GROUNDWATER RESOURCE ASSESSMENT: TASK 1BC … · GROUNDWATER RESOURCE ASSESSMENT: TASK 1BC ......

Viva water pure and clean! • Viva forests rich and green!

DEPARTMENT: WATER AFFAIRS AND FORESTRY

GROUNDWATER RESOURCE ASSESSMENT: TASK 1BC

GROUNDWATER QUANTIFICATION

PROJECT NAME GRA II PROJECT NO. 2003-150 SUBSYSTEM Methodology for Groundwater

Quantification VERSION NO 2.0 Final VERSION DATE 2004-10-10 DOCUMENT TYPE Methodology report COPY PRINTED DATE 2006-06-13

GROUNDWATER RESOURCE ASSESSMENT: TASK 1BC GROUNDWATER QUANTIFICATION

TABLE OF CONTENTS of 41

Department: Water Affairs and Forestry

VERSION: 2.0

1. INTRODUCTION ................................................................................................ 4 1.1 Symbols and Conventions............................................................................................4 1.2 Applicable Documents ..................................................................................................4 1.3 Acronyms And Abbreviations ......................................................................................4

2. EXECUTIVE SUMMARY .................................................................................... 4

3. INTRODUCTION ................................................................................................ 7

4. METHODOLOGY ............................................................................................... 9 4.1 General ...........................................................................................................................9 4.2 The GIS Layers...............................................................................................................9 4.3 Approach used in estimating each aquifer level ........................................................11 4.4 Assigning confidence levels to the data and final result...........................................12 4.5 The procedure for quantifying groundwater...............................................................13 4.6 Method for establishing the GIS layers .......................................................................16 4.6.1 Level 1 – Bottom of the aquifer...................................................................................................... 16 4.6.2 Level 2 – Base of Dynamic Storage under Natural Conditions ..................................................... 25 4.6.3 Level 3 – Current Groundwater Elevation ..................................................................................... 26 4.6.4 Level 4 – Average Groundwater Elevation.................................................................................... 27 4.6.5 Level 5 – Top of the Aquifer .......................................................................................................... 27 4.6.6 Assessment of Aquifer Level 1, Level 3 and Level 5..................................................................... 27 4.7 Provisional National Estimates of volumes of groundwater stored in Aquifer Systems......................................................................................................................................30 4.7.1 Maximum Groundwater Storage.................................................................................................... 30 4.7.2 Average Groundwater Storage...................................................................................................... 31

5. CONCLUSION.................................................................................................... 32

APPENDIX 1: AQUIFER ASSURANCE YIELD APPROACH (AAY)............................ 34

APPENDIX 2 : ANALYSIS OF STRIKE DENSITY DATA............................................. 38

GLOSSARY .................................................................................................................. 39

6. REFERENCES ................................................................................................... 40 TABLES Table 1 : Process for Groundwater Resource Assessment and Allocation.................................... 5 Table 2: Output per GRA II project....................................................................................................... 5 Table 3: Steps to quantify groundwater storage................................................................................ 7 Table 4: Brief description of the approach used to generate the aquifer levels........................... 11 Table 5: Aquifer area : CLs depending on the data source............................................................. 12 Table 6: Storativity or specific yield: CLs depending on the data source..................................... 13 Table 7: Aquifer thickness: CLs depending on the data source .................................................... 13 Table 8: Groundwater storage confidence levels (average of input terms CLs) .......................... 13 Table 9: Procedure for Quantifying Groundwater............................................................................ 13 Table 10: Drilled depths based on Vegter (1995) ............................................................................. 16 Table 11: Description of 64 Groundwater Regions (Vegter, 1995) ................................................. 23 Table 12: Average Groundwater Levels based on Vegter (1995) ................................................... 27


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FIGURES Figure 1: (a) Hydrograph of monitoring borehole M1 (b) Schematic presentation of the physical

layers ............................................................................................................................................. 11 Figure 2: Groundwater Quantification Algorithm/Flowchart .......................................................... 15 Figure 3: Determination of Water-Strike Frequency per 10m Depth Interval below the Waterlevel

........................................................................................................................................................ 18 Figure 4: Strike-Density Curve Granite Gneiss, NW of Pietersberg (Vegter, 1995) ...................... 19 Figure 5: Strike-Density Curve Karoo Sedimentary Rocks (Vegter, 1995) .................................... 19 Figure 6: Strike-Density Curve Granite Gneiss, Dendron (Vegter, 1995)....................................... 20 Figure 7: Strike-Density Curve Table Mountain Group (Vegter, 1995)........................................... 20 Figure 8: Water-strike information from the NGDB classed according to strike-depth below

ground surface overlain over Vegter (2001) Groundwater Regions ....................................... 21 Figure 9: Groundwater Regions of South Africa (Vegter, 2001) ..................................................... 22 Figure 10: Groundwater Regions showing interpolated Depth to Water-Strike ........................... 23 Figure 11: Groundwater Region 42 - Number and Density of Water Strikes per 5m Depth Interval

showing estimated ‘Base of Aquifer’ at 170m.bgl .................................................................... 24 Figure 12: Groundwater Region 22 - Number and Density of Water Strikes per 5m Depth Interval

showing estimated ‘Base of Aquifer’ at 320 m.bgl ................................................................... 24 Figure 13: Groundwater Region 2 - Number and Density of Water Strikes per 5m Depth Interval

showing estimated ‘Base of Dynamic Storage Zone’ at 40 m.bgl........................................... 26 Figure 14: Example of how the base of dynamic storage (28 mbgl) can be obtained from an

unaffected monitoring borehole ................................................................................................. 26 Figure 15: Location of the transects.................................................................................................. 28 Figure 16: Ramatseliso’s Gate to Matatiele ...................................................................................... 28 Figure 17: Ramatseliso’s Gate to Port Edward ................................................................................ 29 Figure 18: Victoria West – Beaufort West – Rietbron – Dyselldorp – George............................... 29 Figure 19: Resultant 'Error Check' Grid of DTM - Maximum Aquifer Storage Level..................... 30 Figure 20: Maximum Volume (m3/km2) of Groundwater Stored in Aquifer Systems in South

Africa ............................................................................................................................................. 31 Figure 21: Average Volume (m3/km2) of Groundwater Stored in Aquifer Systems in South Africa

........................................................................................................................................................ 32


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1. INTRODUCTION

1.1 Symbols and Conventions A surface area of lithological unit AAY aquifer assurance yield

eq. Equation number groundwater level h groundwater abstraction abQ natural input to a groundwater system inQ natural output from a groundwater system outQ change in inQΔ inQ change in outQΔ outQ aquifer storativity S specific yield yS total groundwater volume V change in total groundwater storage volume VΔ summation sign ∑

1.2 Applicable Documents Literature study report

1.3 Acronyms And Abbreviations Acronym/Abbreviation Definition CL Confidence level DEM Digital elevation model GRA II Groundwater Resource Assessment Phase II DWAF Department of Water Affairs and Forestry NGDB National Groundwater Data Base TOR Terms of reference

2. EXECUTIVE SUMMARY The key objective of the GRA II project is to provide an approach to quantifying groundwater resources in South Africa. Together with the approach (or method), the project must provide generic data sets that can be used for rapid and regional-scale groundwater resource assessments. The main purpose for quantifying groundwater in the GRA II project is to provide guidance on how much water can be allocated for use.

