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Simulating solute transport across horizontal-flow barriers using the MODFLOW ground-water transport process

This report describes modifications to a U.S. Geological Survey (USGS) threedimensional solute-transport model (MODFLOW-GWT), which is incorporated into the USGS MODFLOW ground-water model as the Ground-Water Transport (GWT) Process. The modifications improve the capability of MODFLOW-GWTto accurately simulate solute transport in simulations that use the HorizontalFlow Barrier (HFB) Package in MODFLOW. The updated code (called the CHFB Package) (1) allows the user to input dispersivity and diffusion properties of HFB barriers, and (2) modifies the calculation of dispersive solute flux near barriers. The Horizontal-Flow Barrier Package of the MODFLOW model is a valuable tool for simulating ground-water flow through aquifers containing either natural or artificial lowpermeability vertical features. A modification to the Ground-Water Transport (GWT) Process that is compatible with the HFB package has been developed and tested. When HFB is active, the consideration of advective solute flux is straightforward. The consideration of dispersive solute flux is more problematic because the standard evaluation of the crossproduct terms of the dispersion tensor in the finite-difference grid may require the estimation of concentration gradients using nodal values on both sides of the barrier. This standard evaluation may be a poor estimator of concentration gradients at a point on one side of the barrier, however, if the concentrations (and concentration gradients) differ greatly on the other side of the barrier a condition that would be expected for a barrier constructed to contain contaminated ground water. Therefore, this condition could lead to inaccurate estimates of solute flux attributable to the cross-product terms of the dispersion tensor. Thus, for grid locations near a horizontal-flow barrier, the standard finite-difference approximation was modified so that specific components of the concentration gradients for the cross-product terms are calculated over a distance of one cell rather than over two cells. In addition, the CHFB Package allows the dispersive properties of the barrier to have different values than those of the aquifer. Evaluation of numerical experiments indicates that the CHFB Package yields improved results.