the importance of unsaturated zone biogeochemical processes in determining groundwater composition,...

The importance of unsaturated zone biogeochemical processes

in determining groundwater composition, southeastern Australia

Matthew Edwards & John Webb

Abstract Analysis of soil, soil water and groundwater inthe Mount William Creek catchment, southeastern Aus-tralia, shows that Mg2+ and Ca2+ within infiltrating rainfallare rapidly depleted by plant uptake and adsorption onclay minerals. Na+ and K+ may exhibit minor enrichmentat shallow depths but are quickly readsorbed, so thatcation/Cl– ratios typical of groundwater are observed insoil water within the upper 200cm of the soil profile for allspecies. The concentrations of K+ and Ca2+ in soil andgroundwater are more depleted than Na+ and Mg2+ due topreferential uptake by vegetation. Removal of organicmatter results in a continuing, long-term export of allmajor cations from the soil profiles. The processes ofbiogeochemical fractionation within the unsaturated zonerapidly modify the cation/Cl– ratios of infiltrating rainfallto values characteristic of seawater. These mechanismsmay have reached steady state, because groundwaters withseawater ion/Cl– ratios are thousands of years old; theexchange sites on the soil clays are probably saturated, socations supplied in rainfall are exported in organic matterand incorporated into recharge infiltrating into the ground-water. Much of the chemical evolution of groundwatertraditionally attributed to processes within the aquifer iscomplete by the time recharge occurs; this evolutionarymodel may have broad application.

Keywords Unsaturated zone . Cation exchange .Plant uptake . Hydrogeochemistry . Australia

Introduction

The role of soil zone reactions in controlling groundwaterchemistry is often regarded in the hydrogeochemicalliterature as subordinate to reactions occurring in thesaturated zone of aquifer systems. Where aquifers arecomposed of reactive minerals such as carbonates, halidesor unstable silicates, interactions between groundwaterand the host lithologies dominate the evolution ofgroundwater composition (e.g. Garrels and Mackenzie1967; Heathcote 1985; Cardenal et al. 1994; Kimblin1995; Rosen and Jones 1998; Stuyfzand 1999; Toth 1999;Rademacher et al. 2001; Dogramaci and Herczeg 2002;Benedetti et al. 2003).

However, in the semi-confined and unconfined alluvialaquifers of the Murray-Darling Basin, southeastern Aus-tralia, it is generally agreed that groundwater solutes arederived from rainfall and that the dominant evolutionarymechanism affecting groundwater chemistry is concentra-tion by evapotranspiration (e.g. Dyson 1983; Arad andEvans 1987; Macumber 1991; Simpson and Herczeg1994; Herczeg et al. 2001; Cartwright et al. 2004).Despite this, high-resolution rainfall chemistry, datasetsfrom this area (e.g. Hutton and Leslie 1958; Blackburnand McLeod 1983; Simpson and Herczeg 1994; Bormann2004) demonstrate that the ratios of most ions to Cl–

within rainfall differ substantially from the groundwaterratios, indicating that processes apart from evapotranspi-ration are affecting the groundwater chemistry.

Studies of many groundwater systems throughoutAustralia (e.g. Lawrence 1975; Arad and Evans 1987;Macumber 1991; Salama et al. 1993; Acworth andJankowski 2001; Herczeg et al. 2001; Cartwright et al.2004) and in other countries (e.g. Kimblin 1995; Elliot etal. 1999; Guler and Thyne 2004) show that groundwatermajor cation/Cl– ratios typically decline rapidly as thegroundwater salinity increases, from values approximatingthose in local rainfall towards values more characteristicof seawater in many cases. To explain these trends,previous workers have cited mechanisms including min-eral weathering (e.g. Jankowski and Acworth 1993;Cartwright et al. 2004), cation exchange (e.g. Kimblin1995; Toth 1999; Acworth and Jankowski 2001; Bennettset al. 2007), dissolution/precipitation reactions (e.g.Stuyfzand 1999; Herczeg et al. 2001; Guler and Thyne2004), the formation of secondary clay minerals (e.g.

Received: 5 April 2008 /Accepted: 23 February 2009Published online: 27 March 2009

© Springer-Verlag 2009

M. Edwards : J. WebbDepartment of Environmental Geoscience,La Trobe University,Kingsbury Drive, Melbourne, Victoria 3086, Australia

M. Edwards ())ENSR Australia,6/417 St Kilda Road, Melbourne, Victoria 3004, Australiae-mail: [email protected].: +61-3-86992199Fax: +61-3-86992122

Hydrogeology Journal (2009) 17: 1359–1374 DOI 10.1007/s10040-009-0449-8

Blake 1989; Salama et al. 1993) and mixing with connateseawater (e.g. Lawrence 1975; Elliot et al. 1999), with thestated or implied assumption that these processes occurbelow the water table. Until recently, only a few studiesdescribed how reactions within the unsaturated zone couldaffect groundwater major element composition (e.g.Spears and Reeves 1975; Drever and Smith 1978; Mossand Edmunds 1992).

However, it is recognised that many soils are a hot spotof geochemical, particularly organic, activity due to thehigh microbial, fungal and root biomass present (Choroveret al. 2007). Microbial metabolic activity concentrated inthe uppermost few millimeters of the soil strongly affectschemical gradients of nutrient species (Garcia-Pichel et al.2003), and detailed studies on soil water composition(White et al. 2002, 2006) have demonstrated the influenceof biological processes on the concentrations of somespecies. This suggests that soil zone processes caninfluence soil water and therefore groundwater composi-tion to a degree that has often been neglected.

This study assesses the evolution of groundwaterchemistry in a small, partially cleared catchment inVictoria, southeastern Australia, through a detailed studyof soil, soil water and groundwater chemistry. It presentsan alternative explanation wherein most change ingroundwater composition is attributed to soil zone process-es, including plant nutrient cycling, previously neglected inthe study of Australian groundwater chemistry.

Site description

The Mount William Creek catchment is located in theupper reaches of the Wimmera River catchment in westernVictoria, southeastern Australia (Fig. 1), and lies on thesouthern margin of the Murray-Darling Basin, whichoccupies a large part of inland Australia. The study areahas a temperate climate with mean annual rainfall of595.5 mm at Moyston, in the south, and 533.5 mm atStawell, in the northeast of the catchment. Pan evaporationexceeds rainfall in all but the winter months, with a meanannual total of 1501.1 mm at Stawell (Bureau ofMeteorology 2003).

The topography is dominated by the Mount Williamand Mount Difficult Ranges of the Grampians-GariwerdNational Park, which form high mountain escarpments inthe west of the study area, and the relatively subdued,lower elevation Ararat Hills and Black Range in the east.Much of the catchment has been cleared for drylandagriculture (largely sheep grazing), excluding somesignificant tracts of native forest vegetation, particularlyin the Grampians-Gariwerd National Park, which extendsalong the entire western boundary.

The area is geologically complex, with a Palaeozoicbasement consisting of Cambrian greenstones and turbi-ditic metasediments, Silurian schists and sandstones and anumber of Devonian-age granitic intrusives (Cayley andTaylor 2001). The basement is overlain in the MountWilliam Creek valley by Cainozoic fluvial sediments and

colluvium, sub-divided into a basal Tertiary sand andgravel unit overlain by Quaternary sandy clay. Thereforethree aquifers are present: Palaeozoic basement, semi-confined Tertiary sediments and Quaternary alluvium.

Soil profiles analysed throughout the catchment(Fig. 1) fall between two distinct geological and physio-graphic end-members, represented by sites 27 and 5, bothof which were selected for detailed sampling. Site 27(SP27) is located adjacent to the Grampians-GariwerdNational Park (Fig. 1), in Quaternary transported silty-claysbeneath natural Eucalypt forest. The site lies in a localvalley floor at an elevation of ~190 m relative to AustralianHeight Datum (AHD) (equivalent to mean sea level), andslopes gently to the north and east. The soil composition isdominated mineralogically by kaolinite and quartz withminor anorthite, and consists of medium-grey clay gradingdownwards into light brown silty-sand at 2.0 m depth. Thewater table in the vicinity of SP27 is approximately 10 mbelow the ground surface, such that the surficial Quaternaryalluvium is saturated at this locality.

Site 5 (SP5) lies in cleared farmland on deeplyweathered Silurian schist (Fig. 1). It is located on thecrest of a broad ridge at ~240 m AHD with thesurrounding terrain falling gently to the north and south.The water table lies at a depth of 8.5 m. The soil is a paleyellow silty clay with minor quartz gravel and isdominated mineralogically by kaolinite and quartz withminor illite/muscovite and clinochlore. The farmland atsite 5 is used for sheep grazing; no fertilisers are applied,nor is there any evidence of historical fertiliser usage.

