Clearing of native vegetation for agricultural crops and grazing has been the definitive anthropogenic cause for dryland salinity in Australia and has had a devastating effect on catchment hydrology. In a high rainfall area of the Mt Lofty Ranges, land-clearing has caused a groundwater salinity ‘hotspot’ to form on a hill-slope producing transient groundwater salinity disequilibrium. The perched saline groundwater is over ten times more saline than the ambient groundwater. This phenomenon is largely unknown in the hydrologic literature. Furthermore, this saline groundwater was observed to discharge into a nearby intermittent stream thereby greatly increasing its salinity.
This surface water-groundwater interaction was investigated by weekly sampling and analysis of the physical parameters of the stream and the saline groundwater hotspot. Samples were analyzed for major and trace elements, stable isotopes of water and stable isotopes of strontium. These indicate that the probable mechanism for the salinity increase in the stream is a water-table rise during wet season recharge, which causes the saline groundwater to drain into the creek. Major and trace elements and strontium isotopes indicate the stream water is a mix of runoff and saline groundwater that has discharged into the creek. Strontium isotopes also identified that the source of salinity in the saline groundwater is largely caused by salts concentrated during evapotranspiration in the clayey unsaturated zone. Stable water isotopes have no relationship to salinity. CFC data indicates that the saline groundwater hotspot has been recently recharged which supports historic land-clearing as the cause of the high salinity.
The preferred model for the origin of the groundwater hotspot involves flushing of accumulated salts from the thick clayey unsaturated zone due to increase recharge following land-clearing. Depending on the veracity of this model, this process of groundwater salinization could be much more widespread than presently appreciated.