Environmental isotopes as indicators of inter-aquifer mixing, Wimmera region, Murray Basin, Southeast Australia

Chemical Geology 277 (2010) 214–226

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Chemical Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o

Research paper

Environmental isotopes as indicators of inter-aquifer mixing, Wimmera region, Murray Basin, Southeast Australia Ian Cartwright a,b,⁎, Tamie Weaver c, Dioni I. Cendón d, Ian Swane e a

School of Geosciences, Monash University, Clayton, VIC 3800, Australia National Centre for Groundwater Research and Training, Flinders University, Adelaide SA 5001, Australia URS Australia Pty Ltd, 6/1 Southbank Boulevard, Southbank, VIC 3006, Australia d Australian Nuclear Science and Technology Organisation, Kirrawee DC, NSW 2232, Australia e Terrenus Pty Ltd, 12 Granville Street, Wilston, QLD 4051, Australia b c

a r t i c l e

i n f o

Article history: Received 12 December 2009 Received in revised form 29 July 2010 Accepted 4 August 2010 Editor: B. Bourdon Keywords: Inter-aquifer flow 14 C Environmental isotopes Groundwater Murray Basin

a b s t r a c t Complex groundwater flow systems in confined aquifers that result from geological structures, stratigraphic changes, or the absence of efficient aquitards are difficult to constrain using physical parameters alone. Despite a relatively simple aquifer configuration, the distribution of groundwater total dissolved solids (TDS) concentrations, δ13C values, 87Sr/86Sr ratios, and 14C activities (a14C) in groundwater in the Wimmera region of the southern Murray Basin implies that considerable inter-aquifer flow has occurred. Given the presence of both silicate and carbonate aquifers, δ13C values and 87Sr/86Sr ratios are the key parameters that demonstrate interaquifer flow. Locally, between 40 and 95% of water from one aquifer has infiltrated the underlying aquifer homogenising many aspects of the groundwater geochemistry. Groundwater residence times estimated from a14C range from modern to N 30 ka and the distribution of 14C residence times confirm that inter-aquifer flow is regional scale and long term. Recharge of the deepest aquifers occurs across a broad region and not solely at the basin margins. Vertical leakage rates are ~6–10× 10−3 m/year and long-term recharge rates 0.1–0.2 mm/year (b 1% of annual rainfall). Groundwater from this region is a locally valuable resource and failure to recognise that inter-aquifer flow occurs threatens the sustainability of this resource. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Understanding groundwater flow and inter-aquifer mixing is critical to understanding hydrogeological systems and managing groundwater resources. Flow systems in deep aquifers may be complex due to the presence of geological structures (e.g., folds and faults), stratigraphic changes, or the absence of effective aquitards (e.g., Sayed et al., 1992; Macfarlane et al., 1994; Edmunds et al., 2002; Ortega-Guerrero, 2003; André et al., 2005). Sub-horizontal groundwater flow parallel to the main stratigraphic units is reasonably straightforward to constrain using hydraulic heads and hydraulic conductivities. However, because vertical hydraulic conductivities are less commonly measured, the flow of groundwater between and vertically within formations is more difficult to determine using physical hydrogeology (e.g., Sayed et al., 1992). Where low hydraulic conductivity layers exist in a sequence, these may prevent significant groundwater flow between over- and underlying aquifers. Even in aquifer sequences without confining layers, because hydraulic conductivities are higher parallel to bedding, groundwater flow may ⁎ Corresponding author. School of Geosciences, Monash University, Clayton, VIC 3800, Australia. Tel.: +61 3 9905 4887; fax: +61 3 9905 4903. E-mail address: [email protected] (I. Cartwright). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.08.002

still be dominantly parallel to stratigraphy. If flow is largely confined to stratigraphic units, deeper groundwater may be protected from near-surface contamination, and shallow groundwater and surface water systems may be isolated from the impacts of pumping of deeper aquifers. However, the occurrence of significant inter-aquifer flow may compromise both surface water and groundwater bodies and thus needs to be constrained. Recharge of fully confined aquifers also requires interformational flow; in many cases this may occur at the basin margins where the basin sediments are thinner and commonly consist of coarse-grained high hydraulic conductivity breccias and conglomerates (e.g., Chiew et al., 1992). Groundwater geochemistry, especially where aquifers have contrasting mineralogy, is an effective method of determining regional scale inter-aquifer flow and patterns of recharge (e.g., Weaver and Bahr, 1991a,b; Dogramaci et al., 2001; Edmunds et al., 2002; Dogramaci and Herczeg, 2002; Ortega-Guerrero, 2003; Hendry et al., 2004; André et al., 2005; Edmunds, 2009; Cartwright, 2010). Here we use major ion geochemistry and environmental isotopes to constrain inter-aquifer mixing in the Wimmera region of the southern Murray Basin, Australia. Although the use of geochemical tracers in this way is not uncommon, few studies have incorporated radiometric isotopes to delineate not only the patterns but also the timescales of inter-aquifer flow and relatively few have quantified the extent of interformational flow.

I. Cartwright et al. / Chemical Geology 277 (2010) 214–226

141o

138oE

144o

215

147o

33oS SA NSW VIC

Murray

35

o

Pre-Cainozoic Basement

Adl

Extent of Geera Clay

Southern Ocean

Groundwater Flow WR

Rivers 37o

N

Sandy Soils

Fig. 1. Generalised map of the Murray Basin showing the extent of the Geera Clay and the regions of extensive sandy soils. Adl = Adelaide, WR = Wimmera River. Dashed box shows location of Fig. 2. Groundwater flow directions are similar in all aquifers. Modified after Evans and Kellett (1989).

Additionally, we attempt to distinguish between inter-aquifer mixing and calcite dissolution as mechanisms of resetting carbon and strontium isotopes. Understanding the timescales of groundwater mixing is extremely important for assessing the potential impact of groundwater extraction or contamination on water resources in this region. 1.1. Hydrological setting The Murray Basin (Fig. 1) occupies ~ 300,000 km2 of southeast Australia and contains sequences of late Palaeocene to Recent sediments that overlie Proterozoic to Mesozoic basement rocks (Lawrence, 1988; Evans and Kellett, 1989; Brown, 1989; Brown and Radke, 1989; McAuley et al., 1992). The Murray Basin is up to 600 m thick and, except for a small region in the southwest that discharges to the Southern Ocean, is a closed basin with groundwater discharge to rivers, salt lakes, and playas near the basin centre. The shallowest units in the Murray Basin are unconfined and recharge occurs across broad areas, not just at the basin margins (Evans and Kellett, 1989; Herczeg et al., 2001; Cartwright et al., 2008). The geological and hydrogeological framework of the Wimmera region (Fig. 2) is discussed by Evans and Kellett (1989), Lawrence (1988), Brown (1989), and Swane (2004). Annual rainfall in the Wimmera varies from ~550 mm in the south to ~400 mm in the north (Bureau of Meteorology, 2009); rainfall occurs mainly in the austral winter (May–August) and potential evapotranspiration exceeds rainfall for most of the year. The Murray Basin in the Wimmera region deepens northward from the Dundas Plateau to in excess of 300 m (Fig. 2a). The basement topography is irregular and basement highs and outcrops (such as Mount Arapiles) exist in the south of the area. The Shepparton Formation comprises a heterogeneous series of laterally discontinuous fluvio-lacustrine clays, sands, and silts that form a shallow unconfined aquifer in the southeast of the Wimmera region (Fig. 2c). The LoxtonParilla Sands is a sequence of marine sands and silts that underlie the Shepparton Formation in the southeast of the area. Where the Shepparton Formation is absent, the Loxton-Parilla Sands and its aeolian derivative, the Lowan Formation, are the surficial units. The LoxtonParilla Sands in the west of the Wimmera region are underlain by the Murray Group, which comprises up to 130 m of marine and marginal marine limestone with interbedded calcareous sands, marls, and silts (Dogramaci et al., 2001; Dogramaci and Herczeg, 2002: Fig. 2e). The Renmark Formation (Fig. 2d) comprises up to 200 m of Palaeocene to late Miocene fluvial clays, silts, sands, and gravels that grade upwards from largely terrestrial to marine and marginal marine (Brown, 1989). The Ettrick Formation, Geera Clay, Bookpurnong Beds and Winnambool Formation are collectively referred to as Mid Tertiary Aquitard units

