The paper discusses the use of the “pathline counting” approach to estimate solute concentrations at an extraction well using particle-tracking flow models. The pathline counting approach is a unique method for predicting solute concentrations due to mixing. The pathline counting method using the semianalytical module RESSQC, contained within the WHPA model, was compared to MT3D, a numerical mass transport code, for a simple hypothetical problem. Results from the comparison indicate that results from the pathline counting approach were in reasonable agreement with the MT3D results. This approach was then applied to a field study, the Park City Bioremediation Project. The migration of a conservative tracer injected into three upgradient cells was simulated to estimate the concentration of solute at two pumping wells. The calculated concentrations compared favorably with measured field concentrations.