons or nutrient speciation to satisfy the requirement for a cost-effective BMP.

The undertaken analysis allows identification of the main factors influencing water quality within the studied urban drains in the Perth metropolitan area, which include;

1. Nutrient inputs in the urban environment

2. Development stage: effect of legacy nutrients

3. Hydrogeological conditions

4. In-stream processes and residence time prior to discharge to the river

5. Seasonality in concentration and load

The current study concluded that the hydrological regime of the catchment should be considered during selection of BMPs for water quality control.

Storm runoff delivers 20 to 60% of annual loads of predominantly particulate and organic nutrients during the wet season from May to October. Hydrological analysis of urban drains indicates that during storm events water quality is defined by runoff from impervious sources, but can also be influenced by remobilisation of nutrients from sediments accumulated in streams, compensation basins and wetlands. During these events, water quality control initiatives aimed to reduce nutrient loads should be considered:

• Non-structural BMPs, including preventive measures against nutrient accumulation on impervious surfaces such as street and road sweeping, and proper lawn and park management to prevent fertiliser, soil and debris entering waterways;

• Structural BMPs to control water quality in runoff prior to discharge into the drainage network; and

• Drain and structural BMP maintenance including vegetation control such as plant maintenance or sediment removal).

Low baseflow during summer (November to April) delivers less than 15% of annual loads of predominantly particulate and organic nutrients1 and PO4, when the Swan-Canning Estuary is likely to be particularly sensitive to nutrients input. During this period, urban drainage is the main source of fresh water inflow to the estuary. Under these conditions, water quality control initiatives to be considered include:

• Non-structural BMPs which may prevent nutrient release from soil to groundwater. Groundwater is the sole source of baseflow during this period; and appropriate BMPs can include fertiliser control to reduce the nutrient input, and soil amendment, to reduce nutrient transfer from soil to groundwater. The latter is one of most effective non-structural BMPs in this situation;

• Structural BMPs which may reduce nutrient transfer to groundwater. These include BMPs to control water quality in recharge areas such as lining the base of infiltration basins and soak wells with materials which allow nutrient adsorption, such as NUA (Wendling et al., 2009); and

1 Nitrate and Ammonia are also detected in Bayswater, which is related to point sources in this catchment

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• Drain maintenance. Vegetation control, such as plant maintenance or sediment removal can prevent in-stream nutrient generation.

However, wetlands on the outflow from the catchments are unlikely to effectively treat DON, and may be of limited use in SRP removal when the N:P ratio is low. This observation was confirmed by limited monitoring data in the constructed wetlands.

High baseflow in the wet season from June to October delivers 20 to 60% of annual nutrient loads of various proportions of organic (up to 50%) and inorganic nutrients. In addition to the BMPs suggested for water quality control during low baseflow, the following water quality control measures should also be considered during the high flow period:

• Structural BMPs to control water quality in the outflow from the subsurface drains or smaller branch drains prior discharge to main drains. Such BMPs may include, among others, biofiltration systems. These can be small and are particularly effective in NO3, NH4 and PO4 attenuation, levels of which were found to be high during the winter months;

• Wetlands are likely to be an effective treatment for NO3, NH4, SRP and particulate nutrients during this period. However, their effectiveness may be compromised if they are not large enough to allow for the up to 30 days residence time required for attenuation of nutrient species such as NH4 and NO3; and

• Drain maintenance including vegetation control such as plant maintenance or sediment removal.

Point sources generally have the most significant effect on water quality during low baseflow periods. Nutrient forms and associated loads are expected to be site specific. For instance, during summer in Bayswater, two such sources (the CSBP site and the industrial zone) contribute at least 60% N in the form of NO3 and NH4.

Due to variation in land use, hydrological condition and presence of nutrient point sources, the effectiveness of appropriateness of BMP may vary between the catchments, and recommendations for the individual drains were provided.

Based on the analysis undertaken, the following are considered to be the most important future steps in the selection of nutrient BMPs in the Perth metropolitan area:

• Catchment-scale groundwater modelling and better characterisation of groundwater quality through the establishment of a groundwater monitoring network. Monitoring is currently poor.

• The development and implementation of methodologies for effective and quantitative estimation of BMP efficiencies based on high frequency monitoring, selection of in-situ equipment and hydro-biogeochemical modelling.

• The quantification of nutrient leachate from soil under different land and water management practices using field investigations and laboratory trials.

• The definition of in-stream attenuation parameters through the establishment of biological and water quality monitoring in targeted drain reaches and compensation basins within the studied catchments.

In addition, it is required that a better understanding of the role of organic nutrient speciation in the water environment is developed.

Best management practices Part 1: Water quality and water regime in Perth urban drains Page 1

1. INTRODUCTION The efficacy of best management practices (BMPs) to reduce nutrient concentrations and loads in Perth’s urban main drains is poorly known. Results from other cities cannot be reliably extrapolated to the local conditions due to the specifics of nutrient transfer from the catchments to drains within Perth’s sandy catchments and the major contribution of groundwater to nutrient concentrations.

A workshop involving CSIRO, the Water Corporation, the Western Australian Department of Water (DoW) and the Swan River Trust lead to definition of the priority research areas related to urban water issues in the Perth metropolitan area. One of the outcomes was the need to clarify long-term water quality variations in natural streams and urban main drains and to validate the current suite of BMPs for improving water quality in urban main drains. This includes both the technologies and management practices that may be useful in managing drainage water quality.

Drainage water quality can best be evaluated on a catchment-by-catchment basis given each has unique soils, hydrology and land use history. Water quality objectives have to be applied to both existing drainage systems and to those being planned to service new land development. Usually, the latter have higher standards applied because of more stringent regulatory provisions.

1.1. Aim and Objectives The research project funded by the Water Corporation and CSIRO Flagship Water for a Healthy Country has been established with the following objectives:

1) Definition of the main Western Australian issues: nutrient speciation and concentrations, and their relationship to drainage hydrological characteristics;

2) Definition of the relationships between water quality and land use, water management in the catchment and climate and the identification of opportunities for structural interventions; and

3) BMP requirements for the identified issues and available technology (in-situ treatments), in light of:

a) Western Australian experience; and

b) Interstate and international experience, where conditions are similar to Perth local conditions.

The study region covered developed areas where urban main drains contribute to drainage flows to the Swan and Canning Rivers. The project activities were presented quarterly to the project technical working group, which reviewed intermediate results and on-going project activities.

1.2. Water Quality in the Perth Urban Drains According to the latest report (Swan River Trust 2009), annually 826 tonnes of nitrogen and 46 tonnes of phosphorus enter the Swan-Canning Estuary, predominantly from the Avon River (70%TN and 43%TP) and Ellen Brook catchments (9%TN and 22%TP) (Table 1-1). Other rivers and drains contribute 180 tonnes of nitrogen and 16 tonnes of phosphorus. During drier seasons, the nutrient load generated in the Perth metropolitan area dominates. The vast majority of urban drains are perennial, designed to control groundwater table and to convey groundwater discharge to the estuary all year around. In these sub-catchments urban drains contribute greater amounts of nutrients to the system than those with seasonal flow.

The main source of nitrogen and phosphorus in an urban environment is residential and recreational activity, in particular fertiliser application on grassed areas and gardens. Increased urbanisation also results in increased nutrient loads, mainly due to increased


Table 1-1 - Nutrient export (TN and TP) from Avon R iver Catchment and Swan-Canning Catchment, modified from Table 15, Swan River Trust (2009).

Area (km 2)

Average annual

discharge (ML)

Total annual TN load (t)

Average annual TN

yield (kg km -2)

Total annual TP load (t)

Average annual TP

yield (kg km -2)

Avon 123,891 253,934 575 5 20 0.16

Swan-Canning

2090 189,336 251 120 26 12

Ellen Brook 716.4 26,752 71.4 100 10.04 14

Urban 355 84,081 97.5 288 8.4 22.5

Peri-urban 1,019 78,501 82 91 7.5 9.4

runoff when compared with predevelopment conditions even if nutrient concentrations reduce in urban drainage.

According to a DoW review of water quality in urban catchments there is an overall downward trend in nutrient concentration. Although a report on this finding has not been published, the data was presented at the CSIRO Urban Drainage Workshop in March 2008.

Previous studies (GHD 2006), indicated that the total catchment area where Water Corporation drains occur was estimated at 518 km2 and available water quality data was related to 78% of this area (Table 1-2).

• Where data is available, 18% of the catchment area generated runoff which meets long term water targets for TN (<1 mg/L) and 55% of the catchment area generated runoff which meets long term water targets for TP (<0.1 mg/L).

• Where data is available, 21% of the catchment area generated runoff which does not meet short term water targets for TN (>2 mg/L) and 22% of the catchment area generated runoff which does not meet short term water targets for TP (>0.2 mg/L).

• The most extreme water quality was reported for the non-urbanised or partly urbanised catchment (Figure 1-1, Figure 1-2).

Urban catchment is characterised by higher runoff when compared with peri-urban catchments. The drain discharge data, gathered by DoW, is only available for six catchments where Water Corporation drains are established. Only three of these catchments (Bayswater drain, Mill Street drain and South Belmont drain) represent solely urban land use. The others include sub-catchments with a large proportion of hills or other peri-urban sub-catchments.

It is expected that land use, and also its alteration, influence drain discharge and quality. Since a large portion of discharge from urban drains is associated with groundwater, the groundwater management in the catchment (groundwater abstraction, reduction in groundwater table as a result of drains installation, presence or absence of reticulated sewage system) would also have an impact on drain discharge rates and water quality. It appears that nutrient concentrations are somewhat lower in urban drains than in peri-urban ones. Variation in nutrient speciation may also occur as a result of catchment urbanisation.


Figure 1-1 - Mean Total Phosphorus concentrations i n urban streams (data courtesy of WA Department of Water)

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extreme (>>0.5 mg/L)


Figure 1-2 - Mean Total Nitrogen concentrations in urban streams (data courtesy of WA Department of Water)

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high (>2.0 <3.0 mg/L)

extreme (>>4.0 mg/L)


Table 1-2 - Areas of the urban catchments with various mean Tot al Phosphorus and Total Nitrogen concentrations in urban streams

Area (km) % total area

Drains catchment 518

Drains catchment where water quality data are available

405 78%

Low 71 18%

Moderate 247 61%

TP

High 86 21%

Low 222 55%

Moderate 93 23%

TN High 90 22%

1.3. Currently Adopted Water Quality Control Measur es (BMPs) The major water quality control measures in the Perth metropolitan region include end-of-pipe solutions, such as wetlands or gross pollutant traps, and waterways restoration, such as the establishment of “living streams”. These water quality control measures are generally retrofitted. The implementation of BMPs in new developments has been increasing due to legislative requirements for reductions in nutrient export from new urban areas. The BMPs being installed include bio-filtration and swales.

A brief outline of the Swan River Trusts BMPs projects, which are either implemented or in the planning stage, as part of the Drainage Nutrient Intervention Program, are shown in Appendix A (Peter Adkins, SRT, personal communication). These are all retrofit projects; including Liege Street Wetland and Wharf Street Wetland, vegetation of compensation basins, Bickley Road Basin Restoration Project, Anvil Way Basin and Mills and Division Street Works, and one bioretention system; Manley Street.

Community based NRM (natural resource management) groups are also involved in the implementation of BMPs, including those within the sub-catchments described in this report. They include living streams, compensation basin vegetation or wetland conversions and wetland construction in the Bayswater main drain and Bannister Creek sub-catchments. The location of these BMPs can be found in Appendices B and D, respectively (Appendix B, Figure A 8 and in Appendix D Figure A 65).

The addition of a phosphorus adsorbent, Phoslock®, was also trialled at several locations within the Mills Street main drain sub-catchment between 2003 and 2005 (Swan River Trust 2005) (Appendix A). The Water Corporation, in collaboration with GHD, installed a series of permeable reactive barriers (curtains and blankets) in the Mills Street main drain sub-catchment with the initial findings provided in GHD (2007b). A summary of these findings is contained in Appendix A.