(Brown and Radke, 1989), although locally hydraulic conductivities are sufficiently high to allow flow across these units (Dogramaci et al., 2001; Dogramaci and Herczeg, 2002). The Ettrick Formation consists of 10–20 m of glauconitic calcareous clays with silt, sand, and locally limestone layers that underlie the Murray Group (Fig. 2f). The Winnambool Formation comprises up to 35 m of poorly consolidated fossiliferous clays, marls, and silts. It represents a facies change in the Murray Group and is interlayered with limestones of the Murray Group at its western boundary (Fig. 2f). The Winnambool Formation grades eastwards into the Geera Clay, which comprises up to 75 m of massive clays with minor sand and silt layers (Fig. 2b). The eastern margin of the Geera Clay is gradational with the upper part of the Renmark Formation. The Bookpurnong Beds are a discontinuous series of clays, sands, and silts that locally separate the Murray Group from the overlying Loxton-Parilla Sands. The Douglas Depression is a topographic low in the central Wimmera region that is 5–10 km wide and up to 30 m below the general land surface (Fig. 2f) and which contains numerous salt lakes that have deposits of halite and other evaporite minerals. As many of these salt lakes are throughflow lakes and others exhibit reflux behaviour, they contribute saline recharge to the shallow aquifers (McAuley et al., 1992; Swane et al., 2001; Swane, 2004). Lakes outside the Douglas Depression are ephemeral and lack evaporite crusts, indicating that they lie above the water table and are also local recharge points. Elsewhere, as in the Murray Basin in general, recharge of the shallowest unconfined aquifers will occur across the region. The deeper Renmark Formation does not crop out and can only be recharged through the overlying units; this may occur at the basin margins where the Murray Basin sequences are coarser grained and overlie weathered basement and/or via broader scale leakage through the overlying units. Hydraulic heads in the Loxton-Parilla Sands decline from ~160 mAHD (Australian Height Datum) in the south of the region to ~70 mAHD in the northwest (Fig. 2c). Groundwater in this aquifer flows to the north and northwest. Patterns of groundwater flow in the Murray Group and Renmark Formation are similar to that in the Loxton-Parilla Sands (Fig. 2d,e). Lateral hydraulic gradients in all aquifers are ~ 5 × 10−4. Except in the northwest of the area (Fig. 2c), density-corrected vertical hydraulic gradients in nested bores are generally downward with a difference of up to 20 m between the head in the shallowest and deepest aquifers (McAuley et al., 1992). Vertical hydraulic gradients are up to 0.2, especially in the south of the region, although gradients of 0.02 to 0.05 are more common. Horizontal hydraulic conductivities determined from pump or slug tests are 0.5–5 m/day in the Loxton-Parilla Sands, 1–14 m/day in the Murray Group, and 1–30 m/day in the Renmark Formation (Lawrence, 1975; Tickell and Humphries, 1986; Evans and Kellett,

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1989; Swane 2004). Vertical hydraulic conductivities of the Geera Clay are 10−5–10−4 m/day (Lawrence, 1975). Processes controlling groundwater geochemistry are similar throughout the southern Murray Basin and are described in detail by Arad and Evans (1987), Love et al. (1993), Herczeg et al. (2001), Swane et al. (2001), Dogramaci and Herczeg (2002), Cartwright and Weaver (2005), Cartwright et al. (2006, 2008) and Petrides et al. (2006). Major ion geochemistry, 87Sr/86Sr ratios, and stable isotope data imply that the dominant hydrochemical process is evapotranspiration of rainfall during recharge together with minor silicate weathering and minor precipitation and/or dissolution of carbonate, gypsum, and halite; cation exchange modifies the composition of the most saline groundwater. Many of the silicate-dominated aquifers in the southern Murray Basin are unreactive and water–rock interaction during flow is limited; indeed much of the groundwater geochemistry is determined by processes in the unsaturated zone (Herczeg et al., 2001). Carbonate dissolution is locally important in controlling the geochemistry of groundwater in the major limestone units, such as the Murray Group Limestone (Love et al., 1993; Dogramaci and Herczeg, 2002). Because of a simple stratigraphy and the presence of the Mid Tertiary Aquitard units that have relatively low hydraulic conductivities, regional groundwater flow is generally assumed to be dominantly lateral (e.g., Evans and Kellett, 1989), and there has been little consideration of vertical flow. A significant part of the Wimmera region contains groundwater with total dissolved solids, TDS, concentrations b10,000 mg/L that may be used for domestic or agricultural supply (Fig. 3). As most groundwater in the Murray Basin has TDS concentrations N10,000 mg/L (e.g., Evans and Kellett, 1989; Lawrence, 1988; Brown, 1989), documenting groundwater residence times, flow paths, and mixing is important to the sustainable management of this relatively scarce resource.

on mixed carbonate–silicate powders from drill cuttings using the same methodology. Sr concentrations of calcite were determined by ICP-MS on powdered samples leached with 10% HCl. δ34S values were measured via combustion using a Carlo Erba Flash Elemental Analyser of BaSO4 precipitated from filtered samples by the addition of BaCl2; prior to precipitation the samples were acidified to pH b2 and heated to just below boiling to remove DIC. A range of international standards (NBS 18, IAEA CO1, IAEA CO8, NBS 127, IAEA SO5) were used to calibrate the δ34S and δ13C analyses to the V-CDT and V-PDB scales, respectively. Precision (1σ) based on replicate analyses is: δ34S = ±0.25‰; δ13C = 0.2‰. 87Sr/86Sr ratios were measured at the University of Adelaide on a Finnigan MAT 262 thermal ionisation mass spectrometer in static mode. Sufficient water to yield 1–2 μg Sr was evaporated to dryness. The residue was dissolved in 2 ml of 6 M HCl, evaporated to dryness, and redissolved in 2 M HCl. Sr was extracted from centrifuged supernatant using cation exchange columns and Biorad AG50W X8 200–400 mesh resin. 88Sr/86Sr was normalized to 8.375209. 87Sr/86Sr ratios of silicate and carbonate minerals were measured using similar techniques. Analyses of SRM987 gave 87Sr/86Sr ratios of 0.710260 ± 0.000009 (1σ). The Sr procedural blank was taken as 1 ng and not significant. Powdered rock samples were leached with 1 M HCl to extract Sr from carbonates and subsequently digested with concentrated HF and HNO3 at 150 °C to extract Sr from silicate minerals. 87Sr/86Sr ratios of these extracts were determined in an identical way to those of the groundwater. 14C activities (a14C) were measured either using the ANSTO AMS 2MV Tandetron accelerator, STAR (Fink et al., 2004) with an average 1σ error of ±0.3 pMC or by AMS at the National Isotope Centre, New Zealand, with similar 1σ errors; a14C are reported as percent modern carbon (pMC). Major ion geochemistry are presented in Table 1, groundwater stable and radiogenic isotope data in Table 2, and 87Sr/86Sr ratios and δ13C values of solid samples in Table 3.