Site hydrogeology

The Palaeozoic basement aquifer incorporates all thePalaeozoic rock units within the catchment, and is largelycomposed of crystalline rocks characterised by fractureporosity. The aquifer outcrops along the western catch-ment boundary as the densely vegetated GrampiansRanges, which consist of metamorphosed sandstones,through the centre of the catchment as a discontinuousridge of metavolcanic greenstone, and extensively in theeast of the catchment as rolling hills of granite and deeplyweathered metamorphic rocks (Fig. 2). Throughout thecentral lowlands and tributary valleys the aquifer isconcealed and confined to semi-confined beneath theCainozoic sedimentary cover, and has a low hydraulicconductivity of the order of 0.8 m/day (Edwards 2006).

The Tertiary Calivil Formation comprises unconsoli-dated sands and gravels, consisting predominantly ofquartz, that lie in palaeodrainage lines beneath the presentMount William Creek floodplain, where deposits canreach 40 m in thickness and up to 6,000 m in width(Fig. 2). It is the dominant aquifer in the catchmentbecause of the large volume of relatively porous sedimentand is also known as the Tertiary sediments aquifer; it isresponsible for much of the down-basin groundwater flowwithin the catchment (Edwards 2006). The aquifer doesnot outcrop and therefore receives recharge from other

1360


aquifers. It is heterogenous and anisotropic, due to amoderately well defined fining upwards sequence andstrong lateral variability; hydraulic conductivities areestimated to range from 1.0 m/day up to 100 m/day.

The Quaternary alluvial aquifer consists mostly ofextensive alluvium, which blankets much of the lowerrelief area along Mount William Creek and tributaryvalleys to depths of up to 25 m (Edwards 2006; Fig. 2).The sediments are clay-rich and unconsolidated, withminor sand stringers evident during drilling, both in thisstudy and previous investigations (e.g. Harrison 1993).This aquifer is characterised by a low hydraulic conduc-tivity of the order of 1.0 m/day.

Potentiometric and groundwater geochemical evidencepresented by Edwards (2006) generally indicates a lowdegree of cross-formational interaction throughout themajority of the study area where groundwater movementis dominated by lateral flow; however, vertical flow issignificant in areas of relatively high recharge.

Recharge occurs dominantly from direct rainfall acces-sions and is highly variable throughout the catchment,ranging from 50–100 mm/year in the Grampians Rangeand associated colluvial slopes to 0.2 mm/year in areas ofwell-developed regolith beneath remnant native vegetation(Edwards and Webb 2006). Transient and steady-state soilwater Cl– mass balance modelling (Edwards 2006) show

Fig. 1 Location of the Mount William Creek catchment study area, showing sample sites

1361


that recharge at SP5 and SP27 is of the order of 8 and0.2 mm/year, respectively, consistent with the physiogra-phy of the sites.

Sampling and analytical methods

GroundwaterA total of 79 groundwater bores were sampled during thisstudy (Fig. 1), ranging in depth from 5 to 142 m, with atypical depth of ~20 m. Before collection of groundwatersamples, all bores were purged of at least three bore volumesof water using a Grundfos SQ1–80N electric submersiblepump for bores with ≥100 mm diameter casings, and a PVCbailer for bores with smaller diameter casings. Groundwatersamples were collected in 120-ml polyethylene vials follow-ing the stabilisation and measurement of the physicalparameters EC, pH, Eh and temperature; samples wereimmediately refrigerated for transport back to the laboratory.All groundwater samples were filtered through 0.45 µmcellulose nitrate filters prior to analysis.

Ca2+ , Mg2+ and K+ concentrations were determined onacidified samples containing 10% LiCl, using a GBC 933Plus Atomic Absorption Spectrophotometer (AAS) with an

air-acetylene flame. Samples were diluted with deionisedwater tofit the working ranges for Ca2+ ,Mg2+ andK+ of 0.1–4.0, 0.5–2.0 and 0.5–4.0 mg/L, respectively. Na+ wasdetermined on acidified samples using a Sherwood Model410 flame photometer, after dilution to fit the working rangeof 0–30 mg/L. Cl– and other anion concentrations weredetermined on unacidified samples using a PhenomenexA300 anion peak ion chromatograph with a 1.7 mMNaHCO3/1.8 mM Na2CO3 eluent, 25 mM H2SO4 regeneratesolution, and flow rate of 1.5 ml/min. The samples werediluted to fit the working range for Cl– of 0–250 mg/L.Alkalinity was determined by standard titration to pH 4.5with HCl following the method of Rayment and Higginson(1992). Selected groundwater samples were collected andanalysed for 14C concentration. Samples were collected inacid-washed (HNO3) 1-L glass bottles, rinsed three timeswith sample water and filled so that headspace wasnegligible, in order to minimise atmospheric contamination.Analysis was performed using the ANTARES tandemaccelerator at ANSTO, Sydney (Fink et al. 2004). Theaverage error associated with the percent modern carbon(pMC) values here is 0.3%, and the age limit is 50,000±200 years. Age determinations made on the basis of pMCvalues are uncorrected for the addition of radiogenically dead

Fig. 2 Areal extents of the major aquifer systems, Mount William Creek catchment

1362


carbon, due to the paucity of carbonate minerals or preservedorganic material within the aquifers (Edwards 2006).

SoilsA total of 36 soil profiles (Fig. 1) were sampled for aminimum analytical suite of soil moisture, EC1:5, pH1:5 andCl–. Soil samples were collected using a Gemco solid flightauger-drilling rig in the winter of 2005. Sub-samples weretaken by hand at the top, middle and bottom of every 50-cm flight length during drilling, and combined to form asingle sample that represents the entire 50-cm interval. Thistechnique ensured that representative samples were collect-ed and that cross contamination was kept to a minimum.Samples were immediately stored in zip-lock plastic bagsand refrigerated for transport back to the laboratory.

Soil moisture contents were determined gravimetricallyby weighing sub-samples before and after oven drying at105°C. The EC1:5 and pH1:5 were determined by adding25 ml of deionised water to 5 g of air-dried and sieved soil(<2 mm), and agitating the resultant solution for 1 h. Aftersettling, or centrifuging where necessary, the supernatantwas measured for EC1:5 and pH1:5 using a WTW LF330conductivity meter and probe and a Metrohm 704 pHmeter and probe, respectively. The supernatant solutionsfrom a sub-set of 20 soil samples were analysed for Cl–

using a Phenomenex A300 anion peak ion chromatographto give a correlation between Cl– and EC1:5 (r2=0.98),thus providing Cl–1:5 data for all samples.

The exchangeable cation content of the soils and the soilwater composition (i.e. soluble cations) were determined bystandard techniques (methods 15A1 and 15A2; Raymentand Higginson 1992) on samples from soil profiles SP5 andSP27 only. Initially both soluble and exchangeable cationswere extracted from the soil sample by weighing 5 grams ofair dried and sieved soil into a 125-ml polyethylene vial,adding 100 ml of 1M NH4Cl and mechanically shaking for1 h. The soil extract was then passed through a Whatman 52filter paper, collected in a volumetric flask and analysed forK+ , Ca2+ and Mg2+ using a GBC 933 Plus AtomicAbsorption Spectrophotometer (AAS) with an air-acetyleneflame, and for Na+ using a Sherwood Model 410 flamephotometer. Standards were made up using 1M NH4Cl (sothat matrix compositions were uniform) and samples werediluted to fit the working ranges previously specified forgroundwater analyses.

In the second technique, soluble cations were firstremoved by weighing 5 grams of air dried and sieved soilinto a 30-ml polyethylene vial and adding 25 ml of 60%

aqueous ethanol. This extract was then mechanicallyshaken for 1 h, centrifuged and the supernatant solutiondiscarded. A second 25 ml aliquot of 60% aqueousethanol was added to the soil and the process wasrepeated. A third rinse was carried out using 25 ml of20% aqueous glycerol. Ethanol and glycerol are usedrather than distilled water to avoid modifying thedistribution between soluble and exchangeable species inthe soil. Samples were weighed before and after thisprocedure to account for any entrained solvents, trans-ferred to 125 ml polyethylene vials and exchangeablecations then extracted by adding 100 ml of 1M NH4Cl andmechanically shaking for 1 h. The extract was passedthrough a Whatman 52 filter paper, collected in avolumetric flask and analysed as in the previous proce-dure. Thus the detection limits for Na+, Ca2+, Mg2+ andK+ as both exchangeable and soluble cations in the extractsolutions are 0.1, 0.1, 0.15 and 0.5 mg/L, respectively.The second procedure determines only exchangeablecations; the soluble cation content is obtained by thedifference between the results of the two methods. Extractconcentrations were converted mathematically to 1:5equivalents for ease of comparison to soil Cl– data.

