A numerical model is developed to simulate hydrophobic organic compound sorption kinetics, based on a retarded intraaggregate diffusion conceptualization of this solid‐water exchange process. This model was used to ascertain the sensitivity of the sorption process for various sorbates to nonsteady solution concentrations and to polydisperse soil or sediment aggregate particle size distributions. Common approaches to modeling sorption kinetics amount to simplifications of our model and appear justified only when (1) the concentration fluctuations occur on a time scale which matches the sorption timescale of interest and (2) the particle size distribution is relatively narrow. Finally, a means is provided to estimate the extent of approach of a sorbing system to equilibrium as a function of aggregate size, chemical diffusivity and hydrophobicity, and system solids concentration.

Gschwend, P.M., Backhus, D.A., MacFarlane, J.K. and Page, A.L., 1990. Mobilization of colloids in groundwater due to infiltration of water at a coal ash disposal site. J. Contam. Hydrol., 6: 307-320.We investigated groundwaters in the vicinity of a coal ash site near an electric generating station in the western U.S.A. The purpose of the study was to ascertain why fine particles or colloids appear in some subsurface water samples there. If such fine particles are merely introduced during bailing or pumping operations which suspend otherwise immobile soil colloids, we should exclude these particulate materials from the water samples before analysis intended to quantify what is moving through the aquifer. However, if the colloids were truly suspended and moving with the groundwater flow in situ, then we should include their contribution to our assessment of the mobile loads.Application of very careful sampling techniques (slow pumping rates, no atmospheric exposure) did not cause the large quantities of colloids observed previously to disappear from well water in which they occurred. Additionally, the same sampling procedures did not cause similar abundances of colloids to appear in waters collected from neighboring wells installed and developed in the same manner and in the same geologic strata. Thus we believe sampling artifacts do not explain the colloids' presence in the groundwater samples.On the other hand, the groundwater chemistry and the nature of the suspended colloids (size, composition) strongly suggest these fine particles were suspended and therefore moving with the groundwater flow. At wells exhibiting large amounts of suspended colloids (~ 10-100mg L-), the water was enriched in CO, 2 and depleted in 02, relative to nearby locations. The colloids were typically between 0.1 and 2 lm in size and were primarily silicates. These results suggest to us that, where infiltrating water is percolating through a site that has been mixed with coal ash, the secondary carbonate minerals in the soils are being dissolved; removal of this cementing carbonate phase may consequently release soil silicate colloids to be carried in the flowing water.Such processes may enhance contaminant transport in groundwater by augmenting the pollutant load moving in the groundwater, and increasing the permeability of the porous medium to pollutant infiltration with waste water and/or rainwater. . r L