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Constraining groundwater flow, residence times, inter-aquifer mixing, and aquifer properties using environmental isotopes in the southeast murray basin, australia

Contents lists available at Applied Geochemistry Constraining groundwater flow, residence times, inter-aquifer mixing,and aquifer properties using environmental isotopes in the southeast Murray Basin,Australia Ian Cartwright Tamie R. Weaver , Dioni I. Cendón L. Keith Fifield Sarah O. Tweed , Ben Petrides ,Ian Swane a School of Geosciences, Monash University, Clayton, Vic. 3800, Australiab National Centre for Groundwater Research and Training, Flinders University, Adelaide, SA 5001, Australiac URS Australia Pty Ltd., 6/1 Southbank Boulevard, Southbank, Vic. 3006, Australiad Australian Nuclear Science and Technology Organisation, Kirrawee DC, NSW 2232, Australiae Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, ACT 0200, Australiaf School of Earth and Environmental Sciences, James Cook University, Cairns, Qld 4870, Australiag Coffey Environments Pty Ltd., Abbotsford, Vic. 3067, Australiah Terrenus Pty Ltd., 12 Granville Street, Wilston, QLD 4051, Australia Environmental isotopes (particularly d18O, d2H, and d13C values, 87Sr/86Sr ratios, and a14C) constrain geo- Available online 22 February 2012 chemical processes, recharge distribution and rates, and inter-aquifer mixing in the Riverine Province ofthe southern Murray Basin. Due to methanogenesis and the variable d13C values of matrix calcite, d13Cvalues are highly variable and it is difficult to correct 14C ages using d13C values alone. In catchmentswhere d13C values, 87Sr/86Sr ratios, and major ion geochemistry yield similar a14C corrections, 15% ofthe C is derived from the aquifer matrix in the silicate-dominated aquifers, and this value may be usedto correct ages in other catchments. Most groundwater has a14C above background (2 pMC) implyingthat residence times are <30 ka. Catchments containing saline groundwater generally record older 14Cages compared to catchments that contain lower salinity groundwater, which is consistent with evapo-transpiration being the major hydrogeochemical process. However, some low salinity groundwater in thewest of the Riverine Province has residence times of >30 ka probably resulting from episodic rechargeduring infrequent high rainfall episodes. Mixing between shallower and deeper groundwater results in14C ages being poorly correlated with distance from the basin margins in many catchments; however,groundwater flow in palaeovalleys where the deeper Calivil–Renmark Formation is coarser grainedand has high hydraulic conductivities is considerably more simple with little inter-aquifer mixing.
Despite the range of ages, d18O and d2H values of groundwater in the Riverine Province do not preservea record of changing climate; this is probably due to the absence of extreme climatic variations, such asglaciations, and the fact that the area is not significantly impacted by monsoonal systems.
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(e.g., temperature or altitude), or groundwater mixing (e.g., Environmental stable and radiogenic isotopes, especially d18O, d2H, d13C and d34S values, 87Sr/86Sr ratios, and 14C and 3H activities capacity to date groundwater that is up to 30 ka old, and due to are invaluable tracers of regional-scale hydrogeological processes.
the ubiquitous presence of dissolved inorganic C (DIC) in ground- Oxygen and hydrogen isotopes are the only true tracers of the water, 14C is the most widely used radiogenic dating technique water molecule and since all processes in the hydrological cycle in regional aquifers (e.g., fractionate 18O/16O and 2H/1H ratios, d18O and d2H values may be used to determine the extent of evaporation, recharge conditions ) and is invaluable in constraining the timescales ofgroundwater flow and recharge. Stable C and S isotopes trace thesources of dissolved inorganic and organic C and SO4 in groundwa- ⇑ Corresponding author at: School of Geosciences, Monash University, Clayton, Vic. 3800, Australia. Tel.: +61 03 9905 4903; fax: +61 03 9905 4887.
and constrain processes such as bacteriological E-mail address: (I. Cartwright).
0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi: I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 reduction and methanogenesis (e.g., specific regions or concentrated on individual isotopic tracers.
87Sr is produced by the de- The application of geochemical tracers to the adjacent Mallee– cay of 87Rb with a half-life of 48.8 Ga. As Rb substitutes for K and to Limestone Province a) is discussed by a lesser extent Na in minerals, the 87Sr/86Sr ratio of a mineral is governed by its initial 87Sr/86Sr ratio, its Rb/Sr ratio, and its age . A significant part of rural SE Australia depends on groundwater from the Murray Basin for agricultural, industrial, ). Strontium derived from K-rich minerals such as biotite and, increasingly, domestic water supply. This demand will in- and K-feldspar has high 87Sr/86Sr ratios while Sr derived from Ca- crease as population grows, and ongoing development of this re- rich minerals such as calcite or gypsum has low 87Sr/86Sr ratios.
gion relies on the long-term sustainable use of groundwater.
