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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
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Study area
Areas at high risk ofdryland salinity in 2000
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Increasing salinisation of the landscapeIncreasing salinisation of the landscape
Boggy Creek SpringGellerts Seep
19521952 19961996
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Gellerts Swamp today
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TopographyTopography
HamiltonHamilton
WillauraWillaura
Grampians
Stavely Range
Hop
kins
Riv
er
Cockajemmy Lakes
Boggy Creek Spring
Gellerts Seep
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Surface GeologySurface Geology
Greenstone
Sandstone/shale
Grampians Group
Granite
Basalt
Colluvium
Alluvium
Stream Alluvium
Swamp deposits
Cambrian
Silurian
Devonian
Pleistocene
Quaternary
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Hydrogeology Hydrogeology – – flow pathsflow paths
Flow path 1
Flow path 1
Flow path 2Flow path 2
270
260
250
240
250
230
250
220
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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
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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)
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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
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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
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Hydrogeology Hydrogeology – – flow path 1flow path 1
Flow path 1
Flow path 1
270
260
250
240
250
230
250
220
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Hydrogeology – flow path 1 cross sectionHydrogeology – flow path 1 cross section
Mt William SwampHopkins River
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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
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Hydrogeology Hydrogeology – – flow path 2flow path 2
Flow path 2
Flow path 2
270
260
250
240
250
230
250
220
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Hydrogeology – flow path 2 cross sectionHydrogeology – flow path 2 cross section
Lake Muirhead Cockajemmy LakesGellerts Swamp
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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
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•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
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•groundwater becomes more reducing downflow•reflects organic content of shallow alluvial aquifer•oxidising waters downflow in basalt and basement aquifers
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Downflow change from kaolinite to smectite stability fields
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Decrease in Si/Cl ratio with increasing salinity (downflow) probably reflects reaction of groundwater silica with kaolinite to form smectites
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Marked pH increase downflow probably due to H+ removal on clays
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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