2. Sampling and analytical techniques 3. Groundwater geochemistry Groundwater was sampled from 29 monitoring bores at 14 locations that have screen lengths of 3–7 m and which sample only one lithological unit. Sampling of deeper (N25 m) wells used a low flow bladder pump set, where possible, at the screened interval. Groundwater drawdown was minimised during pumping to ensure that water was extracted from the well screen and samples were collected once pH, electrical conductivity (EC), and Eh had stabilised. Shallow wells were purged using an impellor pump. pH, EC, alkalinity (HCO3), and dissolved O2 (DO) were measured in the field. pH with a precision of ±0.1 was measured using Orion or Extech meters and electrodes calibrated against standard solutions. EC was measured using an Orion 240 conductivity meter and probe. HCO3 concentrations were determined using a Hach digital titrator and reagents with a relative precision of ±5%. DO was determined using a Hach drop titrator and reagents and is precise to ± 1 mg/L. Cations were analysed using a Varian Vista ICP-AES at the Australian National University on samples that had been filtered through 0.45 μm cellulose nitrate filters and acidified to pH b2. Anions were analysed on filtered unacidified samples using a Metrohm ion chromatograph at Monash University. The relative precision of anion and cation concentrations is ±2%. Charge balances were all within ±10%, and 90% are within ±5%. Stable isotope ratios were measured at Monash University using Finnigan MAT 252 and ThermoFinnigan Delta Plus gas source mass spectrometers. δ13C values of dissolved inorganic carbon (DIC) were measured by direct acidification by H3PO4 in a He atmosphere in a ThermoFinnigan Gas Bench. δ13C values of carbonates were measured

Groundwater in all aquifers from the west of the Wimmera region is part of an extensive (~ 10,000 km2) region of low salinity groundwater in the western Murray Basin that was largely recharged during wetter climatic periods (N20 ka: Leaney and Herczeg, 1999; Leaney et al., 2003) and which generally has TDS concentrations of b3500 mg/L (Fig. 3). Groundwater in the eastern Wimmera is more saline with TDS concentrations commonly N3500 mg/L. Shallow (b20 m) groundwater from the Loxton-Parilla Sands underlying the Douglas Depression locally has TDS concentrations of N100,000 mg/L (Fig. 3) due to recharge from the saline lakes (Swane et al., 2001). Na is the dominant cation in groundwater from the silicate aquifers, typically constituting 80–90% of cations on a molar basis; relative Na concentrations in groundwater from the limestones of the Murray Group are between 40 and 90% (Fig. 3). Except in the lowest salinity groundwater (TDS b1500 mg/L), Cl is the dominant anion (N90% of total anions on a molar basis). Groundwater from the different aquifers has similar geochemistry (Fig. 4), which is consistent with evapotranspiration of rainfall being the dominant hydrochemical process (c.f., Herczeg et al., 2001; Cartwright et al., 2008). However, there is a broad 2:1 correlation (r2 = 0.56) between molar HCO3 and Ca concentrations in groundwater from the Murray Group (Fig. 5) that most probably reflects calcite dissolution (c.f., Dogramaci and Herczeg, 2002) and which is not observed in the silicate-dominated units, except in the Renmark Formation to the west of the Geera Clay where it underlies the Murray Group (Fig. 5).

Fig. 2. a. Basement elevation in m relative to Australian Height Datum (AHD) in the Wimmera region. A = Mount Arapiles, E = Edenhope, H = Horsham, N = Nhill. Grid references are from the Australian Map Grid (zone 54H). b. Extent and thickness (m) of the Geera Clay. c. Hydraulic heads in the Loxton-Parilla Sands and Shepparton Formation. Dashed line shows approximate transition from downward to upward head gradients between the Loxton-Parilla Sands and the underlying aquifer. d. Thickness and hydraulic heads in the Murray Group. e. Thickness and hydraulic heads in the Renmark Formation. f. Cross section along X–X′ in Fig. 2a. showing distribution of major lithologies. Data from Lawrence (1975), Brown and Radke (1989), Evans and Kellett (1989), McAuley et al. (1992), Swane (2004).

I. Cartwright et al. / Chemical Geology 277 (2010) 214–226

a)

Lake Hindmarsh Wimmera River

20 km -200

60

1

-100

Pre-Cainozoic Basement

5 3

Limit of unit

4 6

h X

7

8

13

A

60

10

E

N

Town

70

0 b

Bore Locality

1

60

H

9

e

Hydraulic Head (mAHD) Groundwater Flow

d)

f d

60

X’

-100

c

N

200 Lithology Thickness N

g

Douglas Depression

Shepparton Formation

2 14

j

i

217

11 100 12 200

a

80

59 90 Dundas Plateau

100

120 130

110

b)

120 75

0

140 150

75

60

130

50

140

59 25 50

e)

70

70

25

60

80 0

59

90 100 110

c) 60

120 130 140

Upward 70 Head Gradient

Downward Head Gradient

150

80

0

59 90 100 110 120

50

60

f)

X’

X mAHD Palaeo-ridges

130 140

200

150

Bookpurnong

Douglas Depression Water Table

160

100

59

Loxton-Parilla Geera / Winambool

MG

Renmark

0

Basement Ettrick

50

60

218

I. Cartwright et al. / Chemical Geology 277 (2010) 214–226

TDS (mg/L) <1000 10013500 350113,000

a)

3.1. Spatial variation in

60

87 Sr/86Sr ratios of Renmark Formation groundwater increase eastwards from 0.709–0.710 to 0.712–0.715 (Fig. 6a,b). Discounting samples with δ13C N 0‰ that, as discussed below, have been impacted by bacteriological DIC reduction, there is also a general decrease in δ13C values in the Renmark Formation groundwater from west to east from −7 to −6‰ to −17 to −13‰ (Fig. 6a,c). The Renmark Formation groundwater with low 87Sr/86Sr ratios and high δ13C values is from the region west of the Geera Clay where the Renmark Formation is separated from the Murray Group only by the Ettrick Formation. Groundwater from the Murray Group generally has 87Sr/86Sr ratios of ~0.709 (typical for groundwater from this unit throughout the southern Murray Basin: Dogramaci and Herczeg, 2002), except at its eastern margins where the 87Sr/86Sr ratios are 0.710–0.711. δ13C values in the Murray Group groundwater vary from −7 to −3‰ in the west of the area to −14 to −12‰ at its eastern extent. Groundwater from the Mid Tertiary Aquitard units has 87Sr/86Sr ratios of 0.710–0.712 and δ13C values of −16 to −14‰, whereas Loxton-Parilla groundwater has 87Sr/86Sr ratios of 0.710–0.714 and δ13C values of −18 to −9‰. The bulk silicate and carbonate fractions in the Renmark Formation from the Wimmera region have 87Sr/86Sr ratios of 0.730– 0.745 and 0.709–0.713, respectively (Table 3); these are within the range recorded in this unit elsewhere in the Murray Basin (silicate 87 Sr/86Sr ratios = 0.711–0.755 and carbonate 87Sr/86Sr ratios = 0.708– 0.714: Dogramaci and Herczeg, 2002; Cartwright et al., 2007). The Murray Group contains calcite with 87Sr/86Sr ratios of 0.708–0.709 and a silicate fraction with 87Sr/86Sr ratios of 0.710–0.713 (Dogramaci and Herczeg, 2002). The Loxton-Parilla Sands contains calcite with 87 Sr/86Sr ratios of 0.711–0.713 and a silicate fraction with 87Sr/86Sr ratios of 0.718–0.743 (Table 3), while the Geera Clay contains calcite with 87Sr/86Sr ratios of ~ 0.709 and a silicate fraction with 87Sr/86Sr ratios of 0.715–0.717 (Table 3). δ13C values of calcite in the Murray Group are −2.4 to + 2.2‰ with an average of + 1‰ (Dogramaci and Herczeg, 2002). The Geera Clay contains calcite with δ13C = −2.1 to 1.7 (average = −0.2‰), the Renmark Formation contains calcite with δ13C = −3.5 to −1.6 (average = −2.1‰), and the Loxton-Parilla Sands contains calcite with δ13C = −3.2 to −0.7 (average = −2.4‰) (Table 3). Sr concentrations of calcite in these lithologies are between 1 and 4 mmol/kg (Table 3), which is similar to the range of Sr concentrations reported for calcite in the Murray Group and Renmark Formation elsewhere (Dogramaci and Herczeg, 2002). The relatively high δ13C values of calcite in the Renmark Formation in the Wimmera region compared with those elsewhere (e.g. Cartwright, 2010) is probably due to the Renmark Formation being deposited in a marginal marine setting (Brown, 1989). The Loxton-Parilla Sands in this area are also marginal marine (Brown, 1989).