The process of establishing this will follow the steps outlined in Table 1 for any given area of interest, be it an aquifer, a catchment or a particular study area. Table 1 also mentions where the information will come from out of the five GRA II projects.




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Table 1 : Process for Groundwater Resource Assessment and Allocation Step Task Project No

1 Establish the volume of groundwater held in storage 1

2

Establish the proportion of this that can feasibly be abstracted, and the proportion that should be abstracted in a single year in order to bridge drought cycles

2 with input from 3

3

Establish the proportion of 2 above that should remain behind in the aquifer in order to meet specific management criteria (eg the Reserve, prevention of land subsidence, maintain water quality in the aquifer, etc)

4 and Reserve Estimates

4 Establish the current abstraction 5

5 Establish the remainder that can be allocated for further use -

In order for the approach to be practical and to meet the objective of supporting groundwater resource quantification per defined management unit, the following factors have been incorporated into the approach: • The data sets are spatially (GIS) based; • The data sets can easily be replaced once new data becomes available; • The approach is applicable at various scales; • The approach is easy to use (ie the answers can be obtained using a hand

calculator and it is not necessary to have GIS-computer skills). The outputs of the five GRA II projects are mostly in volumes of groundwater per grid area (Table 2).

Table 2: Output per GRA II project Key output Form of output for GRA II Project

No

Storage

Available storage in m3 per grid area (this will take the form of rough estimates to be used as country-wide default values; and it will describe an approach to determining current storage should detailed, localized data be available)

1

Natural recharge Mean annual recharge in m3 per grid area (from precipitation & surface water) 3

Natural discharge Mean annual discharge to surface water in m3 per grid area 3

Abstraction potential Allocatable groundwater use in m3 per grid area in order to bridge drought cycles and taking aquifer permeability into account

2

Groundwater use

Average usage in m3 per quaternary catchment or grid area Method to determine annual usage in m3 per quaternary catchment or grid area

5

Aquifer classification Guide on aquifer status to serve as qualitative input into allocating groundwater use 4

Note: All key outputs will cover the entire country – ie default values will be provided should accurate figures not be available.




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The approach adopted in Project 1 (Methodology for Groundwater Quantification) is summarized in Section 4 of this report. It was initially intended that two separate reports would be written to describe “the methodology” and the “rapid approach”. It is, however, preferable to combine these reports into one, since a single method has been developed. What is different between the more detailed approach and the rapid approach is that the input data for the rapid approach are the generic values that have been developed for the entire country, whereas for the more detailed approach, site specific values will be used. A considerable portion of the work done on the project to date has been on developing the default values (and at this stage, they have not been finalized). GIS layers based mostly on a 1 km by 1 km grid have been developed for the various “levels” within an aquifer. These levels are grouped into static storage, which is the volume of groundwater available in the permeable portion of the aquifer below the zone of natural water level fluctuation, and dynamic storage, which is the volume of groundwater available in the zone of natural water level fluctuation. The levels are listed below:

Level 1 - bottom of the aquifer Level 2 - bottom of the natural dynamic groundwater elevation Level 3 - current groundwater elevation Level 4 - average groundwater elevation Level 5 - top of the aquifer

Static storage

Dynamic storage

The volume of aquifer material bound between the layers reduced by an appropriate storage coefficient gives groundwater storage (V). The mathematical expression for groundwater storage is:

hASV Δ=∑ Where

∑ is a summation sign A is the area of each grid S is the storage coefficient

hΔ is thickness between any two layers of interest. By taking different values, various storage volumes can be computed. Project 2, which provides the groundwater planning potential map, will use one or more of these storage values, together with drought cycles and aquifer permeability to determine the practical groundwater resource that should be available for use in any given area on an annual basis.

hΔ

The process required to establish the volumes is summarized in Table 3.




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Table 3: Steps to quantify groundwater storage Steps Method Result

Step 1 Delineate the area to be studied. Area of groundwater unit

Step 2 Establish the coefficient of storage for the aquifer S-value 1. Establish aquifer thickness by taking the difference between

the top of the aquifer (Level 5) and the bottom of the aquifer (Level 1).

2. Apply the coefficient of storage to the aquifer volume.

The maximum groundwater storage

1. Establish the base of the natural dynamic storage (Level 2) and take the difference between this and the bottom of the aquifer.

2. Apply the coefficient of storage. The static storage

1. Establish the average water level (Level 4) and take the difference between this and the bottom of the aquifer.

2. Apply the coefficient of storage.

The average total storage

1. Take the difference between the average water level (Level 4) and base of the natural dynamic storage (Level 2).


The average dynamic storage

Step 3

1. Establish the average current water level (Level 3) and take the difference between this and the base of the natural dynamic storage.


The current available dynamic storage

3. INTRODUCTION This report outlines the approach for quantifying groundwater storage. It is a draft report because the methodology and data sets will only be finalized once they have been tested on an aquifer that is well understood. The purpose of this report is to describe the quantification approach and to elicit comments prior to finalizing it. The development of the methodology follows the literature study on groundwater quantification methods carried out as the first phase of Project 1. The aim of the literature study was to review quantification methods in use locally and internationally, and present a possible approach that would be applicable in South Africa. The underpinning terms of reference for the development of the method are (as in the Project Charter): • A single methodology must be developed to carry out groundwater resource

assessments on a large-scale, where regional groundwater resources need to be assessed, and on a small-scale, where a single aquifer (or closely spaced aquifers) need to be assessed. It needs to be applicable to stressed aquifers.

• It must take into account, and be developed from the wide spectrum of past and current work that looked at topics relating to groundwater resource quantification (including work on “reserve’ quantification, recharge estimation, etc).

• It must be applicable for the whole country and be useable with existing data sets.

• It must be useful for the purpose of authorising water use. • It must be scientifically sound and user friendly for the target audience. • It must be rigorously tested via wide consultation and it must be widely

accepted. • It must be robust. • It must be tested on one or more stressed aquifers. • The study must assist or inform the national groundwater monitoring strategy. • The study must determine the attributes needed to produce the static models

in REGIS Africa and dynamic modelling.




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It was notable during the literature study that all approaches to quantifying groundwater hinge around the water balance equation or some components of this equation. However, different authors implement the water balance equation in different ways depending on the complexity of the problem to be solved. Methods of solutions to the water balance equation vary from simple analytical to complex numerical approaches. Simply put, the long-term water balance for an unexploited aquifer unit is described mathematically by:

0=− outin QQ where:

−inQ total water entering the groundwater system by rainfall recharge, direct stream recharge, inflow through aquifer boundaries;

−outQ total water leaving the aquifer unit through base-flow to rivers, spring-flow, discharge to wetlands and sea, evapotranspiration and discharge through aquifer boundaries to neighbouring aquifers. If there are additional anthropogenic inputs or outputs, such as artificial recharge or groundwater abstraction ( ), the natural balance is upset. The knock-on impacts

include a reduction in the overall discharge (of abQ

outQΔ ) from the unit, modifications to the input terms and a change in storage ( VΔ ). It should be noted that even if there is no artificial recharge, natural recharge increases (by inQΔ ) in response to the lowering of water levels, but discharge decreases dominate over the recharge increases (Wright and Xu, 2000). All these modifications can be represented mathematically if there is no artificial recharge by (modification of eq. 1): ( ) ( ) VQQQQQ aboutoutinin Δ=+Δ−−Δ+ The acceptable portion of recharge available for abstraction, , depends on the

decline in storage, , and decline in natural discharge, , that each groundwater manager is willing to accommodate. This requires decisions based on latest estimates of the water balance (estimates of and ), and the predicted impacts (in response to various pumping schedules) on the reduction in groundwater levels, loss of stream base-flow or spring-flow, land-subsidence, increased salinity, etc. These are scientific issues that can be assessed or solved with varying degrees of certainty. They span many scientific disciplines and may also require input from economists and lawyers. Ultimately, the decision on the proportion of groundwater that will be allocated for use will be based on what level of alteration to the groundwater system (and groundwater dependant systems) is acceptable.

