Groundwater evolution within a catchment affected by dryland salinity, southeastern Australia

1
Groundwater evolution within a catchment affected
by dryland salinity, southeastern Australia
John Webb and Darren Bennetts
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Areas at high risk of dryland salinity in 2000
Study area
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Increasing salinisation of the landscape
Boggy Creek Spring
Gellerts Seep
1952
1996
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Gellerts Swamp today
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Topography
Grampians
Boggy Creek Spring
Willaura
Hopkins River
Hamilton
Gellerts Seep
Cockajemmy Lakes
Stavely Range
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Surface Geology
Quaternary
Swamp deposits
Stream Alluvium
Alluvium
Colluvium
Pleistocene
Basalt
Devonian
Granite
Silurian
Grampians Group
Cambrian
Sandstone/shale
Greenstone
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Hydrogeology flow paths
250
270
240
Flow path 1
260
230
250
220
Flow path 2
250
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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)
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 of
movement (14C)
14C ages may be overestimates, but indicate flow
times of several thousand years
4000 years old
7900 years old
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Hydrogeology flow path 1
250
270
240
Flow path 1
260
230
250
220
250
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Hydrogeology flow path 1 cross section
Mt William Swamp
Hopkins River
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Salinity increases along flow path 1
Mt William Swamp
Hopkins River
3
7
9
13
8
8
5
1
Salinity (mS/cm)

• 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

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Hydrogeology flow path 2
250
270
240
Flow path 2
260
230
250
220
250
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Hydrogeology flow path 2 cross section
Cockajemmy Lakes
Lake Muirhead
Gellerts Swamp
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Salinity increases along flow path 2
Cockajemmy Lakes
Lake Muirhead
Gellerts Swamp
15-30
3
9
22
15

• Increase along flow path again due to addition
  of saline diffuse
• recharge 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