Groundwater Flow Systems and Salinity in the ... - CSIRO · Grant Jones (BRS) and Ron Bellchambers...

Groundwater Flow Systems and Salinity in the Valleys around Jamestown, South Australia: Geochemical and Isotopic Constraints

Richard G Cresswell and Andrew L Herczeg

CSIRO Land and Water Technical Report No. 30/04

June 2004

Copyright and Disclaimer © 2004 CSIRO To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water.

Important Disclaimer: CSIRO Land and Water 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 Land and Water (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: Salt-bush has been planted in the open valleys south of Jamestown to help lower shallow water-tables causing dryland salinity.

Photographer: Richard Cresswell © 2004 CSIRO

JAMESTOWN

Groundwater Flow Systems and Salinity in the Valleys around Jamestown, South Australia:

Geochemical and Isotopic Constraints

Richard G. Cresswell and Andrew L. Herczeg

CSIRO Land and Water Technical Report 30/04 / BRS Report ZZZ

May 2004

This report builds on the South Australian Salinity Mapping and Management Support Project funded by the National Action Plan for Salinity and Water Quality. The National Action Plan

for Salinity and Water Quality is a joint initiative between the State and Commonwealth Governments

SA-SMMSP Jamestown Hydrogeochemistry Report

ii

Addresses and affiliations of authors

Richard G. Cresswell Bureau of Rural Sciences GPO Box 858 Canberra ACT 2601 Australia Present address: Cooperative Research Centre for Landscape Environments and Mineral Exploration c/o CSIRO Land and Water Long Pocket Laboratories 120 Meiers Rd Indooroopilly QLD 4068 Australia

Andrew L. Herczeg Cooperative Research Centre for Landscape Environments and Mineral Exploration c/o CSIRO Land and Water Private mail Bag 2 Glen Osmond SA


iii

Executive Summary This report aims to provide some geochemical constraints for the groundwater systems around Jamestown in the Northern Agricultural District of South Australia, and hence provide further support for models of dryland salinity in this region. These groundwater systems must be placed in the context of the materials comprising the landscape, and a companion report (Wilford, 2004) examines the regolith architecture of the region, with emphasis on the use of airborne geophysics (AG) to help our understanding in three dimensions. That report also examines some of the ramifications of the nature of the landscape on salinity in the area. Local hydrogeological investigations have been summarised in Henschke, et al. (2004) and are combined with the inferences of the airborne geophysics as the basis for examining groundwater connectivity in the sub-surface. Specific objectives of this project were:

1. Evaluate the groundwater pathways as suggested by airborne geophysics; 2. Assess recharge mechanisms for the groundwater systems based on chemical

constraints; 3. Determine the connectivity between the surface waters and the deeper

groundwaters; 4. Determine the source(s) of salt in the groundwater.

The environmental isotopic composition of the groundwaters sampled from the observation bores show minimal divergence from the meteoric water line suggesting that diffuse recharge is the dominant infiltration process for the region. The highest 14C contents were measured form the flank of the central, Bundaleer Valley, sited in the colluvial aprons along the margins of the valleys. This suggest rapid, recent recharge is occurring through these regions. The deep bore in this area penetrates the alluvial sequence, sampling groundwaters at the base of the alluvials. This is the freshest bore sampled in the region, but has a relatively low 14C concentration of 46% modern carbon (pMC) implying older waters (up to 5,000 years). We surmise that there is a significant component of recharge from higher in the valleys contributing to the groundwaters at this site bringing older waters to mix with the young recharge from the valley flanks. Lithology contrasts between the colluvial flank deposits and the valley alluvials may also limit exchange of waters between these two sources. Bores in the eastern, BelalieValley also show low 14C (<55pMC), at levels similar to those in the deep Bundaleer bore. These may all be sampling deeper waters from the bedrock system leaking into the surficial deposits. The lowest 14C concentrations were measured in waters from the northern section of the Belalie Creek, consistent with this view of an older groundwater source to the north and east that is feeding the Belalie Creek area, and thence southwards down the Belalie Valley and west in to the deeper Bundaleer sediments. The dissolved solutes in Jamestown groundwaters are predominantly derived from marine aerosols and are evapo-concentrated prior to recharge to reach the observed salinities. There is no evidence of significant contribution from marine deposits of mineral weathering to the main solute load, which is dominated by Na+ and Cl- ions.


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Contents Addresses and affiliations of authors.............................................................................ii Executive Summary ..................................................................................................... iii Contents ........................................................................................................................iv Figures v Tables vi Acknowledgments........................................................................................................vii 1. Introduction......................................................................................................1 2. Aims and scope................................................................................................6 3. Physiology .......................................................................................................7 4. Climate & Vegetation ......................................................................................8 5. Hydrogeology ..................................................................................................9 6. Hydrogeological model .................................................................................13 7. Groundwater chemistry .................................................................................16

a. Sampling ........................................................................................................16 b. Analytical.......................................................................................................17 c. Field Parameters ............................................................................................17 d. Major Ion Chemistry......................................................................................18 e. Stable isotopes ...............................................................................................26

i. Stable isotopes of water: Oxygen/Deuterium ................................................26 ii. Strontium: 87Sr/86Sr......................................................................................27 iii. Sulphur: δ34SSO4 .......................................................................................27

f. Radioisotopes.................................................................................................28 i. Radiocarbon...................................................................................................28

8. Discussion......................................................................................................32 a. Recharge Mechanisms ...................................................................................32 b. Sources of salt................................................................................................32 c. Hydrogeological models ................................................................................34 d. Aims and Objectives revisited .......................................................................36

References....................................................................................................................37 Appendix I: SA-SMMSP DATA.................................................................................40 Appendix II: Rainfall and seawater data used in this report ........................................44 Appendix III.................................................................................................................45


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Figures Figure 1. Catchments of teh Jamestown region .............................................................2 Figure 2. The Broughton River Catchment....................................................................3 Figure 3. Distribution of saline land ..............................................................................4 Figure 4. Rainfall chart for Jamestown: 1878-2002 ......................................................5 Figure 5. Digital elevation model ..................................................................................7 Figure 6. Modelled rainfall response usinf HARTT......................................................8 Figure 7. Groundwater flownet for the Jamestown region ..........................................10 Figure 8. DEM streams vs airborne magnetics streams...............................................13 Figure 9. AEM, magnetics and streams .......................................................................14 Figure 10. Airborne radiometrics images ....................................................................15 Figure 11. Piezometer locations...................................................................................16 Figure 12. Modified Schoeller plot ..............................................................................18 Figure 13. EC vs TDS..................................................................................................19 Figure 14. Calcium vs a) depth and b) chloride...........................................................20 Figure 15. a) Chloride and b) alkalinity vs depth ........................................................21 Figure 16. Chlordie vs sulphate ...................................................................................22 Figure 17. Chloride vs sodium.....................................................................................22 Figure 18. Chloride vs alkalinity .................................................................................23 Figure 19. Chloride vs potassium ................................................................................23 Figure 20. Sulphur vs sulphate.....................................................................................24 Figure 21. Chloride vs bromide over chloride .............................................................25 Figure 22. Stable isotope plot ......................................................................................26 Figure 23. Radiocarbon vs depth .................................................................................31 Figure 25. Chloride vs bromide ...................................................................................33 Figure 26. Hydrogeology summary from airborne geophysics ...................................35


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Tables Table 1. Details of piezometers ...................................................................................17 Table 2. Three valley chemical comparison ................................................................19 Table 3. Radiocarbon data and calculated ages ...........................................................29


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Acknowledgments This report is but one component of a much larger project looking at methods of salinity mapping in South Australia under the South Australia Salinity Mapping and Management Support Project, funded by the National Action Plan for Salinity and Water Quality. The National Action Plan for Salinity and Water Quality is a joint initiative between the State and Commonwealth Governments. While this report is intended as a stand-alone document, it should also be considered in the broader context of other reports being produced under this Project. In this context, numerous discussions and significant input from a large number of colleagues have helped shaped its structure and content. The following, however, deserve explicit acknowledgement for their help and input to this report: Chris Henschke, Craig Liddicoat and Mary-Ann Young (PIRSA – Rural Solutions); Grant Jones (BRS) and Ron Bellchambers gave freely their assistance with data collection, reporting and field work. Ron also continued to provide local assistance and continues to collect water level and conductivity data for the project. John Dighton (CSIRO) took control of the field sampling operations. Aleks Plazinska and John Spring (BRS) facilitated the ion chemistry analyses. Heather Middleton (CSIRO, North Ryde) carried out the strontium isotope analyses. Megan Lefournor and Michelle Caputo (CSIRO, Urrbrae) performed the 14C, stable isotope and CFC analyses. Phil Bierwirth analysed the radiometrics images. We would also like to thank Fred Leaney (CSIRO, Urrbrae) for his critical review of this report.