Thus Sr isotopes are important tracers of water–rock interaction While this review concentrates on the long-term regional hydroge- and mixing between groundwater from aquifers of contrasting ology, it is also recognised that the salinisation of groundwater, mineralogy. Unlike C, O, H or S isotopes, mineral precipitation soils, and rivers due to rising water tables following land clearing and dissolution does not fractionate 87Sr/86Sr ratios making Sr iso- (e.g., is an important environmental issue.
topes reasonably straightforward to interpret.
When coupled with major ion geochemistry and physical 1.1. Murray Basin hydrogeology hydrogeology, environmental isotopes constrain groundwater res-idence times, distribution and rates of groundwater recharge, The Murray Basin (occupies 300,000 km2 of SE Austra- water–rock interaction, and groundwater flow paths and are thus lia and contains late Palaeocene to Recent sediments that overlie fundamental to understanding hydrogeological systems and man- Proterozoic to Mesozoic basement ( aging groundwater resources. Constraining recharge rates is re- quired to assess the sustainability of groundwater use while The basin is up to 600 m deep and comprises three delineating recharge areas is required to determine potential sub-basins or provinces (Riverine, Scotia, and Mallee–Limestone: threats to groundwater from near-surface contamination. Deter- a) that are separated by basement ridges. Except for a small mining whether there is a climate control on recharge or if landuse region in the SW that discharges to the Southern Ocean, the Mur- changes have altered recharge rates is also important in under- ray Basin is closed and groundwater discharges to salt lakes and standing the long-term and future behaviour of aquifer systems.
rivers near the basin centre. The Murray–Darling river system is Aquifers in northern Europe, Canada, northern China and Africa the only major surface water feature draining the basin.
contain groundwater with distinctive d18O and d2H values that The Riverine Province underlies the Riverine Plain of Victoria were recharged under colder or wetter conditions than present and New South Wales (There are three main strati- graphic units in the eastern Riverine Province b). The lowermost Renmark Group consists of Palaeocene to late Miocene fluvial silts, sands, gravels, and clays that form a con- indicating that recharge rates have fined aquifer system. Overlying the Renmark Formation are the Pli- varied on timescales of thousands of years. In southern Australia, ocene sands of the Calivil Formation. In most of the Riverine land clearing over the last 200 a following European settlement Province, the Calivil Formation is in hydraulic continuity with the has increased recharge underlying Renmark Formation and these formations commonly Thus modern recharge rates form a single aquifer ). The Cal- measured from bore hydrograph fluctuations or lysimeters may ivil–Renmark Formation is thickest in ancestral drainage channels not indicate the long-term behaviour. Documenting inter-aquifer (‘‘deep leads'') of present day rivers (e.g., the Murray, Campaspe, flow is also important. If groundwater flow is largely parallel to Lodden, Avoca, Ovens, and Goulburn Rivers) that were incised after stratigraphy, deeper groundwater may be protected from near sur- the Middle Miocene marine regression and subsequently filled face contamination; likewise, shallow groundwater and connected with sediments (). Groundwater in these deep surface water systems may be isolated from the impacts of pump- leads flows northwards and feeds into the Murray deep lead where ing of deeper aquifers. By contrast, significant inter-aquifer flow groundwater flow is eastwards (Lateral hydraulic may compromise both the quality and quantity of surface water conductivities of the Calivil–Renmark sediments within the deep and groundwater. Groundwater flow parallel to the main strati- leads based on pumping or slug tests are 40–200 m/day (e.g., graphic units is reasonably straightforward to constrain using hydraulic conductivities in the areas be- hydraulic heads and hydraulic conductivities. However, because tween the deep leads are lower vertical hydraulic conductivities are less commonly measured ). The Calivil and Renmark For- groundwater flow between, and vertically within, formations is mations do not crop out and this aquifer is recharged by downward more difficult to determine using physical hydrogeology (e.g., flow through the overlying units.