There is limited published information on the nutrient reduction effectiveness of BMPs implemented in the Perth Metropolitan area. GHD (2007a) reported the performance of the Liege Street Wetland between November 2004 and February 2007.

• The wetland was designed for water quality treatment during autumn’s stormwater, and particularly the first seasonal ‘flush’, and for summer baseflow. Assessment was based on monthly grab sampling and automated stormwater sampling. GHD (2007a) reports a 27% and 45% reduction in total N and total P loads respectively during the summer baseflow period. ‘Limited’ to ‘no’ water quality improvement was observed during storm events (GHD 2007a). No information was reported for the various species of nitrogen and phosphorus.


• Another study of the Liege Street Wetland, Majimbi (2007), observed a net reduction in TP load equivalent to 0.1 kg d-1 and a net loss of TN load equivalent to 18 kg d-1.

• Majimbi (2007) also studied the Tom Bateman Wetland in the Bannister Creek sub-catchment and observed no TP removal, however, an estimated removal of TN of 42 kg d-1 was reported.

• The permeable reactive barrier trials in the Mills Street main drain catchment had varying degrees of success in reducing nitrogen and phosphorus in groundwater discharge to the drain (GHD 2007b, and Appendix A). Generally, the trials showed reduction for inorganic N and SRP species but not organic nutrients.

Most of published reviews and analysis, including SCWQIP reports and consultancy reports for the Water Corporation, give some estimation of the BMP efficiency in terms of total N and P concentrations. Nutrient speciation is not commonly considered, which leads to uncertainty in evaluation of BMPs’ treatment capacity. Often, the references are given only to concentrations, not to nutrient load variation between inflow to, and outflow from, the treatment facility.


2. PROJECT METHODOLOGY The project was structured around the defined project objectives (Figure 2-1) and included the following activities:

1. Selection of drain catchments for more detailed investigation.

2. Collation of the currently available data.

3. Literature review on BMPs and water quality in urban environments.

4. Analysis of the available spatial and time series data related to the:

a. Hydrological and hydrogeological characteristics of the catchment, and

b. Nutrient speciation and their transfer from the catchments (source) to the urban main drainage.

5. Additional water quality sampling program in the selected catchments.

These activities took place in 2008-2009 and are described below.

2.1. Selection of the Drain Catchments for More Det ailed Investigation

In conjunction with the Water Corporation four catchments were selected for investigation following the agreed selection criteria, which included:

1. A continuum of urban development age and extent to define an effect of urbanisation on nutrient concentration in urban drainage.

2. The entire drain catchment should be within the Swan Coastal Plain with no influence of catchments within the Darling Range, where the rainfall-runoff relationship is significantly different to the sandy and mostly flat urban catchments on the plains.

3. Data should be availability for both water quality and drain discharge.

The following catchments were selected (Figure 2-2):

• Bannister main drain: where urban development is ongoing and long-term water quality data and some discharge data were available from DoW.

• Bayswater main drain: well-established urban catchment with long term water quality and flow data from DoW.

• Forrestdale main drain: where the catchment is largely peri-urban, but is currently undergoing urban development. Water quality and flow data were monitored by CSIRO from 2006 to 2009.

• Mills Street main drain: well-established urban catchment with long-term water quality data from DoW and discharge data from the Water Corporation. The catchment differs from the Bayswater main drain due to its larger proportion of industrial and commercial land use.

For the purpose of this report only Water Corporation controlled drains were included in the study and the terms used in the discussion are based on the ‘main’ drain channel and side drain channels hereafter referred to as ‘branch’ drains. Although this study uses data from the monitoring of the Water Corporation controlled drains, a network of small drains controlled by local government authorities feeds into the Water Corporation network. Thus issues raised in this report regarding BMPs for the control of nutrients applies equally to all surface water drainage networks regardless of who manages the drains.


Figure 2-1 - Project activities structure


BAYSWATER

BANNISTERCREEK

FORRESTDALE

MILLSST

ARMADALE

FREMANTLE

PERTH

Meteorological station

Stream gauging

Coastal waterline

Estuarine waters

Inland waters

Catchments assessed

5 Km

Figure 2-2 - Catchments selected for analysis withi n the project


2.2. Collation of the Currently Available Data In order to characterise the selected catchments, the following data was collected and stored in the CSIRO interactive database (Pollock, 2009). The data types and sources are defined in Table 2-1. The data included:

• Drainage water quality and flow.

• Climate data.

• Land use, both current and historical.

• Water management, including groundwater allocation and wastewater management sewage.

• Drainage network.

• Groundwater levels, quality and other hydrogeological information.

Table 2-1 - Data gathered during the project and da ta sources

Custodian Datasets Department of Industry and Resources 1:50,000 Environmental and Urban Geology Department of Land Information

Building Footprints Cadastral Boundaries High Resolution Digital Imagery (2008) Large-scale Topographic Database (1:2000 Relief) Road Centrelines

Department of Water Geomorphic Wetlands Swan Coastal Plain Groundwater Allocations Groundwater Contours, Superficial Aquifers, Metropolitan Area (May 2003) Groundwater Subareas Hydrogeology (1:250,000 Map Series) Hydrography (Linear) Hydrography (Linear Hierarchy) PRAMS Layers Swan-Canning Landuse Swan-Canning Septic Tanks WIN Sites and Observation Records (surface water and groundwater)

Geoscience Australia Gazetteer of Australia 2005 SILO Point Patched Climate Dataset Water Corporation Water Corporation Billing Subgroups

Compensation Basins Drain Catchments Drainage network Water Corporation Sites and Observation Records (surface water quality and discharge) Information on Reticulated Sewage Infill Program

Swan River Trust Information on Drainage Intervention Program BMP projects


2.3. Analysis of the Available Data A number of methods were used for analysis of the data available for selected catchments.

2.3.1. Spatial analysis

Much of the available information was related to spatial data, which was presented in a series of maps for each catchment (Appendix B to E) developed in the ArcGIS-9.3 environment. Spatial analysis of this data allowed the generation of some additional information on each catchment as described below.

Land cover analysis

A land cover dataset was developed to identify six land cover classes: non-vegetated, trees, grass, roads, rooftops and water. The method utilised high resolution digital imagery for year 2008, road centrelines, and building footprints. Open water bodies were digitised based on the 2008 imagery. The maximum-likelihood classification technique applied to the 2008 digital image was able to distinguish the classes: non-vegetated, trees, and grass. The maximum-likelihood classes were then updated with the road centrelines, building footprints, and water bodies to further discriminate the original non-vegetated class. The resulting land cover dataset was combined with the land use data, to produce a single raster with both land cover and land use information for the studied catchments.

Mapping groundwater tables (summer and winter)

Surfaces of the groundwater table in the Superficial Aquifer were developed for summer (minimum level) and winter (maximum level) 2005. The approach utilised the WIN database, surface water hydrology datasets, and groundwater contours for 2003 (Perth Groundwater Atlas). Minimum and maximum groundwater levels were extracted for superficial bores that were observed more than once during 2005. The two sets of groundwater levels were used to produce the corresponding Triangulated Irregular Network (TIN) surfaces, representing minimum (summer) and maximum (winter) groundwater levels for 2005. In addition to the groundwater level data, the TINs were constrained in the following ways:

• Groundwater level was fixed at zero in the Swan-Canning Estuary.

• Groundwater level was fixed near ground level along the lower ends of the Swan River and Canning River.

• Some control of groundwater level peaks near the eastern edge of the surfaces was based on 2003 groundwater level contours (outside the study catchments).

The resultant TIN surfaces were converted to raster surfaces, and compared to a ground surface to produce corresponding depth to groundwater surfaces for summer and winter.

Method for discerning septic tank zones

Septic tank zones were developed to define areas with a significant number and spread of septic tanks. The septic tank zones were derived from the septic tank point locations. The septic tank points were buffered to a distance of 100 m. The buffer polygons were then aggregated, dissolving gaps less than 10,000 m2 in area and discarding results less than 100,000 m2 in area. The resulting septic tank zones were satisfactory; however some small areas with a high density of septic tanks (approximately 3-6 tanks within a few hundred metres) were excluded by this approach.

2.3.2. Catchment water balance

The catchment water balance within Perth’s urbanised area is greatly influenced by catchment hydrogeological conditions. Both groundwater and surface water modelling were outside the scope of the project and the water balance was estimated based on the available drain discharge data and land/water use data. However, PRAMS (Perth Region Aquifers Modelling System) results became available for this project courtesy of the South West Sustainable Yield project, currently underway in CSIRO. Some components of the water


balance in the selected catchments were derived from this regional model and included groundwater throughflow, groundwater abstraction in accordance with current DoW licences, and estimated groundwater use in domestic garden bores. However, following the water balance analysis undertaken for the selected catchments it appears that the groundwater abstraction from domestic bores is likely to be overestimated by PRAMS.

Hydrograph separation

Baseflow estimation was based on the separation of the river hydrograph to define the flow period where variation between sequential hourly flow measurements was typical for baseflow in each catchment (Figure 2-3). A MATLAB routine was developed and used to estimate baseflow for the observation period in the catchment where discharge data was available.

This data was further used to define the hydrological relevance of samples collected at the gauging station.

Figure 2-3 - An example of hydrograph separation (B ayswater main drain 2007)


Catchment water balance

Catchment annual average water balance was estimated based on a conceptual basis, taking into account land cover types, climate parameters and outcomes of PRAMS modelling undertaken under a number of assumptions. The general components of the catchment water balance include:

abstcationgwEVBFiltrationstromkssepticwthroughflogwRF QQQQQQQQ _inftan__ ++++=++

where: RFQ - rainfall over the catchment area,

througflowgwQ _ - net groundwater flow to the catchment,

kssepticQ tan_ - leakage from septic tanks,

stormQ - stormwater;

iltrationQinf - groundwater recharge,

BFQ - baseflow or groundwater discharge to the drains,

EVQ - evaporative losses, and

abstcationgwQ _ - groundwater abstraction.

The following methods for estimation of the individual water balance components were adopted:

1. Stormwater ( stormQ )

It was assumed that the road area and additional impervious surfaces contribute to the stormwater (peak flow) events when rainfall is in excess of 2 mm. The estimation of additional impervious area contributing to stormwater was based on fitting the annual stormwater discharge, defined by hydrograph separation.

2. Infiltration ( iltrationQinf )

Infiltration was estimated as the amount of water passing through the ground surface towards the groundwater table and was equal to a sum of roof runoff infiltration, losses from irrigation, rainfall infiltration. The infiltration rate is expected to be greater than groundwater recharge since it does not account for variation in soil moisture content and some evapotranspiration losses from unsaturated zone. The following assumptions were introduced:

• Roof runoff is directly infiltrated to groundwater and equal to daily rainfall in excess of 1 mm.

• Irrigation occurs over grass and tree areas defined as:

o 10% irrigation applied to this area is lost to groundwater recharge.

o Irrigation is applied at 40 mm/week over a 31.5 week period.

• During the winter months when rainfall is greater than evaporation, the rainfall is fully infiltrated in the areas of grass and trees and those that are un-vegetated (with the exception of the area of impervious surfaces contributing to the stormwater events defined above).

3. Evaporative losses ( EVQ )

Evaporative losses included; evapotranspiration by deep rooted vegetation, evapotranspiration from the area with shallow groundwater table’ evaporation from open water and the evaporative losses associated with rainfall interception by vegetation.

• Evapotranspiration losses occur in the area of deep rooted vegetation (trees) estimated as 40% FAO56 Potential Evaporation (PEV) (SILO data).

• Evapotranspiration losses occur from the area where shallow groundwater is within 2m of the ground surface. The evapotranspiration rates are equal to PEV at the


surface and decline linearly to 0 at 2m depth. The area is estimated with exclusion of impervious surface, as described in section 2.3.1.

• Evaporative losses from open water applying PEV rates were estimated for the area covered by compensation basins.

2. Groundwater abstraction rates ( abstcationgwQ _ ) and annual average volumes of

groundwater throughflow ( througflowgwQ _ ) were obtained from the data set adopted in the

PRAMS model (Cymod Systems Pty Ltd 2004).

