EFFECT OF SORPTION ISOTHERM TYPE ON PREDICTIONS OF SOLUTE MOBILITY INSOIL

Partitioning of a species between aqueous solution and sorbed phases i s commonly described as an equilibrium process and modeled by a sorpti on isotherm. These sorption models may be derived from the cumulative effect of many underlying specific reactions. However, in heterogenous systems, such as soils, the full array of these reactions and the rel ative contribution of each are unknown. This problem may be circumvent ed, without sacrificing consistency with the underlying reactions, by use of a sorption isotherm which faithfully describes measured sorptio n behavior. Since transport predictions based on one or another isothe rm model may be quite different, careful choice of an isotherm is crit ical. The objective of this study was to examine the effect of isother m type on predicted solute mobility in soil. The derivation of several isotherms from either specific reactions or a combination of ''local' ' isotherms with weighting functions is first presented. The use of Fr eundlich and logarithmic Scatchard plots for selecting the proper sorp tion isotherm is discussed. Isotherm models, fit to experimental data from the literature, are then used in a transport simulation model. Di fferences between results are discussed. Also, a brief sensitivity ana lysis which shows the effects of isotherm parameters on transport beha vior is presented.