Hydrochemistry of a groundwater-seawater mixing zone, Nauru Island, central Pacific Ocean

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.

瑙鲁岛（Nauru Island）是一座位于太平洋中部的喀斯特化白云质石灰岩岛（karstified dolomitic limestone island）。一层薄而不连续的淡水层覆盖于厚层咸淡水混合带（brackish water mixing zone）之上。在该混合带中，地下水盐度随深度逐渐升高，直至在海平面以下约70米处接触海水。重碳酸根-钙-镁型淡水地下水会向海水水化学组分演化。特定碳酸盐矿物的饱和指数（saturation indices）随地下水盐度升高而增大：当总溶解固体（total dissolved solids，TDS）为300 mg/L时，白云石达到过饱和状态；5000 mg/L时方解石达到过饱和；6000 mg/L时文石达到过饱和。由于混合带内的地下水已达白云石饱和状态，因此具备白云岩化（dolomitisation）的潜力，且该过程大概率发生在混入海水比例较低的场景中。可基于二氧化碳分压（partial pressure of CO₂）划分开放体系与封闭体系的演化趋势：分压较低的开放体系包含包气带水（vadose waters）、洞穴水（cave waters）以及盐度更高的混合带水，其中后者的化学演化主要受海水混入作用调控；封闭体系则包含混入海水比例较低的淡水层，其水化学特征受CO₂溶入作用以及溶解-沉淀反应共同控制。基于喀斯特地下水与海水简单混合的理论计算，无法准确描述随混合程度动态变化的实际化学过程。