>13,000 Sample Localities Douglas Depression Groundwater Flow

59

b) 60

59

c) 60

3.2. Groundwater

14

87

Sr/86Sr ratios and δ13C values

C activities and residence times

a14C of groundwater in the Wimmera region ranges from 0.54 to 106 pMC (Fig. 7a, Table 2). The determination of groundwater residence times requires that a14C be corrected for the input of carbon from the aquifer matrix and any other processes that may affect the carbon system. If it is assumed that open system equilibration of 14C occurs in the unsaturated zone followed by closed system dissolution of carbonate, the fraction of C derived from the atmosphere (q) may be derived from the δ13C values of DIC (δ13CDIC), carbonate (δ13Ccc), and recharging water (δ13Cr) via:

59

50

60

Fig. 3. Groundwater TDS concentrations of the Loxton-Parilla Sands (a), Murray Group (b), Renmark Formation (c). Note the irregular distribution of salinity with respect to the groundwater flow paths. Data from Department of Conservation and Natural Resources (1995a,b).

q=

δ13 CDIC −δ13 Ccc δ13 Cr −δ13 Ccc

ð1Þ

(Clark and Fritz, 1997). The following δ13Ccc values were used: Murray Group δ13Ccc = +1‰ (Dogramaci and Herczeg, 2002); Mid Tertiary Aquitard δ13Ccc = 0‰ (Table 3); Renmark Formation and Loxton

Table 1 Location and major ion geochemistry of Wimmera groundwater samples. Loc Fig. 2

East1

North1

Unit2

Screen3 m

EC μS/cm

DO mg/L

pH

HCO3 mg/L

TDS mg/L

Cl mg/L

Br mg/L

NO3 mg/L

SO4 mg/L

K mg/L

Na mg/L

Mg mg/L

Ca mg/L

Si mg/L

Sr mg/L

SI (gyp)4

112185 112186 112459 101108 64281 110088 110089 110090 110091 110092 98309 70221 70222 110093 110094 110095 68436 46147 46148 84745 84746 58449 58450 58451 112202 60610 60623 113245 113247

1 1 1 2 3 3 3 4 4 4 5 6 6 7 7 7 8 9 9 10 10 11 11 11 12 13 13 14 14

575392 575392 575392 632734 579750 579750 579750 588669 588673 588674 610053 618950 618950 584105 584105 584105 558148 574727 574726 584250 584255 599696 599696 599696 573303 530530 530530 547418 547415

6009901 6009901 6009901 5997111 5973850 5973850 5973850 5972830 5972827 5972824 5977660 5966200 5966200 5948609 5948609 5948609 5952847 5940533 5940526 5930440 5930439 5922463 5922461 5922458 5913311 5926368 5926368 5986721 5986722

Ren MG LPS Ren Ren MG LPS Ren MG MG Ren Ren MTA Ren MTA LPS Ren Ren LPS Ren LPS Ren MTA LPS MTA MG Ren Ren MG

213.5 97.5 32 233 147.5 86.5 38 96.5 39 25.5 112 101 41.5 165.5 116 27 190.5 78 38 95 22 78.5 42 17 37.5 143.5 95 291.5 123.5

13,080 16,870 4540 27,000 17,500 2780 1805 13,130 12,190 9480 15,890 27,400 20,700 28,900 28,100 138,000 1550 11,010 6380 1512 8640 5930 10,670 28,090 20,400 898 926 2460 1981

1 0 0 0 2 1 3 1 4 0 0 2 0 1 1 1 2 0 0 0 2 0 0 4 1 2 2 0 tr

6.92 7.88 7.34 6.59 8.54 9.05 6.78 6.72 7.84 7.36 7.63 7.27 7.22 7.16 7.57 7.20 7.68 6.75 7.45 7.89 7.16 6.73 7.13 6.25 7.36 8.53 7.26 10.13 7.56

352 208 308 32 44 65 79 387 285 447 111 94 132 40 32 195 113 240 243 301 314 144 268 100 207 67.0 101 258 144

7580 9270 2280 18,100 10,000 1380 847 8600 8220 4900 11,100 17,700 12,100 18,600 18,100 115,000 640 6600 3750 558 5030 3480 6983 18,800 12,800 326 378 1100 877

3920 5560 1120 10,400 6170 801 433 4500 4240 2480 6350 9880 7060 10,400 10,100 63,100 356 3100 1900 241 2510 1760 3720 10,100 6970 159 175 563 489

13.9 17.5 3.30 30.9 16.9 3.00 1.50 12.8 12.2 7.67 22.3 29.0 24.0 30.0 25.0 218 0.65 10.4 5.58 0.95 9.91 5.76 12.7 30.5 20.0 0.50 0.50 1.53 1.63

1.2 1.2 0.11 0.27 bd bd bd 0.35 0.82 0.31 0.36 bd bd bd bd bd bd 0.34 0.69 0.24 0.54 0.95 1.51 0.38 bd 0.80 0.20 2.95 0.20

721 22 214 680 22 29 71 732 790 365 813 1430 1060 1220 1340 8870 39 678 371 24 470 332 477 1530 905 15 8.1 1.6 15

44 81 18 69 79 19 8 29 45 30 74 63 38 105 90 436 14 41 23 8 22 19 32 34 65 12 22 34 18

2290 3110 733 5730 2930 442 242 2910 2810 1780 3080 4960 3200 6000 5650 38,000 175 2240 1160 242 1520 1100 2170 5730 4200 83 145 488 301

288 290 79 709 475 60 33 267 237 148 367 780 390 660 675 3760 35 286 157 12 253 139 303 827 415 23 16 0.64 35.5

290 175 107 404 296 17 49 129 78 78 372 585 310 229 155 556 16 241 119 23 221 105 248 458 178 19 7 0.98 12

9.7 5.6 8.3 bd 1.7 3.3 9.5 15.4 8.5 11.4 3.1 5.8 7.0 1.4 1.7 11.0 2.9 5.5 10.4 5.5 14.0 5.3 11.5 24.4 5.9 12.0 3.1 0.7 3.7

1.4 2.1 3.0 7.1 13.7 1.1 0.5 2.4 2.2 1.6 3.1 10.0 6.0 4.0 2.8 21.6 1.6 4.3 1.8 0.3 5.0 1.5 3.2 7.3 3.0 1.2 0.6 0.1 0.2

−0.88 −2.62 − 1.47 − 1.03 − 2.47 − 2.96 − 2.02 − 1.23 − 1.39 − 1.57 − 0.82 − 0.57 − 0.80 − 1.02 − 1.14 − 0.24 − 2.71 − 0.96 − 1.33 − 2.75 − 1.08 − 1.40 − 1.11 − 0.66 − 1.13 − 2.95 − 3.64 − 5.57 − 3.29

I. Cartwright et al. / Chemical Geology 277 (2010) 214–226

Bore

1

Eastings and Northings (Australian Map Grid, Zone 54). Units: LPS = Loxton-Parilla Sands, MG = Murray Group, MTA = Mid Tertiary Aquitard, Ren = Renmark Formation. 3 Depth to middle of screen below ground surface. 4 Saturation index with respect to gypsum calculated using PHREEQC (Parkhurst and Appelo, 1999). bd = below detection, tr = trace. 2

219

220

I. Cartwright et al. / Chemical Geology 277 (2010) 214–226

Table 2 Stable and radiogenic isotope data from Wimmera Groundwater. Bore1

Locality Fig. 2

Unit2

δ34S ‰

δ13C ‰

87

112185 112186 112459 101108 64281 110088 110089 110090 110091 110092 98309 70221 70222 110093 110094 110095 68436 46147 46148 84745 84746 58449 58450 58451 112202 60610 60623 113245 113247 678475 518465 928085 509465 795305 604365 847415 756695 756515 604505 581115