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1. Introduction The wide valleys and rolling hills of the Jamestwon area belie a potentially complicated regolith of meandering palaeo-channels and intercalated colluvial fan deposits. Using airborne geophysics (AG) and supporting ground information, the South Australia Salinity Mapping and Management Support Project attempts to delimit the nature, extent and origin of these regolith units and help define their relationship with groundwater quality and potential salinity hazards. The Jamestown airborne electromagnetic (AEM) fly zone straddles three broad north-south valleys (Figure 1), which are part of the Broughton River catchment (Figure 2). For the purpose of this report, the valleys have been named from west to east as follows:

Caltowie catchment (headwaters of Yackamoorundie Creek) Bundaleer valley (headwaters of Baderloo Creek) Belalie Creek catchment (feeds into Bundaleer valley via Jamestown) Belalie Plains catchment (headwaters of Freshwater Creek)

There are three main geological zones that influence groundwater flow systems in the catchments:

Basement rock (fractured rock aquifers) Colluvial outwash fans fringing the ridges Valley-fill alluvium (sedimentary aquifers in the broad valleys)

The three catchments occupy about a quarter of the area of the Broughton River catchment. They also contribute water to the Bundaleer Reservoir. This is considered an unreliable reservoir in terms of water quality with a mean salinity of 1100 mg/L. Diversions from some of the streams, including Freshwater Creek, have ceased due to high salinity, indicating that dryland salinity has impacted on this storage (Jolly et al 2000). Old agency reports indicate that dryland salinity started being recognised as an important issue in the Jamestown area in 1982. Farmers reported marked increases in dryland salinity during 1982 and were becoming increasingly concerned with the loss of production particularly in the lower lying areas of the Bundaleer valley (Dept of Ag internal reports, 1982 and 1985). Dryland salinity is expressed as waterlogged and scalded areas in the Caltowie and Bundaleer Valley catchments (Figure 3). Watertables have fallen by a few metres over the past 10 years with corresponding decreases in salinised and waterlogged land. This has allowed the re-establishment of lucerne in some areas of the valley floor.


2

Anecdotal evidence has put some crucial dates on the development of salinity problems in the region. These dates have been compared to the climate record (as indicated by the rainfall cumulative deviation from the mean, Figure 4) by Mary-Anne Young of PIRSA Rural Solutions, Jamestown (pers. comm.). We can note that the observations of salinity in fact merely reflect a change in terminology and the earlier periods of waterlogging would today be referred to as salinity. Virtually no dryland salinity is observed in the Belalie Creek and Belalie Plains catchments. While the present broad Belalie Creek valley “pinches out” at Belalie gorge, the older palaeovalley continues south, crossing a subdued catchment divide (Wilford, 2004; Cresswell, 2004). This southward extension of the valley is underlain by a palaeovalley of the old Broughton palaeochannel (Rowett, 1997).

Figure 1. The three valleys under consideration in this project.


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Figure 2. The Broughton River Catchment within the Northern and Yorke Agricultural District. The sub-catchments covered by this project lie around Jamestown and are ephemeral tributaries to the Broughton River. The fly-zone is marked by the hashed line around the Jamestown area, and encompasses an area of very high salinity risk, moderately saline soils and areas of high salinity identified by the National Land and Water Resources Audit (NLWRA, 2000).


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Figure 3. Distribution of saline watertables, shallow watertables land recharge potential in the Jamestown region and location of the main settlements. (Data from DWLBC, 2002). Areas of severe dryland salinity indicated in orange. See key on next page.


5

Figure 4. Cumulative deviation from the mean rainfall for Jamestown. Significant environmental events are indicated from local knowledge (Mary-Ann Young, pers. comm. 2002).

Key to Figure 3.


6

2. Aims and scope This report aims to put some geochemical constraints on the groundwater systems and hence provide further support for models of dryland salinity in the Jamestown region. We do not concern ourselves here with chemistry of waters that may be associated with other forms of salinity, namely irrigation and transient. The former is not a significant concern in this region, despite some irrigated lucerne in the Bundaleer Valley. The latter has been the focus of a companion report on transient salinity in the region, focussing on a region at the southern end of the flying area, where it is more prevalent (Fitzpatrick, et al., 2003). The groundwater systems must be put into the context of the materials comprising the landscape. Another companion report examines the regolith architecture of the region, with emphasis on the use of the airborne geophysics to help understand the landscape in three dimensions and also examines some of the ramifications for salinity (Wilford, 2004). Hydrogeological investigations have been summarised in Henschke, et al. (2004), and are combined with the inferences from the airborne geophysics (Wilford, 2004) as the basis for examining groundwater connectivity in the subsurface. Specific objectives of this project are:

1. Evaluate the groundwater pathways as suggested by the AG 2. Assess recharge mechanisms for the groundwater systems based on chemical

constraints 3. Determine the connectivity between the surface waters and the deeper

groundwaters 4. Determine the source(s) for salt in the groundwater


7

3. Physiology Stephens, et al. (1945) described the soils and relief of the area, and this has been summarized and refined by Wilford (2004). A digital elevation model (DEM: Figure 5) derived from the airborne survey data, illustrates the landscape. The Jamestown region is characterised by a range of landforms from low relief colluvial and alluvial fans, floodplains and pediments, through to rises, low hills and hills. Depositional materials occur in three main valleys: the Belalie, Bundaleer and Caltowie. Coalescing colluvial and alluvial fans of Quaternary age have filled these valleys to depths of up to 40 metres. The thickest sediments consisting of silt, clay, fine sand and minor gravels occur in the Belalie and Bundaleer valleys. The Caltowie valley has a thinner sediment cover which appear to have a lower electrical conductivity when compared with the other two catchments. Low angle pediments characterise the upper parts of the Caltowie catchment (Wilford, 2004).

Figure 5. Digital elevation model of the airborne geophysics study area. Oblique view of the area looking north (from Wilford, 2004).


8

4. Climate & Vegetation The area has cool wet winters and typically hot dry summers. Jamestown has an average annual rainfall of about 460mm. Evaporation rates exceed rainfall throughout the year (Bureau of Meteorology, unpublished data - cited from Henschke, 1994). The region has undergone cyclic wetting and drying periods (Figure 4) that have largely determined the water tables in the area (Figure 6). Direct correlation between water tables and climate indicate that land-use practice is not the driving force behind salinity. The native vegetation prior to farming in 1870 probably consisted of open savannah (Stephens, et al., 1945). Low-lying areas along valley floors and adjacent low gradient alluvial and colluvial slopes were thought to be dominated by grasses, including Darthonia semi-annularis (wallaby grass), Lomandra dura (iron grass) and Themeda triandra (Stephens, et al., 1945). Elevated sites with better drainage probably supported a woody cover with a grassy understorey. Woody species included; Casuarina stricta (sheoak) Acacia spp (wattle) and Eucalyptus odorata of mallee habit (Henschke, et al., 1994 and Stephens, et al., 1945). Present day land use includes cropping (wheat, field peas) and sheep grazing. Soft pine plantations provide timber for timber milling operations.

Lehmann Bore 17 - Water levels with accumulative annual residual rainfall (0 months delay)

-3.75

-3.5

-3.25

-3

-2.75

-2.5

-2.25

-2

-1.75

-1.5

-1.25

-1

-0.75

-0.5

-0.25

0

Apr

-92

Apr

-93

Apr

-94

Apr

-95

Apr

-96

Apr

-97

Apr

-98

Apr

-99

Apr

-00

Apr

-01

Apr

-02

Apr

-03

Date

Dep

th (m

)

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Effe

ct o

f Rai

n (m

)

Long-term trend with ARR

Water level 17

Fitted for all monthly intervals

Effect of rainfall

Linear (Water level 17)

Figure 6. Modelled water level response based on rainfall for a bore in the Caltowie catchment. Modelled using HARTT (Ferdowsian, 2001). Note the close correspondence between measured water levels (dark blue) and the effect of rainfall (light blue).