and relies on the application of geochemical The uppermost Shepparton Formation comprises fluvio-lacus- trine clays, sands and silts that are laterally discontinuous resulting This paper reviews the contribution of environmental isotopes in a highly heterogeneous aquifer system. to understanding the regional hydrogeology of the Riverine Prov- estimated that lateral hydraulic conductivities are 30 m/ ince of the SE Murray Basin, Australia. In particular, it assesses: day for the coarser units of the Shepparton Formation and substan- patterns and rates of recharge; whether the groundwater preserves tially less in the fine-grained units; vertical hydraulic conductivi- a record of climate change; groundwater flow paths; and the de- ties are 105 to 101 m/day gree of inter-aquifer mixing. There have been few attempts to inte- ). The heterogeneous grate the environmental isotope data from the SE Murray Basin nature of the Shepparton Formation may inhibit lateral flow, pro- and previous studies ( moting downward leakage into the underlying Calivil–Renmark Formation (In the western Riverine Prov- ince, the Loxton–Parilla Sands comprises a sequence of marine sands and silts that underlies the Shepparton Formation. Locally ) have largely discussed processes in in this region, the Shepparton Formation is absent and the

I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 Fig. 1. (a) Map of the Murray Basin (after ) showing depth to basement and groundwater flow paths. MLP = Mallee–Limestone Province, RP = RiverineProvince, SP = Scotia Province. Catchments (west–east) are: W = Wimmera, Ty = Tyrrell, L = Loddon, PH = Pyramid Hill, C = Campaspe, G = Goulburn, B = Benalla, O = Ovens. (b)Stratigraphic cross-section across the Mallee–Limestone and Riverine Provinces at approximately X–X0 (after showing major units in the MurrayBasin.
Fig. 2. Variation in groundwater TDS in the shallowest aquifers of the Riverine Province of Victoria (data from Victorian State Government Groundwater Beneficial Use Maps:). The TDS distributions represent broad averages and many local variations exist.
Catchments are as for . Dotted area denotes coarser-grained sediments in the deep leads ( I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 Loxton–Parilla Sands is the surficial unit. In the far west of the Riv- 2. Sampling and analytical techniques erine Province, the Loxton–Parilla Sands is underlain by the MurrayGroup, which comprises up to 130 m of marine and marginal mar- Sampling and analytical techniques are discussed in detail in ine limestone with calcareous sands, marls and silts the original studies. In summary, groundwater was from monitor- ). The Ettrick Formation, Geera Clay, Bookpur- ing bores that are maintained by the Department of Sustainability nong Beds, and Winnambool Formation envelope the Murray Group; these units are collectively referred to as Mid Tertiary Aqui- that have screen lengths of 1–25 m and tard units ), although hydraulic conductiv- which sample only one lithological unit. The pH, EC, alkalinity, ities are locally sufficiently high to allow flow across these units dissolved CO2 and dissolved O2 were measured in the field using calibrated metres or titration. Cations were determined using Only the Geera Clay, which ICP-AES on filtered and acidified samples. Anions were deter- comprises up to 75 m of massive clays with minor sand and silt mined on filtered unacidified samples using ion chromatography.
layers is an effective aquitard.
Stable isotope ratios were measured using gas source mass The shallowest formations of the Riverine Province are uncon- spectrometers. The 87Sr/86Sr ratios were determined by thermal fined and recharge of groundwater occurs across broad areas ( ionisation mass spectrometry and 14C activities (a14C) were measured using AMS techniques.
Aside from direct recharge,the Murray River and some of its tributaries recharge the shallowaquifer systems, especially at high river stages 3. Sr and C isotopes Additionally, ex-cept in the west of the province, there are few aquitards The 87Sr/86Sr ratios of groundwater in the SE Riverine Province of potentially allowing widespread inter-aquifer flow to occur ( the Murray Basin, Australia are between 0.709 and 0.723 ), and are generally higher than the 87Sr/86Sr ratio of rainfall in southern Australia (0.713: The Riverine Province comprises several catchments As Sr/Cl ratios are also higher than those of the The Ovens, Goulburn, Campaspe, and Loddon catchments are typical deep lead systems that contain lower salinity groundwater there must be additional sources of Sr apart from rainfall containing (total dissolved solids, TDS, typically 500–3500 mg/L: ), while marine aerosols, such as silicate and carbonate weathering in the the Benalla, Lake Cooper, Pyramid Hill, and Wimmera regions out- unsaturated zone and/or mineral reactions in the saturated zone side the deep leads generally contain more saline groundwater of the aquifers. The bulk silicate fraction of the Renmark and Shep- (TDS locally up to 100,000 mg/L and commonly >20,000 mg/L). An- parton aquifers in the Riverine Province has 87Sr/86Sr ratios of nual rainfall in the area depicted in varies from up to 0.718–0.931 while calcite in these aquifers has 87Sr/86Sr ratios of 2000 mm in the SE of the region to 400 mm in the NW; most of the region has 400–600 mm annual rainfall ( The silicate fractions with Rainfall occurs dominantly in the austral winter the highest 87Sr/86Sr ratios are from the Ovens Valley and reflect months (July–September) and for much of the year potential the high degree of immature sediment derived from Palaeozoic evapotranspiration rates exceed rainfall metasediments and granites in this region. The relatively high 87Sr/86Sr ratios of the calcite throughout the Riverine Province re-flect that it is largely non-marine. The d13C values of calcite in most 1.2. Groundwater chemistry aquifers are very variable, ranging between 17‰ and +2‰ which also reflects the largely non-marine nature of the calcite.
