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There is a large portion of the state (more than 50%) where the ground water is located less than three feet from the surface. In these areas it may be impossible to get the three feet of separation from the bottom of an infiltration practice and the seasonally saturated ground water table required under the NPDES Construction General Permit. Other treatment methods need to be considered in these areas.
When constructing a pond that will likely intercept the ground water table, a close examination of the land uses that will contribute runoff to the pond should be the first step in the design process. If a potential stormwater hotspot is identified as a contributor then it is the recommendation of the MPCA that the pond include a liner to protect against ground water contamination.
MPCA is often asked why it would allow a sedimentation pond (no liner) to be constructed that may intercept the water table, but require a minimum of three feet of separation from the bottom of any constructed infiltration practice and the water table. The treatment processes for these two practices are very different and may help to explain the requirements. A sedimentation pond achieves treatment of stormwater runoff through the act of settling out suspended solids before the discharge point. If the basin is large enough and has a long detention time, additional treatment through biological uptake and microbial action can also occur. An infiltration practice removes pollutants through filtering that occurs in the three foot soil layer beneath the practice along with the biologic and microbial activity that takes place in the layer under aerobic conditions. The soils under the practice need time between events to aerate so they function hydraulically as well as provide aerobic treatment.