Contamination of Soil and Groundwater Due to Stormwater Infiltration Practices A Literature Review

The University of Minnesota is committed to the policy that all persons shall have equal Acknowledgements The Minnesota Pollution Control Agency (MPCA) provided funding for this review. The involvement of the project manager from the MPCA, Bruce Wilson, was greatly appreciated. Executive Summary Recently, there has been an increased interest in the use of infiltration as a method of managing stormwater. Infiltration practices promote groundwater recharge, reduce runoff peak flows and volumes, and can lessen the transport of non-point source pollutants to surface water bodies. However, because stormwater infiltration systems are designed to discharge runoff into the soil, there has been concern that pollutants present in stormwater could contaminate groundwater wells. Thus, to understand the relative risks and benefits of infiltration, the fate of stormwater pollutants must be well understood. The fate of contaminants infiltrated from stormwater runoff and the potential for groundwater contamination was investigated by reviewing literature published in peer-reviewed scientific and engineering journals. This review examines common stormwater infiltration techniques, priority pollutants in urban stormwater runoff, and investigates the fate of these pollutants after infiltration. The potential for groundwater contamination is a complex function of soil and contaminant properties and the depth to the water table. Karst geology in particular can provide pathways for rapid and extensive groundwater contamination from infiltration systems. Heavy metals are often present at very low levels in urban stormwater. Fortunately, studies have demonstrated that metals are generally retained in the upper soil layers via adsorption to solid particles. However, eventual breakthrough can occur due to the finite sorption capacities of the soil media. Periodic replacement of the upper soil layer within infiltration systems has been suggested as a method of preventing possible groundwater contamination and maintaining low soil concentrations. Suspended solids are usually removed via straining by the soil. Because they pose little health risk, suspended solids are mainly a concern because they may clog the infiltration system. Phosphorus and nitrogen can also be removed within the soil media; phosphorus by precipitation or adsorption reactions and nitrogen by bacterial denitrification. Phosphorous is a concern because excess quantities cause eutrophication of surface water bodies. Studies have shown varied results regarding phosphorus removal via infiltration. Nitrates present in drinking water supplies can pose a health concern to certain target groups (fetuses, infants). Most studies indicated that nitrate is poorly retained in infiltration devices due to high solubility. However, the low levels typically found in …