Simulation of solute transport: a multinomial model

A direct simulation method has been developed that uses a distribution function to model solute transport along a streamtube. During each timestep, the aqueous solution at each node is transported to the other downstream nodes according to the distribution function. In order to use the distribution function, fractions of solution are computed for movement of 1, 2, ... n nodes along the streamtube. The mean of the distribution represents the advection component, and the spreading about the mean represents the longitudinal dispersion component of transport. The direct simulation method provides a mass-conserving and numerically stable transport model, which is easily adapted for use with coupled hydrologic/geochemical models. The transport model was used to predict chloride breakthrough curves for a set of column experiments (Opitz et al. 1985). In these experiments, coarse aquifer sediment was contacted with a high chloride influent solution. The experimental breakthrough curves showed an extremely early change in chloride concentration as the influent was added (around 0.4 pore volumes). Using a plug-flow (advection only) model, predicted effluent breakthrough occurred much later than in the columns, indicating that dispersion mechanisms were important in describing transport. 12 refs.