Groundwater evolution within a catchment Groundwater evolution within a catchment

affected by dryland salinity, southeastern affected by dryland salinity, southeastern

AustraliaAustraliaJohn Webb and Darren Bennetts

Study area

Areas at high risk ofdryland salinity in 2000

Increasing salinisation of the landscapeIncreasing salinisation of the landscape

Boggy Creek SpringGellerts Seep

19521952 19961996

Gellerts Swamp today

TopographyTopography

HamiltonHamilton

WillauraWillaura

Grampians

Stavely Range

Hop

kins

Riv

er

Cockajemmy Lakes

Boggy Creek Spring

Gellerts Seep

Surface GeologySurface Geology

Greenstone

Sandstone/shale

Grampians Group

Granite

Basalt

Colluvium

Alluvium

Stream Alluvium

Swamp deposits

Cambrian

Silurian

Devonian

Pleistocene

Quaternary

Hydrogeology Hydrogeology – – flow pathsflow paths

Flow path 1

Flow path 1

Flow path 2Flow path 2

270

260

250

240

250

230

250

220

Scholler plot standardised to the highest Cl concentration.

1

10

100

1000

10000

100000

1000000

K+ Ca2+ Mg2+ Na+ Si Cl- SO42- HCO3-

molarity

Groundwater composition dominated by Na and Cl

36Cl analyses from adjacent area

•median of 19 x 10-15 36Cl/Cl-

•consistent with atmospheric precipitation in southwest Victoria

•contributions from connate water and/or basalt weathering unlikely - 36Cl/Cl- ratios from these sources would be zero or ~4 x 10-15

•groundwater Cl- is probably sourced exclusively from cyclic sources (rainfall and/or windblown dust)

Hydrogeology – groundwater age (tritium)Hydrogeology – groundwater age (tritium)

Only samples in the west

contain tritium - recharged

after 1950

Waters in centre

and east contain

no tritium

Hydrogeology – groundwater age & rates Hydrogeology – groundwater age & rates of movement (of movement (1414C)C)

14C ages may be

overestimates, but

indicate flow times of

several thousand

years

4000 years4000 yearsoldold

7900 years7900 years oldold

Hydrogeology Hydrogeology – – flow path 1flow path 1

Flow path 1

Flow path 1

270

260

250

240

250

230

250

220

Hydrogeology – flow path 1 cross sectionHydrogeology – flow path 1 cross section

Mt William SwampHopkins River

Salinity increases along flow path 1Salinity increases along flow path 1

Mt William SwampHopkins River

33 7799

8855

11 Salinity (mS/cm)

1313

• Progressive salinity increase along flow path due to addition

of diffuse recharge from overlying soil zone, where rainfall

concentrated by evapotranspiration• Note dilution along flowpath due to lateral flow from north

88

Hydrogeology Hydrogeology – – flow path 2flow path 2

Flow path 2

Flow path 2

270

260

250

240

250

230

250

220

Hydrogeology – flow path 2 cross sectionHydrogeology – flow path 2 cross section

Lake Muirhead Cockajemmy LakesGellerts Swamp

Salinity increases along flow path 2Salinity increases along flow path 2

Lake Muirhead Cockajemmy LakesGellerts Swamp

3399 2222 1515

15-3015-30

• Increase along flow path again due to addition of saline diffuserecharge from overlying soils, with some addition from salt in bed of Lake Muirhead. • very saline brines beneath Cockajemmy Lakes

•groundwater samples all plot close to local meteoric water line •groundwater stable isotope composition becomes heavier downflow•probably reflects addition of soil water evaporated under high humidities

•groundwater becomes more reducing downflow•reflects organic content of shallow alluvial aquifer•oxidising waters downflow in basalt and basement aquifers

Downflow change from kaolinite to smectite stability fields

Decrease in Si/Cl ratio with increasing salinity (downflow) probably reflects reaction of groundwater silica with kaolinite to form smectites

Marked pH increase downflow probably due to H+ removal on clays

Conclusions

Groundwater chemistry dominated by:•rainfall input•evapotranspiration

Groundwater evolution reflects:

•progressive addition of saline infiltration from soil zone

•interactions with clay minerals•some oxidation of organic matter in aquifer