abQVΔ outQΔ

inQ outQ

Project 1 does not attempt to solve management criteria – its focus (and outputs) hinge around the volume of water stored in the subsurface (V). The question that requires answering is “how much water is held in storage per unit area of interest?” Once this is known (or a reasonable estimate provided), a planning potential map can be developed (Project 2) that provides the basis for allocating groundwater use. The key outputs from Project 1 that will be used for planning purposes are:




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i) A country-wide grid (on a 1km x 1km basis) that shows average groundwater storage. This is a once-off estimate that can be updated as new information becomes available and as the results of detailed studies are added to this first estimate. The purpose of this is to provide an estimate of average available groundwater throughout the country.

ii) A method to establish current groundwater storage. This will be based on the previous year’s input data (especially recharge and abstraction). The purpose of this is to provide an estimate of the available groundwater storage for the upcoming period (eg the next year).

The literature study revealed that there are numerous approaches to quantifying groundwater. However, unlike surface water resource assessments, they were found to lack an assessment of the risk of groundwater resource failure. A method, termed Aquifer Assurance Yield (AAY) was proposed as a means of including the supply assurance concept into the resource assessment. However, on investigation, it was found that this approach was too data intensive for the generalized approach that is required from this project. The AAY approach incorporates aspects of water balance principles as well as a more detailed risk assessment, thereby allowing for reliability during drought, above average availability after major recharge events, and policy requirements. However, the AAY approach requires long time series groundwater level data under pristine aquifer conditions - data that are not available for characterization on a countywide basis. It is an approach that should be considered for future detailed studies, and for this reason it is included in Appendix 1 of this report. Since it uses the same concepts as surface water, resource planners it could also assist in bridging the surface and groundwater disciplines.

4. METHODOLOGY

4.1 General The key guiding principles on which the method was developed are: i) It must be a GIS-based approach. This means that:

a. The default data sets will be in a GIS-format; b. It is easy to update (as new information becomes available); c. It can perform calculations on a country-wide basis (eg to obtain groundwater

storage, etc) ii) It must be useable at all scales (eg for small-scale, regional studies; and for

large-scale, aquifer or quaternary catchment studies). iii) It must be useable by people without GIS skills. Water resource

specialists/managers must be able to obtain the default values from the GIS system, and apply these values using a hand calculator to their areas of interest.

iv) The approach must be scientifically sound. v) The assumptions, data sources and mathematics that are used to determine the

answers must be documented and recorded so that the calculations can be re-evaluated and checked.

4.2 The GIS Layers Raster GIS layers or grids that are used in calculating groundwater storage have been developed to cover the entire country. The numeric values attached to the grids (eg aquifer thickness) form the default values that should be used for the “rapid assessment”. Where detailed assessments are carried out, site specific values will need to replace the default values.




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The GIS layers consist of five physical aquifer levels, namely:

Level 1 - bottom of the aquifer Level 2 - bottom of the natural dynamic groundwater elevation Level 3 - current groundwater elevation Level 4 - average groundwater elevation Level 5 - top of the aquifer

Static storage

Dynamic storage

These levels are described in Section 4.6 together with the way in which they were developed. A schematic representation of these levels is presented in Figure 1 for an elementary grid, abcd, located around a monitoring borehole M1. The elevations of levels 1 to 5 vary spatially and GIS techniques were used to generate the countrywide, 1 km by 1 km grid, for each level (except for Level 3). The volume bound between the layers reduced by an appropriate storage coefficient gives groundwater storage (V). Based on Figure 1, the mathematical expression for groundwater storage is:

hASV Δ=∑ 3 Where

∑ is a summation sign A is the area of each elementary grid S is the storage coefficient

hΔ is thickness between any two layers of interest. If: i) , eq. 3 gives the maximum groundwater storage; 15 hhh −=Δ ii) , eq. 3 gives the average total groundwater storage; 14 hhh −=Δ iii) , eq. 3 gives the average dynamic groundwater storage; 24 hhh −=Δ iv) , eq. 3 gives the current available groundwater storage; 23 hhh −=Δ v) , eq. 3 gives the static storage. 12 hhh −=Δ




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Figure 1: (a) Hydrograph of monitoring borehole M1 (b) Schematic presentation of the physical layers

4.3 Approach used in estimating each aquifer level Table 4 summarises the approach that was adopted for developing the default values for each aquifer level (for rapid studies), and it summarises how the site specific values should be obtained for detailed studies. The detailed description of how each default level was developed is presented in Section 4.6.

Table 4: Brief description of the approach used to generate the aquifer levels

Aquifer levels Description Method or data used to establish default

values

Method or data used to establish

site-specific values

Level 1 The bottom of the aquifer

The depth at which the permeability and storage coefficient of the aquifer is considered to have decreased substantially.

Vegter’s (1995) recommended drilling depth below average groundwater levels, or Vegter’s (1995) strike frequency graphs (modified for TMG). Still to be finalized.

Geophysics & drilling logs.

Level 2 The bottom of natural dynamic storage

This level is taken to be the lowest natural groundwater level. It is commonly the water level at the end of drought periods.

Based on Vegter’s (1995) strike frequency graphs (modified for TMG).

Unaffected water levels after droughts taken from monitoring boreholes

Level 3 Current groundwater level

Average current water levels N/A

Average current water levels for the area under study taken from monitoring boreholes

Level 4 Average groundwater level

The average groundwater level reflects average groundwater storage. This level will be compared to: i) the bottom of natural

Vegter’s (1995) groundwater level map.

Average, unaffected (by abstraction) water levels taken from monitoring




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Aquifer levels Description Method or data used to establish default

values

Method or data used to establish

site-specific values

dynamic storage in order to quantify average storage in relation to the component of storage that is recharge dependant; and to ii) the base of the aquifer to quantify average storage in relation to total groundwater storage.

boreholes.

Level 5 The top of the aquifer

This level is a subdued replica of topography that reflects the water level in the aquifer when it can be considered to be full. If groundwater levels are higher than this level, the aquifer can be considered to be “overflowing” and it will contribute towards baseflow. If they are lower than this level, then the aquifer can be considered to be “less than full”.

A grid comprising the lowest land elevation per 1 km by 1 km area.

Geophysics, drilling logs and rest water levels after major recharge (rainfall) events.

In addition to these levels, it is necessary to know:

The coefficient of storage

Either specific yield or storativity depending on aquifer type.

Vegter’s (1995) values. Pumping tests & numerical models.

4.4 Assigning confidence levels to the data and final result In order to provide the user with an indication of the reliability of the results, confidence levels should be assigned to the values used in each level. Not only is it important to acknowledge when default values have been used for the input

parameters in eq. 3 ( A , or and S yS hΔ ), it is also important to note that there is a

linear relationship between V A , or and S yS hΔ , and thus eq. 3 is equally sensitive to all the input parameters. There is a need, therefore, to describe storage in terms of confidence levels (CLs). The CLs of the input parameters are presented in Table 5 to Table 7. The confidence level in storage is the average of the confidence levels of the input parameters on the scale 1 to 3 (Table 8).