9

5. Hydrogeology Figure 7 shows a groundwater flow map in the AEM fly zone constructed from existing bore data (60 piezometer sites and about 30 farm bore sites) as well as the 7 new piezometer sites (Henschke, et al., 2004). The flownet indicates an intermediate scale groundwater flow system is occurring in each of the three valleys. Groundwater flow paths range from approximately 4 to over 7 km in length. Indeed, these systems are linked to the Broughton System (Figure 2), which extends over 300 km. Groundwater flows in a general northeast to southwest direction in each of the three main valleys. The elevations of the groundwater contours indicate a step-up in elevation from west to east across the three valleys (eg Caltowie: 390-410m, Bundaleer: 430-450m, Belalie Creek: 500-530m). Surface water and groundwater tend to want to flow from east to west, but north-south quartzite barriers force the flows to trend toward a north - south direction. The major recharge areas would appear to be the colluvial outwash fans fringing the ridges on the eastern side of each valley. Recharge also occurs along the rocky ridge lines.

Caltowie and Bundaleer valley catchments In the Caltowie and Bundaleer valleys, groundwater converges toward discharge areas in the valley-fill alluvium represented by waterlogged valleys that have shallow watertables following wet years. Groundwater flow converges towards alluvial “pinch-outs” in both valleys. The alluvial “pinch-outs” occur near Caltowie and south of Jamestown in the Bundaleer valley. These are the areas where dryland salinity would be expected to occur. Dryland salinity is not apparent downstream of the “pinch-outs”. Some smaller areas of dryland salinity are associated with fractured rock aquifers at the break in slope of the ridges and the broad valleys. Groundwater flow paths are expected to be much shorter in these local flow systems (about 1km).


10

Figure 7: Groundwater flownet for the Jamestown airborne electromagnetic (AEM) fly zone (from Henschke, et al., 2004).


12

• Belalie Creek and Belalie Plains catchments Although groundwater data is sparse in the Belalie Creek subcatchment, construction of a flownet has been attempted. The Belalie Creek sedimentary valley “pinches out” at Belalie gorge, while the sedimentary valley continues south, crossing a subdued catchment divide. This southward extension of the valley may indicate the existence of a palaeovalley of the old Broughton palaeochannel, which apparently originated just east of Jamestown (Rowett, 1997). The “Belalie Springs” are an expression of groundwater discharge at the alluvial “pinch-out”. The spring water has an EC of 7 dS/m and this water discharges down the Belalie Creek through Jamestown and disappears underground in the Bundaleer drain some 3 km south of Jamestown. The flownet suggests that groundwater in the north-western part of the Belalie Creek catchment, converges toward the Belalie infilled gorge and discharges as springs. The spring-fed creek, which flows through Jamestown, has been gauged on a number of occasions. Discharge rates range from 25-70 L/s and salt loads from 2 to 20 tonnes per day. Groundwater in the south-eastern half of the Belalie Creek catchment flows south across the subdued catchment divide into the Belalie Plains catchment where groundwater continues to flow south. The groundwater drains out of the catchment to the south due to the presence of the palaeochannel, that prevents the aquifers from filling up. Airborne EM and magnetics data (Wilford, 2004; Cresswell, 2004) confirmed the existence of palaeochannels in the Belalie Creek/Plains catchments. The gravelly transition zone may act as a preferred pathway for groundwater flow. This catchment is not “filling up” like the other two western catchments.


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6. Hydrogeological model The hydrogeological model we wish to investigate is best illustrated by reference to the airborne geophysics results. Using the DEM collected at the time of flying, we can generate theoretical stream lines, representing the flow of water across the land’s surface during times of peak flow. This defines the surface catchments used in the description above. When compared to the image from the aero-magnetics (Figure 8) we see important differences, as alluded to above. In particular, while Belalie Creek feeds into Jamestown and then Bundaleer Valley on the surface, we see that the magnetics indicate a deeper channel that links Belalie Creek directly with Belalie Plains. We can also show that the surface divide between the Bundaleer and Caltowie catchments continues at depth.

Figure 8. Comparison of DEM-derived stream flow (left) with the magnetic image of sub-surface drainage lines (right). Note the incongruence for the Belalie Creek (eastern valley) flow lines. See text for discussion. Red arrow in left figure indicates flow direction depicted by magnetics. Dotted blue line demarks the subsurface watershed.


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This is further emphasised in the AEM image from 10-15mdepth (roughly the base of the valley fill and the site of dominant water flow (Henschke, et al., 2004), which clearly delineates transport of groundwaters beneath the Belalie Plains, derived from the north (Figure 9).

The breakthrough from the Belalie to Bundaleer Valleys is a recent phenomenon, caused by gradual infilling of the Belalie Creek catchment. The trace of the deep lead is clearly seen in the radiometrics image of the weak potassium radiation given off by the near-surface (top 30 cm) soils, while the uranium signal appears to reflect the present day surface drainage (Figure 10).

Figure 9. Airborne electromagnetics (AEM) conductivity depth image (CDI) of 10-15m below the ground surface. Preferential groundwater pathways are delimited by the high conductivity signals (red) indicating higher moisture content in these zones (from Wilford, 2004). Sub-surface drainage (from the magnetics image)is superimposed in white.


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Figure 10. Airborne radiometrics images. Potassium (left) shows the trace of the alluvial groundwaters and can be correlated with higher silt content. The Uranium image (right) appears to reflect present day surface drainage and may represent recent outwash from the granitic outcrops to the northeast. Both images are colour-coded for concentration, ranging from blue (low) to red (high). For potassium, this ranges from 0-2%; for uranium (right) from 0-500ppm.


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7. Groundwater chemistry

a. Sampling Water samples were collected in August 2003 from the ten piezometer sites described by Henschke, et al. (2004) and developed as part of this study (Figure 11). In addition, two private bores were sampled (C19 and Reece’s Bore) from the Caltowie and Belalie Catchments , respectively. An initial sampling was at times unsatisfactory due to the poor water recovering using the airlift method. In a subsequent trip some of the boreholes were re-sampled (using the Grundfos pump – see below) and a complete water sample obtained for 14C analysis. A sample was also taken from the flowing Belalie Creek to the north-east of Jamestown.

Figure 11. Location of piezometers sampled for this project. Locations are overlain on a transparent AEM depth slice for 5-20m, overlain on the magnetic image. All bores are prefixed with JT- in the records.


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Bore details are listed in Table 1. At least one, and preferably three, standing volumes of water in the boreholes were pumped using a Grundfos MP1 pump until the pH, temperature and EC were constant. Water samples were collected for major and minor ion chemistry, stable isotopes of water (δ2H and δ18O) and sulfate (δ34SSO4) as well as 87Sr/86Sr and 14C. The chemical analysis was done at the chemistry laboratories of the Bureau of Rural Sciences located in the Geoscience Australia Building in Canberra; 87Sr/86Sr were analysed at CSIRO Petroleum Resources, North Ryde and all other analyses were carried out at the CSIRO Land and Water Laboratories, Urrbrae.

Hole_ID PVC EASTING NORTHING SLOTS TD SUMP

JT1 50 mm

274326 6321163 10 - 13 13 n.d

JT2_1 50 mm

275087 6321209 8.5 – 11.5 11.5 -

JT2_2 50 mm

275096 6321209 23.5 – 29.5 29.5 -

JT2_3 100 mm

275100 6321210 10 - 13 13 -

JT3 100 mm

276880 6321430 18 - 24 24 -

JT5_1 100 mm

281167 6315593 21 - 27 33 27 -33

JT5_2 50 mm

282188 6315653 11 - 17 17 -

JT7 50 mm

283803 6322704 28 - 34 34 -

JT8 100 mm

280999 6330687 28-34 40 34-40

JT9 50 mm

270165 6324817 7.5 – 10.5 10.72 -

n.d: not developed Table 1. Details of piezometers installed for the SA-SMMSP.

b. Analytical Standard analytical methods were used as described elsewhere (Rayment & Higginson, 1992; Leaney, et al., 1994; Cook and Herczeg, 2000). Results are presented in tabular form in Appendix I, and various relationships are described and plotted in the sections below.

c. Field Parameters Groundwater salinity measured across the three catchments varied from 3,720 to 9,400 µS cm-1. Data from a previous study in 1991 within the Jamestown area showed a much larger range of 930 to 9,430 µS cm-1 (data supplied by C. Henschke). Major ion


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chemistry for the Jamestown groundwaters is brackish and is sodium and chloride dominated with relatively greater abundance of calcium and bicarbonate in the more dilute waters (Figure 12). The majority of waters had circum-neutral pH values, but with a slightly acidic result from the deepest bore, probably representative of waters from the underlying basement aquifer.

d. Major Ion Chemistry Major ion chemistry for the Jamestown groundwaters is illustrated in Figure 12. The 3 valleys may be distinguished by comparison of relative concentrations of major elements and radiocarbon content, as indicated in Table 2. We see that Caltowie Valley has relatively high chloride, sodium and sulphate, while Bundaleer Valley has relatively low salinities (TDS<5,000 mg/L), but moderate to high radiocarbon and Belalie has high calcium, moderately high chloride (but salinities still less than 5,000 mg/L).

