Numerical Model Assessment of the Effects of Drywell Facilitated Stormwater Infiltration on Groundwater Quality

Abstract:

Drywells are gravity-fed, excavated pits with perforated casings used to facilitate stormwater infiltration and groundwater recharge in areas comprised of low permeability soils or cover. Stormwater runoff that would otherwise be routed to streams or drains in urban areas is used as a source of aquifer recharge, potentially mitigating the effects of drought. However, the potential for groundwater contamination caused by urban runoff bypassing surface soil and near surface sediment attenuation processes has prevented more widespread use of drywells as a recharge mechanism. A study is currently underway in Elk Grove, CA, where two drywells have been constructed: one in a preexisting drainage basin fed by residential lots, and one at an industrial site. Both sites are outfitted with one upgradient and two downgradient groundwater monitoring wells, and one vadose zone monitoring well. To assess long-term effects of stormwater infiltration on the surrounding hydrogeologic system, fate and transport models of contaminants in the vadose zone are coupled with groundwater flow models to predict the concentrations of contaminants at the water table and simulate the transport of contaminant plumes to local municipal wells. Results of water quality sampling have identified the local contaminants of interest and the concentrations of these contaminants in the influent stormwater, and the fate and transport of these contaminants have been simulated using a 2D axisymmetrical model with site specific parameters. The output concentrations from vadose zone modeling are used as inputs to simulate contaminant plume migration in groundwater after two, ten, and fifty years. The results of the numerical modeling assessments indicate that the study’s drywells do not pose a long-term threat to groundwater quality and may be an effective source of aquifer recharge and drought mitigation.