GEOCHEMISTRY OF GROUND WATERS FROM THE GREAT ARTESIAN BASIN, AUSTRALIA

Ground waters from the confined Lower Cretaceous-Jurassic aquifer of the Great Artesian Basin, Australia (GAB) are characterised by Na-HCO3 type waters throughout the eastern and central parts of the basin and Na-SO4-Cl type waters in the western part. Sodium and bicarbonate increase in concentration from the northeastern margins to the southwestern discharge areas along the regional groundwater flowlines. A mass-balance and equilibrium model for major dissolved species and stable carbon isotopes of dissolved inorganic carbon show that the chemical trends observed are caused by mass transfer reactions involving cation exchange of Na for Ca-Mg, carbonate dissolution and reactions between Na and kaolinite to form Na-smectite. The stable carbon isotopic composition of dissolved inorganic carbon (DIC) increases from values of about -15 parts per thousand near the basin margins to -6 parts per thousand in the interior of the basin. An inverse relationship exists between delta-C-13 and DIC indicating addition of enriched carbon as the ground waters move basinwards. A C-12-C-13 mass balance indicates that the trend toward heavier delta-C-13 values in the interior of the basin results from bacterial reduction of carbon dioxide to produce methane rather than dissolution of, and equilibration with, carbonate minerals. The GAB aquifer system is apparently open to CO2 which is the product of in situ anaerobic fermentation producing CO2 enriched in C-13. The chemical evolution of the major dissolved species and carbon isotope distribution in the eastern and central parts of the GAB can then be envisaged as an initial pCO2 up to several orders of magnitude above atmospheric level is acquired within the recharge area by plant respiration and oxidation of organic matter in the soil zone. Initially, silicate and carbonate minerals may dissolve, at least in some parts of the basin, resulting in increased alkalinity, higher Ca and Mg concentrations, and a delta-C-13 concentration of the DIC of around -12 parts per thousand. Processes such as cation exchange of Na for Ca and Mg in addition to the removal of some Na by 'reverse weathering' which produces a Na smectite dominate in the interior of the GAB. The chemistry of ground waters derived from the western recharge areas is controlled by evaporite dissolution as indicated by very high Cl/Br ratios.