The relatively high sulphate in Caltowie groundwaters results in an elevated electrical conductivity (EC) relative to total dissolved solids (Figure 13). The high calcium in Belalie Valley may reflect dissolution of carbonates from the underlying sediments and calcrete layers within and beneath the alluvial fill (Wilford, 2004). This could explain the increase in calcium with depth towards the basement rocks (Figure 14a. We also see an excess of calcium relative to chloride (Figure 14b again suggesting the dissolution of carbonates.

0

10

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30

40

50

60

70

80

90

100

K Ca Mg Na Cl Alkalinity SO4 14C

meq

/L

JT1JT2-1JT2-2JT2-3JT3JT5-1JT5-2JT7JT8Belalie Ck.Reece B.JT9C19

Jamestown ValleyJT1, JT2, JT3

Belalie ValleyJT5, JT7, JT8, Belalie CreekReece Bore

Caltowie ValleyJT9, C19

Figure 12. Modified Schoeller plot for the groundwater data.


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Caltowie Bundaleer Belalie Potassium Low Low Low Calcium Low Low High Magnesium Low Low Low Sodium Moderate Low Low Chlorine Moderate Low Moderate Alkalinity Low Low Low Sulphate High Low Low radiocarbon Moderate High & Moderate Moderate Table 2. Relative levels of major elements and radiocarbon for the 3 valleys.

y = 0.6054xR2 = 0.9191

0

1000

2000

3000

4000

5000

6000

7000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

EC (mS/cm)

TDS

(mg/

L)

BundaleerBelalieCaltowieBelalie Creek

Figure 13. Groundwater field electrical conductivity plotted against measured total dissolved solids (TDS). High sulphate in the Caltowie samples gives an elevated response compared to the low sulphate waters elsewhere.


20

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25calcium (meq)

dept

h to

slo

ts (m

)


Figure 14a. Calcium plotted against depth of piezometer slots. The addition of calcium may represent dissolution of carbonates from calcretes through the profile, but this is not matched by an increase in alkalinity (Figure 15a).

0

5

10

15

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25

30

0 10 20 30 40 50 60 70 80 90 100

chloride (meq)

calc

ium

(meq

)

Bundaleer

Belalie

Caltow ie

Belalie Creek

JT1

JT2-2

Belalie signature

rainw ater evaporation

Figure 14b. The increase in calcium with depth is also manifest in an excess of calcium

over chloride in the region’s waters, particularly those of the Belalie Valley. We see evidence of the interchange of waters from Belalie Creek into both Belalie Valley and Bundaleer Valley.


21

The increase in concentration with depth shown by calcium is not matched by an increase in the alkalinity (Figure 15a). Instead, alkalinity shows an apparent decrease with depth, though this probably reflects the differences between the three valleys which have different thicknesses of alluvial fill. No trends with depth are seen for the other major ions (e.g. chloride, Figure 15b).

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6 7 8alkalinity (as meq CaCO3)

dept

h to

slo

ts (m

)


Figure 15a. Alkalinity plotted against depth of slotted interval. The distribution is similar to the conservative ion chloride (Figure 15b, below), not that of calcium

(Figure 14a).

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50 60 70 80 90 100chloride (meq)

dept

h to

slo

ts (m

)


JT2-2

JT1

JT5-1

JT7

JT3

JT2-3

JT2-1

JT8

RB

JT5-2

JT9C19

Figure 15b. Chloride plotted as a function of depth. Note the similarity with the

distribution of alkalinity in Figure 15a.


22

The grouping defined by calcium against chloride is also defined by sulphate (Figure 16) and sodium (Figure 17), as well as alkalinity (Figure 18), magnesium (which mimics calcium) and potassium (Figure 19) This grouping relates to the exchange of waters from the Belalie Creek catchment south into Belalie Valley, and west into Bundaleer Valley.

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50 60 70 80 90 100chloride (meq)

sulp

hate

(meq

)

BundaleerBelalieCaltowieBelalie Creekrain evap

JT1

JT2-2

Belalie signature

Figure 16 Sulphate:chloride plot for waters of the Jamestown region. A grouping suggests a common source for waters of Belalie Creek and Valley and some of the waters from Bundaleer Valley.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100chloride (meq)

sodi

um (m

eq)


JT1

JT2-

Belalie signature

Figure 17. Sodium:chloride plot for the same waters as Figures 15 and 16. A similar grouping is observed.


23

0

1

2

3

4

5

6

7

8

0 10 20 30 40 50 60 70 80 90 100chloride (meq)

alka

linity

(as

meq

CaC

O3)


JT1

JT2-2

Belalie signature

Figure 18. Alklinity plotted against chloride picks out a similar grouping to that seen in calcium, sulphate, sodium and magnesium (not shown).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90 100chloride (meq)

pota

ssiu

m (m

eq)


JT1

JT2-

Belalie signature

Figure 19. Potassium plotted against chloride showsa similar grouping to the other elements, but the Belalie Creek sample is lower than the other samples in the group.


24

Low sodium may reflect water:rock interaction and sodium adsorption onto the clays of the alluvial deposits. Potassium is also affected in this way. Thus, we have cation exchange with calcium and magnesium in feldspars and mafic minerals being released in to groundwaters, while sodium and potassium are absorbed into clays. The low sulphate relative to chloride and rainfall values is suggestive of sulphate reduction and loss of sulphur to reduced species. The correlation of sulphate with sulphur would corroborate this view (Figure 20).

The proximity of some groundwater signatures (e.g. Cl, Na, SO4) to rainfall evaporation curves (e.g. Figures 16 and 17) suggests a rainfall component to the groundwater (we here use the average rainfall chemistry of rainfall from Port Lincoln and the Gawler Ranges (Keywood, 1995)). Bromide to chloride ratios give evidence for this as the groundwaters have Br/Cl signatures intermediate between those of evaporated rains from Port Lincoln to the south, and the Gawler Rangers to the west (Figure 21). Samples JT1 and JT2-2 from the Bundaleer Valley show distinct signatures. Both show chemistry closer to rainfall values and have the lowest salinities in the area. JT1 was drilled up-slope on the western side of the Bundaleer Valley (Figure 11), off the alluvial plain, on a predominantly colluvial apron (Henschke, et al., 2004). JT2-2 was drilled in the deepest part of the Bundaleer Valley sequence and was slotted (i.e. sampled) near the base of the alluvial sequence (Henschke, et al., 2004).

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14sulphur (meq)

sulp

hate

(meq

)


Figure 20. Sulphur and sulphate plot sympathetically with the exception of elevated sulphate in the Caltowie waters.


25

We may surmise from the chemistry that the groundwaters sampled at these sites are different from those sampled at the other Bundaleer sites that are sampling similar waters to those from the Belalie Valley.

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0 10 20 30 40 50 60 70 80 90 100chloride (meq)

brom

ine/

chlo

rine

(meq

)

Bundaleer

Belalie

Pt Lincoln rain evaporation

Gawler ranges rain evaporation

Figure 21. Bromide to chloride ratio plotted as a function of chloride concentration for the Jamestown waters. All samples plot within the area bound by evaporated rainfall from the west and south.


