Science, Technology and Engineering

Environmental Geoscience

Postgraduate Projects

Quantification of future changes in groundwater salinisation in the basalts of the Hamilton area, using hydrogeological, chemical and isotopic techniques

Darren Bennetts

Supervisor: Dr John Webb

A variety of complex groundwater flow systems occur beneath the Hamilton basalt plains, western Victoria, Australia, and each plays a distinct role in the development of dryland salinity. Groundwaters are of meteoric origin, and ³⁶Cl and major element chemistry indicates that dissolved salts are primarily cyclic; salinisation results from the concentration of cyclic salts by evapotranspiration. Water-rock interaction affects only the freshest groundwaters and is of little consequence to groundwater salinisation. Groundwaters in the basalt aquifer are predominantly recharged through volcanic eruption points and areas with skeletal, rocky soils, where infiltration is rapid, so that salts contained within rainfall are not significantly affected by evapotranspiration; the freshest groundwaters (< 300 mg/L Cl^-) typically occur in these areas. Higher salinity groundwaters (> 1000 mg/L Cl^-) are recharged through thick, clay-rich soils, where significant evapotranspiration takes place. This saline water is progressively added to the groundwater system down-gradient, causing an increase in the salinity along flow path. Groundwater flow systems range from local to regional in extent, with groundwaters dated by tritium and radiocarbon ranging from modern (< 50 years) in recharges areas, up to ~20 ka in age, 15-20 km down the flow path. Local groundwater flow systems often overlie regional flow systems in the basalt aquifer. ⁸⁷Sr/⁸⁶Sr, d¹³C and hydrogeochemical data indicate that significant leakage from the basalt into underlying Tertiary sedimentary aquifers occurs, having a major influence on salinisation in these aquifers. Groundwater discharge from the basalt occurs along the edge of individual lava flows at the break of slope, where most streams, lakes and swamps occur, and here direct evaporation further increases the salinity of discharging groundwaters and surface runoff. Salt budget calculations indicate that salts are currently being exported from the study area, resulting in an overall decrease in the salt content of soils and groundwater across much of the basalt plains. However, where the watertable remains within 1.5 m of the surface, salinity will continue as salts accumulate due to evaporation. Most salinity is of primary origin, and salt budget calculations indicate that salinity will a major part of the landscape for tens to hundreds of years.