INFILTRATION AND FILTRATION-BASED STORMWATER CONTROL MEASURES ARE TOP PERFORMERS IF APPROPRIATELY SITED AND A THREAT TO GROUNDWATER WHEN NOT

With the increasing usage of stormwater infiltration practices it is clear there are both immense potential benefits and risks to groundwater that must be carefully considered. An evaluation of 15 different types of stormwater treatment strategies at the UNH Stormwater Center clearly illustrated that treatment strategies involving filtration or infiltration consistently had exceptional removal efficiencies. Infiltration/filtration systems tested included 5 Low Impact Development designs and 2 manufactured systems, each a treatment train. However in order to assure quality performance of these systems, considerable attention must be paid to design and installation of infiltration/filtration practices. An inappropriate installation and its deficiencies are presented here. Common challenges to the use of the larger structural LID systems include the large space requirements. However, manufactured subsurface infiltration chambers, while quite large, are frequently installed below parking lots and thus not large space consumers. The increasingly widespread use of the subsurface infiltration systems at big-box stores, where appropriate subsurface materials and hydrology are present, bodes well for water quality, but where inappropriately sited are a risk to groundwater. Design recommendations for subsurface infiltration systems set groundwater separation at 3-4 feet from estimated seasonal high water tables because this distance is needed to adequately achieve contaminant removal. Distances are typically greater for contaminant hotspots and sole source aquifers. If this distance is not met, or filled with crushed stone (typical of bedding material), limited contaminant removal is observed . The efficacy of infiltration/filtration practices rely upon appropriate design, filter materials and site selection.