1 1 1 2 3 3 3 4 4 4 5 6 6 7 7 7 8 9 9 10 10 11 11 11 12 13 13 14 14 a b c d e f g h h i j

Ren MG LPS Ren Ren MG LPS Ren MG MG Ren Ren MTA Ren MTA LPS Ren Ren LPS Ren LPS Ren MTA LPS MTA MG Ren Ren MG Ren MG MG MG MG MG MG Ren MG MG MG

16.5 7.7 12.3 31.4 51.1 45.6 15.5 18 18.5 17.4 23.2 19.6 17 25.6 15.2 17.8 29.2 14.9 19.5 10.4 19.1 18.9 15.9 20.3 19.2 28.7 45.8 53.1 22.8 32.1 20.8 19.8 19.4 18.6 17.7

− 17.3 − 7.4 − 14.7 − 12.5 19.4 14.1 − 15.3 − 13.4 − 12.3 − 13.9 − 17.1 − 15.4 − 8.8 − 16.2 − 15.4 − 15.4 − 8.7 − 11.1 − 13.8 − 15.5 − 15.1 − 17.1 − 18.4 − 15.9 − 13.1 − 6.6 − 6.7 22.8 − 12.0 − 11.1 − 5.9 − 4.2 − 3.9 − 5.7 − 3.7 − 2.9 − 6.2 − 3.4 − 4.6 − 3.9

0.71072 0.70937 0.71021 0.71247 0.70923 0.70896 0.71389 0.71128 0.71041 0.71046 0.71460 0.71248 0.70953 0.71076 0.71046 0.71018 0.70912 0.70895 0.70955 0.71099 0.70986 0.71210 0.71150 0.71164 0.71115 0.70886 0.70896 0.71016 0.70965 0.70933 0.70923 0.70891

21.0 17.2 14.1 20.0

Sr/86Sr

0.70915 0.70909 0.70899 0.70924 0.70924 0.70939

a14C pMC

q3

Age4 years

40.39 ± 0.21 4.46 ± 0.08 58.45 ± 0.26 68.27 ± 0.29 43.48 ± 0.23 20.82 ± 0.14 54.48 ± 0.26 41.57 ± 022 29.74 ± 0.15 37.48 ± 0.20 20.93 ± 0.15 5.05 ± 0.08 13.45 ± 0.12 49.38 ± 0.23 13.41 ± 0.12 14.01 ± 0.15 2.02 ± 0.04 0.54 ± 0.03 14.05 ± 0.12 67.18 ± 0.29 97.75 ± 0.32 1.08 ± 0.04 15.78 ± 0.12 106.38 ± 0.43 9.96 ± 0.13 2.23 ± 0.05 3.61 ± 0.08 22.97 ± 0.14 1.44 ± 0.04 06 6.0 9.6 9.2 13.6 2.4 10.8 06 5.1 4.6 06

0.93 0.43 0.77 0.90* 0.70* 0.50* 0.81 0.69 0.68 0.76 0.92 0.81 0.53 0.86 0.93 0.81 0.50* 0.55 0.72 0.82 0.79 0.92 0.99 0.84 0.79 0.40* 0.60* 0.85* 0.67 0.73 0.42 0.32 0.30 0.41 0.28 0.24 0.45 0.27 0.34 0.30

6850 18,800 2280 2280 3940 7240 3250 4180 6840 5870 12,200 22,900 11,300 4590 16,000 14,500 26,500 N30,000 13,500 1640 Modern N30,000 15,200 Modern 17,200 23,900 23,200 10,800 N30,000 N30,000 16,100 9830 9700 9030 20,500 6480 N30,000 13,700 16,500 N30,000

1

Bore locations shown on Fig. 2. Units: MG = Murray Group, MTA = Mid Tertiary Aquitard, LPS = Loxton-Parilla Sands, Ren = Renmark Group. 3 Proportion of carbon derived from soil zone, calculated from δ13C values except for * which are estimated from 87Sr/86Sr as discussed in text. 4 Corrected age; uncertainty derived from uncertainty in a14C. An age of N30,000 years has been assigned where a14C b 2 (see text). 5 Samples from Leaney and Herczeg (1999) and Leaney et al. (2003); δ34S values for these samples from Dogramaci et al. (2001), 87Sr/86Sr from Dogramaci and Herczeg (2002). 6 Samples designated as indistinguishable from background (a14C b 3 pMC) by Leaney and Herczeg (1999) and Leaney et al. (2003). 2

Table 3 87 Sr/86Sr ratios, δ13C values, and Sr concentrations of calcite from carbonate and silicate fractions of the aquifer matrix from the Wimmera region. 87 Sr/86Sr carbonate

87 Sr/86Sr silicate

δ13C carbonate

Sr1 (mmol/kg)

Renmark B58449 (65–70 m)2 B58449 (75–77 m) B58449 (80–86 m) B68436 (161–177 m) B68437 (185–198 m)

0.713257 0.711311 0.712870 0.710919 0.712122

0.745120 0.730148 0.737142 0.730577 0.733987

− 3.5 − 1.6 − 2.2 − 1.8 − 1.6

1.2 3.4 2.1

Geera Clay B111094 (110–113 m) B111094 (113–115 m)

0.709118 0.708512

0.715377 0.717540

− 2.1 1.7

Loxton-Parilla B111095 (5–12 m) B111095 (15–17 m) B58451 (10–25 m) B84746 (19–25 m)

0.711321 0.713200 0.712547 0.712051

0.720650 0.718129 0.742988 0.719021

− 0.7 − 3.2 − 3.0 − 2.8

Sample

1

Sr concentration of calcite. Representative of whole core at indicated depth.

2

3.8 3.2 2.7

Parilla Sands δ13Ccc = −2‰ (Table 3). δ13Cr is calculated from the δ13C of the soil carbon, temperature, and pH in the recharge zone. The preland clearing vegetation of the Wimmera region was dominated by eucalypts that have δ13C values of −30 to −27‰ (Schulze et al., 2006) and, assuming a ~4‰ 13C fractionation during outgassing (Cerling et al., 1991), soil CO2 δ13C values would have been −26 to −23‰. At 20 °C and pH 7, δ13Cr calculated from the fractionation data of Vogel et al. (1970) and Mook et al. (1974) is −20 to −17‰; and a value of −18.5‰ is used. Although these calculations require the pH and temperature of recharge and the δ13C of the soil zone CO2 to be estimated, calcite from the regolith elsewhere in the southern Murray Basin has δ13C values of −16 to −12‰ (Cartwright, 2010) that are close to those predicted to be in equilibrium with δ13Cr of −20 to −17‰ at 20 °C, implying that the estimated δ13Cr is reasonable. The 14 C age (t) is given by: t=

lnða = ðq⋅a0 ÞÞ −λ

ð2Þ

where a is the measured a14C, a0 is the initial a14C (assumed to be 100 pMC), and λ is decay constant (1.209 × 10−4 yr−1). For consistency, the residence times of groundwater samples of Leaney and

MG MTA LPS Ren

20

0

40

20

0

60 O 3 40 HC

60

80 80 10

10 0

20

0

0

0

0

0

40

20

20

10

80

60

SO 4 40 60

80

+K 0 6

80

80

60

Na 40

40 Mg 60

0

40

20

100

10

20

0

80

20

20

40

40

60

60

80

80

I. Cartwright et al. / Chemical Geology 277 (2010) 214–226

20

40

Ca

60

80

100

Cl

Fig. 4. Piper diagram of groundwater (1085 samples) from the Wimmera region (MG = Murray Group, MTA = Mid Tertiary Aquitard, LPS = Loxton-Parilla Sands, Ren = Renmark Formation). Data from Table 1, Swane (2004), and Victorian Water Resources Data Warehouse (http://www.vicwaterdata.net/vicwaterdata/home.aspx). Arrows show trends with increasing TDS.