Table 5: Aquifer area : CLs depending on the data source Confidence Level Data source Low (1) Quaternary catchment or catchments. Medium (2) Lithology based on 1:250 000 geological maps High (3) Aquifer boundary as defined by detailed studies



DepartmeWater Affairs and Forestry

N: 2.0

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Table 6: Storativity or specific yield: CLs depending on the data source Confidence Level Data source Low (1) Estimated from other similar aquifer types or geological areasMedium (2) Determined form pumping test data High (3) Determined from SVF methods or numerical models

Table 7: Aquifer thickness: CLs depending on the data source Confidence Level Data source Low (1) Country-wide estimates of saturated thickness based on

water table and bedrock, or generalized water strike frequency analyses (described in Section 4.6.1)

Medium (2) Extrapolated from nearby similar aquifer types or geological areas

High (3) Geophysical data and drilling logs of the area under study, and in certain circumstances, from pumping test analyses or water level fluctuations (where the aquifer has been previously stressed).

Table 8: Groundwater storage confidence levels (average of input terms CLs) Confidence Level Average of CLs of input terms of eq.3 Low (1) 1 Medium (2) 2 High (3) 3

4.5 The procedure for quantifying groundwater The approach to quantifying groundwater follows the procedure in Table 9 and Figure 2. Note that where it is mentioned that a variable needs to be established (eg aquifer thickness), the value should be obtained from the default values provided in this study if a rapid assessment is being done (ie the value will come from the GIS layers that will be available on completion of the project), or, if a detailed study is being done, then the values will need to be obtained from data and reports on the area under investigation.

Table 9: Procedure for Quantifying Groundwater Steps Method Result

Step 1 Delineate the area to be studied. Area of groundwater unit

Step 2 Establish the coefficient of storage for the aquifer S-value 1. Establish aquifer thickness by taking the difference between

the top of the aquifer (Level 5) and the bottom of the aquifer (Level 1).

2. Apply the coefficient of storage to the aquifer volume. 3. Determine the average of input parameters confidence

levels.

The maximum groundwater storage with assigned confidence level

1. Establish the base of the natural dynamic storage (Level 2) and take the difference between this and the bottom of the aquifer.

2. Apply the coefficient of storage. 3. Determine the average of input parameters confidence

levels.

The static storage with assigned confidence level

1. Establish the average water level (Level 4) and take the difference between this and the bottom of the aquifer.


levels.

The average total storage with assigned confidence level

Step 3

1. Take the difference between the average water level (Level The average dynamic




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Steps Method Result 4) and base of the natural dynamic storage (Level 2).


levels.

storage with assigned confidence level

1. Establish the average current water level (Level 3) and take the difference between this and the base of the natural dynamic storage.


levels.

The current available dynamic storage with assigned confidence level

Example where all default values (except current water levels) are used: The area under study = 100 km2 (total of all 1km2 grids) S-value = 0.001 Level 1 - bottom of the aquifer = 40 m Level 2 - bottom of the natural dynamic groundwater elevation = 25 m Level 3 - current groundwater elevation = 20 m Level 4 - average groundwater elevation = 15 m Level 5 - top of the aquifer = 5 m




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Figure 2: Groundwater Quantification Algorithm/Flowchart

The volumes of groundwater in storage are: Maximum storage: 3.5 Mm3 (with low confidence level) Static storage: 1.5 Mm3 (with low confidence level) Average total storage: 2.5 Mm3 (with low confidence level) Average dynamic storage: 1 Mm3 (with low confidence level) Current available storage: 0.5 Mm3 (with low confidence level)




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In reality, if the default values are used, this example would consist of one hundred 1km2 grids, and the values for each Level could differ between grids. The final storage answers would be the sum of the answers of all one hundred grids.

4.6 Method for establishing the GIS layers

4.6.1 Level 1 – Bottom of the aquifer This depth has been taken to be the depth at which the permeability and storage coefficient of the aquifer are considered to have decreased substantially from that part of the aquifer which contains the bulk of readily accessible and exploitable groundwater. South Africa has relatively few primary aquifers and most (90%) of the groundwater occurs in secondary (weathered and fractured) rock aquifers (Vegter, 2001). For primary aquifers, e.g. alluvial aquifers, the base of the aquifer is fairly well defined in the form of solid rock (bedrock) that underlies any unconsolidated sediment or soil. However, for many secondary aquifers, there is no distinct base of the aquifer. Consequently, the definition of base of secondary aquifers is problematic. For this study, the problem has been addressed using groundwater strike information from the National Groundwater database (NGDB). Vegter (1995) and Seymour (1996) observed that there are distinct water strike-density profile curves for the various types of secondary aquifers. A statistical analysis of the water strike density with depth provides a reasonable indirect method of deriving the base of secondary aquifers. Vegter (1995) produced a Saturated Interstices Map of South Africa, which shows the optimum borehole drilling depth obtained from the statistical analysis of the water strikes in boreholes. For the purposes of this study, two approaches were assessed to define the aquifer base GIS grid. The first approach makes use of existing information and is based on the recommended drilling depths contained in Vegter’s Saturated Interstices Map of 1995, which in turn, were based on the water-strike frequency curves obtained from the NGDB in the early 1990s. The approach adopted by Vegter in obtaining drilling depths is not clearly described in his report. Vegter’s optimum drilling depths are presented as depth ranges and not absolute values from area to area, and therefore a single value was ascribed to each depth range for the purpose of developing a single aquifer base layer (Table 10). The drill depths (metres below water level) were then added to the average water levels (metres below ground level) to give the base of the aquifer in metres below ground level.

Table 10: Drilled depths based on Vegter (1995)

Vegter (1995) drill depth range (Metres below groundwater level)

Adopted drill depth (Metres below

groundwater level) <20 20

20-30 25 30-50 40

50-100 75 The second approach is based on the analyses of water-strike information obtained from the NGDB, similar to the approach of Vegter (1995) and Seymour (1996), but in this case all the current data on the NGDB is used – a totally 150,219 records for the entire country. Seymour only used boreholes on the NGDB that contained a single water-bearing fracture and his dataset consisted of 60,000 records. Furthermore, in




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this study the depth to the water-strike in metres below groundlevel is used, as opposed to Seymour who analysed depth of water-strike below water level. Seymour (1996) subdivided the water-strike information for the country into 226 quasi-homogeneous geographic regions or polygons. These regions were derived by superimposing the Exploitability Layer of Vegter’s Borehole Prospects Map over a simplified Lithostratigraphic Map (unfortunately no maps showing the extent of these regions could be located). Seymour generated so-called ‘strike-density profiles’ for each zone as follows: • The number of water-strikes (S) within each consecutive 10m interval below

the waterlevel was determined. In the example in Figure 3, 46 and 173 water-strikes occurred between depths 0 – 10 and 10 – 20m below the waterlevel.

• The number of boreholes (N) passing through a particular 10m depth-interval was calculated. Therefore, the number of holes passing through a particular depth interval decreases with an increase in depth below the waterlevel. In the example in Figure 3, 450 and 441 holes passed through the 0 – 10 and 10 – 20m depth intervals.

