Nauru Island is a karstified dolomitic limestone island in the central Pacific Ocean. A thin, discontinuous freshwater layer overlies a thick brackish water mixing zone. In the mixing zone, groundwater salinity increases gradationally downwards until seawater is encountered at about 70 m below sea level. Fresh HC03-Ca-Mg groundwaters evolve to seawater. Saturation indices for particular carbonate minerals increase with increased groundwater salinity. Supersaturation is achieved with dolomite at 300 mg/L total dissolved solids, with calcite at 5000 mg/L, and with aragonite at 6000 mg/L. As groundwaters in the mixing zone are saturated with dolomite there is potential for dolomitisation, and this probably occurs at low proportions of admixed seawater. Open and closed system trends can be defined, based on the partial pressure of CO2 , The open system, with lower partial pressure, comprises vadose waters, cave waters and the more saline mixing zone waters; in the latter, chemical evolution is controlled mainly by mixing with seawater. The closed system comprises the freshwater layer with low proportions of admixed seawater; its chemistry is controlled by ingassing of CO2 and by dissolution and precipitation reactions. Theoretical calculations based on simple mixing between karst groundwaters and seawater are inadequate to describe actual chemical processes, which change with the degree of mixing.
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