The processes controlling the major ion geochemistry are simi- Only in the Wimmera region is the calcite dominantly marine. Cal- lar throughout the southern Murray Basin and are described in de- cite from the limestones of the Murray Group has d13C values of 2.4‰ to +2.2‰ while Renmark Formation and Loxton–Parilla Sands contain calcite with d13C values of 3.5‰ to 0.7‰ ( and The dominant hydrochemical Groundwater from the various catchments has different process is evapotranspiration of rainfall during recharge with min- 87Sr/86Sr ratios (Ovens: 0.716–0.723; Goulburn: 87Sr/86Sr = 0.716– or silicate weathering and minor precipitation and/or dissolution 0.719; Campaspe: 0.714–0.719; Pyramid Hill: 0.714–0.716; Lake of carbonate, gypsum, and halite. Cation exchange (especially the Cooper: 87Sr/86Sr = 0.715–0.719; Tyrrell = 0.711–0.716; Wimmer- sorption of Na onto clays and the release of Ca and Mg) modifies a = 0.709–0.715: ). Aside from the Wimmera region, the composition of the most saline groundwater. Many of the sili- the Riverine Province aquifers are dominated by silicate minerals, cate-dominated aquifers in the southern Murray Basin are rela- thus carbonate weathering and/or mixing between groundwater tively unreactive and water–rock interaction during groundwater from carbonate and silicate aquifers are not likely to be a major flow is limited; indeed processes in the unsaturated zone probably processes. However, locally, carbonate cements and veins do exist.
control much of the groundwater geochemistry ( Given that carbonate minerals are generally more reactive than sil- ). Carbonate dissolution is locally important in controlling icates, calcite dissolution may still control the 87Sr/86Sr ratios and the geochemistry of groundwater in the Murray Group aquifer d13C values of groundwater. For groundwater from most of the ); however, it is catchments the lack of correlation between 87Sr/86Sr ratios and only a minor process elsewhere. Given that evapotranspiration is d13C values (suggests that there has not been significant car- the dominant process, there in a broad inverse correlation between bonate dissolution. This conclusion is difficult to make from the the TDS concentration of groundwater and recharge rates ( isotope data alone as in some catchments the d13C values of calcite are variable. In addition, methanogenesis has locally increased d13C catchments containing saline groundwater have low- values of DIC in the Campaspe, Wimmera, Pyramid Hill, and Tyrrell er recharge rates than catchments containing lower salinity ). However, the following observations imply that

I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 Fig. 3. Variation in 87Sr/86Sr ratios of Riverine Province groundwater with distance from the basin margins in the Ovens (a), Goulburn (b), Lake Cooper (c), Campaspe (d),Pyramid Hill (e), Tyrrell (f), and Wimmera (g) catchments (data from: ).
calcite dissolution is not a major process: (1) carbonate cements change as a result of cation exchange on clays that are derived from and veins are a minor component of the aquifers, especially in weathering of (and which have the similar 87Sr/86Sr ratios to) the the east of the province; (2) trends in Sr/Cl or Sr/Na vs. 87Sr/86Sr primary silicate minerals ().
are not consistent with carbonate dissolution in most of the catch- The variation of 87Sr/86Sr ratios and d13C values in the Wimmera ments (and (3) as noted earlier, the major groundwater reflects both inter-aquifer mixing and ion geochemistry implies that carbonate dissolution is limited.
calcite dissolution. Calcite in the Renmark Formation and Loxton– The 87Sr/86Sr ratios in the Ovens, Goulburn, Lake Cooper and Parilla Sands in the Wimmera region has 87Sr/86Sr ratios 0.709– Tyrell catchments in both the Shepparton and Calivil Renmark For- 0.713 and d13C = 3.5‰ to 0.7‰ ( mations broadly decrease with distance from the basin margins.
The variation in 87Sr/86Sr ratios and d13C This spatial variation in 87Sr/86Sr ratios reflects variations in the values in groundwater from the Loxton Parilla Sands and in the distribution of minerals within the aquifers. Potassium-rich miner- Renmark Formation where it is not overlain by the Murray Group als, such as biotite and K-feldspar that generally have higher results from the dissolution of calcite by groundwater that initially 87Sr/86Sr ratios are more abundant in the proximal parts of the had high 87Sr/86Sr ratios and low d13C values. However, groundwa- aquifers close to the basin margins, while more distal sediments ter in the Renmark Formation where it underlies the Murray Group contain higher relative abundances of plagioclase that has lower has lower 87Sr/86Sr ratios than those of calcite in the Renmark 87Sr/86Sr ratios The major ion Formation (implying that additional mixing of water from chemistry (especially the low cation/Cl ratios) implies that pro- the Murray Group has occurred. Mass balance calculations based gressive silicate weathering during groundwater flow is only a on the Sr and C isotopes and concentrations suggest that locally minor process (Cation exchange (especially up to 40–70% of the groundwater in the Renmark Formation was the exchange of Na for Ca, Mg, and Sr) is well documented in derived from the overlying Murray Group ( Murray Basin groundwater and the 87Sr/86Sr ratios probably ). In support of this assertion, groundwater from the Renmark

I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 Fig. 4. 87Sr/86Sr ratios vs. d13C values of Riverine Province groundwater in the Ovens (a), Goulburn (b), Lake Cooper (c), Campaspe (d), Pyramid Hill (e), Tyrrell (f), andWimmera (g) catchments. (h) 87Sr/86Sr ratios of the aquifer matrix and predicted trends for carbonate dissolution (dashed lines) (data from ).
