Geochemistry of Carbonate Precipitation from the Ground Waters in Basaltic Aquifers: An Equilibrium Thermodynamic Approach

The geochemistry of ground waters in the basaltic aquifers is considerably modified by the incongruent dissolution of augite and plagioclase from basalts. The analyses of the water chemistry data by using the WATSPEC aqueous-model computer programme for computing the ionic species abundances, the partial pressures of CO₂ (PCO₂) and mineral Saturation Indices (SIc and SId) suggest. that the ground waters are supersaturated with respect to calcite and dolomite. The modeling has indicated a state of equilibrium with respect to dolomite first followed by calcite. This state of equilibrium can be linked with the selective dissolution of augite leading to Mg-rich alkaline environment. which is also favourable for the formation of montmorillonite clays. The clays in turn act as a buffer and as a result the levels of Ca and Na do not exceed 1.8 mmol/L concentration. Due to the early dolomite saturation the Ca/Mg ratios areless than unity, whereas Na/Ca ratios are above unity suggesting precipitation of calcite and possibility of input from zeolites and mixing of difierent sources of water with local ground water.

The precipitation of calcite is under high PCO₂ conditions related to degassing and diffusion of CO₂ at the water-table and along cracks, joints and fissures in the basalts. The high PCO₂, dissolved oxygen content, presence of contaminants and fluctuations in the water table suggest that the ground water has chemically evolved under open system conditions.