3. The water balance component associated with wastewater disposal from septic tanks ( kssepticQ tan_ ) was estimated under following assumptions:

• Prior to sewage infill a number of domestic septic tanks were in operation within some of the selected catchments (Bayswater and Mills Street Main Drains). They were progressively replaced by a reticulated sewage system between 1995 and 2000. It was assumed that 2.7 occupants per household generate waste water use at a rate of 80 kL yr-1 capita-1.

• Currently, there are a number of properties with septic tanks in each catchment. It was assumed 5 people using these generated 50 kL yr-1 of wastewater (this data is arbitrarily defined).

2.3.3. Drainage water quality

Water quality data was available for outflow from the individual catchment and also for a number of snapshot sampling programs, undertaken by DoW and the Water Corporation. A summary of water quality data availability is given in Table 2-2, which shows that water quality samples are routinely collected by DoW on a weekly or fortnightly basis at the gauging stations in each catchment. As the baseflow component dominates the drainage discharge both in terms of volume and its duration, a regular sampling interval leads to a better characterisation of water quality during baseflow samples than storm flow events (Kirchner et al. 2004).

Available water quality data was analysed in the context of the annual hydrological cycle. The water quality data set was separated and analysed for the period of low baseflow, high baseflow and storm flow. The data separation was based on the hydrograph analysis as illustrated on Figure 2-4 and was undertaken as an add-on function of the database querying capability. To follow up on the previous statement, only 4 out of 22 sampling events shown in Figure 2-3 are to some extent related to stormflow.

Loads, expressed in kg or tonnes, were estimated for each time interval between two sampling events as

21−+

= iii

CCVLoad

where (Ci+Ci-1)/2 - an average concentration between two consequent events, V - total drain discharge. When the result is added for all sampling event over a year, the total annual load was calculated. This analysis was undertaken for all sampling date, which is approximately equal to the total annual load, and also for sampling which were related to baseflow period only. The difference between those two annual values was assumed to characterise an annual load associated with storm events. However due to the fact that the sampling regime was bias towards baseflow periods, the annual total loads and annual load associated with stormwater is likely to be less accurate, than the annual baseflow loads.

In addition to routine sampling at the gauging stations, a number of snapshot water quality surveys were also undertaken by the Water Corporation and DoW.


2.3.4. Water quality sampling program (CSIRO snapsh ot)

In addition to available data analysis, a snapshot water quality sampling program was undertaken in the Bayswater, Mills Street and Bannister catchments. Sampling sites were chosen based on water availability during the sampling period and information determined from historical data. One round of sampling for each catchment was undertaken in April 2009. The following parameters were measured:

• Instantaneous discharge.

• Physicals: pH, electric conductivity (EC), dissolved oxygen (DO) and temperature.

Table 2-2 - Water quality data** availability

At gauging station Snapshot

1988-1996 1996-2008 Drain catchment

Winter only (June - October)

Weekly until 2000, then Fortnightly

2006-2008 DoW WC

Bayswater TP, SRP*, TN,

NH4*, NOx*, TKN^

TP, SRP, TN, TKN, NH4, NOx,

DOC#

In addition to main suit: DON, TOC

2007 & 2008

21 sampling events at 9

points throughout the catchment (2002-2004)

Mills Street TP, SRP*, TN,

NH4*, NOx*, TKN^


DOC#


12 sampling events at 21

points throughout the catchment (2001-2003)

Mills Street (high frequency)

TN, TP (2001-2007)

SRP, NH4, NOx, TKN

(2001-2002)

Bannister TP, SRP*, TN,

NH4*, NOx*, TKN^


DOC#


2008

Forrestdale

TP, TDP, SRP, TN, TKN, NH4,

NOx, TOC, DOC (CSIRO data)

2005 & 2006

** TP = total P, SRP = soluble reactive P, TN = total N, NH4 = ammonium N, NOx = nitrate N + nitrite N, TKN = total Kjeldahl N (TKN is a measure of total organic N plus NH4) * not available for periods between 1990 and 1992 ^ available from 1995 # available from 1997


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

01/2007 03/2007 05/2007 07/2007 10/2007 12/2007

Dis

char

ge (m

3 /sec

)

Sampling

Peak flow

Baseflow Low

Baseflow High

Figure 2-4 - The water quality data separation base d on the hydrograph analysis

• Nutrients

o Total P (TP), total dissolved P (TDP), soluble reactive P (SRP), total N (TN), total dissolved N (TDN), ammonium-N (NH4-N), nitrate-N (NO3-N), nitrite-N (NO2-N):

o Dissolved organic N (DON) was derived from the difference between TDN and (NH4-N + NO3-N +NO2-N).

o Particulate N (PN) was calculated as the difference between TN and TDN.

o Dissolved organic P (DOP) was derived from the difference between TDP and SRP.

o Particulate P (PP) was calculated as the difference between TP and TDP.

o Total organic C (TOC), dissolved organic C (DOC):

o The difference between TOC and DOC gave the particulate organic C (POC).

o Note: aside from TP, TN and TOC all nutrients were measured after filtering through 0.45 µm membrane filters.

• Total suspended solids.

• Boron – commonly used as a groundwater tracer for septic tank effluent.

• Microbiological analysis – measured as groundwater tracer for septic tank effluent:

o Total heterotrophic counts.

o Entrococci faecalis.

o Thermotolerant coliforms.

The water quality data from these snapshots are presented in Appendix B to D.

2.3.5. Data limitation

During data collection, consultation with the project technical working group and data analysis, a number of available data limitations were highlighted. These included:

• Water quality data stored in the WIN database may be affected by the changes in water sampling protocols adopted in the 1990s.

• Lack of groundwater quality data.


• Lack of high resolution surface water quality data related to storm events.

• Quality of flow data in Bannister Greek was not sufficient to be included in analysis.

• No long-term water quality and flow data was available for Forrestdale main drain.

These limitations are considered during the data analysis (such as in a case of the DoW surface water quality data were used) or were overcome by employing the alternative methods of analysis (such as generation of data set solely related to storm events adopting baseflow separation techniques described in 2.3.2).

2.4. Literature Review on BMPs and Water Quality in Urban Environment

Over 120 references, both journal papers and reports, and including overview papers and reports, were reviewed to define the current knowledge of water quality BMPs both in Australia and worldwide. The focus of the review of published data was:

• Nutrient sources in urban environments and relevant nutrient speciation.

• Natural processes influencing nutrient attenuation (or enrichment).

• The effectiveness of BMPs for various nutrient species at various catchment scales.

The results are presented in the Part 2 of this report.


3. NUTRIENTS SOURCES, PATHWAYS AND ATTENUATION PROCESSES IN THE URBAN ENVIRONMENT AND BEST MANAGEMENT PRACTICES FOR NUTRIENT REMEDIATION

To determine the most appropriate best management practice (BMP) for in-situ water quality control in a particular catchment the nutrient sources and their forms (dissolved, particulate, inorganic and organic), nutrient pathways in the environment, and possible transformations along these pathways need to be investigated first. A literature review and discussion of current understanding of these processes and their links with the various types of BMPs and the nutrient attenuation mechanisms of the BMPs are given in the Part 2 of this report, with a brief summary provided here.

The main sources of nutrients in an urban environment are wet and dry atmospheric deposition (e.g. rain and dry fallout), inorganic and organic fertiliser application to urban gardens and public open spaces, vegetation decomposition and the in-situ disposal of wastewater (Figure 3-1). Rainwater conveys nutrients from the atmosphere, hard surfaces and vegetation to the urban main drains within surface runoff, the unsaturated zone and groundwater pathways.

Natural nutrient transformation processes convert nitrogen (N) and phosphorus (P) species from one form to another, and attenuation or enrichment of water with nutrients takes place along the pathways. In the context of this document, nutrient ‘speciation’ refers to the chemical forms which make up the total N and total P pools. The major N species present in the urban environment include ammonium (NH4), nitrate (NO3), dissolved organic N (DON) and particulate N (PN). Nitrite (NO2) may also be present though generally only in minor amounts in surface water. Dissolved inorganic N is the sum of NH4, NO2 and NO3. The major phosphorus species present are phosphate (PO4), dissolved organic P (DOP) and particulate P (PP). It is not possible to directly measure only free phosphate and as such soluble reactive phosphate (SRP) is commonly used as a measure of PO4.

The nutrient transformation processes in a catchment are complex, dependent on scale and time, and are influenced by hydrological, hydrogeological and hydrochemical processes as well as biological activity (Figure 3-2 and Figure 3-3).

A successful in-situ water quality control measure should be linked to an understanding of nutrient transformation processes, and BMPs can be selected and designed to intercept the nutrients of concern on the most appropriate pathway to influence overall nutrient export to the receiving waters. Water quality BMPs commonly utilise these natural transformation processes to create conditions which favour the attenuation of nutrients to reduce or delay nutrient exports.

Sources Targets

Urban environmentRainfallDry depositionFertilisersPlants & animalsSeptic tanks

Pathways

Unsaturated zoneGroundwaterSurface water

Urban Main DrainsSwan-Canning Estuary

Natural Attenuationor Enrichment

e.g. unsaturated zonein-stream processes

Sources Targets

Urban environmentRainfallDry depositionFertilisersPlants & animalsSeptic tanks

Pathways


Pathways


Urban Main DrainsSwan-Canning Estuary

Natural Attenuationor Enrichment

e.g. unsaturated zonein-stream processes


Figure 3-1 – Nutrient source, pathway and receiving water continuum

Figure 3-2 – The nitrogen cycle (Environment Canada 2001)

Figure 3-3 – The phosphorus cycle (Environment Cana da 2001)


Until recently, stormwater management practices in urban areas internationally have primarily dealt with issues concerning flood control and attenuation of peak stormwater discharges (Roy et al. 2008). In Perth, urban drainage systems have also traditionally been built for control of the shallow groundwater table (Department of Environment 2004). The importance of BMPs for water quality control in urban environments has become more prominent in the last few decades and BMPs incorporating both quantity and quality control have been collectively known as Water Sensitive Urban Design (WSUD) in Australia, Low Impact Development (LID) in the United States and Sustainable Urban Drainage Systems (SUDS) in the United Kingdom (Revitt et al. 2003; Roy et al. 2008). BMPs are commonly designed for the treatment of a range of pollutants found in the urban environment such as particulates, nutrients, heavy metals and trace organic pollutants.

There are two broad types of BMPs: structural and non-structural. Structural BMPs are defined as engineered and constructed systems that allow in-situ water quality improvement (US EPA 1999). Non-structural BMPs comprise institutional and pollution prevention strategies to preclude or minimise the transport of pollutants in stormwater runoff and/or reduce the volume of runoff generated (Taylor and Wong 2002a; US EPA 1999).

Structural BMPs are designed to enhance the natural processes of the nutrient cycle by employing systems based on physical, chemical or biological attenuation of nutrients. The successful selection of structural BMPs appropriate for an urban catchment relies on the understanding of catchment-specific issues (Revitt et al. 2003; US EPA 1999), including water quality parameters of concern, the landscape, hydrology and hydrogeology of the catchment, BMP maintenance requirements, land availability and construction and maintenance costs.

Structural BMPs can be classified into seven main categories:

• Infiltration systems.

• Detention systems.

• Retention systems.

• Filtration systems.

• Constructed wetlands.

• Vegetated systems.

• Permeable reactive barriers.

Non-structural BMPs are defined as institutional and pollution prevention strategies to prevent or minimise pollutants from entering stormwater runoff and/or reduce the volume of generated runoff (Taylor and Wong 2002a; US EPA 1999). This definition needs to be extended in areas of shallow groundwater and sandy soils as found on the Swan Coastal Plain to include strategies which prevent the leaching of nutrients to the shallow groundwater.

Taylor and Wong (2002a; 2002b; 2002c) found that the disadvantages of non-structural BMP implementation was found to lie in the difficultly in assessing their performance, especially when related to changes in people’s behaviour, the impact on stormwater quality and receiving water health. Unlike structural BMPs, the effectiveness of non-structural BMPs is difficult to measure as many programs they are applied over large areas with no definitive point in the landscape where they can be monitored.