26

e. Stable isotopes

i. Stable isotopes of water: Oxygen/Deuterium The δ2H and δ18O composition of groundwaters from the monitoring bore network range from –29.2 to –34.3 per mille (%0VSMOW) and –4.6 to –5.7 %0VSMOW, respectively (Figure 22). These values are generally more depleted in 2H and 18O compared to the mean annual rainfall composition for Adelaide (-24.5 and -4.5 %0VSMOW), but are similar to mean wintertime rainfall. Most of the data lie on the local meteoric water line, however the sample from Caltowie lies to the right of the meteoric water line, suggesting greater evaporation of rainwaters prior to infiltration. There is a tendency for the groundwaters to be more enriched in 2H and 18O in the boreholes where the screen depths are shallower.

-39

-37

-35

-33

-31

-29

-27

-25-6 -5.8 -5.6 -5.4 -5.2 -5 -4.8 -4.6 -4.4 -4.2 -4

δ18O

δ2 H

BundaleerBelalieCaltowieGMWL

Adelaide meteoric water line

Global meteoric water line

Figure 22. Stable isotope plot of oxygen and hydrogen. The waters plot close to the global meteoric water line suggesting a rainfall-dominated source. Only the sample from the Caltowie Valley plots to the right of the line indicating some evaporation prior to infiltration.


27

ii. Strontium: 87Sr/86Sr Six groundwater samples were analysed for 87Sr/86Sr of dissolved strontium to evaluate this tracer as an indicator of inter-aquifer or vertical mixing and groundwater source. The analytical results for the strontium isotope values reported in Appendix II are significant to six decimal places. Differences between values to the fourth decimal place are highly significant. All samples gave 87Sr/86Sr ratios between a relatively narrow range of 0.7125 and 0.7155. Because the 87Sr/86Sr of dissolved strontium reflects the mineralogy and age of minerals from the aquifer, it can be used to trace mixing where such differences occur. Aquifers with an abundance of potassium-bearing minerals, containing relatively high amounts of rubidium (Rb) (which is the parent of 87Sr), tend to have higher 87Sr/86Sr values than carbonate aquifers, which are consistently around 0.7090. All of the groundwater samples have high 87Sr/86Sr, significantly higher than marine values, and indicate that the strontium in these groundwaters is predominantly derived from local silicate mineral sources, or from waters passing through potassium-rich host rocks. The one sample from the hard rock aquifer (C19) does have a relatively high 87Sr/86Sr value of 0.715164 (±0.0013), though not as high as that from the alluvial waters sampled at JT3). This suggests some mixing between the very high values from silicate mineral dominated sources, and a less radiogenic marine aerosol source. This, in turn, may indicate significant contribution from silicate mineral dissolution providing strontium with relatively high 87Sr/86Sr.

iii. Sulphur: δ34SSO4 The sulphur isotopic composition of dissolved sulphate has a relatively narrow range from 16.6 to 22.4 %0CDT and is close to the seawater value of +19 %0CDT. There are several components that may contribute to dissolved sulphate including: marine aerosols, gypsum in the unsaturated zone, organic matter oxidation and oxidation of reduced sulphur minerals such as pyrite. These data, however, suggest an almost exclusive marine aerosol source, with relatively small component of organic S yielding slightly lower than seawater sulphate isotopic composition. No significant rock sources are evident. Nor are there likely to be significant gypsum sources given that the Jamestown groundwaters are under-saturated with respect to gypsum.


28

f. Radioisotopes

i. Radiocarbon Radiocarbon gives some insights into the recharge processes operating across the region. Eight samples were analysed for radiocarbon content. The hope was to give some indication of the age of different groundwaters and insights into the rates of horizontal flow, the connectivity of the various flow systems and possible connectivity between the alluvial aquifers and fractured rock aquifers in the adjacent highlands. Estimates of groundwater ages from the analytical results for 14C and 13C analysis of 9 groundwater samples from the Jamestown area as part of the SA-SMMSP (Table 1). The water samples were collected by CSIRO and PIRSA staff and the dissolved inorganic carbon (DIC) precipitated as BaCO3 at the CSIRO Land and Water Radiocarbon laboratory. The BaCO3 samples were analysed for 14C activity using the direct absorption method (Leaney et al., 1994). δ13C composition was measured on an aliquot of CO2 produced using a Europa Geo 20-20 mass spectrometer. Results of these analyses were reported during 2003 (job numbers 03100, 0360, 03105). Estimates of groundwater 14C “age” can be made from measured 14C data according to the rearrangement of the basic decay equation according to the following: t = –ln 8033(A/Ao) where t = time or age in years A= measured 14C activity of total dissolved inorganic carbon (TDIC) Ao = estimated initial radiocarbon content of TDIC at the time of recharge, and 8033 is the mean life of a 14C atom. This is what is often referred to as the uncorrected age because there is no consideration for chemical processes within the soil or groundwater affecting 14C concentrations. This age is usually, but not always, an underestimate of the true 14C age. Estimates of 14C groundwater “model” age are usually made by using geochemical and other isotopic data to reconstruct the initial 14C concentrations (A0) using chemical and isotopic data and applying several of the most common correction schemes. A short summary of three common correction methods known as Tamers correction (chemical approach), Pearson correction (isotopic approach) and the Fontes-Garnier correction (a combined chemical/isotope approach) is given in Appendix 1. Table 1: 14C groundwater ‘age’ estimates as uncorrected and two calculated model ages (based on 1mean of Pearson and Fontes & Garnier closed system correction schemes; 2 open system partial CO2 re-equlibration ) for the Jamestown area. Both uncorrected and model ages assume an initial 14C of 95 %mc.


29

Bore ID 14C

(pmC) δ13C Uncorrected

Age (yrs) Model Age1

(yrs) Model age2

(yrs) Closed system Open-system

JT 1 71 -10 2410 Modern Modern JT 2-2 46.4 -9.9 5920 130 2450 JT 2-3 70 -10.7 2530 Modern Modern JT 3 73.8 -10.2 2090 Modern Modern

JT 5-1 52.8 -10.4 4860 Modern 1670 JT 8 41.2 -9 6910 360 2840 JT 9 57 -10 4220 Modern 770

Reece Bore 56.9 -9.8 4240 Modern 430 JT7 43 -10 6550 850 3090

Table 3. Radiocarbon data and calculated ages. The concentrations and mole fraction of the dissolved CO2 and of the HCO-

3 in the bore water samples are required as input data for corrected ages using the Fontes-Garnier models, whereas the Pearson scheme requires only δ13C data. Estimates of these were calculated using field measurements of pH and temperature and laboratory analyses of major anions and cations of water sampled from the bores and running the analyses through a hydrochemical model (PHREEQC). Where chemistry analyses were not available, average values for the molality of the dissolved CO2 and of the HCO-

3 were used in the models. Where there is no temperature data available, a value of 18.5 degrees was assumed. Where no pH data was available, a value of 7.1 was assumed. The results shown in Table 1 indicate apparent ages for uncorrected 14C ages of between 2090 and 6910years for Jamestown area. The large uncertainties are a function of lack of precise knowledge of the initial 14C at the time of recharge, and in this case is estimates to be 95 ± 5 %mc. The closed-system model ages estimates give mostly negative values which correspond to ‘modern’ ages, which means that they must contain some bomb-fallout 14C making them less than 50 years old. This means that recharge is potentially very rapid and that all groundwater ages with the exception of JT2-2, JT8 and JT7 are less than 50 years old. This is unlikely given the depths of the screen intervals below the water table, but could be confirmed with CFC dating. The more probable interpretation is that there is an over-correction due to one or several of the assumptions concerning acquisition of 14C-free DIC in the unsaturated zone being invalid. The open system model age results (last RH column in Table 1) tend to lie in between the uncorrected and closed-system model ages. These values are considered the most appropriate groundwater ages from NAP bores sampled in the Jamestown area based on soil type and relative amounts of soil gas CO2 to soil water DIC. This does not necessarily assume that there is limited interaction between the solution and the carbonate minerals, but only that there has been re-equilibration