Formation where it underlies the Murray Group has Ca:HCO3 ratios corrections are required for input of old 14C-free C from: (1) disso- that are similar to those of groundwater from the Murray Group but lution of carbonate minerals or organic material from the aquifer dissimilar to those of groundwater from the Renmark Formation matrix; (2) deep-seated geogenic CO2 from volcanic activity; and elsewhere (). Similar calculations imply that (3) CH4 generated via the breakdown of organic material in the locally 80–95% mixing from the Loxton–Parilla Formation is re- aquifer matrix. There are numerous schemes for correcting 14C quired to explain the Murray Group groundwater with high ages based on d13C values of DIC (e.g., 87Sr/86Sr ratios and low d13C values ). Comparable estimates of upward leakage between the Ren- ). Most of these schemes assume that DIC mark Formation and the Murray Group NW of the Wimmera region in groundwater is derived largely from open-system dissolution of 15–85% were made by .
of CO2 from the soil zone and that subsequent dissolution of (or ex-change with) carbonates in the aquifer matrix is the main processthat impacts d13C values and a14C in the aquifers.
4. Distribution of 14C ages Despite most aquifers in the Riverine Province of the Murray Basin being dominated by silicate minerals and the input of geogen- summarise the distribution of 14C ages in the Riv- ic CO2 not being likely, the correction of a14C in Murray Basin erine Province. While 14C is the most commonly used tracer to groundwater is not straightforward. In some catchments, such as determine groundwater residence times in regional aquifers, its the Campaspe, the d13C values of DIC range from 18‰ to +2‰ application is not without considerable problems. These include (Using these d13C values to correct 14C ages assuming that the anomalously high a14C activities in groundwater recharged dissolution of matrix calcite had occurred implies that locally since the 1950s due to the atmospheric nuclear tests. Additionally, >90% of the DIC is derived from calcite dissolution. However,

I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 Fig. 5. (a) Distribution of groundwater 87Sr/86Sr ratios and d13C values in the Wimmera region, dashed lines show extent and thickness (m) of Geera Clay. (b) Variation of87Sr/86Sr ratios from east to west across the Wimmera region. (c) Variation of d13C values from east to west across the Wimmera region. Dashed arrows indicate where theGeera Clay is thickest. From : ).
Fig. 6. Variation in 14C ages of Riverine Province groundwater with distance from the basin margins in the Ovens (a), Benalla (b), Goulburn (c), Lake Cooper (d), Campaspe (e),Pyramid Hill (f), Loddon (g), Tyrrell (h), and Wimmera (i) catchments (data from ).
87Sr/86Sr ratios and major ion geochemistry of groundwater imply results in an increase in pH, and groundwater with d13C values that calcite dissolution is only a minor process. DIC with high >0‰ in both the Campaspe and Wimmera catchments have pH d13C values is present in groundwater from other catchments values >7.5 that are substantially higher than most of the other (e.g., DIC from Wimmera groundwater has d13C values up to samples; by contrast, acetate fermentation does not increase pH.
+14‰ while that from the Pyramid Hill and Tyrrell catchments Secondly, acetate fermentation rarely produces DIC with d13C val- has d13C values as high as +1‰). The variable and high d13C values ues >0‰ ), whereas DIC reduction can produce are most probably due to methanogenesis that occurs in the locally d13C values in the range of those observed in the Wimmera and anoxic conditions in these aquifers ( Campaspe groundwater ( ). The following observa- If methanogenesis has occurred, bacteriological reduction tions support that methanogenesis is via the reduction of DIC in of other oxidised species such as NO3 and SO4 is also likely. In the groundwater rather than the breakdown of organic material from Campaspe, Tyrrell, and Wimmera groundwater dissolved SO4 in the aquifer matrix via acetate fermentation. Firstly, DIC reduction the high d13C groundwater has d34S values of 25–50‰ a).