Natural processes of nitrogen and phosphorus transformation can greatly influence the nutrient concentration and speciation in urban drains, which are often utilised for in-situ water quality control systems.

Structural BMPs allows water treatment largely through the re-creation of conditions which enhance one or more processes of natural nutrient transformation and attenuation. A summary of the processes employed in structural BMPs for reduction in nutrient concentrations are given in Table 3-1, showing that vegetated BMPs allow for a wider variety


of processes for nutrient transformation and removal, particularly for treatment of dissolved forms such as SRP, nitrate and ammonium. However, no individual structural BMP can provide the conditions for attenuation or removal of all nitrogen and phosphorus species.

Often different conditions must exist for attenuation of various nutrient forms to take place. For example, in wetlands, anoxic conditions are required for denitrification, but oxic conditions are needed for phosphate. Of particular note is that there is currently limited information related to the effectiveness of existing BMPs in terms of organic nitrogen and phosphorus attenuation.

Non-structural BMPs also play an important role in reducing nutrients load on surface water by better managing the nutrient input in urban environments, and influencing urban planning and people’s attitudes towards land and water management, including fertiliser use. Although the effects of non-structural BMPs are difficult to quantify, they are applicable at catchments or on a city-wide scale.

When a whole-catchment approach is adopted for water quality control in urban drains, the efficiency and effectiveness of BMPs may be enhanced. The BMPs may be established at various scales and they can control water quality at various hydrological stages of urban runoff (Figure 3-4). A whole-catchment approach also allows a better assessment of the efficiency of BMPs when they are designed to control water quality either during storm flow or baseflow.

Table 3-1 – Structural BMP nutrient attenuation pro cesses

BMP ID* BMP type Water quality attenuation

processes

1 Infiltration (discharge to groundwater) Sedimentation Filtration

2 Retention and Detention Sedimentation

3 Filtration (discharge to surface water)

Filtration Adsorption Ion exchange Biological transformation

4 Swales and Filter Strips Sedimentation (surface flow) Filtration (infiltration)

5 Bioretention

Biological uptake Filtration Adsorption Ion exchange Denitrification (Biological transformation)

6 Stream Restoration

Biological uptake Denitrification Filtration Sedimentation (Biological transformation)

7

Vegetated

Constructed Wetlands

Biological uptake Denitrification Adsorption Sedimentation (Biological transformation)

8 Permeable Reactive Barriers Denitrification Adsorption Precipitation


Figure 3-4 - Options for BMPs and their application at catchment areas to control water quality during various hydrological stages


4. CATCHMENT CHARACTERISTICS AND CLIMATE Four main drains catchments were the focus of the current investigations. Details of the analysis undertaken are given in Appendix B-D. A summary of the results is discussed in this section.

4.1. Climate Climate has a significant, but indirect, effect on water quality in urban drainage. Among other factors, rainfall intensity influences stormwater yields, and annual rainfall defines the total annual discharge, which is a dominant factor in the estimation of annual nutrients. Additionally, seasonal temperature influences the biological activities in the water environment, particularly in a Mediterranean climate where mild winter temperatures promote biological activities in the winter months.

Due to the geographical positions of the catchments, data from several meteorological stations was used for rainfall-runoff analysis; Perth Metro data for the Bayswater main drain, Perth Airport data for the Mill Street main drain and Jandakot data for the Bannister and Forrestdale main drains. The records for both Perth Metro and Perth Airport stations show similar rainfall pattern over the period 1940 to 2008, while at Jandakot a recent downward trend in rainfall is less pronounced and there was a period in late 1980s when annual rainfall increased (Figure 4-1).

Annual potential evaporation exceeds annual rainfall (Figure 4-2). However, monthly data (Figure 4-2) shows that rainfall is greater than potential evaporation in May to August. Average maximum winter temperature is above 18oC, and for all three stations is greater during the period 1989 to 2008 than the long-term average (Table 4-1).

Table 4-1 - Rainfall, potential evaporation and win ter temperature at three meteorological stations

Period Perth Metro Perth Airport Jandakot

1940-2008 817 781 840 Annual average rainfall, mm 1989-2008 769 727 806

1940-2008 1836 1951 1778 Annual average potential evaporation*, mm 1989-2008 1886 2054 1818

1940-2008 18.1 18.4 18.1 Average maximum winter temperature (Jun, Jul, Aug), oC 1989-2008 18.6 18.7 18.3

* Measured PEV data is only available from 1970’s, average PEV for 1940-2008 is only indicative

4.2. Land Use Catchment land use has a significant influence on water quality in the catchment, particularly in terms of nutrient input. Predominant land use varies in the studied catchments:

• Residential area in Bayswater catchment (46%, see Figure A3 and Figure A4 in Appendix B).

• Industrial area in Mills Street catchment (42%, see Figure A32 and Figure A33 in Appendix C).

• Agricultural area in Forrestdale main drain catchment (78% in the ‘rural’ and ‘unused’ categories, see Figure A81 and Figure A82 in Appendix E).


(a) Perth Airport

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

Ann

ual R

ainf

all (

mm

)

400

600

800

1000

1200

1400Annual rainfall10-year moving average

(b) Jandakot Aero

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

Ann

ual R

ainf

all (

mm

)

400

600

800

1000

1200

1400

(c) Perth Metro

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

2005

2010

Ann

ual R

ainf

all (

mm

)

400

600

800

1000

1200

1400

Figure 4-1 - Annual rainfall for three meteorologic al stations (Perth Airport, Jandakot, Perth Metro) from 1940 to 2008


(a) Perth Airport

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Rai

nfal

l (m

m)

Pot

entia

l eva

pora

tion

(mm

)

0

50

100

150

200

250

300

350

(b) Jandakot Aero


Rai

nfal

l (m

m)

Pot

entia

l eva

pora

tion

(mm

)

0

50

100

150

200

250

300

350

RainfallPan Evaporation

(c) Perth Metro

Month


Rai

nfal

l (m

m)

Pot

entia

l eva

pora

tion

(mm

)

0

50

100

150

200

250

300

350

Figure 4-2 - Monthly average rainfall and potential evaporation for the three stations from 1975.

In all catchments, 22 to 25% of the catchment area is covered by irrigated grass or woody vegetation, except for the Mills Street catchment (16%). In all urbanised catchments, 20 to 25% of the catchment area is related to various transport uses, with roads comprising 6 to 7%. These values are much lower for the peri-urban Forrestdale main drain catchment.

The history of urban development is illustrated by Figure 4-3, which shows the increased roof areas as buildings have been established in these catchments over the years. Urbanisation began in the late 1950s in the Bayswater catchment, in the mid-1960s in the Mills Street catchment and in late the 1970s in the Bannister main drain catchment, where further


residential development is taking place. The spatial distribution of buildings and the period of their construction in the four studied catchments are also shown on Figure A5, Figure A34, Figure A62 and Figure A83 in Appendixes B-E.

Some catchment activities may be associated with point sources of nutrients. The CSBP site in the Bayswater main drain catchment and the milk processing plant in the Mills Street catchment are examples of such sources. They can significantly influence nutrient fluxes, as will be discussed in the following chapter.

1940

1950

1960

1970

1980

1990

2000

2010

Roo

f are

a as

pec

ent o

f tot

al c

atch

men

t are

a

0

5

10

15

20

25

Bayswater MDMills St MDBannister CreekForrestdale MD

Figure 4-3 – Catchment urban development indicated by the increase in roof area over time

4.3. Drainage Drainage networks in all catchments were significantly modified during catchment urbanisation (such as in the Bayswater and Mill’s Street main drains), and in agricultural catchments to control water logging (Forrestdale main drain). Drainage density, expressed in terms of drain length per unit area (m km-2), is greater in the Mills Street main drain (Figure 4-4). In all three urban catchments, the proportion of piped drains is greater than open drains, and this is likely to influence water quality due to the absence of vegetation and light within the piped sections.

Compensation basins, designed to control storm peak flow, cover 1.5% of the Mills Street main drain catchment (Figure 4-4). These retention basins also influence water quality in the drains, as discussed in the following chapter.

Three of the studied drains discharge directly into the Swan-Canning Estuary (Bayswater main drain – Swan River, Mills Street and Bannister Creek – Canning River), while the Forrestdale main drain is a tributary of the Southern River, which eventually discharges into the Canning River (Figure 4-5).


(a)

BA

NN

IST

ER

BA

YS

WA

TE

R

FO

RR

ES

TD

ALE

MIL

LS S

T

Dra

inag

e de

nsity

(m

km

-2)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Open drainPiped drainPressure drain

(b)

BA

NN

IST

ER

BA

YS

WA

TE

R

FO

RR

ES

TD

ALE

MIL

LS S

T

Per

cent

of c

atch

men

t are

a

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Figure 4-4 - Drain density (a) and the area of comp ensation basins (b)

Figure 4-5 – Studied catchment locations and the gr oundwater table (vertical scale is exaggerated, red dots indicate location of bores wh ere observation data was available for groundwater table interpretation)

Bayswater MD

Mills St MD

Bannister Creek

Forrestdale MD


4.4. Water Balance Table The catchment water regime has a significant influence on water quality in the drains, defining nutrients pathways from the catchment to surface water network and water residence time. The summary of the catchments’ water balance is given in Table 4-2, while details are given in the Appendix B-E (see Table A 1, Table A 5, Table A 9 and Table A 12).

Table 4-2 Summary of the catchment water balances

Bayswater Mills Street Bannister Forrestdale

Rainfall total, GL 20.9 8.9 15.7 75.8

total, GL 2.19 1.9 1.8* 1.1

% rainfall 11% 21% 12% 1.5% Storm runoff % annual discharge 24% 40% 50%

total, GL 12 4 7 48 Infiltration

% rainfall 55% 46% 47% 63%

total, GL 7.1 2.8 n/a 1.1

% rainfall 34% 32% 1.5% Baseflow % annual discharge 76% 60% 50%

Underlined by thick quaternary sand deposits (up to 60m), all drains receive groundwater discharge which comprises a significant potion of the drain annual flow. This highlights the importance of groundwater in the catchment water balance and its effect on drain water quality. Based on the catchment’s positions within the regional hydrogeological system, it is expected that the Bayswater main drain and Bannister Creek receive regional groundwater contributions from recharge zones at the Gnangara Mound and Jandakot Mound respectively. Regional groundwater discharge rates are likely to be semi-constant throughout the year and have the most significant effect on water quality during summer baseflow.

Groundwater discharge to Mills Street and Forrestdale main drains is mainly associated with the local shallow groundwater intercepted by the drainage networks. This process also takes place in the upper sub-catchments of the Bayswater main drain and Bannister Creek catchments during the winter months, when the rising groundwater table reaches the bases of the drains. The effect of the shallow groundwater on water quality in drains within those catchments is most significant during winter baseflow.

One of the specifics of the water regime in the urban catchments, which differentiates them from peri-urban catchments, is the existence of the summer baseflow. This does not occur in the Forrestdale main drain. Here, the flow is limited to the wet winter period (from June to September), with some baseflow observed in spring (until December).

Stormwater volumetric runoff is greater in the Mills Street main drain where the proportion of impervious surfaces is larger than elsewhere. In older urban areas, the runoff from roads is directed to the drains. The storm runoff analysis, based on the assumption that 99% runoff is common over hard surfaces, suggests that 28% of the Mills Street catchment consists of impervious surfaces directly connected to the drainage network (for comparison, this value is 13% in the Bayswater main drain).


5. DEFINITION OF THE RELATIONSHIP BETWEEN WATER QUALITY AND CATCHMENT CHARACTERISTICS

Water quality and its temporal and seasonal variability in each catchment are discussed in the appendix B-E, and only a summary of results is presented in this section. Long-term water quality data was not available for the Forrestdale main drain.

It was found that water quality characteristics change seasonally and between the catchments, and that there were reductions in nutrient concentrations and changes in their speciation from 2000 to the present day.