30

between the soil CO2 and the dissolved bicarbonate, which essentially resets the Ao value to 80-90 %mc prior to recharge. When determining corrected ages using the Pearson and Fontes-Garnier models, it is necessary to estimate the δ13C composition of carbonate material in the aquifer, and of the soil gas in the unsaturated zone. The δ13C composition of carbonate material in the aquifer is primarily determined by the origin of carbonate material in the aquifer and the unsaturated zone. Marine carbonates usually have δ13C compositions ~ 0 ‰ rel PDB while non-marine carbonate material may be as negative (depleted) as –8 ‰ rel PDB. For this study, a value of 0 ‰ rel PDB was used. CO2 gas in the unsaturated zone is generated primarily by root respiration. Hence, the δ13C composition of the soil CO2 gas will be dependent on the δ13C composition of the vegetation in the area. δ13C composition of vegetation ranges from approximately –10 ‰ PDB for plants that have a C4 metabolism to approximately –30 ‰ PDB for peat and humus derived from plants that have a C3 metabolism. In tropical parts of Australia, the values lie somewhere between the two because of a mixture of plants with C3 and C4 type metabolism. A δ13Csw value of –20 ‰,PDB was assumed given the absence of measured soil gas data for the study area. The most negative value measured for the δ13C composition of the DIC for the groundwater in this study is –12.9 ‰ rel PDB (with an average composition of approximately –10.8‰ rel PDB). It is probable that the δ13C composition of the soil gas would be at least as negative as these values. When using the Pearson and Fontes-Garnier models, we have used –20 ‰ rel PDB for the δ13C composition of the soil gas for all groundwater samples. This value is consistent with plants having a predominantly C3 metabolism, at least at the time when recharge took place. Measured values for the δ13C composition of the JT1 and JT9 where the average value for the study (-13.3 ‰ rel PDB) was used instead. The average value for A0 is approximately 43 – 60 %mC for the closed system correction methods and 58 – 66 %mc for the open system correction method. It is clear that if measured values are higher than this, then the apparent ages will be negative and reinforces the fact the assumption of closed system behaviour is not valid in this system, and that re-equilibration has occurred. Improvements in the accuracy of the ages reported here could be made by some measurements of soil gas CO2 14C concentrations at or above the water table to verify that the value of Ao is 100%mc and not some number lower than that. For every 5%mc less than 100 %mc for Ao, the calculated ages would be younger by about 530 years. In general, however, samples from deeper in the regolith give lower 14C concentrations (Figure 23). These low 14C samples are sampling waters running along the base of the alluvial sequence immediately above the weathered bedrock. This is commonly marked by the development of calcretes and silcretes (Wilford, 2004), but we see no evidence of signal dilution by dissolution of these carbonates (Figure 24), despite the apparent increase in calcium seen with depth (Figure 14a) further suggesting cation


31

exchange as the dominant process controlling the concentration the elements in the groundwaters.

0

5

10

15

20

25

30

35

40

45

30 35 40 45 50 55 60 65 70 75 80radiocarbon (pMC)

dept

h to

slo

ts (m

)

BundaleerBelalieCaltowie

C19 JT1

JT3JT5-1

JT2-2

JT7

JT8

Figure 23. Radiocarbon plotted as a function of the depth to slotted interval

0

5

10

15

20

25

30 35 40 45 50 55 60 65 70 75 80radiocarbon (pMC)

calc

ium

(meq

)

BundaleerBelalieCaltowie

C19

JT1

JT3JT5-1

JT2-2

JT7JT8

Figure 24. Radiocarbon plotted against calcium showing the distinct groupings of groundwaters.


32

8. Discussion

a. Recharge Mechanisms Differences in stable isotopic composition of water can often distinguish diffuse and direct recharge mechanisms due to the latter’s propensity to display relative enrichment in 2H and 18O because of evaporation prior to recharge. Diffuse recharge shows less evaporative signature because once water infiltrates below a metre into the soil it is not affected as much by direct evaporation. The isotopic composition of the groundwaters sampled from the observation bores show minimal divergence from the meteoric water line (Figure 22) suggesting that diffuse recharge is the dominant infiltration process for the region. There are too few radiocarbon measurements to allow us to constrain rates of recharge. We do note some important variations in the 14C content of the different waters (Figure 23a). JT1 and JT3 have the highest 14C content and occur on the flank of the Bundaleer Valley, sited in the colluvial aprons that occur along the margins of the valleys. This suggests rapid, recent recharge is occurring through these materials. The deep bore between these bores (JT2) penetrates the alluvial sequence, sampling the groundwaters at the base of the alluvials. This is the freshest bore sampled, but has a relatively low 14C concentration. We surmise that there is a significant component of recharge from higher in the Bundaleer Valley contributing to the groundwater at this site, and this water may be several thousand years old, possibly coinciding with the wetter climatic period 5,000-7,000 years ago (de Deckker, et al., 1988). Bores in the Belalie Valley have generally low 14C content (<55pMC), at levels similar to those in the deep Bundaleer Valley bore. These may all be sampling deeper waters from the bedrock system leaking into the surficial deposits. The lowest 14C waters are from the northern section of Belalie Creek and may indicate derivation from an older groundwater source to the north and east that is feeding the Belalie Creek area, and from there into the Bundaleer Valley and the Belalie Valley to the south.

b. Sources of salt The origin of dissolved salts in the groundwaters can be divided into four possible sources:

1. dissolution of salt deposits within the sedimentary formations; 2. entrainment of seawater from the marine sediments; 3. weathering of aquifer mineral, or 4. concentration of marine derived atmospheric aerosols.

The relationship between bromide and chloride can be used to evaluate whether salt (halite) or rock minerals contribute significant amounts of dissolved solutes to the salt load. Given that sodium and chloride are the dominant ions in solution, and chloride is the most hydrophilic of all the dissolved halides, the source of chloride (and by association sodium) determine the predominant salt load.


33

Bromide versus chloride for the Jamestown groundwaters plot within the bounds of evaporation of a rainfall source for these halides. This is consistent with conservative concentration of salts derived from marine aerosols, or possibly residual seawater (Figure 25). The data are not consistent with either salt dissolution or mineral weathering because halite (NaCl) and most rock forming minerals are deficient in bromide, which would yield Br/Cl ratios in water much less than the seawater value of 2.45 x 10-3. Evaporation affects the composition of the waters, but to a relatively small degree.

The isotopic and chemical data all indicate the predominant influence of either recent rainfall (δ2H, δ18O, 14C, Br) or water-rock interaction (Ca, SO4, Na, δ13C, 87Sr/86Sr) and not the presence of residual connate waters. We may thus assess the chemistry of the waters at each of the bores in relation to the dominant processes operating in the area, namely:

1) Direct rainfall recharge on the ridges 2) Diffuse rainfall recharge (following floods and sheet wash) 3) Water:rock interactions of deeper groundwaters 4) Dissolution of basement carbonates and salts

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 10 20 30 40 50 60 70 80 90 100

chloride (meq)

brom

ide

(meq

)


JT1

JT2-2

Belalie signature

Gawlers rainfall evaporation

Pt Lincoln rainfall evaporation

Figure 25. Chloride against bromide plot. Bounds on the data canbe set be evaporation of rainwaters from the south and west, and there is no recourse to dissolution of halites in the profile to account for the chloride to bromide ratio.


34

While there is a suggestion of evaporation as a driver for solute concentration from the chemical data, the isotopic data suggest that this is a minor process in the area.

c. Hydrogeological models To understand the hydrogeology of salinity we must understand the processes operating at the catchment scale. This scale can be relatively small, dealing with a sub-catchment (Henschke, et al., 1994) but often this must be combined with broader-scale studies to encompass the entire region affected by a groundwater flow system. Around Jamestown, South Australia, consideration of sub-catchments defined by the surface hydrology can be misleading and we need techniques that can rapidly survey large areas (100km2) to evaluate whether we are investigating at the appropriate scale. Airborne geophysics provides technologies that are well suited to this type of investigation. Across the Jamestown region 4 technologies were used and assessed. Airborne magnetics revealed the extensive network of prior stream channels, defining the extent of sub-surface catchments, which, in places, do not coincide with present-day surface watersheds. Airborne radiometrics highlight the surface distribution of materials and can be used to evaluate soils and present-day surface features. Airborne electromagnetics (AEM) expose the sub-surface conductivity structure that relates to salt, water and sediment type. In this region, water is the defining parameter and the course of groundwaters can be delineated at depth. This has to be confirmed by on-ground drilling, sampling and chemical analyses, but provides a framework on which to develop hydrogeological models. The different expressions of salinity were explained: Caltowie Valley suffers from surface scalding, infrastructure damage and very shallow, saline watertables caused by too much water blocked by a bedrock constriction in a shallow system. Jamestown/Bundaleer Valley has regions of crop failure across the valley floor, seasonal waterlogging and periodic shallow, saline watertables also caused by ponding of groundwaters behind a bedrock constriction, but enhanced by overflow of waters from the adjacent, Belalie valley. In Belalie Valley isolated soil degradation and scalding occurs only on north-facing slopes, with minimal salinity issues on the valley floor. The hill-slope salinity is due to sub-soil salinity effects exacerbated by over-stocking and poor land management. This has all but been eliminated in this region by contour banking and refined stocking practices. The valley floor is unlikely to suffer from water-table salinity as the deeper valley fill provides a ready conduit for waters, flushing the saline groundwaters out of the system towards the opening valley to the south. With our improved understanding of the 3-dimensional variability in aquifer materials Wilford, 2004; Cresswell, 2004) and appreciation of water transport in the region, we can develop new hydrogeological models that may better evaluate the relationships between natural recharge, discharge and salinity around the region (Figure 26).


