CONTAMINANT DISPLACEMENT IN AQUIFERS - COUPLED EFFECTS OF FLOW HETEROGENEITY AND NONLINEAR SORPTION

An analytical solution is developed for displacement of a nonlinearly sorbing solute in a three-dimensional heterogeneous aquifer. Nonlinear sorption is modeled by means of different isotherm equations includin g the commonly used Langmuir and Freundlich isotherms, but also the La ngmuir-Freundlich, Toth, and two-site Langmuir isotherms. Evaluation o f cleanup times, defined as the times required to reduce the concentra tion to a given fraction of its initial value, for different isotherms and combinations of input parameters demonstrate the effects of the p roperties of the isotherms. In particular, the asymptotic behavior as the aqueous concentration approaches zero is important for transport p redictions, where Freundlich sorption leads to infinitely increasing c leanup times as the initial plume concentration decreases. The effect of the choice of sorption isotherm, evaluated by using ''best-fit'' pa rameters for four different isotherms, is found to be large and depend ent on heterogeneity for the particular data set considered.