Overall, it was noticed that P and N concentrations in branch drains are much greater than in main drains. In the areas outside of point sources contributions, the Nitrogen pool during summer baseflow is dominated by organic N forms, while in the winter baseflow, the inorganic N increases up to 50% of the total nitrogen load. During storm flow events DON and particulate N comprise the larger portion of total N. Phosphorus is largely present in particulate forms, but the SRP proportion increases between 5 and 10% in Bayswater, between 30 and 40% in the Mills Street main drain, and between 40 and 60% in Bannister Creek. SRP can be as high as 90% during higher baseflow in the Forrestdale main drain or less than 30% during low baseflow.

For the three urban drains, the available data2 suggested that a reduction in nutrient concentrations has occurred since 2000, which may be indicative of:

• The installation of reticulated sewage systems undertaken by the Water Corporation in late 1990s to early 2000s. This is indicated by total P and N concentration, and also changes in speciation such as reduction in Nitrate concentration at the outflow from the Bayswater main drain catchment or a significant reduction in Ammonium concentration, particularly in summer, in the Mills Street main drain.

• An overall increase in water residence time due to reduced rainfall and therefore discharge, which can lead to a reduction in inorganic nitrogen forms.

• The remediation of point pollution sources, such as the CSPB site in the case of the Bayswater main drain catchment. This is indicated by a reduction in ammonium as observed at the outflow.

Overall nutrient concentrations in the undeveloped catchment (Forrestdale main drain) are higher, with organic N forms and SRP dominating the nutrient pool. A summary of the annual nutrient load in 2007 for the studied catchments during baseflow is given in Tables 6-1 – 6-4.

The analysis undertaken allowed the identification of the main factors which influence water quality in urban drainage in the Perth metropolitan area. These included:

• Nutrient inputs to the urban environment.

• The development stage defining the release of legacy nutrients.

• Hydrogeological conditions.

• Hydrological conditions.

• In-stream processes and residence time prior to discharge to the river.

• Seasonality.

These factors are explored in greater detail in the tables below.

2 See comments regarding quality control of the collected water quality data in Section 2.3.5


Table 5-1 – Indicative sources and speciation of ba seflow nitrogen export from the Bayswater main drain catchment (baseflow* as 79% annual load in 2007 as 7.6 t-N)

Annual N speciation load as precent of total load

High Baseflow

TN load 5.1 t (65%

annual)

DON 38%

NO3 32%

NH4 25%

PN 5%

Shallow groundwater in residential area intercepted by drains or subsurface drains in the

catchment upstream from the industrial zone Indicative sources

In-stream processes

Groundwater affected by the industrial zone

CSBP

In-stream processes

Low baseflow

TN load 1.1 t (14%

annual)

NH4 36%

DON 33%

NO3 23%

PN 8%

Indicative sources

CSBP site

Regional groundwater discharge in the lower catchment,

potentially affected by the industrial zone;

In-stream processes

In-stream processes

* N load during peak flow is expected to be mainly related to particulate and organic N forms ** TP 95% PP

Table 5-2 – Indicative sources and speciation of ba seflow nitrogen export from the Mills Street main drain catchment (baseflow* as 45% annual load in 2007 as 3.8 t N)

Annual N speciation load as precent of total load

High Baseflow

TN load 0.8 t (21%

annual)

DON 37%

NO3 32%

NH4 18%

PN 13%

Shallow groundwater in the industrial zone, affected by septic tanks, and residential area,

intercepted by drains or subsurface drains Indicative sources

In-stream processes

Milk factory? Hospital?

In-stream processes

Intermediate baseflow

TN load 0.6 t (16%

annual)

DON 53%

NO3 21%

NH4 13%

PN 13%

Shallow groundwater in the industrial zone, affected by septic tanks, and residential area,

intercepted by drains or subsurface drains Indicative sources

In-stream processes

Milk factory?

Hospital?

In-stream processes

Low baseflow

TN load 0.2 t (5% annual)

DON 64%

PN 33%

NO3 2%

NH4 1%

Indicative sources

In-stream processes

* N load during peak flow is expected to be mainly related to particulate and organic N forms


Table 5-3 – Indicative sources and speciation of ba seflow phosphorus export from the Forrestdale main drain catchment (baseflow* as 42% annual load in 2007 as 0.36 t P)

Annual P speciation load as precent of total load

High Baseflow

TP load 0.14 t (38%

annual)

PP 63%

SRP 37%

Indicative sources

Groundwater in proximity to of the drain or subsurface drains and septic tanks in residential area

In-stream processes

Low baseflow

TP load 0.02 t (4%

annual)

PP 72%

SRP 28%

Indicative sources

Groundwater in proximity to of the drain or subsurface drains and septic tanks in residential area

In-stream processes

Table 5-4 - Indicative sources and speciation of ba seflow nitrogen export from the Bannister Creek catchment

N speciation

High Baseflow

DON 45%

NO3 40%

NH4 10%

PN 5%

Groundwater in proximity to of the drain or subsurface drains and septic tanks in residential

and industrial areas Indicative sources In-stream

processes

In-stream processes

Low baseflow

DON 72%

PN 24%

NO3 2%

NH4 2%

Indicative sources

In-stream

N load during peak flow is expected to be mainly related to particulate and organic N forms

5.1. Nutrient Inputs in the Urban Environment Nutrient inputs in Perth’s urbanised catchments are related to atmospheric deposition, fertiliser application, wastewater management, and in some catchments, site-specific point sources.

Atmospheric deposition

According to the review of published data undertaken, nutrient concentrations in urban runoff, which associated only with the atmospheric deposition of nutrients in urban environments (such as roof runoff, Table 2.1 in Part 2 of this report), can be compatible with currently adopted water quality targets for urban land use in Swan Canning catchment (Swan River Trust 2009). Both particulate and dissolved nutrient species can be associated with atmospheric inputs. The most direct effect of this nutrient source, particularly Nitrogen, on water quality in drains in Perth is associated with road runoff, which has not been well characterised in terms of water quality. However, as shown in Table A 1, Table A 5 and Table A 9, the groundwater recharge associated with roof runoff may exceed the volume of groundwater recharge from the pervious areas of gardens and public open space (POS), and as such may significantly influence groundwater quality.


Fertiliser application

In a survey of Perth households by DoW, it was found that the application of fertilisers in residential areas is comparable to that of agricultural use. Both organic and inorganic fertilisers are used in Perth, which may lead to the leaching of excess organic and inorganic nutrients to groundwater. However, there is limited published data related to the rates of nutrient leaching from soils to groundwater water. The available data indicates that P leachate is limited; while approximately 50% of N applied in fertilisers may be detected at 1m below ground level. The use of iron-rich groundwater for irrigation of POS and domestic gardens increases the retention of P in soil as a result of P adsorption on iron oxide particles formed in the presence of atmospheric oxygen.

Wastewater management

Disposal of septic tank effluent generates the highest input concentrations of NH4 and SRP, at an order of magnitude greater than leachate from other sources. P concentration may be 1.8 to 29 mg L-1 as SRP, while N may reach 350 mg L-1 as NH4 (see Table 2.4 in Part 2 of this report). It was observed that the installation of reticulated sewage systems in 1994 to 2000 coincided with water quality improvement in the Bayswater and Mills Street main drains.

Point source contributions

According to the analysis undertaken, point sources may have significant impacts on water quality in urban drains.

• The CSBP (Bayswater main drain) site is located within 1 km of the confluence with the Swan River and 500m upstream from the monitoring site at the Bayswater gauging station. This site currently represents a large source of NH4, contributing approximately 25% of the annual catchment TN load. However, discharge from this site also influences the attenuation of some nutrient forms. Based on the available information, groundwater discharge from the site is likely to contain dissolved iron (Fe) and aluminium (Al). The Fe and Al form precipitates Fe/Al oxides in the drain, which appear as fine sediments (Figure 5-1). In the drain reaches affected by discharge from CSBP, SRP, DON and DOC concentrations decrease, presumably as a result of adsorption on the freshly precipitated oxides.

• Based on available data for 2001 and 2002, the Masters milk processing plant area was also indicated as a potential source of the high ammonium concentration in one of the northern tributaries of the Mill Street main drain, which discharges to the main drain at 6163043 (Figure A 57). Discharge from this drain provides the main contribution of ammonium, comprising 15% of the annual catchment N load.

The effect of a point source on water quality in the outflow from a catchment is also dependent on its location within the catchment. For instance, an unidentified source of high nutrient concentrations in the upper part of the Mills Street main drain catchment appears to have no impact on the water quality in the lower reaches of the drain.


Figure 5-1 - Sediments observed in Bayswater main d rain in the (a) western and eastern branches (upstream [b] and downstream [c] from the outfall from the CSBP site), (d) at the confluence and (e) in the main channel downstream f rom the gauging station. These show the level of suspended materials contributed by the CSB P site. They are evident along the drain channel up to the confluence with Swan River.

(a)

(b)

(d)

(c)

(e)


5.2. Development Stage: Legacy Nutrients As mentioned above, Perth urban catchments are mainly underlain by thick, sandy deposits. Predevelopment conditions in areas where urban drainage is required are characterised by shallow groundwater tables and frequently, inundation. In undeveloped catchments, evaporation and evapotranspiration are the main influences on water balance, affecting up to 97% of rainfall (as was estimated for the Forrestdale main drain catchment).

These factors, as well as low hydraulic gradients, lead to the accumulation of nutrients and other solutes in shallow groundwater. High residence time and vegetation access to shallow groundwater are likely to be reasons for inorganic nutrient depletion, but organic nutrient concentrations are particularly high in drains. This is the case in the Forrestdale main drain, where N, P and organic carbon concentrations are greater than in other three catchments studied.

During urbanisation, the introduction of drainage networks, particularly sub-surface drainage, greatly increases the groundwater discharge to the drainage network, subsequently causing ’flushing‘ of legacy nutrients. For instance, in Bannister Creek where the catchment is only partly urbanised, the DOC concentration has gradually decreased between 2001 and 2007 (Figure 5-2). Since it is expected that the effect of new urban development in this catchment on the drain water quality is still in a transient state, the downward trend in DOC concentration with time, shown on Figure 5-2, may be interpreted as ’legacy DOC flushing’.

In well-established urban catchments such as Mills Street and Bayswater main drains the effect of the urban development is likely to have reached equilibrium, and DOC concentrations are the lowest of the four drains studied.

Roof Area:Catchment Area (%)

0 5 10 15 20 25Ann

ual m

ean

DO

C c

once

ntra

tion

(mg

L-1)

0

10

20

30

40

50

60

70

BayswaterMills StBannisterForrestdale

Figure 5-2 – Changes in dissolved organic carbon (D OC) with catchment urban development

2001

2007


5.3. Hydrogeological Conditions Groundwater plays a significant role in nutrient transfer from soils to urban drains. Baseflow, as an indicator of the groundwater discharge to urban drains, varies between 40 and 80% of annual discharge in the studied catchments. The baseflow is highest in the Bayswater main drain where the groundwater catchment, which extends further north to the Gnangara Mound, is much greater than the surface water catchment. Baseflow also contributes up to 80% of the annual nutrient load. However, nutrient speciation and their concentrations in baseflow vary seasonally.

It appears that the groundwater systems play a significant role in nutrient attenuation. The unsaturated zone provides a media where attenuation processes takes place in the presence of oxygen and interaction of solute with the soil minerals:

• The thickness of the unsaturated zone between the source and groundwater table influences the transformation of ammonium (N as NH4) to nitrate (N as NO3) as a result of nitrification. This process is a likely to occur in the Bayswater main drain catchment, where groundwater discharge to the drain is affected by the high density of septic tanks within the industrial zone. The deeper groundwater occurrence in this area is likely to support the nitrification of ammonium leaching from the septic tanks. This leads to increased nitrate concentrations in the groundwater which was indirectly confirmed by nitrate enrichment the drains in this area.

• The ammonium (N as NH4) leachate in the areas of shallow groundwater occurrence may not undergo nitrification, and in these areas high ammonium concentration may be detected in groundwater. In an anoxic groundwater environment, NH4 is largely stable, and NH4 can be detected in the surface water receiving groundwater discharge. This is likely the case in the area down gradient from the Masters milk processing plant in the Mills Street main drain catchment, where ammonium concentration in bore 4790 (WIN Id) reaches up to 6.2 mg L-1.