I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 Fig. 7. Variation in 14C ages of Riverine Province groundwater with depth in the Ovens (a), Benalla (b), Goulburn (c), Lake Cooper (d), Campaspe (e), Pyramid Hill (f), Loddon(g), Tyrrell (h), and Wimmera (i) catchments (data from Fig. 8. d34S values vs. d13C values (a) and S/Cl ratios (b) in groundwater from the Wimmera (Wim), Campaspe (Cam), and Tyrrell (Tyr) catchments. S concentrations areexpressed as ratios to Cl to remove the effects of variable salinity in the groundwater. Data from ( These values are much higher than those of gypsum in the local reg- olith or playas (d34S = 15–24‰: ) and/or atmo- were calculated using the statistical correction, with spheric sources (d34S = 15–23‰: ) and the exception of the Wimmera region where the corrected ages reflects bacteriological SO4 reduction based on d13C values are shown. Calculating ages in groundwater The high d13C groundwater also has lower SO4 with low a14C is difficult due to analytical uncertainties and the and NO3 concentrations (that are also consistent with bac- possibility of contamination from the atmosphere during sam- teriological reduction. Methanogenesis by this mechanism has only pling; groundwater that has a14C <2 pMC is considered to be indis- a minor impact on a14C. estimated that tinguishable from background and is assigned an age of >30 ka.
the mass-dependant fractionation of 14C relative to 12C is 2.3 that Groundwater with calculated a0 values that are >100 pMC contains of 13C; thus, an increase in d13C of 10‰ should result in an increase a component of water that was recharged during or following the in a14C of only 2.3%.
atmospheric nuclear tests in the 1950s and 1960s ( In addition to methanogenesis, the d13C of calcite in the aquifer While it is possible to use high 14C groundwater to constrain matrix is not always well constrained and may be variable. This is modern recharge processes for the especially the case in the eastern part of the Riverine Province purposes of this regional study this groundwater is considered to where the sediments are largely non-marine. Given these uncer- be modern with an effective age of 0 a.
tainties, a statistical correction which assumes that in silicate aqui-fers 15% of the C is derived from the aquifer matrix 4.1. Contrasting age distributions between catchments has been applied. In the Wimmera and Goulburn catchmentswhere calcite in the silicate aquifers has a restricted range of There is a distinct difference between the patterns of ground- d13C values this statistical correction produces ages that are similar water ages in the different catchments. Ages of the deeper to those calculated using the d13C values Calivil–Renmark Formation groundwater in the Goulburn, Loddon, I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 and Campaspe catchments increase with distance from the basinmargins (). Groundwater with residence times of 9–13.5 kais recorded 70 km from the basin margin in the Campaspe catch-ment, groundwater with residence times of up to 16.5–18.5 ka ispresent at 85–110 km in the Goulburn catchment, and groundwa-ter with ages of 21 ka is present at 125 km in the Loddon catch-ment. The age of groundwater from the Pyramid Hill Catchmentalso increases away from the basin margins (ages are up to 20 kabetween 40 and 55 km); however, this trend is defined by rela-tively few samples. One groundwater sample from the distal partof the Goulburn catchment yields an anomalously young age(13 ka). This sample, which has higher NO3 concentrations anddifferent d18O values to adjacent groundwater (), is from the vicinity of a groundwater mound andmay be recording the mixing of recently recharged groundwaterwith deeper groundwater. Overall, the distribution of ages in thesecatchments implies a relatively simple pattern of lateral ground-water flow in the deeper Calivil–Renmark Formation with littlemixing from the overlying Shepparton Formation. Elsewhere,groundwater ages do not increase with distance from the basinmargins (). Indeed, in the Lake Cooper, Tyrell, and Wimmeracatchments the oldest groundwater from the Renmark Formationis commonly close to the basin margins. In none of the regionsdo the ages of Shepparton or Loxton–Parilla groundwater increasewith distance from the basin margins ); however in severalof the catchments there is an irregular increase in age with depth.
5. Oxygen and hydrogen isotopes The d18O and d2H values of groundwater from all catchments are similar and cluster around the global and Melbournemeteoric water lines at approximately the composition of modernprecipitation for Melbourne (d18O = 5.0‰, d2H = 28‰). Theoccurrence of samples to the left of the Melbourne meteoric waterline is probably due to climatic differences between Melbourne(which is coastal) and the Riverine Province (which is inland andmore arid). Groundwater with similar d18O and d2H values occurselsewhere in the Murray Basin The groundwater as a whole defines an array witha slope of 5 that is probably due to evaporation in a semi-aridenvironment ). However,most samples show an increase in d18O of <3‰ and there is no cor-relation of d18O values with TDS. A 5‰ increase in d18O values isproduced by 20% evaporation (which is far less than that required to produce the high Fig. 9. (a) d18O vs. d2H values of Riverine Province groundwater by aquifer. Datacluster around the global (GMWL) and Melbourne (MMWL) meteoric water lines at TDS contents of the Murray Basin groundwater. Thus transpiration, about the value of modern rainfall in Melbourne. The arrowed line is a linear best fit which does not significantly affect d18O values, is probably the to the entire dataset and the inset shows changes in d18O vs. d2H resulting from more important process in concentrating solutes in these waters.
