10/30/2002 Env Bd Packet - Laserfiche WebLink

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{ in ativaanasinss ir ti2Epfcgagisecrits • Treatment of a small drainage area (lower sediment loadings) • Careful consideration of depth of ponding and inundation times that reflect plant tolerances • Good construction techniques that prevent smearing, over - compaction, and operation of the basin during the construction period • Performance of regular maintenance All of these topics are discussed in further detail below. Site Sensitivity Analysis Before an infiltration system can be designed, a site sensitivity analysis must be performed. This evaluation may eliminate an infiltration practice from consideration because of soil characteristics or potential effects on ground- water. Because of varying geologic settings, a site evaluation needs to be tailored to the specific site conditions. A team approach to this evaluation is recommended where various disciplines such as engineering, hydrogeology and soil science are represented. The applicability of infiltration basins on a site depends on numerous site factors, including soils, slope, depth to water table, depth to bedrock or impermeable layer, contributing watershed area, land use, proximity to wells, surface waters, foundations, and others. Generally, infiltration basins are suitable to sites with gentle slopes, permeable soils, relatively deep bedrock and groundwater levels, and a small contributing watershed area (less than 2 acres, ideally). When performing a site evaluation, the following items should be considered: • Runoff water quality: If runoff water will contain a significant concentration of soluble pollutants that could contaminate groundwater, an infiltration basin should not be used. Specifically, infiltration basins are not recommended for industrial and commercial land uses since there is a high potential for groundwater con- tamination from chemical spills and maintenance (salting and sanding) activities. In site - specific cases where infiltration basins are deemed acceptable for these land uses, the design must be located off -line and incorpo- rate some form of upstream treatment (e.g., an upstream oil -grit separator or sand filter). • Degree of detail: The level of detail required for the study should be considered. For instance, a small struc- ture receiving runoff from a rooftop will not require as much detail as a structure serving a larger area and having a higher potential pollutant load. • Geologic (groundwater) sensitivity: A site with a highly sensitive geology, such as one with a carbonate or surficial sand aquifer, may eliminate this practice from consideration. • Depth to water table and bedrock: The seasonally high water table must be far enough below the bottom of the infiltration basin (at least 3 feet) to allow the structure to function hydraulically and to allow trapping and treatment of pollutants by the soil. Similarly, the bottom of the infiltration basin should be at least 3 feet from bedrock, although in the case of fractured bedrock, separations up to 10 feet may be required. This minimum separation distance is required to trap or treat pollutants before they reach the groundwater or bedrock and to maintain vegetation in the basin (MPCA, 2000). Metropolitan Council / Barr Engineering Co. 3 -159