Modeling solute transport in a water repellent soil

Abstract

A bromide tracer was applied on a 2.2m long and 0.4m wide plot at the location of a hydrophobic soil in the southwest of The Netherlands. At the end of the experiment, the plot was excavated to a depth of 0.7m using 100cm3 samples, yielding a total of 1680 samples to quantify the three-dimensional spatial distribution of water content, pH, bromide concentration and degree of water repellency. Measured water content and solute distributions indicated that unstable (fingered) flow prevails. It is considered that contaminant transport under such conditions can proceed at rates that are higher than that which would normally occur if the flow were stable. This article illustrates an attempt at modeling contaminant transport under unstable flow conditions using measurements obtained from the experimental plot. A finite element solution of the two-dimensional Richards equation forms the basis for the unstable flow simulation, while a particle tracking random walk solution of the two-dimensional convection–dispersion equation forms the basis of the transport simulation. The water flow simulation and the solute transport simulation were compared with the measured data. Initial results indicate that model predictions compared fairly well with measured water content and solute transport data.