• The strike-density (SD) per 10m depth interval is calculated as follows: SD = S / N

Seymour noted that curves often show a relatively low strike-density just below the waterlevel, which he ascribed to infilling of fractures with clayey weathering products - thereby reducing fracture permeability (see ‘effective weathered storage’ in Figure 3). The strike-density maximum or peak represents the main water-bearing fractured portion of the aquifer (see ‘effective fracture storage’ in Figure 3). Vegter (1995) found that fresh rocks typically showed strike-densities of < 0.15 per 10m interval.




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Figure 3: Determination of Water-Strike Frequency per 10m Depth Interval below the Waterlevel




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Vegter (1995) and Seymour (1996) identified four typical or characteristic strike-density curves. Figure 4 and Figure 5 are the most common types of curves, where the initial increase in strike-density is interpreted as an indication of the existence of saturated decomposed rock. In the case of Figure 6, saturation is restricted to the fractured rock only. Competent Table Mountain Group quartzites do not decompose and there appears to be little near surface enhancement of fracturing (Figure 5), whilst despite the low strike-density, open water-bearing fractures occur uniformly to depths in excess of 100m to zoom and even deeper.

Vegter (1995) postulated that the nature of the saturated zone in hard rock formations, whether groundwater is stored in the fractures only or in fractured as well as decomposed rock, may be derived from strike-density curves – with the proviso that the data are adequate and contain a representative number of boreholes. He states that the ‘mean thickness of that part of the saturated zone which contains the bulk of the readily accessible and exploitable groundwater’, may be taken as half the depth to where strike-densities typical of fresh rock make their appearance – except in the case of the Table Mountain Group Aquifers. For example, in Figure 4, Figure 5 and Figure 6 these depths may be taken as 30, 50 and 40m, providing a corresponding mean thickness of 15, 25 and 20m.

Figure 4: Strike-Density Curve Granite Gneiss, NW of Pietersberg (Vegter, 1995)

Figure 5: Strike-Density Curve Karoo Sedimentary Rocks (Vegter, 1995)




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Figure 6: Strike-Density Curve Granite Gneiss, Dendron (Vegter, 1995)

Figure 7: Strike-Density Curve Table Mountain Group (Vegter, 1995)

The shallow portions of these strike-density curves are typically smooth, whilst the deeper portions of the curve are jagged as a result of insufficient number of deeper boreholes on which to base the statistics. Vegter (1995) states that, with the exception of the TMG Aquifer systems, the thickness of the saturated zone wherein the bulk of the readily accessible groundwater is stored is seldom greater than 25m. An approach similar to Seymour’s (1996) of utilising strike-density curves was adopted to broadly define the base of the secondary aquifers (Level 1 in Figure 1), except in this case a 5m depth interval below ground surface was used below 10m. The NGDB water-strike information (Figure 8) was imported into ArcGIS and analysed according to Vegter’s (2001) 64 groundwater regions of South Africa (Figure 9, Table 11). Vegter defined these regions based on the type of ‘openings’ (secondary or primary) in the aquifer, lithostratigraphy, physiography and climate. Groundwater regions 59, 60, 61 and 64 consist mainly of Tertiary-Quaternary primary aquifers.




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The resultant 64 strike-density curves were then visually analysed and the ‘base of the aquifer’ (Level 1) defined in metres below ground surface – the base of the aquifer being the point at which the strike density initially approaches zero. For instance, base of the secondary aquifers in groundwater region 42 is estimated at 170 m.bgl by visually fitting a linear-line to the strike-density recession curve (Figure 11). It must be noted that this depth to base of the aquifer does not imply that no water-bearing fractures are likely to occur below this level, but rather refers to the zone in which the bulk of the readily available and exploitable groundwater is stored.

Figure 8: Water-strike information from the NGDB classed according to strike-depth below ground surface overlain over Vegter (2001) Groundwater Regions




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Figure 9: Groundwater Regions of South Africa (Vegter, 2001) The strike-density graphs vary from region to region. In contrast to Groundwater Region 42, Figure 12 shows the strike-density curve for Groundwater Region 22, where the thick deposits of the Kalahari Group dominate the curve and where the ‘Base of Aquifer’ is estimated at 320 m.bgl. Inspection of the water-strike information in this region (Figure 8) also indicates that the defined regions are not homogenous in terms of water-strike depths, as water strike depths are generally < 60m along the southern margin of this region. The variability of water-strike depths within the Groundwater Region is indicated in Figure 10. A shortcoming of this approach is that the ‘Base of the Aquifer’ is determined visually from the strike-density curve. Work is currently being undertaken in an attempt to define a more objective method to defining the base of the aquifer.




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Figure 10: Groundwater Regions showing interpolated Depth to Water-Strike

Table 11: Description of 64 Groundwater Regions (Vegter, 1995) Groundwater

Region ID Area km2

Groundwater Region Name

GroundwaterRegion ID

Area km2

Groundwater Region Name

1 5692 Makoppa Dome 33 45349 Northeastern Upper Karoo 2 3790 Waterberg Coal Basin 34 49135 Bushmanland Pan Belt 3 13915 Beauty-Messina Granulite Gneiss Belt 35 11329 Hantam 4 1499 Limpopo Karoo Basin 36 12283 Tanqua Karoo 5 5710 Soutpansberg Karoo Troughs 37 31876 Western Upper Karoo 6 19391 Waterberg Plateau 38 35921 Eastern Upper Karoo 7 11982 Pietersburg Plateau 39 27402 Southeastern Highland 8 6124 Soutpansberg 40 19803 Western Great Karoo 9 19790 Western Bankeveld and Bushveld 41 42370 Eastern Great Karoo

10 7862 Zeerust-Delmas Karst Belt 42 57370 Ciskeian Coastal Foreland and Middleveld 11 4132 Middelburg Basin 43 30578 Transkeian Coastal Foreland and Middleveld 12 15259 Eastern-Northeastern Bankeveld 44 32874 Northwestern Middleveld 13 9399 Springbok Flats 45 25630 Northeastern Middleveld 14 12473 Western Bushveld Complex 46 21948 Kwazulu-Natal Coastal Foreland 15 16797 Eastern Bushveld Complex 47 17934 Northwestern Cape Mountain Ranges and 16 2449 Northern Bushveld Complex 48 6945 Southwestern Cape Mountain Ranges 17 16929 Central Highveld 49 33792 Southern Cape Mountain Ranges 18 29939 Western Highveld 50 2662 Oudtshoorn Basin 19 35475 Lowveld 51 13225 Willowmore-Grahamstown Belt 20 10136 Northern Lebombo 52 9348 Ruensveld 21 10114 Southern Lebombo 53 1342 Intermontane Tulbagh-Ashton Valley 22 50091 Eastern Kalahari 54 6197 Far Northwestern Coastal Hinterland 23 58253 Western Kalahari 55 7641 Knersvlakte




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Groundwater Area Groundwater Groundwater Area Groundwater Region ID km2 Region Name Region ID km2 Region Name

24 19957 Ghaap Plateau 56 7559 Swartland 25 17971 West Griqua Land 57 1497 Outenikwa Coastal Foreland 26 54978 Bushmanland 58 5539 Southwestern Cape Coastal Sandveld 27 32633 Namaqualand 59 192 Die Kelders 28 57805 Southeastern Highveld 60 1122 Bredasdorp Coastal Belt 29 19225 Taung-Prieska Belt 61 1798 Stilbaai Coastal Belt 30 25120 Northeastern Pan Belt 62 669 Lower Gamtoos Valley 31 41672 Central Pan Belt 63 4471 Algoa Basin 32 8014 Northeastern Highland 64 9654 North Zululand Coastal Plain