• Replacement of septic tanks, the main source of ammonium, with reticulated sewage is likely to significantly reduce ammonium concentration in both groundwater and receiving drains. As was observed in the Mills Street drain in the 2000s, ammonium concentration reduced from on average 1 mg L-1 prior to 2000 to 0.3 mg L-1 post-2000. This is particularly evident during the summer months (see Figure A 44 and Figure A 47 in Appendix C). During these periods, the water quality in the drain at the gauging station is mainly related to the groundwater contribution in the lower sub-catchment, where the majority of septic tanks were removed.

• DOC (and potentially TON) can be reduced due to the faster DOC decomposition rates under oxic conditions in the unsaturated zone. A thicker unsaturated zone provides longer residence times for soil leachate under oxic conditions and as a result DOC (and TON) concentrations are likely to be lower in the areas with deeper groundwater occurrence. This was evident in the Bayswater main drain, where DOC concentrations are greater during high baseflow, mainly associated with shallow groundwater discharge in the upper residential sub-catchment (Figure 5-3).

• P adsorption in soils is strongly related to the soil mineralogy. P adsorption in Perth’s sandy soils is low compared to soils with higher clay and silt contents. Soils of the younger Spearwood and Quindalup dune systems contain high levels of carbonate and iron minerals. They tend to have higher P adsorption than the older soils of the Bassendean dune system which has low or no carbonate and iron minerals.


1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

Bas

eflo

w (

m3 s

-1)

0.0

0.2

0.4

0.6

0.8

1.0

DO

C (

mg

L-1)

0

5

10

15

20

25

30

BaseFlow DOC

Figure 5-3 - Baseflow rates and seasonal variation in DOC concentration

Nutrients attenuation may also occur in saturated zone (aquifer).

• In the presence of organic carbon, which is typical in Perth aquifers, nitrate (N as NO3) is largely removed from groundwater due to the process of denitrification, providing that the residence time groundwater permits the completion of this process. As a result, and in accordance with available data on groundwater quality in Bayswater, nitrate concentrations are low in areas which are not influenced by septic tanks.

• However, in groundwater discharge areas or during the high baseflow period where shallow groundwater is intercepted by sub-surface drains, the low groundwater residence time prohibits denitrification. As a result, the nitrate (N as NO3) concentration is greater during winter baseflow in all studied drains. Overall, 25 to 30% of annual N load in these drains is nitrates, with the majority related to the winter months.

• Denitrification also appears to be limited in the groundwater down gradient from the Bayswater industrial zone, where groundwater discharge to the drain causes an increase in nitrate concentration. Here, the limitation to denitrification may be due the higher nitrate input and the proximity of the source to the receiving drain, which reduces the residence time of leachate in the groundwater.

• The hydrogeochemical (anoxic) conditions in aquifers do not allow significant attenuation of the DON, DOC and ammonium (N as NH4). Some DOC is utilised during denitrification, which requires a 5:4 ratio of C to NO3. In conditions of high nitrate input, such as in the area of the industrial zone in Bayswater, the potential denitrification demand for high DOC concentrations may be the reason for lower DOC in this drain than elsewhere. Overall, it is expected that groundwater may potentially be the major source of organic nutrient species in drainage, however, this statement requires further clarification.

5.4. In-Stream Processes In Perth’s climatic conditions, it is expected that a high level of biological and biochemical activity takes place in the water environment all year round. During summer the presence of water in drains, even during the late hot summer and autumn, allow significant plant and algae growth. In Perth’s mild winters, when maximum daily temperature is close to 19oC (see Table 4-1), the conditions are still suitable for biological activity. As a result, it is expected


that by-products of such activities can influence water quality. Plants and algae can be a sink for nutrients entering the surface water network as they require nutrients (both N and P) for growth, but without proper maintenance (such as plant removal) they can also become a nutrient source when decayed.

Biological activity in the drainage network provides significant attenuation potential for inorganic nutrient forms, particularly ammonium, nitrate and phosphate. As shown on Figure 5-4, high SRP concentrations in the sources and headwaters are reduced to the much lower concentrations within the main drain channel.

Ammonium is generally the preferred N form for uptake by plants (Nasholm et al. 2009), thus may account for the observed reduction in NH4 observed (Figure 5-5). As described above, it is also unstable in the presence of oxygen, when as a result of the nitrification process, ammonium is converted to nitrate (see equations 3 and 4). Rapid reduction in ammonium concentrations is illustrated by Figure 5-5, which shows a reduction in NH4 concentration at the average rate of 15 g/m of drain length.

source branch main

TP

(m

g L-1

)

0

1

2

3

4

5

6

Figure 5-4 – TP concentrations in leachate from sou rces (such as septic tanks or soil as shown in Table 2.3 and Table 2.4 in Part 2 of this report ), branch drains and main drains, indicating high level of TP attenuation in Mills Street main d rain

Distance from the point source (m)

0 200 400 600 800 1000 1200 1400 1600 1800

NH

4 lo

ad (

kg d

ay-1

)

0

2

4

6

8

10

12

14

16

18

20

Figure 5-5 – Ammonium attenuation in the Mills Stre et main drain channel


During low summer flow, anoxic conditions promote nitrate attenuation within the compensation basins and wetlands due to denitrification in the presence of organic carbon. This is commonly abundant in Perth’s surface water, or derived from sediments. Figure 5-6 (a) shows the changes in surface water N concentration between the inflow and outflow for a compensation basin in the Mills St catchment (see Figure A 57). However, it appears that the same basin causes enrichment in TON concentration in surface water.

Attenuation of both organic carbon and organic nitrogen is limited within the in-stream environment. Accumulation of organic matter in the surface water network, particularly in compensation basins and wetlands, is likely and may potentially become a nutrient source in the future (Figure 5-7).

(a)

in out

NO

3 (m

g L-1

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4(b)

in out

DO

N+P

N (

mg

L-1)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Figure 5-6 – Nutrient attenuation effect of a Mills Street main drain compensation basin (a) nitrate (NO 3) and (b) dissolved organic nitrogen plus particula te N (DON+PN)

Figure 5-7 – In-stream processes – organic matter b uild-up in Brown’s Lake wetland (9 April 2009)


It was also observed that there is a relationship between SRP concentration and N:P ratio (N as inorganic N and P as SRP) in surface water, as shown on Figure 5-8. It appears that in surface water the higher SRP concentrations are more common when the N:P ratio is low, and conversely, SRP concentrations are lower for high N:P ratio. This potentially indicates that in-stream plant or algae P uptake is limited when the N:P ratio is lower than value of 7 to 10. This ratio is typical for P and N in plants (algae) tissues.

DIN:SRP ratio

0 50 100 150 200 250 300

SR

P (

mg

L-1)

0.0

0.1

0.2

0.3

0.4

Mills StBannisterBayswater

Figure 5-8 – Variation in soluble reactive P (SRP) concentration with the ratio of dissolved inorganic nitrogen (DIN) to SRP for three catchment s

This relationship between P and N concentrations may explain the high and consistent SRP concentrations in the Mills Street main drain during summer baseflow. When inorganic N concentration is particularly low, this may also be a limiting factor for N uptake by plants where N:P ratio is much greater than 10 as in the Bayswater main drain.

5.5. Seasonal Concentration and Load Overall, it appears that the greater the residence time of nutrient-rich water in the catchment, the more likely it is that inorganic nutrients are removed from the solution as a result of nitrification, denitrification (both in surface and groundwater) and plant uptake. Under such conditions, mostly organic nutrients forms remain in solution. Even particulate forms of N and P in such conditions are likely to be of organic origin (e.g. algae), since the flow rates during summer are low and therefore mineral particulates with high specific density will constitute a low proportion of suspended solids.

As a result, organic nutrients are likely to dominate in the surface water during low flow periods, as in the Mills Street and Bannister main drains. Such conditions are typical for summer baseflow, but may also prevail in the winter baseflow during drier years.

However, inorganic nutrient species may be also present during low flow periods in certain circumstances, such as:

• When the N:P ration is lower than value of 10, SRP may be present.

• When inorganic N is related to point sources located in a close proximity to the sampling site (such as in the Bayswater main drain, where discharges from the industrial zone and CSBP sites occur 500 m upstream from the gauging station).


During winter baseflow the water residence time decreases both in groundwater and in surface water network:

• In groundwater: due to the interception of shallow groundwater by subsurface drainage; and

• In the surface water network: due to the higher flow velocity in the main drains and relatively lower biological activity during the winter period.

During high baseflow periods, both nitrate and ammonium comprise a greater proportion of the total N pool and the SRP proportion in the total P pool also increases (Figure 5-9 and Figure 5-10).

During storm events, particulate and organic nutrient forms dominate, with the greater proportion of particulate matter during first seasonal flush events (as in Mills Street main drain, Figure A 48 in Appendix C). It is likely that both stream sediments and runoff from the impervious surfaces are the sources of these N/P forms.

(a)

Discharge (m3 s-1)

0.0 0.3 0.6 0.9 1.2 1.5

SR

P (

mg

L-1)

0.00

0.02

0.04

0.06

0.08

0.10

0.12 BaseflowPeak flow

(b)

Discharge (m3 s-1)

0.0 0.3 0.6 0.9 1.2 1.5

NH

4 (m

g L-1

)

0.0

0.2

0.4

0.6

0.8

1.0

Figure 5-9 – Variation in nutrient concentrations w ith discharge separated for baseflow and peak flow periods, (a) soluble reactive phosphorus (SRP) and (b) ammonium (NH 4)


(a)

TP (mg L-1)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

SR

P (

mg

L-1)

0.00

0.02

0.04

0.06

0.08

0.10

0.12BaseflowPeak flow

(b)

TN (mg L-1)

0 1 2 3 4 5 6

NH

4 (m

g L-1

)

0.0

0.2

0.4

0.6

0.8

1.0

Figure 5-10 – Variation in (a) soluble reactive pho sphorus (SRP) with total P concentration and (b) ammonium (NH 4) with total N for baseflow and peak flow periods.

The sources of nutrients loads in the studied urban catchments are summarised in Figure 5-11 to 6-16, indicating that during storm events the nutrients in surface water are associated with impervious surfaces (including roads) and in-stream processes. During baseflow, they are fertilised and/or irrigated areas, septic tank zones and some identified point sources (such as the CSBP site in the Bayswater main drain catchment).


Figure 5-11– Main sources of nutrient in surface wa ter during baseflow period in Bayswater main drain catchment; septic tanks area; CSBP area; irrigated and fertilised area

Figure 5-12 - Main sources of nutrients in surface water during storm flow period (peak flow) in the Bayswater main drain catchment: roads and in-st ream features (compensation basins and open drains)


Figure 5-13 – Main sources of nutrient in surface w ater during baseflow period in the Mills Street main drain catchment: septic tanks area (mai nly industrial) and irrigated and fertilised area

Figure 5-14- Main sources of nutrient in surface wa ter during storm flow period (peak flow) in the Mills Street main drain catchment: roads (and o ther hard surfaces in the industrial area) and in-stream features (compensation basins and ope n drains)

Milk processing

plant


Figure 5-15– Main sources of nutrient in surface wa ter during baseflow period in Bannister main drain catchment: septic tanks area; irrigated and fertilised area

Figure 5-16- Main sources of nutrient in surface wa ter during storm flow period (peak flow) in Bannister main drain catchment: roads and in-stream features (compensation basins and open drains)


6. IDENTIFICATION OF THE MOST APPROPRIATE BMPS FOR WATER QUALITY IMPROVEMENT IN URBAN DRAINAGE IN THE PERTH METROPOLITAN AREA

Following the discussion on nutrient forms and seasonality of water quality variation, this section sets out to explore which BMPs are likely to be more effective in controlling water quality in Perth urban drains. Nitrogen and phosphorus species are the only water quality parameters considered in this discussion.

For a BMP to be effective, the following should be considered:

• It is important to identify the sources of nutrients in the catchment, their pathways in transfer from the catchment to surface water drainage, and the process which influence in-stream nutrient transformation (as discussed in the earlier sections).

• Not only total concentrations, but P and N speciation and their seasonal variation must be identified and linked to the relevant sources and pathways.

