Modeling Water Table Mounding and Contaminant Transport beneath Storm-Water Infiltration Basins

Abstract

The objectives of this study were to link an unsaturated and saturated flow model for the purpose of evaluating mounding and contaminant transport beneath an infiltration basin, to calibrate and test the combined water table flow model using experimental data collected from an infiltration basin, and to evaluate the potential for contaminant transport with a numerical fate and transport model. Mound formation may reduce the thickness of the soil available to retard pollutant movement, reduce the infiltration rate of the basin if the mound intersects the basin bottom, and facilitate contaminant movement away from the basin. A 0.10-ha infiltration basin serving a 9.4-ha residential subdivision in Oconomowoc, Wisconsin, was instrumented. Two storm events were modeled using the three-dimensional saturated numerical model MODFLOW. Recharge used in MODFLOW was taken from the seepage flux of the unsaturated one-dimensional model HYDRUS. A good fit was achieved between modeled and measured timing and magnitude of water table rise for both storms. The three-dimensional saturated fate and transport code MT3D was used to simulate a tracer study. Mounding caused more rapid tracer transport away from the basin compared to the natural gradient.