Figure 11: Groundwater Region 42 - Number and Density of Water Strikes per 5m Depth Interval showing estimated ‘Base of Aquifer’ at 170m.bgl

Figure 12: Groundwater Region 22 - Number and Density of Water Strikes per 5m Depth Interval showing estimated ‘Base of Aquifer’ at 320 m.bgl




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4.6.2 Level 2 – Base of Dynamic Storage under Natural Conditions Under natural conditions groundwater level fluctuates with time in response to groundwater inputs ( inQ ) and outputs ( ) as shown in Figure 1 (a). Based on these fluctuations, the groundwater resource can be classified as dynamic or static. The dynamic groundwater resource can be defined as the volume of groundwater available in the zone of natural water level fluctuation. These fluctuations are a function of natural outflows and annual or periodic recharge.

outQ

The absolute minimum unaffected (by abstraction) groundwater-level for a chosen representative monitoring borehole hydrograph marks the bottom of the dynamic groundwater storage. Below this level is static groundwater storage. The static storage can be defined as the volume of groundwater available in the permeable portion of the aquifer below the zone of natural water level fluctuation. Static storage is not affected by recharge and other seasonal or periodic influences. If the aquifer is to be utilized in a sustainable way, the total amount of groundwater available for abstraction should not exceed the total dynamic storage in the long term. If abstraction exceeds the dynamic storage groundwater will come from static storage. Extended and/or long-term abstraction from static storage will be accompanied by hazards such as groundwater mining (over-exploitation), geo-technical problems (such as land subsidence), salinisation (if poorer quality water is induced into the aquifer) and other environmental problems (such as the reduction or stoppage of spring flows, etc). The acceptable portion of static storage that should be abstracted depends on the management criteria for each area (which is an issue beyond the scope of this project). Default values for the base of the ‘Base of the Zone of Dynamic Storage’ (Level 2 in Figure 1) have been visually established using the water-strike density curves for each of Vegter’s (2001) 64 Groundwater Regions (Figure 9). Where the strike-density curve exhibits a clear ‘peak’ or maximum, the depth to the base of this zone is taken as the lower depth range at which the peak occurs. For example, the depth to the base of the zone of dynamic storage is estimated at 40m below surface for Groundwater Region 2 and Region 22 (Figure 12), whilst 25 m.bgl is estimated for Region 42 (Figure 11). This depth, although defined slightly differently, is equated to Seymour’s (1996) boundary between ‘effective weathered’ and ‘effective fractured’ zone of storage (Figure 3). Site-specific values, for detailed studies, need to be obtained from water level hydrographs of monitoring boreholes that are unaffected by abstraction or other anthropogenic activities that could affect groundwater levels. An example of a monitoring borehole that gives a good indication of the base of dynamic storage is monitoring borehole B5N018 in the Limpopo Province (Figure 14). Here the base of dynamic storage can be taken to be 28 m below ground level.




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Figure 13: Groundwater Region 2 - Number and Density of Water Strikes per 5m Depth Interval showing estimated ‘Base of Dynamic Storage Zone’ at 40 m.bgl

-40

-35

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l)

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Jan-62 Mar-64 May-66 Jul-68 Sep-70 Nov-72 Jan-75 Mar-77 May-79 Jul-81 Sep-83 Nov-85 Jan-88 Mar-90 May-92

Water Level Rainfall

Monitoring Borehole B5N018

Base of dynamic storage

Figure 14: Example of how the base of dynamic storage (28 mbgl) can be obtained from an unaffected monitoring borehole

4.6.3 Level 3 – Current Groundwater Elevation The current groundwater levels will be used to establish current available groundwater storage for the following year. Values will have to come from boreholes that reflect, as best as possible, the average water level conditions in the area under study. Preferably, they should be monitoring, non-production boreholes, but if such




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boreholes are not available, then the levels used must be rest-water levels as opposed to pumping water levels. No default values can be given.

4.6.4 Level 4 – Average Groundwater Elevation The average groundwater level to be used for the default values was adapted from Vegter’s (1995) in the manner outlined in Table 12.

Table 12: Average Groundwater Levels based on Vegter (1995)

Vegter (1995) water level range (Metres below groundwater level)

Adopted average water level

(Metres below groundwater level)

<10 8 10-20 15 20-30 25 30-50 40 50-75 60

75-100 85 100-125 110

>125 130

For detailed studies, the average water levels will need to be established from water level monitoring data.

4.6.5 Level 5 – Top of the Aquifer A minimum topographical elevation surface (grid) was developed to form the top of the aquifer layer. This surface is the lowest topographical elevation on a 1km,x,1 km grid. The,1km grid was created using a MIN aggregate function in the Raster Calculator of ArcGIS Spatial Analyst, and was based on the 90m Shuttle Radar Topography Mission elevation data provided by DWAF IWQS. Examples of this surface are represented in Figures 3 to 5.

4.6.6 Assessment of Aquifer Level 1, Level 3 and Level 5 Three transects were developed to provide a graphical view of the aquifer-layer (Level 1 based on the first (Vegter, 1995) approach Figure 15. They are (i) Eastern RSA: Ramatseliso’s Gate (~28.8º E; 30º S, at the border between RSA and Lesotho) to Port Edward; and Ramatseliso’s Gate to Matatiele (~50 km); and (ii) Southern RSA Victoria West - Beaufort West - Rietbron - Vermaaks River (Dysseldorp) – George. The cross-sections of the default physical levels (excluding the base of the dynamic storage, as this has not been finalized) were plotted on the same graph (Figures 16 to 18). The plots assist in assessing the accuracy of the estimated aquifer-levels relative one another. The topographic surface was developed using a 1 km grid created by aggregating 90m Shuttle Radar Topography Mission elevation data provided by DWAF IWQS using a MEAN aggregate function in the Raster Calculator of ArcGIS Spatial Analyst.




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Figure 15: Location of the transects

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Distance (km)

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atio

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Top of Aquifer

Average Waterlevel

Base of Aquifer

Figure 16: Ramatseliso’s Gate to Matatiele




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Figure 17: Ramatseliso’s Gate to Port Edward

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1 17 33 49 65 81 97 113 129 145 161 177 193 209 225 241 257 273 289 305 321

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Figure 18: Victoria West – Beaufort West – Rietbron – Dyselldorp – George




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The integrity of the various aquifer-level grids was also checked using geospatial mathematical operations in the GIS. For instance, the elevation of the maximum aquifer waterlevel (level 5) should not exceed the ground surface, unless the groundwater contributes to river baseflow or wetlands. In order to check for such ‘errors’ aquifer-level 5 was subtracted from the DTM (Figure 19), where the maroon pixels represent potential ‘overflow’ errors. Such errors in the default national datasets still need to be removed.

Figure 19: Resultant 'Error Check' Grid of DTM - Maximum Aquifer Storage Level

4.7 Provisional National Estimates of volumes of groundwater stored in Aquifer Systems The volumes of groundwater stored in the aquifer systems of South Africa were provisionally estimated using the default national aquifer storativity and aquifer storage-levels.

