Quantitative Studies in the Geological Sciences
The mixing of sea water and (or) saline subsurface water with fresh calcium carbonate ground water can be of major importance in the chemical diagenesis of carbonate rocks and sediments. A computational scheme for calculation of the theoretical distribution of dissolved species in mixtures of two solutions was developed and used to evaluate the effect of mixing saline water with fresh calcium carbonate ground water. The calculations include data on (1) mixtures of surface sea water (pH 8.15) with solutions in equilibrium with calcite at CO2 partial pressures from 10−4 to 1.0 atm at 5°, 15°, 25°, and 35° C; (2) mixtures of surface sea water with solutions twofold saturated with calcite; and (3) mixing of actual saline water and fresh carbonate ground water from central Florida and the Yucatan Peninsula.
The results show that the amount of undersaturation in mixtures is a function of P CO2 , temperature, ionic strength, degree of calcite saturation, and pH of end-member solutions prior to mixing. All mixtures of surface sea water and saturated calcium carbonate ground water investigated containing 0 to 10 percent sea water by volume were undersaturated with calcite. The range of undersaturation increased greatly with increasing PCO2 (above 10−3 atm) in calcite-saturated water and with decreasing temperatures. No undersaturation occured in mixtures of surface sea water and supersaturated calcium carbonate ground water (twofold saturated) having CO2 partial pressures less than 10−2 atm at 25° C. More than 10 percent surface sea water mixed with supersaturated calcium carbonate ground water (twofold saturated) at CO2 partial pressures greater than 10−2 atm was required to cause undersaturation. Arbitrarily lowering the pH of sea water increased the range of undersaturation in mixtures, but this range decreased in mixtures of carbonate ground water and sea water at pH values lower than 8.15 when the calcite pH 8.15.
The results of these calculations define geochemical environments that favor (1) development of increased porosity and permeability in limestone aquifers and (2) fresh-water dolomitization of calcium carbonate rocks.