Herczeg (1999) and Leaney et al. (2003) were recalculated in the same way as samples from this study (Table 2); however, the ages are similar to those reported in the original studies. Groundwater in the deeper aquifers in the Wimmera region is commonly anoxic and has Eh values of −4 to +2 V relative to the Standard Hydrogen Electrode (Swane, 2004; Dogramaci et al., 2001). Several samples have high δ13CDIC values (up to + 22.8‰) that are almost certainly the result of methanogenesis in these reduced waters via either the breakdown of long chain organic molecules that leads eventually to acetate fermentation: 2CH2 O ¼ CO2 þ CH4 ;

ð3Þ

or by the direct reduction of dissolved CO2 in the groundwater: þ

CO2 þ 4H ¼ CH4 þ 2H2 O

ð4Þ

221

(Aravena et al., 1995; Clark and Fritz, 1997; Valentine et al., 2004; Leybourne et al., 2006). Both processes increase δ13C values of DIC but have different impacts on a14C. Acetate fermentation generally breaks down 14C-free organic carbon from the aquifer matrix and the subsequent dissolution of the 14C-free carbon in groundwater reduces a14C. By contrast, reduction of DIC results in a small increase in a14C (~2.3 times the increase in δ13C values: Saliege and Fontes, 1984) due to mass-dependant fractionations. The following observations imply that methanogenesis occurs by DIC reduction. Firstly, DIC reduction results in an increase in pH and the three samples with δ13C values N0‰ (64,281, 110,088, and 113,245) have pH values of 8.5–10.1 that are substantially higher than most of the other samples (Table 1); by contrast, acetate fermentation does not increase pH. Secondly, acetate fermentation rarely produces DIC with δ13C values N0‰ (Aravena et al., 1995; Clark and Fritz, 1997), whereas DIC reduction can produce δ13C values in the range of those observed in the Wimmera groundwater (Valentine et al., 2004; Leybourne et al., 2006). If methanogenesis has occurred, bacteriological reduction of other oxidised species such as NO3 and SO4 is also likely. Much of the sulfate in the groundwater has δ34S values of 15–24‰, consistent with it being derived from dissolution of gypsum from the local regolith or playas (δ34S = 15–24‰: Chivas et al., 1991) and/or atmospheric sources (δ34S = 15–23‰: Dogramaci et al., 2001). Groundwater with δ34S values N24‰ has lower S/Cl ratios (Fig. 8a) and lower gypsum saturation indices (Table 1), implying that it has been impacted by bacteriological sulfate reduction. The broad correlation between δ34S and δ13C values (Fig. 8b) implies that reduction of both C and S in individual samples has occurred and allows assessment of whether δ13C values may be used to correct a14C. For samples with δ34S values N24‰, q values were estimated using the 87Sr/86Sr ratios (Table 2), which are not impacted by bacteriological reduction. As discussed below 87Sr/86Sr ratios are well correlated with δ13C values allowing Sr isotopes to be used in this way. All three samples with δ13C values N0‰ and δ34S values N40‰ are from the same area (Fig. 6a), implying that sulfate and carbon reduction is spatially restricted. The minor increase in a14C due to methanogenesis (Saliege and Fontes, 1984) has little impact on calculated residence times and has not been accounted for. Calculating ages in groundwater with low a14C is sensitive to the uncertainties in the corrections, analytical uncertainties, and any minor contamination from the atmosphere during sampling; groundwater that has a14C b2 pMC is considered as indistinguishable from background and assigned an age of N30 ka. Groundwater with calculated a0 values that are N100 pMC contains a component of water that was recharged during or following the atmospheric nuclear tests in the 1950s and 1960s (Clark and Fritz, 1997); this groundwater is labelled

35

20

MG 30

MTA

15

LPS

HCO3 (mmol/L)

25

Ren

10 5

20

0

15

0

5

10

10 5 0

0

5

10

15

20

25

30

Ca (mmol/L) Fig. 5. Molar Ca vs. HCO3 concentrations from groundwater in the Wimmera region. Dashed line shows the expected ratios for calcite dissolution. Inset compares Renmark Formation groundwater from the west of the Geera Clay region with groundwater from the Murray Group (shaded field). MG= Murray Group, MTA= Mid Tertiary Aquitard, LPS = Loxton-Parilla Sands, Ren= Renmark Formation. Data from Table 1, Swane (2004), and Victorian Water Resources Data Warehouse (http://www.vicwaterdata.net/vicwaterdata/home.aspx).

222

I. Cartwright et al. / Chemical Geology 277 (2010) 214–226

0.715 0

60 0.7092,-4.6

0.7102, -14.7 0.7094, -7.4 0.7107, -17.3 0.7097, -12.0 0.7102, 22.8

0.7090,-2.9

0.7125,-12.5

0.7139, -15.3 0.7105, - 13.9 0.7090, 14.1 0.7104, -12.3 0.7092, 19.4 0.7114, -15.3 0.7146, -17.1

0.7093,-3.9 0.7092,-3.4 -6.2

N

50 0.7091,-8.7

0.7092,-5.7 0.7096, -13.8 -5.7 0.7090, -11.1 0.7089, -6.6 -3.2 0.7090, -6.7

(0.7095, -8.8) 0.7125, -16.2

0.7102, -15.4 (0.7105, -15.4) 0.7108, -16.2 0.7099, -15.1 0.7110, -15.5

0.709 0.708 0 -2

c

60 87Sr/86Sr,

δ13C

0.7102, -14.7 : Loxton-Parilla Sands 0.7094, -7.4 : Murray Group Limestone (0.7105, -15.4) : Mid Tertiary Aquitard 0.7107, -17.3 : Renmark Group

δ13C (‰ PDB)

-4 0.7093,-11.1

50

Basement

0.711

H 0.7121, -17.1

(0.7112, -13.1)

0.712

0.710

0.7092,-5.9 E

MG MTA LPS Ren

0.713

0.7116, -15.9 (0.7115, -18.4)

0.7089,-4.2

59

b

0.714

87Sr/86Sr

a

-6 -8 -10 -12 -14 -16 -18 -20 50

52

54

56

58

60

62

64

Easting

Fig. 6. a. Distribution of groundwater 87Sr/86Sr ratios and δ13C values, dashed lines show extent and thickness (m) of Geera Clay. b. Variation of 87Sr/86Sr ratios from east to west. c. Variation of δ13C values from east to west (samples with δ13C N 0‰ not shown). Dashed arrows indicate where the Geera Clay is thickest. Data from Table 2.

modern (M) in Table 2 and Fig. 7a. The calculated groundwater residence times have the following distribution (Fig. 7a): 1. Groundwater residence times in both the Murray Group and the Renmark Formation in the region west of the Geera Clay are 6.5 to N30 ka.

2. Renmark Formation groundwater in the east of region has residence times of 5.1 to N30 ka. Groundwater with the longest residence times in this area is relatively close to the basin margins. 3. Where the Geera Clay is present, Renmark Formation groundwater has shorter residence times (1.6–6.9 ka) than groundwater from the overlying Murray Group (6.8–18.8 ka) or Mid Tertiary Aquitard

Fig. 7. a. Distribution of 14C activities and calculated groundwater ages, dashed lines show extent and thickness (m) of Geera Clay. b. Variation in groundwater residence times with distance north from the basin margins. c. Variation in groundwater residence times of the Murray Group with depth. Data from Tables 1 and 2.

I. Cartwright et al. / Chemical Geology 277 (2010) 214–226

60

a

MG MTA LPS Ren

50 Bacteriological Sulfate Reduction

40 30

Atmospheric/ Gypsum

20

δ34S (‰ V-CDT)

10 0

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0

S/Cl (molar) 60

b

Reduction of Sulfate & DIC

50 40 30 20 10 0 -20

-15

-10

-5

0

5

10

15

20

25

δ13C (‰ V-PBD) Fig. 8. a. Variation in δ34S values and molar S/Cl ratios in Wimmera groundwater. Box shows the δ34S values of atmospheric deposition and dust; arrowed trend is that expected for bacteriological reduction of sulfate. b. Variation in δ34S and δ13C values in Wimmera groundwater. Arrowed trend is that expected for combined bacteriological reduction of sulfate and DIC. Data from Tables 1 and 2.

units (16.0–17.2 ka). Groundwater residence times in the Renmark Formation in this area increase northward. 4. The Loxton-Parilla Sands groundwater has variable groundwater residence times (13.5 ka to modern) that show no consistent variation with either location or depth. At all localities groundwater from this unit is younger than that in the underlying units.