The bulk chemical composition of soil samples fromprofiles SP5 and SP27 were determined by X-rayflorescence (XRF) using 0.75±0.00001 g of sample (ovendried at 40°C until constant mass) mixed with 6.75±0.00001 g of 66:34 lithium tetraborate:lithium metaborateflux (oven-dried at 400°C for 2 h) and fused at 1,050°Cinto a glass button using an automated fusion machine.Glass buttons were analysed using a Siemens SRS 303 ASWavelength Dispersive XRF spectrometer. Elements wereoriginally reported as wt% oxides. The mineralogicalcomposition of the soil samples was determined byqualitative X-ray Diffractometer (XRD) analysis, per-formed from 4–70° for 90 min, using a Siemens D5000X-ray Diffractometer on dried sub-samples after milling to~60 μm. Quantitative XRD analysis was undertaken usingSIROQUANT (Taylor and Hinczak 2001).

Local rainfall composition

Rainfall was not analysed in the present study; however,Victorian rainfall has been studied in detail by Hutton andLeslie (1958) and Bormann (2004), with both studiesincluding sample locations (Beaufort and Ararat) near theMount William Creek catchment (Fig. 1). Comparison ofthe two average weighted datasets (Table 1) reveals

Table 1 Rainfall chemistry for locations proximal to the Mount William Creek catchment study area

Locality Na+

(mg/L)Ca2+

(mg/L)Mg2+

(mg/L)K+

(mg/L)Si(mg/L)

Cl–

(mg/L)Br–

(mg/L)SO4

2–

(mg/L)HCO3

–

(mg/L)

Beauforta 0.9 0.8 0.2 0.1 n/a 1.1 n/a n/a 3.1Araratb 1.9 1.2 0.5 0.9 0.05 4.4 0.01 1.9 n/a

a Hutton and Leslie (1958)b Bormann (2004)n/a denotes a parameter not analysed

1363


Tab

le2

Major

elem

entdata

forgrou

ndwatersfrom

theMou

ntWilliam

Creek

catchm

ent,southeastern

Australia

Bore

IDAqu

ifer

screened

Eastin

g(m

)Northing

(m)

Elevatio

n(m

AHD)

Na+

(mg/L)

Ca2

+

(mg/L)

Mg2

+

(mg/L)

K+

(mg/L)

Si

(mg/L)

Cl–

(mg/L)

Br–

(mg/L)

SO42–

(mg/L)

HCO3–

(mg/L)

NO3–

(mg/L)

pMC

(%)

14Cage

(years)

