Water Table Mounding Beneath Stormwater Infiltration Basins

Abstract

The objectives of this study were to monitor water table levels in response to infiltrating stormwater from an infiltration basin, to calibrate and test a numerical water table flow model using data obtained, and to evaluate the primary factors that affect water table mounding. Infiltration basins are often utilized in urban settings to recharge stormwater to the water table. Localized recharge by these relatively small basins can cause a water table mound to form below the basin. Mound formation may reduce the thickness of the soil available to filter pollutants and may reduce the infiltration rate of the basin. Therefore, an accurate understanding of water table mound formation is important in the proper design of infiltration basins. A 0.10-hectare infiltration basin serving a 9.4-hectare residential subdivision in Oconomowoc, WI, was instrumented for this study. Three storm events were modeled using the two-dimensional variably saturated numerical model HYDRUS-2D. A good fit was achieved between modeled and observed data for the timing and magnitude of water table rise for all three storms. Mound height was most sensitive to the thickness of the basin sedimentation layer and the saturated hydraulic conductivity. Ideal basin siting conditions to minimize mounding were found to occur in a material with high saturated hydraulic conductivity and with thick unsaturated and saturated layers.