• It is important to define the impact of nutrient export on ecological conditions in the receiving water environment. Risk level needs to be established for various seasons and decisions made as to which P and N species need to be targeted.

The latter is related to the health of the Swan-Canning Estuary and the risk of eutrophication, which is the main constraint to the control of nutrient export from the catchments. However, currently knowledge of the Swan-Canning system does not allow quantitative risk assessment of the following:

• Are summer nutrient fluxes of the greatest importance in terms of estuary health?

• What is more crucial; total nutrient load or nutrient concentrations?

• Do organic nutrients have a similar impact on the receiving environment as inorganic nutrients?

Answering these questions is one of the most important steps in defining BMPs for water quality control in Perth urban drains.

According to the review of published literature, there are a number of issues to consider when BMPs are selected :

1. No single BMP will treat all nutrient forms, however, vegetated systems are more effective in nutrient control than non-vegetated BMPs.

2. Badly planned BMPs can become a source of nutrients due to:

a. Poor investigation of materials used in construction, for example the incorporation of soil materials with high nutrient or organic carbon contents.

b. Vegetation build-up and seasonal variability, for example, high biomass accumulation during summer contributes to high nutrient removal while reduction of productivity, or even dieback, of vegetation during the winter months, may contribute to nutrient export.

c. Eutrophication of BMPs, that is, the increase in nutrient content within the BMP to such levels that conditions develop that are detrimental to the continuing functioning of the ecosystem.

3. There are limited options for in-situ groundwater treatment.

4. There are limited options for the treatment of organic nutrients.

5. Assessment of the effectiveness of non-structural BMPs is difficult.

6. The efficiency of BMPs may reduce with time if their maintenance is not adequate. When the cost of a BMP is determined, maintenance should also be accounted for.


In addition, the hydrological regime of the catchment should also be considered during selection of BMPs for water quality control, as illustrated in Figure 3-4. Based on the analysis undertaken, the following considerations could be applied in the selection of BMPs in the Perth metropolitan area for nutrient control in urban drains.

6.1. Storm Water Runoff Storm runoff delivers 20 to 60% of the annual loads of predominantly particulate and organic nutrients during the wet season (May to October).

Hydrological analysis of discharge from urban drains indicates that during storm events water quality is predominantly related to runoff from impervious sources, but can be also influenced by remobilisation of nutrients from sediments accumulated in streams, compensation basins and wetlands. Therefore, water quality control options to be considered for reducing nutrient loads during these events should include:

• Non-structural BMPs aimed at preventing the accumulation on impervious surfaces of nutrients which may potentially be washed out to the drains with storm runoff. These may include:

o Street and road sweeping.

o Appropriate lawn and parks management to prevent fertiliser, soil and debris accumulation on impervious surfaces.

• Structural BMPs aimed at control of water quality at the storm runoff discharge to the drains. In older suburbs this is directly piped to drains as shown in Figure 6-1. Appropriate measures may include:

o Filtration or biofiltration systems which can provide runoff treatment prior to discharge into the drainage network (e.g. as shown in Figure 6-2). These do not occupy a large area and are cost-effective.

• Drain maintenance, such plants maintenance and sediment removal. This can avoid remobilisation of nutrients stored in drains and compensation basins, such as that shown in Figure 5-7.

Figure 6-1 - Storm runoff discharge point (Mills St reet and Bayswater main drains)


Figure 6-2 – Filtration/biofiltration system, which allows the discharge of road runoff

over a grassed are prior to discharge to the drains (Bletchley Park, Southern River)

6.2. Baseflow Water quality control in the summer (low baseflow) and winter (high baseflow) may require different considerations for BMPs selection.

During low baseflow periods, nutrient sources are related to the groundwater, which provides the sole source of water in the urban drains during summer, and also to internal sources of nutrients with the drain network. High water residence time allows forced attenuation of inorganic nutrient speciation, and predominately organic nutrient forms (particularly nitrogen) are detected in the drains during this period.

Low baseflow during summer (November to April), when the Swan-Canning Estuary is particularly sensitive to nutrient input, delivers less than 15% of annual loads of predominantly particulate and organic nutrients3 and Phosphate. During this period, urban drainage is the main source of water inflow to the of Swan-Canning system.

Under low baseflow conditions, water quality control should include both preventive measures, which limit the nutrient input to the catchment and subsequently to groundwater, and in-situ water quality control BMPs, including the following:

• Non-structural BMPs directed at:

o Fertiliser control to reduce the organic nutrients input to gardens and POS and their potential transfer to groundwater and subsequently to surface water.

o Soil amendment to bind nutrients in the soil and prevent their leachate to groundwater. A number of suitable materials are currently available to improve the capacity of soil to retain nutrients, and these are summarised in (Wendling and Douglas 2009).

• Structural BMPs can also be used to reduce nutrient transfer to groundwater. These include:

o BMPs to control water quality in recharge, such as lining the bases of infiltration basins and soak wells with materials which allow nutrient adsorption (Wendling and Douglas 2009).

• Drain maintenance (plant maintenance or sediment removal).



However, attenuation of organic forms of nutrients appears to be limited, and wetlands on the outflow from catchments are unlikely to effectively treat organic N, and may also be limited in terms of SRP removal when the N:P ratio is low. This conclusion is supported by limited observational data in constructed wetlands.

High baseflow during the wet season (June-October) delivers 20 to 60% of annual loads of various organic (up to 50%) and inorganic nutrients. Baseflow during this period is also sourced from groundwater. Additionally, delayed discharge from compensation basins and wetlands may contribute to baseflow. Due to a rising groundwater table, the area where sub-surface drains intercept the shallow groundwater table expands, which also leads to an increase in drainage discharge rates. Concurrently, groundwater residence time reduces and inorganic nutrient concentrations increase.

Under such conditions, water quality control should include both preventive measures which limit the nutrient input to the catchment and subsequently to the groundwater, and in-situ water quality control BMPs, including:

• Similarly to preventive measures identified for the low baseflow period, non-structural BMPs, which are expected to be particularly effective for these conditions, should be directed towards fertiliser control and soil amendments to reduce both inorganic and organic nutrient load transfer to groundwater and then to surface water.

• Again, as in low baseflow water quality control, structural BMPs, which are expected to be particularly effective for these conditions, should be applied to control nutrient transfer with groundwater recharge. These include:

o BMPs to control water quality in the branch drains prior to discharge to main drains (particularly at the discharge point of subsurface drains to the main drains to prevent NO3 transfer to surface water).

o Wetlands are likely to be effective in the treatment of NO3, NH4, SRP and particulate nutrients during these flow conditions, but their effectiveness may be compromised by:

� Inadequate infrastructure size to allow for up to 30 days residence time.

� The requirement to incorporate processes for the attenuation of different nutrient species. For example, NH4 attenuation requires the presence of oxygen, whereas NO3 attenuation requires anaerobic conditions.

� A reduction in biological activity during the winter period, which is likely to reduce the wetland’s efficiency in promoting inorganic nutrient uptake by plants.

• Drain maintenance, which may include plant maintenance or sediment removal.

Point sources influence water quality mainly during low baseflow periods. In some cases, point sources may affect the water quality in the receiving environment, particularly when they are located in the lower catchment areas close to the discharge zone, such as the CSBP site and the industrial zone in the Bayswater main drain. Nutrient forms and associated loads are expected to be site-specific. For instance, in Bayswater these two sources contribute at least 60% N during summer as nitrate and ammonium.


6.3. Individual Drains Some additional recommendations were made in relation to the individual drains.

Bayswater main drain

• The focus of rehabilitation should be on the lower catchment to control summer nutrient export:

o In the septic tanks zone, this will allow reduction in nitrate, particularly in summer.

o The CSBP site, which will allow reduction in ammonium concentration, but may lead to increases in nitrate, organic matter and dissolved P.

• Focus on the upper catchment to control winter nutrient export:

o Fertiliser management.

o BMPs for the discharge areas of sub-surface drains.

Mill Street main drain

• BMPs during summer baseflow should be directed at the control of particulate and organic forms (>90% load) and SRP. These are not likely to be treated by concentrated wetlands.

• During winter baseflow, the focus should be on the septic tanks zone to improve water quality in winter baseflow (>50% load).

• Maintenance of compensation basins is required to reduce the output of organic nutrients from the infrastructure.

• Fertiliser management will allow control of water quality mainly in the low (residential) sub-catchment, where high SRP and NO3 input was identified.

Bannister Creek

• BMPs during summer baseflow should be directed at controlling particulate and organic forms (>90% load) and SRP, as these are not likely to be treated by concentrated wetlands.

• Fertiliser management will allow water quality control mainly in the low (residential) sub-catchment.

• A continuing reduction in organic nutrient concentration is expected with further urbanisation of this catchment.

Forrestdale main drain

• Summer baseflow is likely to become a feature of the drain water regime after further urbanisation. Currently, this does not occur.

• Export of legacy nutrients is expected during this catchment urbanisation.

An outline of a framework which may be adapted to the selection of water quality BMPs for the control of nutrients is given in Figure 6-3. This flow diagram is shown as an example for water quality control measures during baseflow in Bayswater main drain. As indicated, firstly appropriate BMPs must be identified for hydrological significant areas of the catchment (such as groundwater recharge zones, groundwater discharge zone etc). Then, targeted flow conditions or sources and relevant nutrient species are identified and the BMPs with most appropriate water treatment capacity can be identified. This is expected to be an initial stage in BMPs selection, as additional criteria should be considered including BMPs’ cost, maintenance requirements and land availability. Consideration should also be given to prioritising water quality requirements within receiving environments, such as the Swan-Canning Estuary.


Figure 6-3 – Decision tree for determining the most effective BMPs for the Bayswater main drain catchm ent


7. CONCLUSIONS This main research results are related to the factors influencing water quality in urban drainage, overall BMP efficiency, effectiveness of BMPs in Perth, and the needs for future research.

It was concluded that for any catchment the success of in-situ water quality control measures is greatly dependent on the following:

1. Identification of the sources of nutrients in the catchment, their transfer pathways from the catchment sources to surface water drainage, and the processes which influence in-stream nutrient transformation; this allows a most appropriate intervention both in terms of BMP location and technologies deployed.

2. Identification of not only total concentrations, but P and N species and their seasonal variation, as well as their links to the relevant sources and pathways: similarly to the previous statement, this provides a guide to the most appropriate treatment technology.

3. Definition of the significance of nutrients load in terms of ecological conditions in the receiving water environment, when risk level needs to be established for various seasons and for P and N species: allows the effort to be directed to the priority seasons or nutrient speciation to satisfy the requirement for a cost-effective BMP.

In Perth, the latter point is predominantly related to the health of the Swan-Canning Estuary and the risk of eutrophication. However, currently the following issues are not fully clarified:

1. Are only summer nutrient fluxes important or are winter nutrient fluxes also likely to cause damage to the estuary?

2. Do total annual nutrient loads or their concentration pose equal levels of concern?

3. What is the impact of organic nutrients and is it similar to inorganic nutrients?

Defining the answers to these questions is one of the important steps in defining BMPs for water quality control in Perth urban drains, which may determine priorities for water quality control measures.

The undertaken analysis allows identification of the main factors influencing water quality within the studied urban drains in the Perth metropolitan area.

1. Nutrient inputs in the urban environment

It was confirmed that fertiliser application is a major source of nutrient load to groundwater. However, wastewater was the major contributor of nutrients in the Mills Street and Bayswater catchments prior to the Water Corporation sewage infill program.

Point sources may have a significant impact on water quality in urban drains, such as:

• The CSBP site (Bayswater main drain) is contributing approximately 25% of the annual TN load, predominantly NH4-N. However, discharge from this site also contributes to the attenuation of some other nutrient forms including SRP, DON and DOC. This is likely due to the presence of dissolved iron (Fe) and aluminium (Al) in discharge from the CSBP site. In surface water, and in the presence of oxygen, these ions form Fe/Al oxides that promote nutrient adsorption or co-precipitation.