35

Chemistry of groundwaters and environmental isotopes help us understand the origins of the groundwaters and salts and can be used as checks on our models of groundwater movement. Samples were taken for radiocarbon, stable isotopes, strontium isotopes and ion chemistry, and we find some important differences between them. Thus, Caltowie Valley has higher chloride, sodium and sulphate; Bundaleer Valley generally has the lowest levels of all ions and Belalie Valley has high calcium and chloride. The stable isotopes (18O, 2H, 34S) all show a seawater signature that suggests all waters were derived from rainwaters, with no input from any connate waters. Strontium ratios are high (87Sr/86Sr~ 0.714) are reflect dissolution of plagioclase from the source granites to the north-east. Radiocarbon gives two signals. A modern signature for waters sampled above 25m, and roughly 50 percent modern for deeper waters. This appears to be independent of valley and aquifer type and does not correlate with calcium or alkalinity, indicating that carbonate dissolution is not diluting the signal.

Figure 26. Magnetic image (left) showing subsurface drainage lines. Blue lines indicate main water channels and we see the confluence of multiple channels at salinity prone areas, upstream of bedrock constrictions. Airborne electromagnetic (AEM) depth slice from 15-20m overlain on the magnetic image (right) shows the palaeochannels and preferential groundwater flow in the Belalie Valley and confirms the existence of constrictions in the Bundaleer and Caltowie Valleys.


36

d. Aims and Objectives revisited

1. Evaluate the groundwater pathways as suggested by the AG Groundwater flow systems indicated by the airborne geophysics and the hydrogeological model indicate flow systems that are focused towards the valleys from the highlands and then converge and continue southwards. The spatial continuum in chemical composition, and similarity of all groundwaters in stable isotope composition suggest that the groundwater pathways are all hydraulically connected. 14C compositions, though few in number and sampled from variable depths, are broadly consistent with such a conceptual model. Estimated groundwater 14C ages are of the order of a few thousand years and increasing ‘age’ correlates with greater depth below groundwater, and with distance along inferred flow paths.

2. Assess recharge mechanisms for the groundwater systems based on chemical constraints

Recharge is predominantly by diffuse recharge, possibly at higher rates through the alluvial fans. The stable isotope concentrations in the groundwater are similar to mean winter rainfall compositions for Adelaide indicating that there is no preferred flood-out recharge and that evaporation of surface floodwater prior to recharge is minimal. The high Cl- concentrations throughout the area (1,000 – 2,000 mg L-1) suggest low historical recharge rates in the vicinity of 0.5 – 2 mm yr-1.

3. Determine the connectivity between the surface waters and the deeper groundwaters

While connectivity between the surface and shallow groundwaters can be surmised through evaluation of rainfall and stream flow records and groundwater levels, which indicate a direct causal impact on alluvial groundwater levels through rainfall infiltration, the lack of either surficial or deep, bedrock water samples does not allow us to comment on the chemical connectivity, or interaction between the surface, shallow and deep groundwater systems.

4. Determine the source(s) for salt in the groundwater

The chemical composition of the groundwaters show some subtle variations amongst the different valleys, and over the range of observed TDS. But the dominant ions (Na+ and Cl-) have ratios identical to seawater, as do the Br-/Cl- ratios, indicating a predominant marine aerosol origin for the salts. Mineral weathering plays a subordinate role and there are no residual salts in the formation that contribute Cl- or SO4

2- to the groundwaters and to the temporary surficial salt efflorescence.


37

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Chemical Methods. Inkata Press, Melbourne


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Rowett, A.I. 1997. Preliminary report of palaeodrainage in the St Vincent Basin and Mount Lofty Ranges. Mines and Energy Resources SA. Report Book 97/25.

RUST PPK Pty. Ltd. 1994. Bundaleer Valley Landcare Group dryland salinity

investigation. Rust PPK Environment and Infrastructure Report no 94/562 27E259A. August 1994.

Schofield, J. 1999. Ground conductivity survey of a catchment north east of

Jamestown. Draft DEH Report

Stephens, C.G., Herriot, R.I., Downes, R.G., Langford-Smith, T. and Acock, A.M. (1945) A soil, land-use; and erosion survey of part of County Victoria, South Australia. CSIR Bulletin 188, Melbourne.

Tamers, M.A. (1975) Validity of radiocarbon dates on groundwater: Geophysical surveys, v 2 p217-239

Walker G.R. and Cook, P.G. 1991 The importance of considering diffusion when

using carbon-14 to estimate groundwater recharge to an unconfined aquifer. J Hydrol, 128, 4-48

Williams, M.A.J. 2001. Chapter 1 – Quaternary Climate changes in Australia and their

environmental effects. Geol. Soc. Australia, Sp. Pub., 21, 3-11 Wilford, J.R. 2004 3D regolith architecture of the Jamestown area – implications for

salinity. Report to SA-SMMSP/ CRC LEME restricted report.


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Appendix I: SA-SMMSP DATA Jamestown Water Sampling: Field

Data Valley

Bore number

Easting Northing pH T (oC)

EC (mS/c

m)

DTW (m, gr)

Bore depth (m)

Slotted interval (m)

Sample date Comments

Bundaleer JT1 275096 6321230 6.96 5.48 8.64 13 11.5

JT2-1 275096 6321230 6.82 6.75 4.72 11.5 10 JT2-2 275096 6321209 7.24 18.2 3.21 4.66 29.5 26.5 14/08/2003

JT2-3 275100 6321210 7.08 17.9 6.59 4.72 13 11.5 14/08/2003

JT3 276880 6321430 7.16 18.5 7.43 1.61 24 21 13/08/2003 Belalie JT5-1 282167 6315593 7.25 18.6 5.44 13.24 33 24 13/08/2003

JT5-2 282188 6315653 7.05 6.96 13.17 17 14 JT7 283803 6322704 6.84 5.78 21.4 34 31 13/08/2003 sediment laden sample, decant 2 x 500mL JT8 280999 6330687 7.12 18.6 6.4 22.64 40 31 14/08/2003 Reece B. 6.97 17.7 4.85 40 13/08/2003

Belalie Ck. 6.80 7.01 0 Caltowie JT9 270165 6324817 6.63 8.19 9 11 9

C19 6.65 9.4 11


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Jamestown Major Ions Valley Sample Alkalinity Cl Br NO3 SO4 Ca Mg Na K S Si Sr sum sum imbalance Br/Cl

id. meq meq meq meq meq meq meq meq meq meq meq anions cations % x10-3

Bundaleer JT1 7.06 44.00 0.07 0.06 6.60 3.61 13.00 37.48 0.50 6.63 1.79 0.032 51.2 54.6 -3.22 1.53

JT2-1 4.76 60.08 0.10 0.01 4.52 11.70 23.46 31.74 0.60 4.51 1.20 0.091 65.0 67.5 -1.92 1.74 JT2-2 4.86 28.49 0.05 0.05 3.06 5.85 10.45 18.83 0.40 2.88 1.22 0.042 33.4 35.5 -3.02 1.67 JT2-3 4.1 56.69 0.10 0.11 4.35 11.55 22.06 29.70 0.56 4.31 1.24 0.086 61.0 63.9 -2.30 1.68 JT3 4.78 63.75 0.09 0.38 5.19 13.25 20.82 37.04 0.63 5.14 1.54 0.141 69.0 71.7 -1.95 1.46

