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United States Department of Agriculture

Agricultural Research Service

Title: The Cxtfit Code for Estimating Transport Parameters from Laboratory Or Field Tracer Experiments, Version 2.0, Research Report No. 137

Authors
item Toride, N - U.C. RIVERSIDE
item Leij, F - U.C. RIVERSIDE
item Van Genuchten, Martinus

Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: August 1, 1996
Publication Date: N/A

Interpretive Summary: There is great interest to quantify the fate and transport of chemicals in porous media out of concern for contamination of the subsurface environment. The program CXTFIT 2.0 can predict the chemical concentration versus depth and time for several different transport scenarios involving stead water flow. The program also allows the estimation of transport parameters by fitting analytical solutions to experimental observations. The program also includes a stream tube model, which attempts to simulate transport in heterogeneous media by assuming that the field is made up of transport in heterogeneous media by assuming that the field is made up of parallel stream tubes, each with different transport properties. Averages solute concentrations for the field are obtained by averaging the results for all stream tubes. Examples are given on how transport parameters may be determined from laboratory or field tracer experiments for several types of initial and boundary conditions, as well as different zero-order production profiles. A detailed description is provided of the computer program, including the subroutines used to evaluate the analytical solutions for optimizing model parameters. Input and output files for all major problems are included in this manual.

Technical Abstract: Successful predictions of the rate and transport of solutes in the subsurface hinges on the availability of accurate transport parameters. WE modified and updated the original CXTFIT code of Parker and van Genuchten of 1984 for estimating solute transport parameters using a nonlinear least-squares parameter optimization method. The program may be used to solve the inverse problem by fitting mathematical solutions of theoretical transport models, based upon the convection- dispersion equation (CDE), to experimental results. This approach allows parameters in the transport models to be quantified. The program may also be used to solve the direction or forward problem to determine the concentration as a function of time and/or position. Three different one-dimensional transport models are included: (I) the conventional CDE; (ii) the chemical and physical nonequilibrium CDE; and (iii) a stochastic stream tube model based upon the local-scale CDE with equilibrium or nonequilibrium adsorption. The two independent stochastic parameters in the stream-tube model are the pore-water velocity, v, and either the dispersion coefficient, D, the distribution coefficient, Kd, or the nonequilibrium rate parameter, a. These pairs of stochastic parameters were described with a bivariate lognormal probability density function (pdf).

Last Modified: 10/21/2014
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