Occurrence of high bicarbonate groundwater in Victoria, Australia

Abstract

Determination and collection of major ion chemistry for Victorian groundwater was initiated by the Geological Survey in about 1855. The Survey collection is the major source of groundwater information, although in the last two decades water authorities and industry bodies have gathered large amounts of information. The data set compiled for examination of the occurrence of the bicarbonate ion is a synthesis of data from mainly government sources and represents samples mainly from farm bores, government investigation holes and petroleum exploration drilling. Overall bicarbonate is the second most abundant anion in Victorian groundwater with a salinity above 300 mg/L Total Dissolved Salts (TDS). A concentration of 800 mg/L bicarbonate was selected to define the class of high bicarbonate alkalinity groundwater. The occurrence of high bicarbonate groundwater in Victoria has been examined in relation to hydrogeological setting. The chemical evolution to a high bicarbonate groundwater type is observed in a number of different geological systems. Different facies may be identified based on the pH and the predominance of anions and cations including iron. Within each aquifer type, high bicarbonate groundwater can occur within the flow system: in mixing and flushing zones, in hypersaline environments, and spectacularly associated with waste disposal plumes. Geological associations include: fluviatile and alluvial aquifers, extensive aquifers of the coastal sedimentary basins, and the low flow fissure networks in bedrock of the gently undulating ranges of Central Victoria. In the sedimentary coastal basins a common association is with aquifers containing accessory concentrations of carbonate, carbonaceous matter and pyrite. The facies in these aquifers can develop over a short distance and may occur in strata enveloped within other strata containing significantly different water quality. In Central Victoria, Lower Palaeozoic rocks frequently exhibit an association between the occurrence of high bicarbonate groundwater and labile interbedded, lithic and arkosic sandstones with pyritic carbonaceous shales that exhibit deep kaolinised weathering profiles. In deep bedrock mining activities of nineteenth century in Central Victoria dewatering of large volumes of the rock mass took place. Gradually this resulted in a freshening of the groundwater extracted and suggested a localized increase in flow and recharge to the bedrock. A century later and more than 70 years after the dewatering operations stopped, mineral exploration bores encountered groundwater on the same gold fields with increased sulphate and bicarbonate concentrations. The high bicarbonate concentrations had been re-established as a consequence of reaction with sulphide and carbonate minerals.