223

area, groundwater in the shallower aquifers has higher hydraulic heads than that in the deeper aquifers (Fig. 2); hence any inter-aquifer flow will be downwards. The observation that 14C residence times are irregularly distributed with respect to position in the basin (Fig. 7b) also implies that there has been significant inter-aquifer flow. In particular, the occurrence of groundwater in the Renmark Formation and Murray Group aquifers with ages of b10 ka several kilometres from the basin margin implies leakage of younger waters from the shallow formations into the deeper aquifers. The variation of 87Sr/86Sr ratios and δ13C values may be used to further constrain groundwater mixing. The eastward increase of 87Sr/ 86 Sr ratios of the Murray Group groundwater and the concomitant decrease in δ13C values (Fig. 6) are best explained by the downwards influx of groundwater from the Loxton-Parilla Sands through the Bookpurnong Beds into the Murray Group near its eastern margins. Likewise the lower 87Sr/86Sr ratios and higher δ13C values of groundwater from the Renmark Formation in the west of the region indicate that downwards leakage has occurred from the Murray Group into the Renmark Formation through the Ettrick Formation. Groundwater from the Renmark Formation where it underlies the Murray Group has Ca:HCO3 ratios that are similar to those of groundwater from the Murray Group but dissimilar to groundwater from the Renmark Formation elsewhere (Fig. 5), which also implies leakage from the Murray Group into the Renmark Formation. The trend of 87Sr/86Sr ratios vs. δ13C values in the Wimmera groundwater (Fig. 9) is characteristic of those that occur by mixing of groundwater from a silicate aquifer that has high 87Sr/86Sr ratios and low δ13C values and groundwater from a carbonate aquifer that has low 87Sr/86Sr ratios and high δ13C values (e.g., Faure, 1991). However, similar trends may be produced by progressive dissolution of calcite with low 87Sr/86Sr ratios and high δ13C values. Isotopic ratios (Rf) resulting from either of these two scenarios are given by: Rf =

xRA CA + ð1−xÞRB CB xCA + ð1−xÞCB

ð5Þ

(Faure, 1991), where, x is the proportion of A in the mixed sample, CA and CB are the concentrations of Sr or C in component A and B, and RA and RB are the isotopic ratios of Sr or C in component A and B. The effects of calcite dissolution on groundwater from the silicate aquifers were calculated assuming the following. Firstly, based on the data in Tables 1 and 2, the groundwater initially contained 0.03 mmol/kg Sr

There is no correlation between 14C residence times and distance from the basin margin (Fig. 7c). In both the Murray Group and Renmark Formation, groundwater with long residence times occurs near the basin margin whereas younger groundwater occurs within the basin. 4. Discussion The geochemistry of the groundwater from the Wimmera region allows the pattern of inter-aquifer mixing to be constrained. The pattern of TDS variation is unrelated to position in the basin and TDS concentrations both increase and decrease along groundwater flow paths (Fig. 3). These flow paths are constructed using hydraulic heads and only constrain the component of sub-horizontal flow within individual formations. Either the salinity of the recharging groundwater has varied over time or, more likely, the variation in TDS concentrations across the region reflects inter-aquifer flow. Despite the variation in lithology in the Wimmera region, groundwater from the different units has similar major ion geochemistry (Fig. 4), which is characteristic of groundwater systems that exhibit inter-aquifer flow (Edmunds, 2009). Except for the very northwest of the study

Fig. 9. Variation in δ13C values and 87Sr/86Sr ratios from Wimmera groundwater. Arrowed line show predicted trend for carbonate dissolution by a groundwater from a silicate aquifer (Silicate GW). Dashed lines show mixing between Renmark Formation and Murray Group groundwater, values are proportion of the Murray Group groundwater. Samples with δ13C values that may have been elevated by methanogenesis are not shown. MG =Murray Group, MTA= Mid Tertiary Aquitard, LPS = Loxton-Parilla Sands, Ren= Renmark Formation. Data from Table 2, Leaney and Herczeg (1999), and Dogramaci and Herczeg (2002).

224

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with 87Sr/86Sr = 0.715 and 2.5 mmol/kg C with a δ13C = −18‰ (Silicate GW in Fig. 9). These 87Sr/86Sr ratios and δ13C values are close to the highest and lowest, respectively, recorded from groundwater in the silicate aquifers and are appropriate for groundwater that has derived the majority of Sr from silicate minerals and DIC from carbon in the soil zone. Secondly, the calcite contained 2.5 mmol/kg Sr with 87Sr/86Sr = 0.710 and 25 mmol/kg C with a δ13C value of −2‰ (Table 3). The predicted trend implies that calcite dissolution can explain at least some of the variation in 87Sr/86Sr ratios and δ13C values, especially in groundwater from the Loxton Parilla Sands and in the Renmark Formation to the east of the Murray Group. However, groundwater in the Murray Group where it underlies the Renmark Group has lower 87Sr/86Sr ratios than those of calcite in the Renmark Formation, implying that additional mixing of water from the Murray Group has occurred. The mixing line on Fig. 9 is constructed between Renmark Formation groundwater with the typical composition of that in the east of the area (87Sr/86Sr= 0.7115, δ13C = −16‰, Sr/DIC = 0.005) and groundwater from the Murray Group limestone (87Sr/86Sr = 0.709, δ13C = −2‰, Sr/ DIC = 0.04) and shows the relative proportion of the limestone end member in the mixed water. Although the variation in groundwater geochemistry makes these calculations subject to uncertainty, they indicate that locally substantial inter-aquifer mixing has occurred. The low 87Sr/86Sr ratios and high δ13C values of the Renmark Formation groundwater in the west of the region imply the addition of 40–70% water from the overlying Murray Group. Similar calculations using 87Sr/ 86 Sr = 0.7110, δ13C = −15‰ to represent the Loxton-Parilla Sands groundwater imply that 80–95% mixing is required to explain the Murray Group groundwater with high 87Sr/86Sr ratios and low δ13C values. Even if it were assumed that the Loxton-Parilla groundwater that infiltrated the Murray Group had dissolved no carbonate prior to infiltration (e.g., using the Silicate GW composition in Fig. 9), it would still imply 60–70% mixing. If re-equilibration of Sr and C isotopes due to mineral dissolution has occurred subsequent to mixing, the calculated relative volumes are minima. These estimates are similar to the volumes of upward leakage between the Renmark Formation and the Murray Group northwest of the Wimmera of 15–85% (Dogramaci and Herczeg, 2002). It is difficult on the basis of major ion geochemistry or Sr and C isotopes to assess leakage between the Loxton-Parilla Sands and the Renmark Formation in the east of the region, as both have similar mineralogies and contain groundwater with similar geochemistry. However, the distribution of groundwater salinities and the very heterogeneous distribution of a14C in the Renmark groundwater imply that there must be leakage through the Loxton-Parilla Sands into the Renmark Formation. 4.1. Rates of vertical leakage Groundwater residence times within the Murray Group in the Wimmera region broadly increase with depth (Fig. 7c), implying a flow system with a considerable vertical flow component. The distribution of residence times with respect to the basin margin (Fig. 7b) shows that this is not an artefact of deeper groundwater being sampled in the north of the region. This flow system probably results from the high hydraulic conductivities in the Renmark Formation that allows northward lateral groundwater within this unit generating high vertical hydraulic gradients between the deep and shallower aquifers and resulting in the groundwater flow paths in the shallower aquifers being steeply inclined. The distribution of ages in Fig. 7c implies a vertical flow rate of ~6 × 10−3 m/year. If downward flow predominates, for a porosity of 0.2 and head gradients of 0.02–0.05 it would require vertical hydraulic conductivities of 2.4–6.0 × 10−2 m/year (6.6 × 10−5 to 1.6 × 10−4 m/day). Although these calculations are subject to significant uncertainties (such as assuming that hydraulic gradients have remained similar over time), they show that the

