Porosity development in coastal carbonate aquifers

Geochemical mixing theory suggests that the mixing of seawater and calcite-saturated fresh ground water can result in a solution that is undersaturated with respect to calcite. Previous studies of the mixing of such waters in carbonate rocks along certain coastlines have indicated that this mixing effect may be responsible for significant amounts of calcite dissolution and porosity development. In this study, potential rates of porosity development by calcite dissolution are assessed by combining geochemical mixing theory with the hydrodynamics of fresh-water-salt-water mixing zones in a coupled reaction- transport model. Results from the reaction-path model PHREEQE are used with a variable-density ground-water flow and solute-transport model to simulate an idealized cross section of a coastal carbonate aquifer. Results of the simulations indicate that the dissolution process is sensitive to fresh-water chemistry, ground-water velocities, and sea-level movement. Dissolution potential was evaluated at three field sites, and evidence from those sites is in general agreement with the simulation results. Dissolution rates indicated by the model show that under the proper conditions this dissolution mechanism can produce significant increases in porosity over relatively short spans of geologic time (tens of thousands of years).