Acidification of salinised waterways by saline groundwater discharge in south-western Australia

SummaryRegional acidification of surface waters is occurring in south-western Australia that is coupled with dryland salinity and acidity carried by rising groundwater. We report on basin-scale influences of acidic saline groundwater discharge on surface water chemistry in the Swan-Avon Basin investigated through 7 years of annual base-flow and lake water quality snapshots. This is linked with analysis of temporal hydrochemistry and discharge at two sites in the basin to assess historic patterns. At least 350 km of major waterways and tributaries in the Swan-Avon Basin were found to exhibit base-flow acidity (pH < 4.5) along with low-level waters in a number of lakes. Acidity appears linked with saline groundwater discharge, contains high concentrations of aluminium, iron and manganese and a range of trace elements (e.g. Pb, Cu, Ni, Zn). There is evidence that acidification has been occurring for at least 30 years and is linked with increased diffuse discharge of saline groundwater in the salinising landscape. However, base-flow acidity in some sub-catchments can be attributed to point discharge from agricultural groundwater drains used to mitigate salinsation of land. Managing acidification of base-flows requires containing diffuse acidification in salinising landscapes and practical options so that landholders can treat point discharge of acidic groundwater from drains. This investigation highlights that the influence of groundwater discharge on stream water quality in salinising landscapes is not limited to salinity and can also include acidification, particularly in deeply weathered regolith.

► Snapshots of base-flows show wide-spread acidification in a salinising basin.
► Acidification is linked with saline groundwater discharge and acidity.
► This is evident for >30 years and can be influenced by drainage of salt-land.