1434

67Quaternary

647,77

95,88

1,88

521

2.00

1,79

413

028

27

20.0

3,10

010

.536

451

7bd

lnm

nm14

3468

Quaternary

647,77

95,88

1,88

521

2.00

2,07

513

531

76

21.3

3,68

411.7

427

590

bdl

nmnm

1434

71Quaternary

649,48

05,87

4,96

123

4.00

2,13

884

251

423

.73,46

311.3

388

474

bdl

nmnm

1434

75Quaternary

650,55

05,87

5,54

823

9.00

2,37

5117

363

1519

.64,40

013

.644

939

3bd

lnm

nm14

3476

Quaternary

652,19

85,88

0,45

722

6.50

1,62

556

303

935

.43,20

17.1

266

0bd

lnm

nm14

3477

Quaternary

652,22

45,88

1,35

122

5.50

2,10

054

397

637

.14,01

66.9

272

10bd

lnm

nm14

3478

Quaternary

647,73

45,87

6,80

422

8.00

695

6816

05

12.8

1,55

62.8

5277

bdl

nmnm

2544

Quaternary

648,51

25,89

8,35

019

0.00

2,85

015

540

312

21.5

5,54

719

.459

517

bdl

nmnm

5314

Quaternary

654,52

75,88

1,08

024

5.00

2,56

373

348

2227

.24,41

614

.965

735

9bd

lnm

nm48

884

Quaternary

644,58

15,89

4,40

319

2.20

1,21

010

621

08

8.2

2,51

58.3

414

147

3.8

nmnm

4888

5Quaternary

640,65

15,90

0,76

218

3.44

1,39

514

332

29

19.2

2,98

59.1

419

13bd

lnm

nm48

887

Quaternary

640,52

35,89

8,75

620

3.85

2,87

018

944

212

33.6

5,77

212

.655

926

325

.2nm

nm48

888

Quaternary

641,011

5,89

7,87

519

9.89

803

2487

432

.41,10

22.8

142

486

bdl

nmnm

4888

9Quaternary

640,55

65,90

0,30

020

3.09

1,40

014

732

436

21.7

3,07

37.7

280

99bd

lnm

nm48

890

Quaternary

640,53

25,89

9,27

418

5.28

2,15

065

344

3424

.93,96

26.4

371

329

bdl

nmnm

4889

1Quaternary

640,66

65,89

8,31

818

9.59

1,36

844

159

1640

.32,14

45.2

367

408

bdl

nmnm

5125

0Quaternary

638,90

05,90

0,25

018

1.90

1,91

5111

369

4713

.83,85

98.3

471

32bd

lnm

nm1106

74Quaternary

642,65

05,88

9,60

019

7.07

112

314

47.7

113

0.3

1413

35.6

nmnm

1106

77Quaternary

650,15

05,88

2,20

021

7.88

324

311

326

.033

60.8

7522

8bd

lnm

nm1106

78Quaternary

649,45

05,88

2,00

021

6.56

2,98

021

641

663

22.4

5,62

915

.573

713

010

.6nm

nm1106

80Quaternary

648,90

05,88

2,55

021

3.95

1,94

521

341

629

20.1

4,21

211.1

472

337

bdl

nmnm

1106

81Quaternary

642,55

05,89

3,55

019

4.00

485

4387

97.4

948

2.1

7059

2.4

nmnm

1138

87Quaternary

645,00

05,89

7,48

019

9.00

808

414

88

20.2

1,64

83.0

7436

2.0

nmnm

5125

2Quaternary

636,80

55,89

7,23

819

4.00

651

72

11.1

102

0.6

732

bdl

nmnm

5125

4Quaternary

635,80

05,89

7,20

022

2.60

618

6718

418

16.7

1,47

22.5

6960

2.6

nmnm

5309

Quaternary

654,10

85,88

1,21

023

3.00

3,17

570

478

524

.55,93

117

.870

490

bdl

nmnm

5310

Quaternary

654,09

25,88

1,05

823

7.50

3,46

397

514

1120

.86,48

527

.182

014

7bd

lnm

nm53

11Quaternary

654,09

25,88

0,88

824

3.00

1,07

039

152

1511.4

1,91

46.0

231

181

bdl

nmnm

1434

74Quaternary

652,19

85,88

0,45

722

6.50

2,96

316

446

214

17.7

5,45

517

.856

253

9bd

lnm

nm1121

99Tertiary

646,48

25,87

8,20

422

6.21

558

692

412

.21,15

42.1

162

0.6

67.52

3,15

514

3479

Tertiary

647,45

45,87

7,05

422

6.00

373

5277

925

.084

91.5

2483

bdl

nmnm

1434

73Tertiary

650,55

05,87

5,54

823

9.00

2,18

862

211

433

.13,119

9.7

354

1,02

4bd

lnm

nm48

883

Tertiary

644,58

15,89

4,40

319

2.20

683

126

158

87.1

1,64

74.2

134

112

1.9

32.07

9,14

048

886

Tertiary

640,53

65,89

9,76

418

4.55

515

642

424

.277

71.2

111

118

bdl

12.76

16,540

4890

3Tertiary

645,20

75,88

7,24

120

4.77

409

964

211.6

780

1.4

319

4.1

nmnm

4890

4Tertiary

642,97

15,88

9,07

420

3.85

226

844

46.7

449

1.0

4636

bdl

nmnm

6787

0Tertiary

646,71

65,88

1,98

8211.80

338

7854

918

.976

02.1

6115

50.9

37.67

7,84

01106

76Tertiary

648,75

05,87

2,45

023

6.07

458

4773

513

.382

61.6

4067

bdl

31.40

9,31

01116

93Tertiary

648,42

65,86

5,84

427

0.78

8612

323

11.8

244

0.4

617

0.5

nmnm

1122

00Tertiary

646,48

25,87

8,20

422

6.21

936

162

8.2

190

0.4

717

0.8

nmnm

1130

08Tertiary

644,76

75,88

6,59

820

4.43

8545

145

14.8

156

0.4

117

6bd

l17

.42

14,040

1138

85Tertiary

636,80

05,89

7,14

419

3.63

2,01

013

336

248

7.4

3,63

69.0

440

648

bdl

48.65

5,79

01141

74Tertiary

640,45

65,89

1,80

219

9.61

134

1022

27.5

276

0.6

1135

1.1

60.64

4,02

01141

75Tertiary

640,45

65,89

1,80

219

9.61

307

336

39.3

532

1.5

398

bdl

61.29

3,93

01141

73Tertiary

642,63

45,89

3,44

319

3.98

710

55119

77.0

1,35

44.5

152

118

bdl

68.97

2,98

51138

86Tertiary

645,011

5,89

7,37

019

9.00

925

6215

38

7.4

1,80

55.7

187

56bd

lnm

nm67

750

Tertiary

651,64

35,90

0,01

519

2.50

4,31

326

172

819

17.7

8,02

027

.61,02

520

8bd

lnm

nm48

905

Tertiary

645,17

45,88

6,22

019

9.89

249

938

210

.148

71.0

2218

2.1

75.23

2,28

548

906

Tertiary

642,55

05,88

8,30

020

3.09

212

bdl

324

11.2

399

0.8

251

bdl

nmnm

1364


5125

5Basem

ent

634,20

05,89

7,20

027

7.70

401

bdl

bdl

4.8

520.2

48

0.5

nmnm

8218

1Basem

ent

646,12

55,86

6,90

728

0.00

9bd

l1

20.9

14bd

l3

1bd

lnm

nm1121

97Basem

ent

648,75

05,87

2,45

023

6.05

345

7466

1417

.783

71.6

3371

1.3

27.40

1,04

001121

98Basem

ent

646,48

25,87

8,20

422

6.08

7234

155

15.6

131

0.4

214

70.6

nmnm

1122

01Basem

ent

636,79

85,89

7,14

419

3.57

170

724

38.2

353

0.8

1425

bdl

nmnm

1209

69Basem

ent

659,40

05,87

0,90

027

0.00

3,16

021

842

030

21.2

5,80

817

.371

424

8bd

lnm

nm12

0970

Basem

ent

659,50

05,87

0,95

026

1.50

1,46

084

166

720

.12,45

46.2

390

240

8.3

nmnm

1209

71Basem

ent

659,70

05,87

1,00

026

1.50

863

5396

621

.71,44

63.8

183

211

17.4

nmnm

1209

73Basem

ent

657,96

85,86

9,15

627

1.00

2,51

098

312

117

22.1

4,30

611.0

375

416

bdl

nmnm

1209

75Basem

ent

656,116

5,87

4,49

026

2.00

3,00

560

413

7713

.25,19

912

.447

575

4bd

lnm

nm12

0976

Basem

ent

659,40

05,87

3,30

026

8.00

321

818

523

.128

01.0

6825

747

.1nm

nm12

0980

Basem

ent

664,48

05,87

8,80

033

0.00

690

8510

38

16.3

1261

3.1

3840

22.0

nmnm

1209

81Basem

ent

664,48

05,87

8,80

033

0.00

793

3587

1127

.21,311

3.1

7333

6bd

lnm

nm12

0982

Basem

ent

664,60

05,87

9,28

035

5.00

380

839

1523

.662

21.5

3016

511.9

nmnm

1209

84Basem

ent

664,10

05,87

3,10

034

0.00

2,46

014

334

542

18.6

4,29

210

.945

190

910

.6nm

nm14

3469

Basem

ent

644,44

95,87

6,79

327

5.00

462

44

7.2

760.2

33

1.2

nmnm

2543

Basem

ent

648,51

25,89

8,35

019

0.00

2,71

315

839

014

12.0

5,12

115

.759

829

bdl

nmnm

5224

Basem

ent

658,96

75,88

5,90

830

8.00

318

924

148

.545

21.5

8172

1.8

nmnm

5308

Basem

ent

654,10

85,88

1,29

423

8.50

3,55

015

852

533

17.1

6,50

722

.179

030

5bd

lnm

nm53

13Basem

ent

654,52

75,88

1,25

223

5.00

3,10

017

051

734

21.7

5,80

514

.874

056

9bd

lnm

nm12

0978

Basem

ent

660,99

75,87

7,88

928

0.00

294

6816

010

15.3

675

2.3

7153

65.7

nmnm

1209

79Basem

ent

660,99

75,87

7,88

928

0.00

278

1969

621

.035

20.8

4442

514

.6nm

nmBurrum

Basem

ent

664,98

95,86

4,52

633

1.00

2,22

558

444

5342

.44,52

911.4

382

59bd

lnm

nm48

882

Basem

ent

642,89

85,90

3,05

120

3.00

823

2289

1217

.41,35

84.4

179

61bd

lnm

nmMarriot1

Basem

ent

658,48

75,88

6,22

937

3.00

432

11

35.5

280.2

1634

16.8

nmnm

5312

Basem

ent

654,52

75,88

1,25

223

5.00

4,70

046

662

1638

.58,09

425

.61,08

573

bdl

nmnm

6784

9Basem

ent

642,33

45,88

0,31

631

3.89

151

25

6.0

20bd

l4

12bd

lnm

nm67

749

Basem

ent

650,40

05,90

0,30

019

1.30

4,28

824

474

423

10.9

8,02

630

.01,10

017

4bd

lnm

nm52

23Basem

ent

658,85

55,88

6,06

831

9.00

415

324

938

.561

61.7

6169

8.6

nmnm

bdlbelowdetectionlim

it(groun

dwater

detectionlim

itforanions

is0.1mg/L,and

is0.1,

0.1,

0.15

and0.5mg/LforNa+

,Ca2

+,M

g2+andK+,respectively);n

mdeno

tesaparameter

not

measured

1365


variability imparted by differences in the climatic con-ditions between the two sampling periods. In the 1958study, which was based on monthly analyses over an18-month period, there was above-average rainfall(770.9 mm/year), and the lower rainfall salinity (Table 1)can be attributed to increased rainout effects anddecreased dust activity. By contrast, the 2004 study, whichwas based on monthly analyses over a 12-month period,was conducted during a sustained drought (449.8 mm/year), where reduced rainout and a greater dust entrain-ment produced the higher salinities observed (Table 1).The apparatus used in both studies were standardmeteorological rain gauges that collect wet depositionand any dry deposition falling directly onto them. Thesetwo datasets are likely to encompass most of thecompositional variability in modern rainfall in the area,and rainfall composition will be expressed as a range,where possible, throughout the following discussion.

Groundwater composition

Groundwaters of the Mount William Creek catchmentrange in salinity from very fresh (TDS <100 mg/L) to

relatively saline (TDS ~20,000 mg/L; Table 2). Mostgroundwaters analysed (94%) are dominated by Na+ andCl–, with a small proportion (6%) dominated by Na+ andHCO3

–. The dominance of Na+ and Cl– in the groundwatersolute load increases with increasing salinity, from ~40%by weight in the lowest salinity samples (TDS < 500 mg/L)to ~85% by weight at higher salinities (TDS > 8,000 mg/L). This trend, together with the commensurate decline inthe relative abundance of other cations and bicarbonate,corresponds to a progression from a composition with ion/Cl– ratios reflecting local rainfall to one with ratios moreclosely resembling those of dilute seawater, particularly forCa2+ and Na+ (Fig. 3), and is similar to trends observedelsewhere in southeastern Australia (e.g. Arad and Evans1987; Tickell and Humphrys 1987; Macumber 1991;Cartwright et al. 2004) and throughout the world (e.g.Elliot et al. 1999; Guler and Thyne 2004). This progressionfrom rainwater to seawater ion/Cl– ratios occurs at lowsalinities (TDS < 2,000 mg/L; Fig. 3), such that the vastmajority of groundwaters, which are moderately to verysaline, have approximately the same relative distribution ofdissolved species as seawater. Geochemical calculationsusing PHREEQC (Parkhurst and Appelo 1999) indicatethat groundwaters are typically undersaturated with respect

Fig. 3 Ion/Cl– vs. TDS plots for groundwaters from all aquifers of the Mount William Creek catchment, southeastern Australia

1366


to calcite and other common minerals including dolomiteand gypsum.

Soil and soil water chemistry

Ca2+The soluble calcium content of the soil (i.e. within soilwater) is extremely low (below detection limit) at both sites

(Figs. 4a, e and 5), indicating a very rapid attenuation ofthe calcium supplied in rainfall, and this loss must occur inthe upper 50 cm of the profile in both cases. It is due partlyto adsorption onto the exchange sites of clay minerals, asdemonstrated by the much higher concentrations of Ca2+ asexchangeable cations in the soil (Fig. 4a, e). In bothprofiles, the highest exchangeable Ca2+ values occur in theupper 50 cm, complementing the observed loss in solubleCa2+ (Fig. 4); the concentration of exchangeable Ca2+ is

Fig. 4 Soluble and exchangeable cation/Cl ratios with depth for soil profiles SP5 and SP27

1367


much higher in SP5 than SP27 (Table 3), consistent withthe higher clay mineral (kaolinite) content of this profile(Table 4). Ca2+ is typical of Group II cations in that it isweakly adsorbed onto clay minerals as outer-spherecomplexes (Sparks 2005), making it susceptible to ex-change for other cations (as discussed in the following).There is some evidence that adsorption of Ca2+ causesdesorption of H+ in the soil profiles, as pH1:5 typicallydeclines with depth (Table 4).

