Groundwater Mounding and Contaminant Transport Beneath Stormwater Infiltration Basins

Background / Need

Increased impervious areas resulting from urbanization cause an increase in stormwater runoff and a decrease in infiltration to the groundwater table. Infiltration basins are often required to recharge a portion of the pre-development infiltration volume. The localized recharge by these relatively small basins can cause a groundwater mound to form below the basin. Mound formation is important as it may reduce the ability of the soil to filter pollutants, and may reduce the infiltration rate of the basin. Therefore, an accurate understanding of groundwater mound formation is important in the proper design of infiltration basins. Analytical solutions to estimate maximum groundwater mounding have been shown to suffer from many limiting assumptions. Predictions for mound height have generally been much higher with analytical methods than with numerical methods. As over estimation of mound height can have basin siting implications, rendering an accurate estimation of mound formation important.

Objectives

The goal of this study was to increase our understanding of the causes of groundwater mounding beneath stormwater infiltration basins. By understanding the relative importance of factors affecting groundwater mounding, the potential mound formation at future sites can be evaluated with greater confidence. The main objectives of the project were: 1) To monitor groundwater levels and changes in soil moisture in the unsaturated zone in response to infiltrating stormwater from an infiltration basin, 2) To calibrate and validate a groundwater flow and contaminant transport model using data obtained under objective one, and 3) To use the model to extrapolate field data to other hydrogeologic settings.

Methods

A 0.10 hectare infiltration basin serving a 9.4 hectare residential subdivision in Oconomowoc, Wisconsin was used in this study. Subsurface conditions included sand and gravel material and a groundwater table at 2.3 meters below grade. Three storm events between August 2006 and April 2007 were modeled using the two-dimensional numerical model HYDRUS. Inverse modeling was performed with HYDRUS to estimate soil and aquifer parameters. The model was calibrated to heads recorded at the basin center. Model validation was accomplished by interchanging fitted parameters between storms. The model was then applied to various basin designs and subsurface conditions to determine their effect on mound height.