Belalie JT5-1 2.62 53.03 0.08 0.19 2.15 11.95 19.18 24.52 0.44 2.00 0.94 0.083 55.9 56.1 -0.15 1.56 JT5-2 2.92 63.75 0.10 0.18 3.88 13.35 21.40 33.83 0.49 3.78 1.14 0.083 66.9 69.1 -1.56 1.63 JT7 2.98 53.59 0.09 nd 2.44 19.90 19.09 17.35 0.40 2.36 1.33 0.074 56.7 56.7 -0.07 1.65 JT8 1.95 58.95 0.10 0.01 1.22 19.05 17.04 22.83 0.43 1.01 2.23 0.078 61.0 59.3 1.39 1.72 Reece

Bore 3.7 62.05 0.11 0.15 5.23 22.05 24.77 22.17 0.45 5.21 1.04 0.107 66.0 69.4 -2.53 1.80

Belalie Ck. 3.78 63.18 0.11 0.26 4.90 18.10 22.06 29.43 0.35 4.79 1.26 0.114 67.3 69.9 -1.90 1.67Caltowie JT9 5.86 72.77 0.12 0.20 13.23 5.55 13.66 62.61 0.72 12.44 1.01 0.039 79.0 82.5 -2.22 1.66

C19 5.52 88.00 0.15 0.29 14.13 12.05 19.42 63.91 0.67 13.06 1.71 0.065 94.0 96.1 -1.11 1.65nd - not detected


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Jamestown Minor Ions Valley Sample F PO4 Al B Ba Cu Fe Li Mn Zn

id. meq meq meq meq meq meq meq meq meq meq

Bundaleer JT1 0.072 n.d. <0.01 0.164 0.0010 <0.003 <0.003 <0.007 <0.004 <0.003

JT2-1 <0.1 n.d. 0.013 0.158 0.0012 <0.003 <0.003 <0.007 0.039 <0.003 JT2-2 0.053 n.d. <0.01 0.125 0.0009 <0.003 <0.003 <0.007 <0.004 <0.003 JT2-3 <0.1 n.d. 0.011 0.139 0.0013 <0.003 <0.003 <0.007 <0.004 <0.003 JT3 <0.1 n.d. 0.014 0.172 0.0019 <0.003 <0.003 <0.007 <0.004 <0.003

Belalie JT5-1 <0.1 n.d. 0.013 0.089 0.0018 <0.003 <0.003 <0.007 <0.004 <0.003 JT5-2 <0.1 n.d. 0.013 0.097 0.0010 <0.003 <0.003 <0.007 <0.004 <0.003 JT7 <0.1 n.d. 0.020 0.075 0.0061 <0.003 <0.003 <0.007 0.5 <0.003 JT8 <0.1 n.d. 0.021 0.081 0.0059 <0.003 <0.003 <0.007 0.0092 <0.003

Reece B. <0.1 n.d. 0.020 0.106 <.0005 <0.003 <0.003 <0.007 0.1114 0.0058 Belalie Ck. <0.1 n.d. 0.017 0.097 0.0014 <0.003 <0.003 <0.007 <0.004 <0.003 Caltowie JT9 <0.1 n.d. <0.01 0.375 <.0005 <0.003 <0.003 <0.007 <0.004 <0.003

C19 <0.1 n.d. <0.01 0.361 0.0008 <0.003 <0.003 <0.007 <0.004 <0.003 nd - not detected


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Jamestown Environmental Isotopes

Valley Bore d18O d2H d 34S 14C 87Sr/86Sr error id. ‰ rel SMOW ‰ rel SMOW ‰ rel CDT pMC

Bundaleer JT1 -5.02 -30.6 71

JT2-1 -4.92 -29.3 JT2-2 -5.51 -33.4 19.1 46 0.714463 0.0018 JT2-3 -5.06 -30.6 JT3 -5.38 -32.7 22.4 74 0.715542 0.0016

Belalie JT5-1 -5.34 -34.2 18.6 53 0.714371 0.0017 JT5-2 JT7 -5.7 -34.3 17.5 43 0.714313 0.0019 JT8 -5.32 -32.6 16.6 41 0.712557 0.0012 Reece Bore

Belalie Ck. Caltowie JT9 -4.64 -29.9

C19 18.3 71 0.715164 0.0015


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Appendix II: Rainfall and seawater data used in this report Rainfall data from Keywood, 1986. C h e m istry

pH f ield A lkalinity F Cl Br SO4 Ca K Mg Na Si Ba Sr U Br/Cl(f ield) meq meq meq meq meq meq meq meq meq meq meq meq meq x10-3

CaCO3

s eaw ater 8.22 2.84 0.07 545.79 0.84 56.46 20.55 10.23 107.50 467.83 0.29 0.00029 0.183 0.013 1.54

rain (Pt Lincoln) 3.55 0.00 0.55 0.45 0.09 0.99 3.55 0.00000 0.000 1.35

En viron m e n tal Isotope sδ2Η δ18Ο δ 34S δ13C 87Sr/86Sr

‰ rel S M O W

‰ rel S M O W

‰ rel C D T

‰ rel P D B

s eaw ater 0 0 19 0 0.70906


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Appendix III. Hydrochemical/Isotopic Models For Correction of 14C in Groundwater Radiocarbon laboratories routinely report results of 14C analyses as years before present (yr BP), percent modern carbon (pmC or %mc) or Delta (∆14C). The convention most often used for reporting 14C activity in groundwater is as pmC, with 100%mc equivalent to the 14C concentration of the atmosphere. For groundwater samples, the results refer to the age or activity of the dissolved inorganic carbon (DIC) in the groundwater and rarely to the age or activity of the groundwater itself. The principle behind 14C dating of groundwater is that carbon dioxide containing a known amount of 14C decays at a well known rate (half life of 5730 years) and the age calculated using equation 1: t = –ln 8033A/Ao (1) where t = time in years A= measured activity of total dissolved inorganic carbon (TDIC) Ao = Initial radiocarbon content of TDIC at the time of recharge High concentrations of CO2 build up in the unsaturated zone due to root respiration and microbial oxidation of organic matter. This CO2 is considered to be modern (100 pmC) or near modern (if there is oxidation of old carbon within the unsaturated zone). In areas where there are appreciable amounts of carbonate minerals in the soil zone, dissolution of carbonate (CaCO3) takes place according to reaction 2: CO2(aq) + H2O + CaCO3 → Ca2+ + 2HCO3

- (2) Two sources of TDIC are involved during the recharge process, namely the 14C active component from the soil and the 14C-free carbonate. If reaction 2 proceeds to completion, about half the resultant HCO3

- will be derived from soil CO2, and half from the CaCO3 that is free of 14C. This occurs in what is generally known as dissolution under closed conditions and the initial 14C composition of the water (A0) is considered to be ~50%. At the other extreme is dissolution under open conditions where the TDIC of soil water is continually exchanging with a very large reservoir of 14C active soil CO2. In this case, the initial 14C activity of the TDIC, A0 remains unchanged at or near 100 pmC modern. In practice, what happens is somewhere in between the two extreme conditions and most groundwaters reach calcite saturation somewhere during transition from open to closed conditions. There have been numerous ‘correction schemes’ developed over the past 40 years to estimate A0. One of the simplest methods, presented by Tamers in 1975, is known as


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the “chemical” correction approach. It is based on the initial and final DIC concentrations and nearly always results in an A0 value of ~50 pmC because it assumes fully closed conditions. It is possible to use the δ13C composition of the DIC in groundwater to indicate the fraction of open and closed dissolution during the evolution of groundwater. The δ13C mixing model, as suggested by Ingerson and Pearson in 1964, uses the δ13C composition of the groundwater relative to the soil CO2 and the CaCO3 to determine the extent of dissolution. Hence, if the δ13C composition of the groundwater is half way between that of the soil CO2 and the CaCO3, A0 is estimated to be ~50 pmC. The decision on which method to use for any particular situation is based on some knowledge of the climatic regime and soil type. Each of the methods summarised, along with others not discussed here, have strengths and weaknesses. Clark and Fritz suggest that “the best approach is to collect as much field data as possible, including samples from the recharge area, and compare results with various models”. A summary of the relative merits of the different models is given in various reviews, the most recent of which is by Kalin (1999). Many of these models are incorporated in the USGS Computer code NETPATH (Plummer et al., 1994) which can be down-loaded from the WEB at http://water.usgs.gov/software/geochemical.html.

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