Evaluation of salinisation processes in the Spicers Creek catchment, central west region of New South Wales, Australia.

Spicers Creek catchment is located approximately 400 km west of Sydney in the Central West region of New South Wales, Australia. Dryland salinity has been recognised as a major environmental issue impacting soil and water resources in the Central West region of NSW for over 70 years. Due to the geological complexity of the catchment and the presence of high salt loads contained within the soils, groundwater and surface waters, the Spicers Creek catchment was identified as a large contributor of salinity to the Macquarie River catchment. Over fifty-two dryland salinity occurrences have been identified in the Spicers Creek catchment and it appears that dryland salinity is controlled by the presence of geological structures and permeability contrasts in the shallow aquifer system. Combinations of climatic, geological and agricultural factors are escalating salinity problems in the catchment. The main aim of this thesis was to identify the factors affecting salinisation processes in the Spicers Creek catchment. These include the role of geological structures, the source(s) of salts to the groundwater system and the geochemical processes influencing seepage zone development. To achieve these aims a multidisciplinary approach was untaken to understand the soils, geology, hydrogeology and hydrogeochemistry of the catchment. Investigative techniques employed in this project include the use of geophysics, soil chemistry, soil spectroscopy, hydrogeochemistry and environmental isotopes.Evaluation of high-resolution airborne magnetics data showed a major north-east to south-west trending shear zone. This structure dissects the catchment and several other minor faults were observed to be splays off this major structure. These structures were found to be conducive to groundwater flow and are influencing the groundwater chemistry in the fractured aquifer system. Two distinctive groundwater chemical types were identified in the catchment; the saline Na(Mg)-Cl-rich groundwaters associated with the fractured Oakdale Formation and the Na-HCO3-rich groundwaters associated with the intermediate groundwater system. The groundwater chemistry of other deep groundwaters in the catchment appears to be due to mixing between these end-member groundwaters within the fractured bedrock system. The spatial distribution of electrical conductivity, Cl-, Sr2+ and 87Sr/86Sr isotopic ratios showed the correlation between saline groundwaters and the location of faults. Elevated salinities were associated with the location of two crosscutting fault zones. The spatial distribution of HCO3-, K+, Li+ and ?Ô±3CDIC highlighted the extent of Na-HCO3-rich groundwaters in the catchment and showed that these groundwaters are mixing further east than previously envisaged.These findings show that Na(Mg)-Cl-rich groundwaters are geochemically distinctive and have evolved due to extensive water-rock interaction processes within the fracture zones of the Oakdale Formation. These saline groundwaters contain elevated concentrations of trace elements such as As, V and Se, which pose a potential risk for water resources in the area. 87Sr/86Sr isotopic ratios indicated that the source of salinity to the Na(Mg)-Cl-rich groundwaters was not purely from marine or aerosol input. Salt is most likely contributed from various allochthonous and autochthonous sources.This research found that the main mechanism controlling the formation of dryland salinity seepage zones in the Spicers Creek catchment is due to the presence of geological structures. These groundwater seepage zones act as mixing zones for rainfall recharge and deeper groundwaters. The main sources of salt to the seepage zones are from deeper Na(Mg)-Cl-rich groundwaters and rainfall accession. The major importance of this research highlights the need for an integrated approach for the use of various geoscientific techniques in dryland salinity research within geologically complex environments