In an effort to keep surface waters clean, a wide variety of stormwater practices have been developed and installed throughout the metro in recent years. Many of these, such as rain gardens and infiltration basins and trenches, are intended to reduce the total runoff volume by infiltrating stormwater. There are concerns that contaminants commonly found in stormwater are infiltrating into groundwater supplies. Working with the University of Minnesota – Saint Anthony Falls Laboratory, this project investigated the impact of stormwater infiltration practices on groundwater quality at three sites in the metro area. The project included a literature survey, laboratory studies, and field measurements. Chloride, nitrate, phosphorus and numerous metals were measured in the subsurface below the three stormwater infiltration practices.
This work indicates that the impacts of nitrate and most of the metals analyzed from stormwater infiltration, at these three sites, are low. The impact of chloride was significant, with higher than expected concentrations measured in the summer and autumn. Lead was measured in the soil water beneath the infiltration devices, as were total petroleum hydrocarbons (the latter at one site). Results for these analytes were inconclusive.
Local information about stormwater infiltration impacts on shallow groundwater is available to inform recommendations for stormwater infiltration BMPs.