A pilot study to assess the influence of infiltrated stormwater on groundwater: Hydrology and trace organic contaminants

Underground infiltration basins (UIBs) mimic the natural hydrologic cycle by allowing stormwater to recharge local groundwater aquifers. However, little is known about the potential transport of organic contaminants to receiving groundwater. We conducted a pilot study in which we collected paired grab samples of stormwater runoff flowing into two UIBs (inflow) and shallow groundwater adjacent to the UIBs. Samples were collected coincident with three rain events and analyzed for volatile organic compounds, semi‐volatile organic compounds, pharmaceuticals, and pesticides. Few contaminants were detected in groundwater, compared with inflow, and groundwater concentrations were typically an order of magnitude less. With one exception (trichloroethene), all groundwater concentrations were at least two orders of magnitude below available guidance or screening values. This short communication highlights information gaps in understanding the hydrologic connectivity between UIBs and receiving groundwater and potential consequent contaminant transport to the subsurface from varying climatic conditions. Practitioner Points Urban stormwater contains organic contaminants including pharmaceuticals, pesticides, and semi‐volatile organic compounds that may be transported to groundwater via infiltration. In general, fewer contaminants were detected in groundwater and at lower concentrations, compared with urban stormwater runoff. Trace organic contaminant concentrations in groundwater were much lower than drinking water guidance/screening values. Stormwater runoff is a source of trace organic contaminants, but the implications of contaminant loading to receiving groundwaters from infiltration practices are rarely studied. Results from our pilot study indicate that although relatively few contaminants were detected in groundwater near underground infiltration basins (UIBs), many questions are still unanswered, and more detailed studies would be beneficial to understand the implications of stormwater infiltration.