CATION AND PROTON-EXCHANGE, PH VARIATIONS, AND CARBONATE REACTIONS IN A FRESHENING AQUIFER

Freshening of aquifers is accompanied by sequential elution of the saltwater (seawater) cations from the sediment's exchange complex. The resulting chromatographic patterns are modeled with a one-dimensional geochemical transport model that can handle the complex interplay of transport and mineral and ion exchange equilibria. The transport part is based on the mixing cell approach, with different time steps for advective and diffusive transport used when required by small grid size. The chemical reactions are calculated explicitly after each time step with the geochemical model PHREEQE. Ion exchange is included in the form of association half reactions, which allows simulation of the dynamic nature of the exchange process. The variation in the constant for proton association is obviated with an activity coefficient for H-X that is derived from the constant capacitance model. All coefficients for the exchange model are obtained by fitting to literature data to be able to perform the modeling as realistically as possible. The code is applied to a laboratory column experiment, and subsequently used to demonstrate chromatographic development of solute profiles in a freshening aquifer. Sequential peaks of Mg2+, K+, and Na+ along a flow path in the Aquia aquifer in Maryland are modeled, and the results confirm that the variation of water qualities in this aquifer has basically a chromatographic origin. Proton exchange acts here as a source of acid in NaHCO3 water in which calcite dissolves. This explains the Na+ to HCO3- ratio and high delta13C observed in these waters.