In the upper part of the profile at both sites, Ca2+

storage as exchangeable cations is considerably lower thanstorage of exchangeable Na+ or Mg2+, particularly in SP27(Table 3), even though rainfall delivers Ca2+ and Na+ invery similar quantities, and much more Ca2+ than Mg2+

(Table 1). This indicates that an additional sink for Ca2+

exists within the profile.Calcite precipitation does not provide this sink,

because soil waters in which the Ca2+ loss is observedhave low salinity and are undersaturated with respect tocalcite. No calcite was encountered during sampling, noridentified by XRD, and there is very low total Ca2+ in theprofiles (Table 3). Testing of soils for calcite usinghydrochloric acid yielded negative results in all cases.

Nutrient uptake by vegetation is the most likelymechanism responsible for the Ca2+ removal from theVictorian profiles; this process has been identified assignificant in determining groundwater composition else-where (e.g. Hudson and Golding 1997; Moulton et al.2000; Benedetti et al. 2003), Ca2+ is a macro-nutrient formost plant species: plants take up relatively largequantities of this element directly from the soil solutionand/or from the cation exchange sites of clay minerals(Sutcliffe 1962). In contrast, plant uptake of Na+ is muchless. This mechanism can also explain why Ca2+ isrelatively more depleted than Na+ within groundwatersin the area (Table 5).

The uptake of nutrients by vegetation is also supportedby the depletion of the groundwater HCO3

–/Cl– ratios inthe more saline groundwaters in the study area (Fig. 3e).This most likely reflects the addition of saline rechargefrom the unsaturated zone that has been depleted inHCO3

– by plant uptake; bicarbonate is a macronutrient for

all plants (Sutcliffe 1962), and no other carbon sinks suchas calcite precipitation, exist in the soil profiles (seeprevious discussion). It is unlikely that significant plantuptake of bicarbonate occurs directly from the saturatedzone due to the water table depths in the areas investigated(>8 m).

K+The concentration of K+ in soil water is only detectable inthe upper 50 cm of the profiles at both sites (Table 3;Fig. 4). The soluble K+/Cl– ratio at the top of the profileapproximates that of rainfall in SP27 and exceeds that ofrainfall in SP5, indicating an additional source of K+ inSP5. This may represent the input of K+ from thebreakdown of organic material and/or desorption of K+

from clays by cation exchange with Ca2+ and/or Mg2+.Because soluble Ca2+ is strongly depleted in the shallowsubsurface (<50 cm; Figs. 4 and 5), the latter explanationmay be more likely; however, both processes are probablyoccurring to some extent. There is also a considerableaccumulated storage of K+ on cation exchange sites in theupper profile (Fig. 4; Table 3). The elevated levels ofsoluble and exchangeable K+ are not derived from theweathering of primary K+ -bearing minerals, because apartfrom some illite, none are present in the upper profiles.

K+ is rapidly removed from the soil water as it migratesdownwards, and at depths of less than 100 cm the levelsfall below the limits of detection, the soluble K+/Cl– ratiosapproximating those of average groundwater (Fig. 4b, f).As with Ca2+ , the removal of K+ is partly the result ofadsorption to the cation exchange sites of clay minerals(Table 3), as shown by the elevated ratio of exchangeableK+ to Cl– compared to the soluble and rainfall K+/Cl–

ratios, particularly in the upper profile (Fig. 4b, f). Theelevated exchangeable K+/Cl– ratio may also be due torecycling from vegetation, which has been shown to causea surface enrichment of essential elements like Ca2+ andK+ that cannot be explained by abiotic processes (Jobbagyand Jackson 2004).

The amount of K+ stored on clay minerals in the upperprofiles is relatively low compared to Na+ and Mg2+

Fig. 5 Total cation concentrations in soil water (soluble cations). Where solute concentrations are below detectable limits, they are plottedat the detection limit

1368


(Table 3). This partly reflects the smaller amounts of K+

delivered to the soil in rainfall (Table 1), but is probablyalso due to plant uptake, as discussed for Ca2+ ; K+ is alsoa macronutrient for most plant species (Sutcliffe 1962).This is consistent with the very strong depletion of bothCa2+ and K+ relative to other major cations in localgroundwater (Table 5). Conversion of kaolinite to illitewill remove K+ , but is unlikely to be occurring in thepresent case due to the low K+/H+ ratios of the soil water(Table 3). Some illite is present in the profile of SP5(Table 4), but is most probably derived from the basementschist.

Na+Groundwater Na+ concentrations are not depleted relativeto rainfall to the same extent as the other cations. Theaverage groundwater Na+/Cl– mass ratio is 0.57, whilethat of local rainfall ranges from 0.44 (Bormann 2004) to0.87 (Hutton and Leslie 1958). This lower degree ofdepletion is shown by the fact that the soluble Na+/Cl–

ratios approximate or exceed those of rainfall at mostsampling depths (Fig. 4c, g). However, there is a minoroverall declining trend in soluble Na+/Cl– ratios withdepth at both sites, so that the soil water Na+/Cl– ratiostowards the base of the profiles are indistinguishable fromthe groundwater ratio (Fig. 4c, g). This minor depletion ofNa+ relative to Cl– is controlled by cation adsorption toexchange sites on clay minerals, as clearly demonstratedby the elevated exchangeable Na+ contents (Fig. 4c, g;Table 3).

In SP27, and to a lesser extent SP5, some of thesoluble Na+/Cl– ratios exceed those of local rainfall,probably due to desorption of Na+ from the exchangeablesites on clay mineral surfaces. Na+-bearing minerals werenot detected by XRD in the soil profiles at either location,and indeed, the amount of total Na+ present in bothprofiles (as determined by XRF) is approximately equal tothe sum of soluble and exchangeable Na+ (Table 3). Thedesorption of Na+ is most likely controlled by cationexchange for Ca2+ and/or Mg2+, as previously discussedfor K+.

Plant uptake appears to play no significant role in thedistribution of Na+ in the soils of the study area, in contrastto Ca2+ and K+. This is probably because of thecomparatively low uptake of Na+ by plants (Sutcliffe1962) and the lower proportion of this species in thebiomass; White et al. (2006) found that grassland biomassin California has an order of magnitude less Na+ than Ca2+

and K+ in most cases.

Mg2+Groundwaters and soil waters are depleted in Mg2+

relative to local rainfall; the soil waters acquire Mg2+/Cl– ratios similar to those of groundwater within the top50 cm of the soil profile (Fig. 4d, h). The very low Mg2+

content of soil water (below detectable limits in somecases) is accompanied by very high storage of exchange-T

able

3Abu

ndance

ofsoluble,

exchangeable

andtotalbu

lksoilcatio

ns(w

eigh

tpercentdrysoil)

Cl

KCa

Na

Mg

Soil

profi

leSam

ple

interval

(cm)

Water

content

(wt%

)

Solub

lewt%

(x10

–3)

Solub

lewt%

(x10

–3)

Exchang

eable

wt%

(x10

–3)

Total

wt%

a

(x10

–3)

Solub

lewt%

(x10

–3)

Exchang

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wt%

(x10

–3)

Total

wt%

(x10

–3)

Solub

lewt%

(x10

–3)

Exchang

eable

wt%

(x10

–3)

Total

wt%

(x10

–3)

Solub

lewt%

(x10

–3)

Exchang

eable

wt%

(x10

–3)

Total

wt%

(x10

–3)

50–

5023

.910

.83.2

17.7

493.9

bdl

43.0

71.5

8.6

28.5

44.5

3.0

80.7

265.3

550

–100

20.9

28.1

bdl

16.6

585.2

bdl

28.6

50.1

11.7

52.8

48.2

1.1

113.8

379.

95

100–

150

11.7

41.9

bdl

7.4

639.1

bdl

16.2

35.8

24.9

60.4

48.2

bdl

84.0

319.6

515

0–20

011.1

26.1

bdl

3.6

1153

.7bd

l7.7

28.6

13.1

31.4

29.9

bdl

17.4

313.6

270–

5012

.713

.62.0

5.2

240.7

bdl

2.3

28.6

19.4

41.7

92.8

11.4

55.4

78.4

2750

–100

13.2

69.4

bdl

5.5

195.1

bdl

2.3

28.6

47.9

59.6

141.0

8.9

61.6

66.3

2710

0–15

015

.112

0.1

bdl

6.8

182.6

bdl

2.3

28.6

70.2

83.7

137.3

7.3

90.7

162.8

2715

0–20

06.0

47.5

bdl

3.2

199.2

bdl

2.9

21.5

30.5

29.2

26.0

1.0

30.9

bdl

bdlbelow

detectionlim

itaBulkchem

ical

compo

sitio

nas

determ

ined

byXRFanalysis

1369


able Mg2+ throughout the profiles (Table 3), so the loss ofsoluble Mg2+ is at least partially the result of adsorption toclay mineral surfaces in the shallow soil. In SP27, the totalMg2+ content of the soil approximates the sum of solubleplus exchangeable Mg2+; there is no other significantsource of Mg2+ within the soil (Table 3). The very largestore of Mg2+ on exchange sites has built up despite therelatively low amounts of this ion in rainfall. This reflectsthe preference for divalent cations by clays in these soilprofiles.