4.7.1 Maximum Groundwater Storage A 1 x 1 km grid of the maximum volume of groundwater storage (MAS) was generated as follows: MAS = (Level5 – Level1) x STOR




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3

Where: MAS = Maximum Aquifer Storage (m /km2) STOR = Aquifer Storativity.

The maximum volume of groundwater stored in the aquifer systems is estimated at 112.8,x,109 m3 (Figure 20). A number of anomalies are evident in Figure 20 and are currently being checked and corrected – e.g. the Eastern Karoo aquifer systems appear to have a greater maximum volume of groundwater stored per unit area than the Table Mountain Sandstone aquifer systems.

Figure 20: Maximum Volume (m3 2/km ) of Groundwater Stored in Aquifer Systems in South Africa

4.7.2 Average Groundwater Storage A 1 x 1 km grid of the ‘average’ volume of groundwater storage (AAS) was generated as follows: AAS = (Level4 – Level1) x STOR Where: MAS = Average Aquifer Storage (m3/km2)

STOR = Aquifer Storativity. The volume of groundwater stored in the aquifer systems under ‘average’ waterlevel conditions is estimated at 65.0,x,109 m3 (Figure 21). The input datasets are currently being checked to assess the anomalous results already discussed above, which are also evident in Figure 21.




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3 2/kmFigure 21: Average Volume (m ) of Groundwater Stored in Aquifer Systems

in South Africa

5. CONCLUSION The GRA II project aims to provide an approach and generic data sets that can be used in quantifying groundwater resources in South Africa. The main purpose is to provide guidance on how much water can be allocated for use. Project 1, Methodology for Groundwater Quantification, provides the method and data sets for quantifying storage. Once this has been done, the proportion of storage that can feasibly be abstracted, and the proportion that should be abstracted in a single year, in order to bridge drought cycles can be determined (and falls under Project 2). In order for the approach to be practical and user friendly, the following factors have been incorporated into the approach: • The data sets are spatially (GIS) based; • The data sets can easily be updated or replaced once new data becomes

available; • The approach is applicable at various scales; • The approach is easy to use (i.e. the answers can be obtained using a hand

calculator and it is not necessary to have GIS-computer skills). The key outputs of Project 1 are: • A description of the method used to quantify groundwater storage

• Default storage values in m3 2 per km grid area.




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The method of determining storage is the same whether a rapid or a detailed assessment is done; the difference lies in the input data. In the case of rapid assessments, the default values provided by this project should be used; and in detailed assessments, site-specific data will need to be used. In order to generate default values, GIS layers based mostly on a 1x1 km grid have, been developed for the various “levels” within an aquifer. These levels are grouped into static storage, which is the volume of groundwater available in the permeable portion of the aquifer below the zone of natural water level fluctuation; and dynamic storage, which is the volume of groundwater available in the zone of natural water level fluctuation. The levels are listed below:

Level 1 - Base of the Aquifer. Level 2 - Base of Zone of Ambient Dynamic Groundwater Elevation. Level 3 - Current Groundwater Elevation. Level 4 - Average Groundwater Elevation. Level 5 - Top of Aquifer.

Static storage

Dynamic storage

By taking the difference between these layers various storage volumes can be calculated. Project 2, which provides the groundwater planning potential map, will use one or more of these storage values, together with drought cycles and aquifer permeability to determine the practical groundwater resource that should be available for use in any given area on an annual basis. The basic methodology for determining the various volumes of groundwater stored in an aquifer system has been defined. The default national datasets required to apply the proposed methodology to determine aquifer storage volumes, namely Aquifer-Levels 1, 3, 4 and 5, have been compiled in the GIS and provisional estimates of the total volume of groundwater stored in the aquifer systems completed. A number of anomalies have been detected in the output grids of aquifer storage – the sources of which are currently being assessed and corrected. The groundwater strike-density approach proposed to determine Aquifer-Levels 1 and 2, both on a local and regional scale, needs to be further evaluated before it can be widely applied. The approach is currently being refined using the water-strike information contained in the NGDB for each of Vegter’s (2001) 64 groundwater regions. Groundwater-strike variability within these regions indicates that these broadly defined regions may not be the ideal basis for preparing strike-density curves for the default national Aquifer Level 1 and 2 datasets.


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APPENDIX 1: AQUIFER ASSURANCE YIELD APPROACH (AAY) The AAY approach is based on the concepts used in the behavior (simulation) analysis used in sizing reservoirs for surface water resources (McMahon and Mein, 1978). The procedure models the behavior of stored water over time taking into account the serial correlation, seasonality and all the inputs to the groundwater mass storage equation:

abbfnoutinttt QQQQQRZZ −−−−++=+ %1 1-1

subject to CZt ≤≤ +10 where = storage at the end of the time period (= storage at 1+tZ tht beginning of period) tht 1+

tZ = storage at the beginning of time period tht

tR = recharge during time period tht

inQ = groundwater inflow through aquifer boundaries during time period tht

outQ = groundwater inflow through aquifer boundaries during time period tht

%nQ = aquifer yield at a given reliability or assurance, , e.g 95% %n

ep−1

assurance (assurance is defined as where is the acceptable probability of failure for instance, at 95% assurance, probability of failure is 5%)

ep

bfQ = base-flow during the time period tht

abQ = ongoing groundwater abstraction during the time period thtC = active storage. The upper limit of the active capacity is the top of the aquifer described in section 4.2. The AAY approach is essentially based on the law of mass conservation. If the monitoring water levels are available, the calculation storage, , will be based on groundwater hydrographs in the area of interest. The calculations of storage will be based on Eq. 3. The great advantage of storage calculations based on the groundwater hydrographs is that, it is not necessary to understand or quantify all the inputs and outputs of Eq. 1-1, because the method presupposes that rises in groundwater level represent the integration of these processes.

1+tZ

The steps for implementing AYY approach are outlined below. i) Delineate area to be studied (e.g Figure 1-1 (a)). ii) Plot groundwater hydrographs for the delineated area on the same plot and choose the

groundwater levels at matching times at the desired time intervals (e.g Figure 1-1 (b)). Produce in GIS the layers for the respective groundwater levels at different time intervals (t1, t2, t3,….tn as shown in Figure 1-1 (b).

iii) Determine the S or values for the delineated areas. yS


APPENDIX of 41


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M1

M9 M8

M7

M4

M5

M6

M2

M3

M10

t1 t2 t3 t4 tn

M4

M2

M7

M3

M1

M6

M8

M9

M10

Time

(a)

(b)

Figure 1-1 (a) Study area (b) Groundwater hydrographs for study area


APPENDIX of 41


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15 hhh −=iv) Apply Eq. 3 using the parameters of steps i to iii above with Δ and call this total time series storage (see also section 4.2). Present it as a plot (Figure 1-2).

v) Decide on the minimum aquifer storage (reserve) and mark it on the plot of step iv as shown in Figure 1-2. The reserve determination falls under Project 4.

vi) Decide on the acceptable probability of failure as described in the next paragraph.

vii) Choose the desired yield (Q ) of the aquifer and deduct it from the storage values of step iv. Plot the result net storage on the same plot and check if the desired aquifer yield satisfy the probability of failure criteria. If not choose another value of aquifer yield and repeat the process until the desired probability of failure is achieved.

%n

For example, consider an aquifer system with storage shown Figure 1-2. The left hand side shows the storage before the aquifer is developed. The total of dynamic storage and static

gives the storage described in step iv above. Arbitrarily choose the first estimate of Q and apply Eq. 1-1 and plot the resulting storage on the same plot.

