The software package includes a modified and updated version of the CXTFIT code of Toride et al. for estimating solute transport parameters using a nonlinear least-squares parameter optimization method. This code may be used to solve the inverse problem by fitting a variety of analytical solutions of theoretical transport models, based upon the one-dimensional advection-dispersion equation (ADE), to experimental results. The program may also be used to solve the direct or forward problem to determine concentrations as a function of time and/or position. Three different one-dimensional transport models are considered: (i) the conventional equilibrium ADE; (ii) the chemical and physical non-equilibrium ADEs; and (iii) a stochastic stream tube model based upon the local-scale equilibrium or non-equilibrium ADE.
This behaviour assessment model of Jury et al. (1983) describes the fate and transport of soil-applied organic chemicals. The model assumes linear, equilibrium partitioning between vapour, liquid, and adsorbed chemical phases, net first order degradation, and chemical losses to the atmosphere by volatilization through a stagnant air boundary layer above the soil surface. The model is intended to classify and screen organic chemicals for their relative susceptibility to different loss pathways (volatilization, leaching, degradation) in soil and air. SCREEN requires knowledge of the organic carbon partition coefficient (K_oc), Henry's constant (K_h), and a net first-order degradation rate coefficient or the chemical half-life. These parameters for selected chemicals provided in the STANMOD software are taken from Jury et al. (1984).