vertical component of flow may be accommodated by modest vertical hydraulic conductivities that are far less than the estimated lateral hydraulic conductivities (1–14 m/day: Lawrence, 1975). Similar calculations may be made for leakage through the Loxton-Parilla Sands into the Murray Group in the centre of the region (e.g. Locs 3 and 4). Groundwater in the Murray Group at these localities has 14C residence times of 6.8–7.2 ka at ~ 39 and 87 m depth, which implies vertical leakage rates of 6–10 × 10−3 m/year and similar hydraulic conductivities to those calculated above. This range of porosities and velocities implies that the long-term average recharge rates are ~ 0.1–0.2 mm/year, which is b1% of total annual rainfall. The Wimmera region originally contained mallee vegetation that is very efficient in utilising most of the available rainfall, leading to very low recharge rates. As evapotranspiration of rainfall is the dominant process that increases Cl concentrations in Murray Basin groundwater (Herczeg et al., 2001; Cartwright et al., 2008), a broad estimate of regional recharge rates may also be made from groundwater salinities. For rainfall of 400–500 mm with a Cl concentration of 1.5 mg/L: Blackburn and McLeod, 1983) a recharge rate of 0.1–0.2 mm/year implies concentration by evapotranspiration of around 2000–5000. This degree of evapotranspiration would be expected to produce groundwater with Cl concentrations of 3500– 7500 mg/L, which is within the range of that of much of the groundwater in the Murray Group (Table 1). 4.2. Role of the Geera Clay There are two interpretations of relatively young 14C ages in the Renmark Formation beneath the Geera Clay (Fig. 7a). Firstly, the observation that 14C ages increase along the direction of flow could be interpreted to indicate that recharge occurs in the south of the region where the Geera Clay grades into the Renmark Formation and contains sand and silt layers, with subsequent northward lateral flow beneath the Geera Clay. Alternatively, the relatively short residence times may reflect vertical leakage through preferential pathways in the Geera Clay. It is difficult to explain the geochemistry of the Renmark groundwater underneath the Geera Clay by vertical leakage. Specifically, 87Sr/86Sr ratios and δ13C values of Renmark Formation groundwater are significantly different to those in the overlying Murray Group and Geera Clay (Fig. 6a); if significant leakage occurred, there would be a greater similarity between the 87Sr/86Sr ratios and δ13C values in groundwater from these two aquifers as is the case further west. The vertical distribution of groundwater residence times is also difficult to reconcile with vertical leakage, as the Renmark groundwater beneath the Geera Clay is consistently younger than groundwater from the overlying Murray Group or Mid Tertiary Aquitard. Rather the geochemistry indicates that flow in this region is dominantly sub-horizontal. The distribution of groundwater residence times implies northward lateral velocities of ~20 m/year, which for a porosity of 0.1–0.3 implies a flux of 2–6 m3/m2/year. As hydraulic gradients in the Renmark Formation are ~ 5 × 10−4 (Fig. 2) it requires that hydraulic conductivities are 4–12 × 103 m/year or 10– 30 m/day. These are within the range estimated for the Renmark Formation, implying that the proposed flow regime is plausible. The flow regime requires that recharge to this area is focussed through a relatively small region to the south of where the Geera Clay occurs. Shallow sediments in this region include debris flows and alluvial fans derived from the exposed basement that have high hydraulic conductivities (Swane, 2004) which permit relatively rapid recharge. The relatively young groundwater in the Renmark Formation close to the western margin of the Geera Clay (e.g. loc. 8) may reflect northwest lateral flow of groundwater within this aquifer from beneath the Geera Clay; in support of this the 87Sr/86Sr ratios and δ13C values of the Renmark groundwater at loc. 8 are more similar to those from groundwater in this aquifer further east than they are to those of Renmark groundwater in the west of the Wimmera region.

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5. Conclusions The combined use of major ion geochemistry and environmental isotopes has allowed groundwater flow in the Wimmera region of the Murray Basin to be constrained. In particular, these tracers have permitted assessment of both the pattern and timescale of interaquifer mixing and allowed the relative impacts of inter-aquifer flow and calcite dissolution on δ13C values and 87Sr/86Sr ratios to be assessed. Despite the relatively simple stratigraphy and the presence of several putative low hydraulic conductivity layers, there has been widespread long-term inter-aquifer flow in this region. Most of the groundwater in the Murray Group and Renmark Formation to the west of the Wimmera region has 14C residence times N30 ka that do not vary with position in the basin (Leaney and Herczeg, 1999; Leaney et al., 2003) and which result from the low recharge rates and relatively slow inter-aquifer mixing. Only the Geera Clay acts as an efficient aquitard and prevents significant inter-aquifer flow. The long-term vertical mixing in a layered aquifer system is unexpected, even given that there are few effective confining units. Also the degree of inter-aquifer mixing in the Wimmera region where locally between 40 and 95% of water from one aquifer has infiltrated the underlying aquifer is far higher than in many other regional aquifer systems (c.f., Weaver and Bahr, 1991a,b; Edmunds et al., 2002; Ortega-Guerrero, 2003; André et al., 2005). The inter-aquifer flow is driven by high hydraulic conductivities in the deeper Renmark Formation that generates high vertical hydraulic gradients between the deep and shallower aquifers. Recharge occurs across the Wimmera region with recharge of the deeper confined Renmark Formation occurring through the overlying units not solely at the basin margin. The inter-aquifer mixing and downward flow implies that any increase in the salinity of shallow groundwater, as may happen with climate or landuse change (Herczeg et al., 2001; Leaney et al., 2003), may impact the Murray Group or Renmark Formation groundwater from the west of the Wimmera that represents a potentially useful water resource. Although the long residence times suggest that any changes to groundwater quality will be slow unless rates of groundwater extraction increase; the old ages also imply that it will be difficult to use the resource sustainably. Renmark Formation groundwater beneath the Geera Clay locally has TDS b3500 mg/L (Fig. 3) and is a viable source of irrigation or stock water. Although the Geera Clay protects this groundwater from potential contamination from overlying units, protection of the specific recharge area of this groundwater is required to ensure its long-term viability. Acknowledgements We thank Massimo Raveggi and Rachelle Pierson for stable isotope and anion analyses, Linda McMorrow for cation analyses, David Bruce for Sr analyses, Fred Leaney (CSIRO) and the staff of the NIC and ANSTO for 14C analyses. Fred Leaney also provided data collected by the CSIRO group. This work was funded by an ARC Large Grant to T Weaver, AINSE grant AINGRA09053 to I. Cartwright, and Monash University. Considerate and helpful comments by M. Leybourne, B. Bourdon, and an anonymous reviewer helped improve the manuscript considerably. References André, L., Franceschi, M., Pouchan, P., Atteia, O., 2005. Using geochemical data and modelling to enhance the understanding of groundwater flow in a regional deep aquifer, Aquitaine Basin, south-west of France. Journal of Hydrology 305, 40–62. Arad, A., Evans, R., 1987. The hydrogeology, hydrochemistry and environmental isotopes of the Campaspe River aquifer system, North-Central Victoria, Australia. Journal of Hydrology 95, 63–86. Aravena, R., Wassenaar, L.I., Plummer, L.N., 1995. Estimating 14C groundwater ages in a methanogenic aquifer. Water Resources Research 31, 2307–2317. Blackburn, G., McLeod, S., 1983. Salinity of atmospheric precipitation in the Murray Darling Drainage Division, Australia. Australian Journal of Soil Research 21, 400–434.

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