Investigation of unconsolidated sedimentary units and their role in the development of salinity in Snake Gully Catchment, central New South Wales

Abstract

The Snake Gully catchment, central New South Wales, Australia, has significant erosion and salt affected land, with surface water flows commonly exceeding 6,000 mS/cm (6 dS/m), and shallow groundwater salinity up to 14000 mS/cm (14 dS/m). A combination of geoscientific techniques including geological mapping, drilling, radiocarbon dating, particle size analysis, geophysics, groundwater monitoring and hydrogeochemistry have been applied to investigate fine grained unconsolidated sedimentary units and their role in the development of salinity in the catchment. Gully erosion in the valley floor has exposed a series of sedimentary units with sharp textural contrasts traditionally known as 'duplex' soils thought to have been formed in-situ over 20 to 30,000 years. The units unconformably overlie slightly weathered Palaeozoic bedrock. Three of the sediment units are massive and exhibit strong bimodality, with large angular clasts of local basement supported in a silt-clay matrix, consistent with deposition as debris flows. Radiocarbon dating of the second unit from surface indicates an age of deposition between 180 and 330 years. Particle size analyses of the sub-50 micron component of the units indicate significant sorting consistent with an aeolian origin. The sediment units contain salt and are highly dispersive. Hydrogeochemical and isotopic analyses do not support evaporative concentration of salt, and indicate that sodium chloride dissolution and ion exchange are the main processes affecting the ionic concentration of shallow groundwaters. It is estimated that 1930 tonnes of salt are stored in the unconsolidated sediment units within this 22.6 km2 catchment, and erosion of a small section of these sediments of 20m x 20m and 2m thick would release 772kg of salt into the surface water system.A number of possible conceptual models have been proposed for salinity in the Snake Gully catchment, and these are assessed against the findings of this study. The results exclude deep groundwater discharge through faults and evaporative concentration of shallow groundwater through in-situ soils as major salinity processes; and support a shallow exotic source of salt in partially confining sedimentary clays mobilised by a shallow pressurised water table. An alternative conceptual salinity model is presented in which salt and dust were deposited in the catchment during the dry and windy glacial periods of the Quaternary. These sediments have been reworked and deposited as viscous sheets of unconsolidated sediment across the valley floor. Since deposition, these clay-rich units have controlled catchment salinity processes by impeding groundwater discharge, promoting shallow water tables; and by providing a source of sodium chloride salt, generating dispersive sodic soils. Inappropriate land management practices compounded by severe environmental events have resulted in the problems of shallow water tables, dryland salinity, and erosion that are now apparent in the catchment.Although the model presented is based upon studies in one catchment, the Quaternary dust transport paths covered a large part of south-eastern Australia, and aeolian deposits are increasingly being recognised across the slopes and tablelands of NSW. The findings of this study clearly demonstrate that clay-rich aeolian derived sediment units have a major role in the development of land and water salinisation in the region.The management implications of this work are discussed, including a shift in focus to incorporate fine grained salt-rich valley slope and floor sediments in salinity management planning. Protection of the salt-rich sediments from erosion and release of salt should be the priority management strategy; along with on-going actions to minimise local groundwater recharge. In combination, these measures will considerably reduce salt export so that downstream salinity targets can be realised.This research report is based upon a Masters of Engineering Science Project report by Ann Smithson supplemented by additional material provided by the authors.