Transport with spatially variable kinetic sorption: recursion formulation

A recursion formulation for the transport of linearly sorbing solutes undergoing nonequilibrium sorption is developed. Constant or spatially varying sorption kinetics can be modeled using the recursion approach. The sorption and desorption rates are modeled as two independent random processes with a prescribed mean and covariance structure with spatial variability in the rate parameters included as well. The recursion solution, in terms of the probability density function for solute travel times, is derived by specifying transition probabilities for moving between the aqueous and sorbed phases. A few simple examples are used to illustrate the approach. The computer implementation leads to a very rapid algorithm that is easily extended to cover cases beyond the basic model presented here. q 1999 Elsevier Science Ltd. All rights reserved.