Hydrochemistry of groundwaters within the Murray and Otway Basins in the Victorian Designated Area, Victorian-South Australian border

Field: Environmental Science (Physical Sciences)

An assessment of the hydrochemistry of groundwaters in the Murray and Otway \nBasins along the Victorian - South Australian border has been conducted. This area is \nknown as the Victorian Designated Area (VDA) and contains three main aquifers, the \nTertiary Limestone Aquifer, Tertiary Confined Sand Aquifer, and Pliocene Sands \nAquifer. The area extends from the Murray River in the north to the Bass Strait \ncoastline in the south. Agricultural development and increasing reliance on \ngroundwater resources in the region have prompted the need for an investigation into \nthe processes affecting groundwater chemistry. Particular emphasis has been placed \non the causes of groundwater salinisation. \nData sourced from the Victorian Department of Natural Resources and Environment \nStatewide Groundwater Database were used to assess temporal and spatial \ngroundwater salinity trends in these aquifers. Groundwater salinity in all aquifers \ngenerally increases northwards, and corresponds to generally decreasing recharge \nnorthwards in the VDA. Groundwater salinity also increases along inferred flow \npaths for all aquifers. Both increasing and decreasing temporal groundwater salinity \ntrends are observed within parts of the study area. \nThe overall groundwater composition of the Tertiary Limestone Aquifer was also \nconsidered, as this aquifer is the most extensively exploited in the VDA. The \ngroundwater type varies from generally Ca,Mg-HC03 type waters in the south, to NaCl, \nS04 type waters in the north. \nExamination of the major ions showed several systematic associations; there is a \nstrong linear relationship between sodium, chloride and electrical conductivity; a \ncharge balance between cations and anions exists for some groundwaters; there is \nincreased bicarbonate with respect to chloride concentrations, and increased calcium \nwith respect to magnesium, when compared with theoretical mixing concentrations \nbetween rain water and sea water. Generally low sulphate to chloride ratios exist \nthroughout the study area, and sodium adsorption ratios generally increase \nnorthwards. Tertiary Limestone Aquifer groundwaters are undersaturated with respect to gypsum, \nsupersaturated with respect to calcite and dolomite, and close to equilibrium or \nsupersaturated with respect to aragonite. \nThe factors affecting groundwater composition vary throughout the study area, and \nbetween aquifers. They include: \na) concentration of salts by evapotranspiration, \nb) water-rock reactions such as de-dolomitization, and water-soil interactions \nthrough cation-exchange processes, \nc) changes in land use, and \nd) possible movement of groundwater of variable quality between aquifers through \nlow-permeability layers. \nRainfall, evaporation and evapotranspiration rates, soil type, groundwater flow \ndirection, length of groundwater residence time, hydraulic conductivity, flow velocity, \ngeology and subsurface structural barriers to regional groundwater flow, all contribute \nto the processes controlling groundwater composition in this region.