Geochemistry of groundwater from Upper Cretaceous-Lower Tertiary sand aquifers in South-Western Victoria, Australia

Abstract

Several chemically different types of groundwater have been discovered in confined sand aquifers of the Upper Cretaceous-Lower Tertiary sediments in the Otway Basin, southern-western Victoria. The main geochemical variables involved in the genesis of these groundwaters are considered separately. They are: 1. (a) cation exchanges which cause softening or hardening of the water; 2. (b) availability of carbonic acid within the aquifer and the formation of sodium bicarbonate; 3. (c) addition of different amounts of oceanic salts mainly from saline connate water in the sediments; 4. (d) variations in the ratio of sulphate to chloride as a result of sulphate accumulation or bacterial sulphate reduction. These variations can be followed more easily from water analyses when the ionic concentrations are expressed as chloride ratios (concentrations of particular ions as percentages of the chloride concentration in equivalents per million). Some of the variations are fairly regular in their distribution in the basin while others are more irregular. Four main classes of water based on sodium and magnesium-chloride ratios are described, with subclasses based on chlorinity and alkalinity. Waters representative of three of these main classes have been described in similar aquifers in other sedimentary basins.