The Impact of Stormwater Infiltration Practices on Groundwater Quality

Establishing enhanced infiltration sites for the control of stormwater runoff in developed areas is becoming a common practice among municipalities, departments of transportation, other government entities, and private entities. One of the purposes of enhanced filtration is to control the volume and rate of runoff from developed areas in order to keep the level of runoff close to that of undeveloped conditions. Another purpose is to capture the stormwater so that contaminants contained in the stormwater will be treated through the process of water infiltrating and flowing through the soil profile. With the increasing adoption of enhanced infiltration practices there is a growing concern that these practices might be putting the quality of the groundwater resources underlying some of these practices in jeopardy. In recent years there have been some efforts to quantify the potential for degradation of groundwater resources by infiltrated stormwater. The study described in this report is an effort to quantify the potential for contaminants to reach the groundwater resources in the Twin Cities Metropolitan Area, Minnesota. A literature review was first conducted to identify concerns with regard to groundwater contamination and the extent of the research that has been completed with regard to these issues. The field study was then conducted over a period of 18 months in which three sites containing enhanced infiltration practices were monitored to quantify the amount of contaminants reaching depths large enough to infer that the contaminants would eventually reach the underlying surficial aquifer. The sites investigated included an infiltration basin near the St. Paul campus of the University of Minnesota (referred to as the Sheep Pasture, SP), a large infiltrating rain garden located within Como Park (referred to as the Hamline-Midway Ave. site, HM), and an infiltration gallery constructed in a formerly industrial area located at Beacon Bluff (referred to as BB) in the eastern side of St. Paul. These sites were selected from a number of potential sites based on the criteria: 1) should have been functioning long enough to contain contaminants; (2) be local for convenience of access; and (3) represent one of the infiltration practices that are of primary concern with regard to groundwater pollution.