various hydrological processes. (b) d18O vs. d2H values for Riverine Province Prior to land clearing, the southern Murray Basin was dominated groundwater from the different catchments. (c) d18O vs. 14C ages for RiverineProvince groundwater (data from by native vegetation (particularly eucalypts) that was an efficient user of available rainfall leading to significant transpiration Despite groundwater recharge occurring in both high rainfall areas (e.g. the Ovens catchment) and low rainfall inland areas limited there is no change in the d18O and d2H values of groundwa- (e.g. Tyrell and Wimmera catchments), there are no spatial varia- ter along the catchment, and in no catchment is there a correlation tions of d18O and d2H (b). Most recharge in the Murray Basin between 14C age and d18O or d2H values (c).
occurs on the Riverine Plains that has limited topographical varia-tion and there is no altitude affect in the d18O and d2H data. Even inthe Ovens catchment, there are no major differences between thed18O and d2H values from groundwater in the more elevated upper catchment and that in the lower catchment. As discussed above,groundwater in many of the catchments (particularly those that Environmental isotope geochemistry has allowed an under- are not deep leads) has undergone mixing that may have homoge- standing of hydrogeochemical processes and regional groundwater nised the d18O and d2H values. However, even in the Goulburn and flow in the Riverine Province that was not possible from a consid- Campaspe catchments where (as discussed below) mixing is more eration of physical parameters and major ion chemistry alone.
I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 6.1. Groundwater ages, salinity, and recharge rates ing upwards mixing from the Calivil–Renmark Formation. A similardifference between 87Sr/86Sr ratios of the Calivil–Renmark ground- Recharge in all catchments occurs across broad areas as the ba- water and most of the groundwater in the Shepparton Formation is sin is unconfined. In most of the Riverine Province, catchments observed in the Pyramid Hill region ), which also precludes outside the deep leads contain higher salinity groundwater with significant inter-aquifer mixing in that area. In many of the other older 14C ages while lower salinity groundwater from the deep catchments, there is not sufficient distinction in the 87Sr/86Sr ratios leads is relatively young. This inverse correlation is expected in ba- to use these tracers to test whether mixing has occurred.
sins where the dominant hydrogeochemical process is evapotrans- As discussed above, the increase in 14C ages with depth in the piration. The coarser-grained sediments of the deep leads result in Shepparton Formation and the Loxton–Parilla Sands indicates that lower degrees of evapotranspiration and higher recharge rates, and flow in these aquifers has a strong downward component. Down- as a first approximation groundwater salinity is an indica- ward flow in the shallow aquifers probably results from the much tion of relative recharge rates. Recharge rates in the Shepparton higher hydraulic conductivities of the deeper Calivil–Renmark For- Formation may be calculated using the 14C data. Although the mation compared with the near surface units that results in refrac- trends of increasing age with depth are irregular in many of the tion of groundwater flow paths. That significant mixing of this catchments, groundwater with 14C ages of 15–25 ka commonly oc- groundwater with the deeper groundwater is not observed in all curs at 40–60 m depth (). The general trends of age with catchments is probably a result of the relative hydraulic conductiv- depth imply infiltration rates of approximately 1–4 mm/a with ities. In the deep leads the high groundwater fluxes within the higher rates in the deep leads than in the intermediate areas. For higher hydraulic conductivity Calivil–Renmark Formation effec- porosities of 0.2–0.3, these infiltration rates equate to recharge tively dilutes the relatively minor leakage from the overlying units.
rates of 0.3–1.2 mm/a (1% of modern rainfall). These are similar The distribution of ages (allows estimation of hydraulic to recharge rates estimated in the Murray Basin by Cl mass balance parameters in the Campaspe, Goulburn, and Loddon catchments.
(0.03–2 mm/a: ). For an The increase in groundwater ages in the Calivil–Renmark Forma- average vertical hydraulic gradient of 0.05 and porosities of 0.2– tion of 9–13.5 ka over 60 km in the Campaspe catchment implies 0.3, vertical hydraulic conductivities calculated using Darcy's Law flow velocities of 4.4–6.7 m/a, which for a porosity of 0.2-0.3 are approximately 105 to 104. These are within the range of ver- equates to a groundwater flux of 0.89–2.0 m3/m2/a. Lateral hydrau- tical hydraulic conductivities for the Shepparton Formation of 105 lic gradients in the Murray Basin are typically 104 to 5  104 and to 101 m/day reported by and Darcy's Law yields lateral hydraulic conductivities of 4000– 8900 m/a (11–24 m/day). Similar calculations in the Loddon catch- Groundwater from the Renmark Formation and Murray Group ment assuming an increase in groundwater age of 21 ka over in the west of the Riverine Province and in the adjacent Mallee 120 km yields hydraulic conductivities of 9–31 m/day, while Limestone Province commonly has residence times of >30 ka ( assuming that the age increase in the Goulburn catchment is Figs. 6 and 7), implying 18.5 ka over 80 km yields hydraulic conductivities of 7–25 m/day.
