Trace elements as an indicator of redox processes in a fractured aquifer system, Ballimore, central New South Wales, Australia

摘要

Hydrogeochemical interpretation of minor and trace element data from the Ballimore region, central New South Wales, Australia suggests that the regions fractured groundwater system transports metals which are variably mobilised and/or precipitated by changes in the aquifers redox conditions. The groundwa-ters pH, Eh conditions and S~(2-) concentrations influence the amount of Fe, Mn, Zn and Ba in solution. Maximum concentrations of these elements are 48 mg/1 of Fe, 2 mg/1 of Mn, 1.2 mg/1 of Zn and 18 mg/1 of Ba. Mixed and discharging groundwaters are strongly buffered with respect to pH (by the H_2CO_3/HCO_3~- couple at 6.2-6.6) and do not attain carbonate equilibrium. This pH buffering suppresses the precipitation of FeCO_3, MnCO_3, ZnCO_3 and BaCO_3 and enables redox processes to control trace element concentrations. Iron and manganese are only present in the moderately reduced mixed and saline groundwaters. In the reduced waters where SO_4~(2-) is not available Ba behaves as a conservative element. Redox potential in some samples is below -350 mV, and in these samples S~(2-) concentrations are up to 8 mg/1 through the reduction of SO_4~(2-) to S~(2-). Redox buffering by this couple controls the concentration of Fe and Mn because the availability of S~(2-) leads to the removal of Fe and Mn through the precipitation of metal-sulphides.