The kinetics of mineral dissolution in carbonate aquifers as a tool for hydrological investigations, I. Concentration-time relationships

Abstract

Groundwater chemical analyses, water level maps, and results of pumping tests are sufficient for studying the relationships between the major geochemical and hydrologic processes in a groundwater system. The importance of available groundwater chemical data for hydrologic investigations has been relatively neglected. Two general courses exist for use of groundwater chemical data and dissolution kinetics in hydrologic investigations: (1) estimating relative ages according to precalibrated concentration-time relationships similar to that of tritium and 14C dating techniques; and (2) integrated study of both hydrologic and geochemical phenomena with the aid of combined hydrogeochemical models. In this paper, a kinetic model for the dissolution of multimineral assemblages in porous media is derived with special emphasis on the simultaneous dissolution of calcite, dolomite and gypsum in some carbonate aquifers. Supersaturation of groundwater samples with respect to calcite and dolomite frequently occurs and is here explained by the relatively high solubility of gypsum, resulting in kinetic competition among dissolving minerals. The applicability of a computerized kinetic model is demonstrated by the evaluation of hydrogeochemical data for the limestone aquifer of central Florida. The kinetic model was calibrated with the aid of available hydrologic estimates and chemical data for part of the aquifer. Chemical-age estimates for the remaining aquifer were obtained by using the calibrated concentration-time relationships of aqueous species. Available 14C data in the limestone aquifer of Florida were then considered and compared to the chemical age estimates. In general, the trend of the two estimates is in reasonable agreement.