There is no direct evidence of the influence of plantuptake on the distribution of Mg2+ in the soils of the studyarea, but studies elsewhere have demonstrated thatbiological cycling can remove Mg2+ from the systemalong with Ca2+ and K+ (White et al. 2002), despite thecomparatively low uptake of Mg2+ by plants (Sutcliffe1962). Considerations of the long-term effect of unsatu-rated zone processes on groundwater composition in thestudy area (see the following) also suggest that uptake ofMg2+ by vegetation is occurring.

Discussion

Is the removal of ions in the unsaturated zonea steady-state situation?The results of this study clearly show that ions supplied inrainfall are being removed in the unsaturated zone,particularly Ca2+ , Mg2+ and K+ , both by cation exchangeon clays and uptake by plants. As previously discussed,these processes cause the higher salinity groundwaters(>2,000 mg/L TDS) in the study area to have ion/Cl–

ratios typical of seawater; groundwaters sampled have 14C

ages ranging from ~2,500 up to ~16,500 years (Table 2).This implies that the cation-removal processes in theunsaturated zone have been operating at a more or lessconstant rate for thousands of years. Despite 14C agedeterminations being derived from groundwater in the semi-confined Calivil Formation, these waters have the samerelative ion concentrations as those waters in the overlyingalluvium, due in large part to the fact that recharge occurs viathe overlying unit.

The exchangeable cation content of the soils in thestudy area is less than might be expected from this timeframe. Assuming a bulk soil density of 1.6 tons/m3

(typical of soils in this part of Australia; Dyson andJenkin 1981), 1 m2 of the upper 50 cm of the soil profilesat both sites contains ~800 kg of soil with 35–50 moles ofexchangeable cations (K+ , Ca2+ , Na+ and Mg2+; fromTable 3); assuming all exchangeable cations were derivedfrom infiltrating pore waters, this represents 500–900 years’ supply of cations in rainfall (from Table 1).The exchangeable cation content of the entire 200 cmthickness of both profiles is 160–190 moles/m2, represent-ing 2,000–3,500 years’ supply of cations in rainfall. Sincethe unsaturated zone processes responsible for the seawa-ter-type ion/Cl– ratios in the higher salinity groundwatersappear to have been operating for many thousands ofyears, this indicates that the concentration of exchangeablebase cations in the soil profiles may have reached a steadystate, implying that continuing net removal of base cationsby cation exchange is no longer occurring. The totalexchangeable cation content of the kaolinite in the upper50 cm of soil profile SP5 (0.08 mol kg–1; from Tables 3and 4) lies within the range of experimental cationexchange capacities for kaolinites in southeastern Aus-

Table 4 Soil profile pH and clay mineralogy

Soil profile Sample interval (cm) pH1:5 % quartz % kaolinite % illite/muscovite

5 0–50 6.46 23.3 76.7 -5 50–100 6.64 14.8 85.2 -5 100–150 5.56 19.8 55.5 24.65 150–200 5.92 35.1 24.2 40.627 0–50 5.72 75.6 8 16.427 50–100 5.16 87.9 4.8 7.427 100–150 5.00 84.9 8.5 6.627 150–200 5.25 96.2 2.5 1.3

Table 5 Depletion of cation species in groundwater relative to a theoretical composition derived by the concentration of local rainfall,using Cl– as the reference ion

Na+ Ca2+ Mg2+ K+ Cl–

Average rainfall (mg/L)a 1.3 0.9 0.3 0.4 2.3Highest salinity groundwater (mg/L)b 4,700 46 662 16 8,094Normalised rainfall (mg/L)c 4,562 3,271 1,146 1,391 8,094Depletion (%) –3d 99 42 99 0

Shows strong depletion of Ca2+ and K+ in Mount William Creek catchment groundwaters relative to other cationsa Volume weighted average composition from Table 1b Cation and Cl– analysis of highest salinity groundwater (Bore 5312)c Average rainfall concentrations normalised to Cl– of highest salinity groundwaterd Negative value indicates enrichment of Na+ , due to small errors

1370


tralia (0.028–0.244 mol kg–1; Ma and Eggleton 1999), sothe clays in the upper parts of the soil profiles in the studyarea may be saturated with respect to base cation storage.

However, continuing, long-term removal of ions byplant uptake is occurring. Ions taken up by vegetationfrom both the soil solution and exchange sites are returnedto the soil through the continual breakdown of organicmatter, and this may be responsible for the occasionalgroundwater cation/Cl– ratios exceeding those in localrainfall (Fig. 3), as shown elsewhere (Jobbagy andJackson 2004). However, this return of ions to the soil isdisrupted by the net erosional loss of particulate organicmaterial through overland transport (sheetflow and rivers,e.g. Hopmans et al. 1987; White et al. 2002); White et al.(2006) reported high Ca2+ concentrations in surface watersdraining grassland terraces in coastal California, whichthey attributed to seasonal biomass cycling. In addition,organic material is lost as smoke during bushfires (e.g.Stewart and Flinn 1985), and, in farmed areas, throughremoval of agricultural products (animals and crops).Significant amounts of both Ca2+ and K+ can also be lostby accumulation on plant leaves during photosynthesisand dispersion by wind (Artaxo and Orsini 1987; de Mello2001). Therefore the removal of organic matter will resultin a continuing, long-term export of ions, particularly Ca2+

and K+, from soil profiles, and the ions most stronglydepleted in Mount William Creek groundwaters are Ca2+

and K+ (Table 5).It is therefore possible that a steady state scenario has

become established at the study site; the soil clays areprobably saturated with respect to base cation storage, andthe cations supplied in rainfall are partly exported inorganic matter and partly incorporated into rechargeinfiltrating downwards into the groundwater. These pro-cesses are apparently sufficient to maintain the patternsobserved.

Relationship between unsaturated zone processesand groundwater salinity and ion/Cl– ratios acrossthe catchmentThe results of this study demonstrate that, in the studyarea, much of the chemical evolution of the groundwatersnormally attributed to groundwater-aquifer interactions infact occurs prior to recharge. How do these processescause the variability in the major ion/Cl– ratios across thecatchment and why, once the salinity exceeds ~2,000 mg/L, is there a further downgradient increase in salinitywithin the aquifers in the study area without any change inion/Cl– ratios (Fig. 3)?

The elevated parts of the study area have relatively thinsoil cover and are characterised by rapid recharge (asshown by groundwater Cl– mass balance and hydrographdata; Edwards 2006; Edwards and Webb 2006). Becauseof the limited time that this recharge spends in theunsaturated zone, it undergoes relatively little evapotrans-piration and interaction with plants and clays, so that thebase cation/Cl– ratios are often only slightly or moderatelydepleted compared to rainfall. This is clearly shown by the

Ca2+/Cl– and K+/Cl– plots (Fig. 3). In contrast, over thelow relief remainder of the catchment, where soil andregolith profiles are thicker and better developed (e.g. SP5and SP27), recharge is slow and diffuse (Edwards 2006;Edwards and Webb 2006), allowing sufficient time forevapotranspiration to increase the salinity and for unsat-urated zone processes to modify the ion/Cl– ratios so thatthey resemble those of dilute seawater (Figs. 3 and 4).

There is a concomitant downflow increase in ground-water salinity and decrease in ion/Cl– ratios across MountWilliam Creek catchment up to salinites of ~2,000 mg/LTDS (the value at which seawater ion/Cl– ratios becomeestablished; Fig. 3). This is caused by mixing of moresaline diffuse recharge that has undergone evapotranspi-ration and ion depletion in the unsaturated zone withfresher, less depleted groundwater recharged in the higherelevation parts of the catchment. Mass balance mixingcalculations using Na+ and Cl– demonstrate the amount ofmixing. Adding dilute groundwater with relatively highion/Cl ratios (<200 mg/L TDS; Table 2) to the very salinesoil water at the base of profiles SP5 and SP27 (1,180 and2,750 mg/L Na+ and 2,350 and 4,280 mg/L Cl–

respectively; from Table 3) shows that groundwater with~2,000 mg/L TDS contains 25–70% soil water; the rangelargely reflects the variability in salinity of the dilutegroundwater compositions used in the calculation.

There is a further downgradient increase in groundwatersalinity from 2,000 to 20,000 mg/L without any significantchange in ion/Cl– ratios (Fig. 3), due to the continuingaddition down the groundwater flow path of saline rechargewith seawater ion/Cl– ratios. This will progressivelyincrease the groundwater salinity but not alter its ion/Cl–

ratios, because these already match the recharge values.Groundwater studies elsewhere in southeastern Australia

have demonstrated the same general process. Bennetts et al.(2006) used major ion and stable isotope data to show thata downflow increase in groundwater salinity within anunconfined aquifer is due to the continued addition alongthe flow path of saline diffuse recharge that has undergoneevapotranspiration in the unsaturated zone.