that recharge rates in this region are also low. By contrast with These estimates are slightly below those estimated from pumping groundwater elsewhere in the Riverine Province, this groundwater tests in the deep leads (typically 40–200 m/day: has low TDS contents implying that the low recharge rates are not The difference may relate to the result of extreme degrees of evapotranspiration. Rather, signif- errors in the assumed hydraulic gradients; due to the increase in icant recharge may occur mainly during infrequent higher-precip- recharge rates following the land clearing over the past 200 a, itation periods in this low rainfall region. If this is the case, rapid the present-day hydraulic gradients may be higher than those that recharge through the sandy soils that dominate this region may ac- were typical in the southern Murray Basin over the length of time count for the low salinity of the groundwater that these flow systems have operated. Alternatively, as pump tests record hydraulic conductivities over a relatively small region, theymay not be representative of the aquifer as a whole. Nevertheless, 6.2. Groundwater flow and inter-aquifer mixing the broad agreement between the hydraulic conductivities calcu-lated from the 14C ages and those measured using pump tests im- The increase in 14C ages with distance () implies that that plies that the interpretation of groundwater ages and flow systems groundwater flow in the Calivil–Renmark Formation in the deep leads is relatively simple with little leakage from overlying units.
By contrast, the variation in 14C ages in the other regions implies 6.3. Variations with climate that flow paths are complex and that there is considerable inter-aquifer mixing. This conclusion is supported by the distribution Unlike large basins elsewhere (e.g., of TDS contents in groundwater. The highest salinity groundwater in the Lake Cooper, Wimmera and Tyrrell catchments commonly occurs close to the basin margins precluding simple lateral groundwater flow, whereas TDS contents of Calivil–Renmark is no correlation between 14C ages and d18O values in the Riverine groundwater in the deep leads are relatively constant or increase Province, and deeper groundwater has similar d18O values to shal- along the flow paths. The variation of 87Sr/86Sr ratios in the Campa- lower groundwater and to modern surface water and rainfall. The spe catchment is also consistent with dominantly lateral lack of variation in d18O and d2H values is surprising given that groundwater flow in the Calivil–Renmark Formation with little groundwater present in the basin was recharged over at least vertical leakage. The 87Sr/86Sr ratios of the Calivil–Renmark 30 ka and that palaeoclimate studies show that between approxi- groundwater within the Campaspe Valley are 0.7159–0.7165 while mately 30–22 ka and 7–4 ka rainfall was higher than at present, those of the Shepparton groundwater are 0.7141–0.7148 (If while between 20 and 10 ka, conditions were considerably drier inter-aquifer leakage were widespread, 87Sr/86Sr ratios in the Cali- (). There is little evidence for these cli- vil–Renmark groundwater should decrease along the flow path, matic changes in the stable isotope data in the Riverine Province which is not observed. The higher 87Sr/86Sr ratios of the Shepparton groundwater, nor are there any obvious gaps in the 14C age spec- groundwater at 75 km from the basin margin are probably pri- trum that might result from extended periods of little recharge mary as hydraulic gradients in this region are downwards preclud- during periods of drier climate.
I. Cartwright et al. / Applied Geochemistry 27 (2012) 1698–1709 Rainfall in SE Australia derives from a variety of sources (mainly laide), Geraldine Jacobsen (ANSTO), and Stuart Fallon (ANU) for the Southern, Indian and Pacific Oceans) rather than dominantly the 14C determinations. Marcus Onken and Kaye Hannam helped from a single weather system ).
collect the samples. Ongoing funding by the Australian Research While there are differences between the d18O values of rainfall de- Council, the National Centre for Groundwater Research and Train- rived from these systems (notably heavy winter rains from the ing, and Monash University is gratefully acknowledged.
Southern Ocean have low d18O values) the variations in climatemay have been too slight to produce major differences in the over- Appendix A. Supplementary data all weather patterns (or at least in the resultant d18O values). Thisis in contrast to areas such as northern China that lie at the margins Supplementary data associated with this article can be found, in of the current monsoon systems where older groundwater has dis- the online version, at tinct d18O values due to variations in monsoon intensity (). Likewise, there was not any dramatic change in hydrogeological conditions follow-ing glaciations, such as occurred in the higher latitudes of the Allison, G.B., Colin-Kaczala, C., Filly, A., Fontes, J.C., 1987. Measurement of isotopic northern hemisphere. Groundwater elsewhere in southern Victoria equilibrium between water, water vapour and soil CO2 in arid zone soils. J.
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