Geochemical calculations and observations on salt water intrusions, I. A combined geochemical/minxing cell model

Abstract

A combined geochemical/mixing cell flow model has been developed. The geochemical model is based on the computer program eq3/6 (Wolery, 1982), has been modified to include cation exchange reactions and is incorporated in a one-dimensional mixing cell model for stationary one-dimensional. transport. Dispersion or diffusion is calculated by mixing the cell contents with neighbouring cells. Relationships between mixing factors and dispersivity and diffusion coefficients are derived. The model is used to simulate concentration changes during salt water intrusions. Fundamental differences are shown to exist between diffusion of salt water from a stagnant water body and dispersive intrusions during marine transgressions or flooding events. With dispersive flow, characteristic water compositions develop in space and time as a result of cation exchange. With diffusion, these steep concentration gradients are smoothed out and “mixed” compositions occur. The concept that characteristic concentrations occur during dispersive flow is used to obtain paleohydrological information in a Dutch polder. The depth of the fresh/salt water interface before polder development is deduced from a simulation of observed exchange reactions in the polder groundwater.