This explanation of groundwater chemical trends doesnot require that cation exchange and/or mineral precipitationoccur within the saturated zone of the aquifer. There is noevidence for these processes in the Mount William Creekcatchment: no valence co-dependent cation enrichment anddepletion is observed in chemical data (e.g. ion ratio plots,Fig. 3), and groundwaters are typically undersaturated withrespect to calcite and other common minerals, particularly atthe low salinities where ion depletion is observed.

Mixing between two sources of recharge in this contexthas been proposed before (e.g. Tickell and Humphrys1987; Love et al. 1994; Cartwright et al. 2004; Bennetts etal. 2006); however, this study differs from previous workon a number of points. Firstly, it explicitly emphasises theunsaturated zone as the medium in which groundwaterchemical characteristics are imparted. Secondly, it assignsthe often-quoted process of cation adsorption and ex-change to a specific location in the flow-path, usingempirically-derived evidence. Thirdly, it provides evi-

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dence that plant nutrient cycling affects soil water andgroundwater composition.

Effect of unsaturated zone processeson groundwater compositionwithin the Murray-Darling Basin and elsewhereThe groundwater chemistry trends in the study area(Fig. 3) have been observed by numerous authors acrossthe Murray-Darling Basin, of which the Mount WilliamCreek catchment is a part (e.g. Lawrence 1975; Arad andEvans 1987; Tickell and Humphrys 1987; Macumber1991; Herczeg et al. 2001; Cartwright et al. 2004).Groundwaters in this basin have a distinct oceaniccharacter that is achieved by the time salinites reach~2,000 mg/L in terms of down basin evolution of thecomposition. The trends responsible for this compositionhave been interpreted in terms of mineral weathering inhigher elevation recharge areas of exposed/thinly coveredbedrock to produce the high cation/Cl– and HCO3

–/Cl–

ratios in low salinity waters, and some combination ofcation exchange, mineral precipitation and/or evaporation/transpiration within shallow unconfined/semi-confinedaquifers to produce the depleted base cation/Cl– ratios inhigh salinity groundwater. These mechanisms have alsobeen implicated in inverse modelling simulations (e.g.Herczeg et al. 2001). However, the present study showsthat processes within the unsaturated zone can exertultimate control on the groundwater composition, andwater-rock interaction within the aquifer may play little orno part. In fact, much of the chemical evolution of thegroundwaters normally attributed to groundwater-aquiferinteractions occurs prior to recharge. Furthermore, the lowsalinity groundwaters in the Murray-Darling Basin withhigh base cation/Cl– ratios are commonly viewed ascation-enriched due to mineral weathering, but the presentstudy demonstrates that they are probably, in fact, cation-depleted compared to local rainfall.

The general trends from rainfall chemistry to ground-water chemistry observed in the study area (Fig. 3) arecommon phenomena elsewhere in Australia and through-out the world (e.g. Salama et al. 1993; Kimblin 1995;Elliot et al. 1999; Guler and Thyne 2004). In addition, theprocesses occurring in the soils of the Mount WilliamCreek catchment have been identified in detailed studieson soil composition elsewhere, e.g. in the United States(Fryar et al. 2001; White et al. 2002, 2006). Combinedwith the non-unique nature of the physiographic andhydrogeological setting of the study area, this suggeststhat unsaturated zone processes may be responsible for theevolution of groundwater chemistry in shallow aquifers insimilar contexts world-wide.

Conclusions

Cation/Cl– ratios in the groundwaters of the MountWilliam Creek catchment are depleted compared torainfall, and this is caused both by plant uptake and

adsorption to the exchangeable sites of clay minerals inthe shallow sub-surface. In the case of Ca2+ , Mg2+ andK+, this occurs with remarkable rapidity, such that cation/Cl– ratios are at or below those in groundwater within theupper 50 cm of the soil profile; for Na+ , cation/Cl– ratiosapproximating those in groundwater are still achievedwithin the upper 200 cm.

Enrichment of the major cations within groundwater,above those ratios observed in rainfall, is very rare, and maybe due to return of ions through breakdown of organicmatter and desorption of monovalent ions consistent with acation exchange process favouring divalent cations.

The concentrations of exchangeable Na+ and Mg2+ onclay mineral surfaces are substantially greater than theconcentrations of K+ and Ca2+ . This does not correlate withthe original concentration of these species in rainfall, nor doesit correlate with any valence-dependent cation exchangepreferences. Both K+ and Ca2+ are important plant nutrients;groundwater data show that both K+ and Ca2+ are moredepleted than Na+ and Mg2+ , indicating that plant uptake ofK+ and Ca2+ exerts substantial control over groundwatercompositions. Removal of organic matter through erosionalloss and other mechanisms will result in a continuing, long-term export of these ions from the soil profiles.

These processes of biogeochemical fractionation meanthat recharge through the unsaturated zone is rapidlymodified so that ion/Cl– ratios are depleted relative torainfall but characteristic of seawater. In the higherelevation parts of the catchment with thinner soils therecharge may be fresher and less depleted; mixing of thisrecharge with more saline diffuse recharge that hasundergone evapotranspiration and ion depletion in thickersoils causes a concomitant downflow increase in ground-water salinity and decrease in ion/Cl– ratios up to salinitesof ~2,000 mg/L TDS (the value at which seawater ion/Cl–

ratios become established in the catchment). There is afurther increase in groundwater salinity without anysignificant change in ion/Cl– ratios due to the continuingaddition down the groundwater flow path of salinerecharge with seawater ion/Cl– ratios.

This explanation of groundwater chemical trends doesnot require that cation exchange and/or mineral precipita-tion occur within the saturated zone of the aquifer, anddemonstrates that much of the chemical evolution ofgroundwaters normally attributed to groundwater-aquiferinteractions in fact occurs prior to recharge.

The higher salinity groundwaters in the study area withion/Cl– ratios typical of seawater are thousands of yearsold, implying that the cation-removal processes in theunsaturated zone have been operating for this time period.It is therefore possible that a steady-state scenario hasbecome established at the study site; the soil clays areprobably saturated with respect to cation exchange, andthe cations supplied in rainfall are partly exported inorganic matter and partly incorporated into rechargeinfiltrating downwards into the groundwater.

The soil processes in the Mount William Creekcatchment have been identified elsewhere, and there areclear analogies between groundwater evolution in the

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study area and that in the Murray-Darling Basin and otherpredominantly unconfined alluvial aquifer systemsthroughout the world, suggesting that the evolutionarymodel presented here may have broad application.

Acknowledgements This research was supported by the WimmeraCatchment Management Authority and an Australian postgraduateaward from the Australian Government Department of Education,Science and Training. Groundwater dating was supported by AINSEGrant 05/172. The authors also acknowledge the contribution of threeanonymous reviewers whose comments substantially improved the finalmanuscript.

References

Acworth I, Jankowski J (2001) Salt source for dryland salinity:evidence from an upland catchment on the Southern Tablelandsof New South Wales. Aust J Soil Res 39:39–59

Arad A, Evans R (1987) The hydrogeology, hydrogeochemistry andenvironmental isotopes of the Campaspe River aquifer system,north-central Victoria, Australia. J Hydrol 95:63–86

Artaxo P, Orsini C (1987) Pixe and receptor models applied toremote aerosol source apportionment in Brazil. Nucl InstrumMethods Phys Res B22:259–263

Benedetti MF, Dia A, Riotte J, Chabaux F, Gerard M, Boulegue J,Fritz B, Chauvel C, Bulourde M, Deruelle B, Ildefonse P (2003)Chemical weathering of basaltic lava flows undergoing extremeclimatic conditions: the water geochemistry record. Chem Geol201:1–17

Bennetts DA, Webb JA, Stone DJM, Hill DM (2006) Understandingthe salinisation process for groundwater in an area of south-eastern Australia, using hydrochemical and isotopic evidence. JHydrol 323:178–192

Bennetts DA, Webb JA, McCaskill M, Zollinger R (2007) Drylandsalinity processes within the discharge zone of a local groundwatersystem, Southeastern Australia. Hydrogeol J 15:1197–1210

Blackburn G, McLeod S (1983) Salinity in atmospheric precipita-tion in the Murray-Darling drainage basin, Australia. Aust J SoilRes 21:411–434

Blake R (1989) The origin of high sodium bicarbonate waters in theOtway Basin, Victoria, Australia. In: Miles (ed) Water-rockinteraction. Balkema, Rotterdam, The Netherlands

Bormann ME (2004) Temporal and spatial trends in rainwaterchemistry across central and western Victoria. Honours Thesis,La Trobe University, Melbourne, Australia, 86 pp

Bureau of Meteorology (2003) Climate data for stations 079105 and079034. Climate and Consultancy Section, Victorian RegionalOffice, Bureau of Meteorology, Melbourne

Cardenal J, Benavente J, Cruz-Sanjulian JJ (1994) Chemicalevolution of groundwater in Triassic gypsum-bearing carbonateaquifers (Las Alpujarras, southern Spain). J Hydrol 161:3–30