%n

Active capacity

Static storage

Dynamic Storage

Reserve %95Q

Initially full

Simulation period

Before aquifer development

After aquifer development

Explanation

Storage variation before aquifer development (step iv)

Theoretical storage variation after aquifer development (step vii) Line marking zero % active capacity

Figure 1-2: Schematic representation of the aquifer assurance yield approach


APPENDIX of 41


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Compute the probability of failure by dividing the total number for which the aquifer is empty (i.e. the total number of time the thick line in Figure 1-2 touches the line of zero % active capacity) by the total number of time periods in the simulation time. If the probability of failure calculated in step iv can not meet the desired assurance, a new value of is chosen and the process is repeated until the desired assurance is achieved. In this if the total number of time periods shown in Figure 1-2 were 100, then the probability of failure this diagram shows is 5%. 5% probability of failure represents 95% assured yield.

%nQ

Advantages • The approach simulates closely the behavior of storage in a given aquifer. • The approach is simple and can be easily understood even by non-technical persons. • The approach takes into account serial correlation insofar as it is included in the

historical observations. Therefore statistically this approach is valid. • The method can also be applied to stressed aquifers. • The method can be used to assess aquifer management options eg the effect of

artificial recharge on live storage. • Even enacted policies on the abstraction can be modeled in this approach. Disadvantages • It is data intensive.


APPENDIX of 41


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APPENDIX 2 : ANALYSIS OF STRIKE DENSITY DATA


GLOSSARY of 41


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GLOSSARY TERM DEFINITION Assurance level The reliability of the resource expressed in terms of the likelihood of frequency

of failure. For example, 4% assurance level means that failure to supply the quoted yield occurs in one year out of twenty-five years on average.

Aquifer A geological formation (or one or more geological formations) that is porous enough and permeable enough to transmit water at a rate sufficient to feed a spring or a well.

Base-flow Streamflow derived mainly from ground water seepage into the stream. The base-flow in Case Study 5 of this study is defined as the annual equivalent of the average low flow that is equaled or exceeded 75% of the time during the 4 driest months of the year (Schultz and Barnes, 2000).

Confidence level An assessment of the quality of input data based on the source from which it is obtained.

Dynamic storage The volume of groundwater available in the zone of natural water level fluctuation (related to natural recharge).

Fracture Any break in rock whether or not it causes displacement, owing to mechanical failure by stress. Fractures include cracks, joints and faults, (from the glossary of Vegter, 2000).

Groundwater Underground water that is generally found in the pore space of rocks or sediments and that can be collected with boreholes, wells, tunnels, or drainage galleries, or that flows naturally to the earth's surface via seeps or springs.

Groundwater availabilty Total volume of groundwater that is available for use from an aquifer. Groundwater mining Pumping ground water from a basin at a rate that exceeds safe yield, thereby

extracting ground water that had accumulated over a long period of time. The result is that, the aquifer may eventually be pumped dry (from the glossary of Schloss et al., 2000).

Hydraulic conductivity Factor of proportionality in Darcy's equation defined as the volume of water that will move through a porous medium in a unit time under a unit hydraulic gradient through a unit area at right angles to the direction of flow.

Lithological domains Distinct areas which bear the same description of rocks on the basis of physical characteristics, such as color and mineral composition.

Numerical model Model that uses numerical methods to solve the governing equations of the applicable problem.

Optimal yield An enhancement of the safe yield concept, which is determined by optimization theory considering socio-economic issues. The optimal yield is determined by selecting the optical management approach that best meets socio-economic goals (Freeze and Cherry, 1979).

Overdraft (1) pumping of ground water for consumptive use in excess of safe yield (from the glossary of Schloss et al., 2000).(2) the condition of a groundwater basin where the volume of water withdrawn exceeds the amount of water captured over the basin over a period of time. The use of water in excess of the perennial yield (from the glossary of Schloss et al., 2000).

Overexploitation A situation in which, for some years, average aquifer abstraction rate is greater than, or close to the average recharge rate (Custodio, 2002)

Recharge The replenishment of ground water in an aquifer. It can be either natural, through the movement of precipitation into an aquifer, direct stream recharge, or artificial-the pumping of water into an aquifer.

Saturated thickness The vertical thickness of an aquifer that is full of water. For the unconfined, unconsolidated aquifers, with distinct boundaries at their bases (eg alluvium overlying bedrock) and those that have a fairly distinct interface between the weathered zone and the underlying fresh rock, the saturated thickness is equal to the difference in elevation between the bedrock surface and the water table. For aquifers with poorly defined bases such fractured and weathered aquifers where the frequency of fracturing changes with depth. Under these conditions, Vegter (1995) defined the saturated thickness as the difference between median regional water strike depth minus median rest water level.


GLOSSARY of 41


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TERM DEFINITION Safe yield (1) Rate of groundwater extraction from a basin for consumptive use over an

indefinite period of time that can be maintained without producing negative effects; (2) the annual extraction from a ground-water unit which will not, or does not, (i) exceed the average annual recharge; (ii) lower the water table that permissible cost of pumping is exceeded; (iii) lower the water table as to permit intrusion of water of undesirable quality; or (iv) lower the water table as to infringe upon existing water rights; (3) the attainment and maintenance of a long-term balance between the volume of ground water withdrawn annually and the annual amount of recharge; (4) the maximum quantity of water that can be guaranteed from a reservoir during a critical dry period. Synonymous to firm yield. (All definitions from the glossary of Schloss et al., 2000).

Specific yield The quantity of water given up by a unit volume of a substance when drained by gravity.

Static storage The volume of groundwater available in the permeable portion of the aquifer below the zone of water level fluctuation.

Storage coefficient A more general term encompassing both or either storativity and/or specific yield. See storativity and specific yield.

Storativity Volume of water released per unit area of aquifer and per unit drop in the potentiometric surface. It is the product of the saturated thickness and the specific storage.

Sustainable Yield Volume of ground water that can be extracted annually from a ground water basin without causing adverse effects (from the glossary of Schloss et al., 2000).

Transmissivity Flow capacity of an aquifer measured in volume per unit time per unit width equal to the product of hydraulic conductivity times the saturated thickness of the aquifer.

Water balance A mathematical construction that shows the amount of water leaving and entering a given watershed or aquifer.

6. REFERENCES McMahon TA and Mein RG (1978) Reservoir capacity and yield. Developments in

water science. Elsevier. Seymour A (1996) Explanation Report - Groundwater Harvest Potential Map of South

Africa, with notes on Secondary Aquifer System Classification. Unpublished Internal Document, January 1996, Department of Water Affairs and Forestry, Pretoria.

Vegter JR (1995) Groundwater harvest potential map of South Africa and explanation

report. Department of Water Affairs Vegter JR (2001) Groundwater Development in South Africa and an Introduction to

the Hydrogeology of Groundwater Regions. WRC Report no TT 134/00. Water Research Commission, Pretoria.

Wright KA and Xu Y (2000) A water balance approach to the sustainable

management of groundwater in South Africa, Water SA Vol. 26 (also available on website http://www.wrc.org.za)


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DOCUMENT HISTORY

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COMMENTS PAGE NUMBERS OF PAGES CHANGED FOR AMENDMENTS

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DOCUMENT ACCEPTANCE Role Name Signature Date Producer1 R Murray 2004-10-10 Accepted by2 E v Wyk Project Manager J Girman Project Sponsor H Pienaar

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