• The Masters Milk Processing Plant area is indicated as a potential source of the high NH4-N concentration in one of the northern tributaries of the Mill Street main drain. According to 2001-2002 observation data, discharge from this site comprises 15% of the annual catchment N load.

During storm events particulate and organic nutrient forms dominate, with the greater proportion of particulate matter during seasonal first flush events (as in Mills Street main drain). It is likely that runoff from impervious surfaces is the main source of these N/P forms; however, some remobilisation of sediments stored in-stream is also possible.


2. Development stage: effect of legacy nutrients

Peri-urban catchments in the Perth metropolitan area which are underlined by sandy aquifers with a shallow groundwater table are commonly characterised by the accumulation of nutrients and other solutes in shallow groundwater and, as a result, in the drains. Inorganic nutrients are largely absent in the nutrient pool due the processes of attenuation or uptake by vegetation, resulting in a high proportion and concentration of the organic nutrient species. The Forrestdale catchment is one such example, where N, P and DOC concentrations in drains are generally greater than in the Mills Street Main Drain, Bayswater Main Drains and Bannister Creek catchments.

In well-established urban catchments, such as Mills Street and Bayswater main drains, where the effect of the urban development has reached equilibrium, DOC concentrations are lowest.

In catchments currently undergoing urbanisation, such as Bannister Creek, the introduction of a drainage network, particularly sub-surface drainage in new urban developments, greatly increases the rate of shallow groundwater discharge and therefore flushing of the legacy nutrients. As a result, the nutrient concentrations, particularly of organic species, are lower than in peri-urban catchments but higher than in well-established urbanised areas. Further reduction in organic nutrients, particularly TOC/DOC, is expected in catchments undergoing urbanisation.

3. Hydrogeological conditions

Groundwater plays a significant role in nutrient transfer from soils to urban drains. Baseflow, as an indicator of the groundwater discharge to urban drains, varies between 40 and 80% of annual discharge in the studied urban drains. It also appears that the groundwater systems play a significant role in nutrient attenuation. The following are the key characteristics of hydrogeological conditions influencing the process of nutrient transport and transformation:

• The unsaturated zone provides a media where attenuation processes take place in the presence of oxygen and as a result of interaction between solute and the soil minerals. Reduction in DOC, and potentially DON, concentrations and transformation of NH4-N to NO3-N may occur in the unsaturated zone.

• Soil and aquifer minerals may provide a media for P adsorption. Soils such as those of the Spearwood and Quindalup dune systems, which contain higher contents of carbonate and iron minerals than Bassendean sands, tend to have higher P adsorption.

• Anaerobic conditions and the presence of organic carbon are typical of the superficial aquifer in Perth. These provide a favourable environment for further attenuation of nutrients and NO3-N is largely removed from groundwater due to the process of denitrification.

• Groundwater residence time may constrain the process of denitrification, particularly in the areas where shallow groundwater is intercepted by sub-surface drains. During winter, this leads to low groundwater residence time which prohibits, or limits, denitrification. As a result, the NO3-N concentration is greater during winter baseflow in all studied drains and may comprise 25 to 30% of the annual N load.

Overall, it is expected that groundwater may potentially be the major source of organic nutrient species in drainage. However, this hypothesis requires further clarification, and in-stream processes may also contribute to organic nutrients load.

4. In-stream processes and residence time prior to discharge to the river

Biological activity in the drainage network, which in Perth’s climatic conditions occurs throughout the year, provides significant attenuation potential for inorganic nutrient forms, including NH4, NO3 and phosphate (PO4). These lead to a reduction of concentrations from often extremely high in the branch drains (e.g. Mills Street total P median 0.43 mg/l with


maximum up to 9.5 mg/L) to the much lower concentration in the main drain (e.g. Mills Street total P median 0.17mg/l).

It was also observed that a relationship exists between SRP concentration and N:P ratio in surface water (N as inorganic N and P as SRP). It appears that the higher SRP concentrations are more common when the N:P ratio is low, and conversely, SRP concentrations are lower for a high N:P ratio. This may indicate that in-stream plant or algae P uptake is limited when the N:P ratio is lower than value of 7 to 10. This may further indicate that the lack of the inorganic nitrogen forms may be a limiting factor to SRP uptake by living organisms. This should be considered when BMPs are selected for such conditions.

5. Seasonality in concentration and load

The seasonal variation in surface water quality was found to be related to the variation in residence time of the water entering drains, the effect of point sources during high and low flow period and also, potentially, to biological activity within the stream.

It can be concluded that the greater the residence time of nutrient-rich water in the catchment, the more likely it is that the inorganic nutrients are removed from the solution as a result of nitrification, denitrification (both in surface and groundwater) and plant uptake. Under such conditions, mostly organic nutrient forms remain in solution.

Subsequently, during low flow periods, organic nutrients dominate in the surface water, such as that in the Mills Street and Bannister main drains. Such conditions are typical for summer baseflow, but may also occur in the winter baseflow in drier years. It is important to mention here that there are limited options for the treatment of organic nutrients.

During winter baseflow, the water residence time decreases both in groundwater, mainly collected in sub-surface drains intercepting a shallow groundwater table, and in the surface water network, due to the higher flow velocity in the main drains and lower biological activity.. As a result, both nitrate and ammonium comprise a greater portion of the total N pool during this period, and the SRP proportion in the total P pool also increases.

The current study concluded that the hydrological regime of the catchment should be considered during selection of BMPs for water quality control.

1. Storm runoff

Storm runoff delivers 20 to 60% of annual loads of predominantly particulate and organic nutrients during the wet season from May to October.

Hydrological analysis of urban drains indicates that during storm events water quality is defined by runoff from impervious sources, but can also be influenced by remobilisation of nutrients from sediments accumulated in streams, compensation basins and wetlands. During these events, water quality control initiatives aimed to reduce nutrient loads should be considered:

• Non-structural BMPs, including preventive measures against nutrient accumulation on impervious surfaces such as street and road sweeping, and proper lawn and park management to prevent fertiliser, soil and debris entering waterways;

• Structural BMPs to control water quality in runoff prior to discharge into the drainage network; and

• Drain and structural BMP maintenance including vegetation control such as plant maintenance or sediment removal).

2. Low (summer) baseflow


Low baseflow during summer (November to April) delivers less than 15% of annual loads of predominantly particulate and organic nutrients4 and PO4, when the Swan-Canning Estuary is likely to be particularly sensitive to nutrients input. During this period, urban drainage is the main source of fresh water inflow to the estuary. Under these conditions, water quality control initiatives to be considered include:

• Non-structural BMPs which may prevent nutrient release from soil to groundwater. Groundwater is the sole source of baseflow during this period; and appropriate BMPs can include fertiliser control to reduce the nutrient input, and soil amendment, to reduce nutrient transfer from soil to groundwater. The latter is one of most effective non-structural BMPs in this situation;

• Structural BMPs which may reduce nutrient transfer to groundwater. These include BMPs to control water quality in recharge areas such as lining the base of infiltration basins and soak wells with materials which allow nutrient adsorption, such as NUA (Wendling et al., 2009); and

• Drain maintenance. Vegetation control such as plant maintenance or sediment removal can prevent in-stream nutrient generation.

However, wetlands on the outflow from the catchments are unlikely to effectively treat DON, and may be of limited use in SRP removal when the N:P ratio is low. This observation was confirmed by limited monitoring data in the constructed wetlands.

3. High (winter) baseflow

High baseflow in the wet season from June to October delivers 20 to 60% of annual nutrient loads of various proportions of organic (up to 50%) and inorganic nutrients. In addition to the BMPs suggested for water quality control during low baseflow, the following water quality control measures should also be considered during the high flow period:

• Structural BMPs to control water quality in the outflow from the subsurface drains or smaller branch drains prior discharge to main drains. Such BMPs may include, among others, biofiltration systems. These can be small and are particularly effective in NO3, NH4 and PO4 attenuation, levels of which were found to be high during the winter months;

• Wetlands are likely to be an effective treatment for NO3, NH4, SRP and particulate nutrients during this period. However, their effectiveness may be compromised if they are not large enough to allow for the up to 30 days residence time required for attenuation of nutrient species such as NH4 and NO3; and

• Drain maintenance including vegetation control such as plant maintenance or sediment removal.

4. Point sources

Point sources generally have the most significant effect on water quality during low baseflow periods. Nutrient forms and associated loads are expected to be site specific. For instance, during summer in Bayswater, two such sources (the CSBP site and the industrial zone) contribute at least 60% N in the form of NO3 and NH4.

Based on the analysis undertaken, the following are considered to be the most important future steps in the selection of nutrient BMPs in the Perth metropolitan area:

• Catchment-scale groundwater modelling and better characterisation of groundwater quality through the establishment of a groundwater monitoring network. Monitoring is currently poor.



• The development and implementation of methodologies for effective and quantitative estimation of BMP efficiencies based on high frequency monitoring, selection of in-situ equipment and hydro-biogeochemical modelling.

• The quantification of nutrient leachate from soil under different land and water management practices using field investigations and laboratory trials.

• The definition of in-stream attenuation parameters through the establishment of biological and water quality monitoring in targeted drain reaches and compensation basins within the studied catchments.

In addition, it is required that a better understanding of the role of organic nutrient speciation in the water environment is developed.


REFERENCES

Cymod Systems Pty Ltd (2004) 'Calibration of the coupled Perth Regional Aquifer Model. PRAMS 3.0.' Department of Environment (2004) 'Introduction, Stormwater Management Manual for Western Australia.' Department of Environment, Perth, Western Australia. Environment Canada (2001) 'Nutrients in the Canadian environment: Reporting on the state of Canada's environment.' prepared by Indicators and Assessment Office, Ecosystem Science Directorate, Environmental Conservation Service, Environment Canada, Ottawa. GHD (2006) 'Consolidation of monitoring information for metro Perth - Drainage monitoring data.' Water Corporation, Perth. GHD (2007a) 'Liege Street Wetland Performance Report 2005 to 2006: Community Summary Report.' Swan River Trust, Perth. GHD (2007b) 'Report for Mills Street groundwater treatment trials - Final evaluation report.' Water Corporation, Perth. Kirchner JW, Feng XH, Neal C, Robson AJ (2004) The fine structure of water-quality dynamics: the (high-frequency) wave of the future. Hydrological Processes 18, 1353-1359. Majimbi AA (2007) An assessment of the nutrient stripping function of two constructed wetlands in the Swan-Canning Estuary. Masters of Science thesis, Curtin University of Technology. Nasholm T, Kielland K, Ganeteg U (2009) Uptake of organic nitrogen by plants. New Phytologist 182, 31-48. Revitt M, Ellis B, Scholes L (2003) 'Report 5.1. Review of the Use of stormwater BMPs in Europe.' Middlesex University. Roy AH, Wenger SJ, Fletcher TD, Walsh CJ, Ladson AR, Shuster WD, Thurston HW, Brown RR (2008) Impediments and solutions to sustainable, watershed-scale urban stormwater management: Lessons from Australia and the United States. Environmental Management 42, 344-359. Swan River Trust (2005) 'PhoslockTM application in drains.' www.swanrivertrust.wa.gov.au. Swan River Trust (2009) 'Swan Canning Water Quality Improvement Plan - Draft for public comment.' Swan River Trust, Perth. Taylor A, Wong T (2002a) 'Non-structural stormwater quality best management practices – A literature review of their value and life-cycle costs.' Cooperative Research Centre for Catchment Hydrology, Technical Report 02/13. Taylor A, Wong T (2002b) 'Non-structural stormwater quality best management practices – A survey investigating their use and value.' Cooperative Research Centre for Catchment Hydrology, Technical Report 02/12. Taylor A, Wong T (2002c) 'Non-structural stormwater quality best management practices – An overview of their use, value, cost and evaluation.' Cooperative Research Centre for Catchment Hydrology, Technical Report 02/11.


US EPA (1999) 'Preliminary Data Summary of Urban Stormwater Best Management Practices.' United States Environmental Protection Agency, Report No. EPA-821-R-99-012. Wendling L, Douglas G (2009) 'A review of mining and industrial by-product reuse as environmental amendments.' CSIRO: Water for a Healthy Country National Research Flagship.

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