Cartwright I, Weaver TR, Fulton S, Nichol C, Reid M, Cheng X(2004) Hydrogeochemical and isotopic constraints on theorigins of dryland salinity, Murray Basin, Victoria, Australia.Appl Geochem 19(8):1233–1254

Chorover J, Kretzschmar R, Garcia-Pichel F, Sparks D (2007) Soilbiogeochemical processes within the Critical Zone. Elements3:321–326

Cayley RA, Taylor DH (2001) Ararat: 1:100,000 map areageological report. Geological Survey of Victoria Report 115,Geological Survey of Victoria, Melbourne, 324 pp

de Mello WZ (2001) Precipitation chemistry in the coast of themetropolitan region of Rio de Janeiro, Brazil. Environ Pollut114:235–242

Dogramaci SS, Herczeg AL (2002) Strontium and carbon isotopeconstraints on carbonate-solution interactions and inter-aquifermixing in groundwaters of the semi-arid Murray Basin,Australia. J Hydrol 262:50–67

Drever JI, Smith CL (1978) Cyclic wetting and drying of the soilzone as an influence on the chemistry of groundwater in aridterrains. Am J Sci 278:1448–1454

Dyson PR (1983) Dryland salting and groundwater discharge in theVictorian Uplands. Proc R Soc Vic 95(3):113–116

Dyson PR, Jenkin JJ (1981) Hydrological characteristics of soilsrelevant to dryland salting in central Victora. Soil ConservationAuthority of Victoria, Melbourne

Edwards MD (2006) A hydrological, hydrogeological and hydro-geochemical study of processes leading to land and watersalinisation in the Mount William Creek Catchment, southeasternAustralia. PhD Thesis, LaTrobe University, Melbourne, 263 pp

Edwards MD, Webb JA (2006) The effects of lithology, soil andvegetation on recharge estimates in an upland catchmentaffected by dryland salinity: Mt William Creek, westernVictoria. 10th Murray Darling Basin Groundwater Workshop,Canberra, September 2006

Elliot T, Andrews JN, Edmunds WM (1999) Hydrochemical trends,palaeorecharge and groundwater ages in the fissured Chalkaquifer of the London and Berkshire Basins, UK. ApplGeochem 14:333–363

Fink D, Hotchkis M, Hua Q, Jacobsen G, Smith AM, Zoppi U,Child D, Mifsud C, van der Gaast H, Williams A, Williams M(2004) The ANTARES AMS facility at ANSTO. NIM B

Fryar AE, Mullican WF, Macko SA (2001) Groundwater rechargeand chemical evolution in the southern high plains of Texas,USA. Hydrogeol J 9:522–542

Garcia-Pichel F, Johnston SL, Youngkin D, Belnap J (2003) Smallscale vertical distribution of bacterial biomass and diversity inbiological soil crusts from arid lands in the Colorado Plateau.Microb Ecol 46:312–321

Garrels RM, Mackenzie FT (1967) Origin of the chemicalcompositions of some springs and lakes. In: Gould RF (ed)Equilibrium concepts in natural water systems. AmericanChemical Society, Washington, DC, pp 222–242

Guler C, Thyne GD (2004) Hydrologic and geologic factorscontrolling surface and groundwater chemistry in IndianWells-Owens Valley area, southeastern California, USA. JHydrol 285:177–198

Harrison A (1993) Hydrogeological assessment of salinity process-es: Mount William Creek Catchment. Report 1993/21, RoyalWater Commission, Melbourne

Heathcote JA (1985) Carbonate chemistry of recent chalk ground-water in a part of East Anglia, UK. J Hydrol 78:215–227

Herczeg AL, Dogramaci SS, Leaney FWJ (2001) Origin ofdissolved salts in a large, semi-arid groundwater system:Murray Basin, Australia. Mar Freshw Res 52:41–52

Hopmans P, Flinn DW, Farrell PW (1987) Nutrient dynamics offorested catchments in southeastern Australia and changes inwater quality and nutrient exports following clearing. For EcolManage 20:209–231

Hudson RO, Golding DL (1997) Controls on groundwater chem-istry in subalpine catchments in the southern interior of BritishColumbia. J Hydrol 201:1–20

Hutton JT, Leslie TI (1958) Accession of non-nitrogenous ionsdissolved in rainwater to soils in Victoria. Aust J Agric Res 9:59–84

Jankowski J, Acworth I (1993) The hydrogeochemistry of groundwaterin fractured bedrock aquifers beneath dryland salinity occurrencesat Yass, NSW. AGSO J Aust Geol Geophys 14:279–285

Jobbagy EG, Jackson RB (2004) The uplift of soil nutrients byplants: biogeochemical consequences across scales. Ecology85:2380–2389

Kimblin RT (1995) The chemistry and origin of groundwater inTriassic sandstone and Quaternary deposits, northwest Englandand some UK comparisons. J Hydrol 172:293–311

Lawrence CR (1975) Geology, hydrodynamics and hydrochemistryof the southern Murray Basin. Memoirs 30, Geological Surveyof Victoria, Melbourne

Love AJ, Herczeg AL, Leaney FW, Stadter MF, Dighton JC,Armstrong D (1994) Groundwater residence time and palae-ohydrology in the Otway Basin, South Australia: 2H, 18O and14C data. J Hydrol 153:157–187

1373


Ma C, Eggleton RA (1999) Cation exchange capacity of kaolinite.Clay Clay Miner 47(2):174–180

Macumber PG (1991) Interaction between groundwater and surfacesystems in northern Victoria. Department of Conservation andEnvironment, Melbourne

Moss PD, Edmunds WM (1992) Processes controlling acidattenuation in the unsaturated zone of a Triassic sandstoneaquifer (U.K.), in the absence of carbonate minerals. ApplGeochem 7:573–583

Moulton KL, West J, Berner RA (2000) Solute flux and mineralmass balance approaches to the quantification of plant effects onsilicate weathering. Am J Sci 300:539–570

Parkhurst DL, Appelo CAJ (1999) User’s Guide to PHREEQC(Version 2): a computer program for speciation, batch-reaction,one-dimensional transport, and inverse geochemical calcula-tions. US Geol Surv Water Resour Invest Rep 99–4259, 310 pp

Rademacher LK, Clarke JF, Bryant Hudson G, Erman DC, ErmanNA (2001) Chemical evolution of shallow groundwater asrecorded by springs, Sagehen basin: Nevada County, California.Chem Geol 179:37–51

Rayment GE, Higginson FR (1992) Australian laboratory handbookof soil and water chemical methods. Inkata, Melbourne

Rosen M, Jones S (1998) Controls on the chemical composition ofgroundwater from alluvial aquifers in the Wanaka and Wakatipubasins, Central Otago, New Zealand. Hydrogeol J 6:264–281

Salama RB, Wells ASM, Farrington P, Bartle GA (1993) Thechemical evolution of groundwater in the aquifer systems of theYilgarn Craton of Western Australia, CSIRO Division of WaterResources, Perth

Simpson JH, Herczeg AL (1994) Delivery of marine chloride inprecipitation and removal by rivers in the Murray-DarlingBasin, Australia. J Hydrol 154:323–350

Sparks DL (2005) Metal and oxyanion sorption on naturallyoccurring oxide and clay mineral surfaces. In: Grassian VH(ed) Environmental catalysis, Taylor and Francis, London, pp3–36

Spears DA, Reeves MJ (1975) The influence of superficial depositson groundwater quality in the Vale of York. Q J Eng Geol8:255–269

Stewart HTL, Flinn DW (1985) Nutrient losses from broadcastburning of Eucalyptus debris in north-east Victoria. Aust ForRes 15:321–332

Stuyfzand PJ (1999) Patterns in groundwater chemistry resultingfrom groundwater flow. Hydrogeol J 7:15–27

Sutcliffe JF (1962) Mineral salts absorption in plants. Pergamon,London, 194 pp

Taylor JC, Hinczak I (2001) Rietveld made easy: a practical guideto the understanding of the method and successful phasequantifications. Sietronics, Canberra, Australia

Tickell SJ, Humphrys WG (1987) Groundwater resources andassociated salinity problems of the Victorian part of the RiverinePlain. Department of Industry, Technology and Resources

Toth J (1999) Groundwater as a geologic agent: an overview of thecauses, processes, and manifestations. Hydrogeol J 7:1–14

White AF, Blum AE, Schulz MS, Huntington TG, Peters NE,Stonestrom DA (2002) Chemical weathering of the Panolagranite: solute and regolith elemental fluxes and the weatheringrate of biotite. In: Hellmann R, Wood SA (eds) Water-rockinteractions, ore deposits and environmental geochemistry: atribute to David A Crerar. Geol Soc Spec Publ 7:37–60

White AF, Schulz MS, Vivit DV, Blum AE, Stonestrom DA (2006)Controls on soil pore water solutes: an approach for distinguish-ing between biogenic and lithogenic processes. J GeochemExplor 88:363–366

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the importance of unsaturated zone biogeochemical processes in determining groundwater composition,...

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