Determining the salt store and mobility in the regolith, bland sub-catchment, central NSW

Abstract

To manage salinity risk for a sub-catchment or a region, quantifying the salt store and degree of salt mobility are two fundamental issues to address. In a thick (e.g. 70 m) and complex aquifer system, where sand bodies are semi-confined to confined within spatially dominating mud and clay, quantifying the salt store and mobility based on information collected from sparsely and unevenly distributed water bores and exploration drill holes is a significant challenge. Airborne electromagnetic (AEM) data, using Fugro?s time-domain TEMPEST fixed wing system, were collected over ~ 1,500 km2 of the Bland sub-catchment in central NSW (Lawrie et. al., 2002). Conductivity responses depict the electrolyte concentrations (mainly sodium chloride) and water content of the regolith and basement rock (Lawrie et al., 2002). Statistical analyses of water content, salinity and electrical conductivity logs show that conductivity is a measure of salt load. Non-porous bedrock is resistive whereas porous and moist sediments with saline pore fluids are conductive. Amongst the sediments, less porous coarse textured palaeo-valley basal gravely sand is less conductive than highly porous clay-rich colluvium and lacustrine mud. Using conductive depth slices (CDI), the spatial salt load distribution was mapped and quantified using ArcGIS (Tan et al., 2005a).