Surface water acidification arising from aquifers impacted by agricultural practices: sources, processes and treatment options

[Truncated] Groundwater is an important resource to humans and ecosystems, especially in arid and semiarid environments where it may be the only source of water. Its value as a resource, however, is reduced when it becomes contaminated through anthropogenic activities. This research focuses on acidic groundwater which remains poorly understood but can have a significant impact on surface water environments, leading to both direct and indirect stress on organisms and ecosystem health. Acidification of groundwater can be caused by atmospheric deposition (acid rain), acid mine drainage and disturbance of acid sulfate soils. Often as a result of agricultural practices. Agricultural practices have caused acidification of groundwater dependent environments in south-western Australia (SWA) where the draining of coastal wetlands for horticulture exposed naturally occurring sulfides that oxidised to generate acid. Additionally, within the inland areas of SWA, ephemeral aquifers provided low salinity neutral baseflow to waterways, however following the clearing of native vegetation for agriculture, naturally occurring deeper acid-saline groundwater now discharge to waterways. It is estimated that there are over 350 km of acidified waterways in the Avon catchment of SWA that have resulted from agricultural practices, though the acidification of surface waters is spatially and temporally heterogeneous.

This research aimed to further our understanding of SWA acidic groundwater sources, controls and impacts, through a series of field investigations, statistical analysis of historical groundwater and mapping data, and hydrological and geochemical modelling. Finally, since the management of acidification impacts remains challenging within broad-acre semi-arid landscapes, a pilot scale passive treatment system was built to test its suitability for remediating acidified stream waters.

This thesis firstly presents an investigation into acidification processes and their impacts on a coastal wetland disturbed by agricultural practices, with a particular focus on surface water - groundwater interactions and the resulting hydrological and geochemical processes. These investigations found that the lowering of watertables, especially with the aid of drainage, over the dryer summer months led to oxidation of sulfides within surficial soil profiles, generating substantial acidity within the unsaturated zone. With the onset of winter rains, the watertable rose redissolving the acid products. The groundwater continued to rise to form a series of disconnected pools, some of which were extremely acid due to the redissolved salts. The pools eventually linked, establishing surface water connectivity, acid dispersion and subsequent discharge from the site. A water balance model for the site was developed and it was identified that manipulation of watertables through control of drainage and placement of organic mulches over sulfidic sediments and soils to minimise evaporation are possible management options for the site.