A model which includes the transport and retardation mechanisms of advective flow, axial dispersion, liquid-phase mass transfer, diffusion into immobile liquid, and local adsorption equilibrium was developed to describe the migration of nondegradable, organic chemicals through a column of saturated• aggregated soil. A range of simplifying assumptions were explored to assess the relative importance of the various mechanisms. Solutions to the model were either adapted from the literature or derived from mass balances and mass transfer principles. The most general form of the model required the development of numerical solutions which employed orthogonal collocation. Soil column breakthrough predictions in terms of relative concentration as a function of total column pøre volumes fed can be characterized by seven independent dimensionless parameters: the Peclet number, the Stanton number, a pore diffusion modulus, a surface diffusion modulus, an adsorbed solute distribution ratio, an immobile fluid solute distribution ratio, and the Freundlich parameter 1In. For a strongly adsorbed chemical in long soil columns, a fifteen fold decrease of the Peclet number, a fivefold decrease of the Stanton number, or a onefold decrease in either the pore diffusion modulus or the surface diffusion modulus have an equivalent effect on the spreading of the breakthrough curve. The breakthrough curve tends to sharpen for favorably adsorbed chemical species (1In < 1.0) and spread when adsorption is unfavorable (1In > 1.0). The movement of chemical is retarded as the solute distribution ratios increase. A sensitivity analysis of model parameters, which were derived from literature correlations, column geometry, soil adsorption isotherms, and breakthrough curves, showed that adsorption capacity, adsorption intensity, and aggregate geometry have the greatest effect on chemical retardation and spreading, while liquid-phase mass transfer has little effect. INTRODUCTION The evaluation of the extent of groundwater pollution requires an understanding of the transport mechanisms which are involved and the ability to incorporate these mechanisms into a predictive mathematical model. A predictive model for the transport of organic chemicals may also be useful in designing landfills or wastewater land application systems. Many of the hazardous chemicals that originate from waste disposal and are subsequently found in groundwater are slightly degradable, weakly hydrophobic, organic compounds which are not readily removed as they travel with the groundwater {'Roberts et al., 1982-1. Many of these compounds are also volatile i-Bouwer and Rice, 1984-1, posing an additional problem because they may diffuse through the soil atmosphere at a much faster rate than with advective transport in the aqueous phase. Much of the understanding of the behavior of organic chemicals in soil and groundwater has been derived from work performed on pesticides •Bailey and White, 1970; Goring and Hamaker, 1972; Helling et al., 197!•. Several mechanisms are responsible for ...