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• • In atn�i�traitgems asIONHARierts • Proximity to drinking water wells and building foundations: Basins should be located at least 150 feet away from drinking water wells to limit the possibility of groundwater contamination, and should be situated at least 10 feet downgradient and 100 feet upgradient from building foundations to avoid potential seepage problems. • Soil infiltration rate: The infiltration rate of the soil must be great enough to drain the structure in a reason- able amount of time, generally 72 hours or less. Sites with clayey soils are not appropriate for infiltration basins. Infiltration rates are discussed in further detail below. If the infiltration rate of the site's soils are not acceptable, the filtration family of BMP systems should be considered. • Size of the tributary drainage area: Although infiltration basins were originally designed to accommodate larger drainage areas, research which has been undertaken to date indicates that large -scale infiltration is not feasible. One of the main problems with centralized infiltration basins is that water from a large area is expected to infiltrate into a relatively small area. This does not reflect the natural hydrologic cycle and generally leads to problems (groundwater mounding, clogging, compaction). For these reasons, the contribut- ing drainage area to any individual infiltration basin should be restricted to 2 acres or less. General Design Considerations Design Volume Infiltration basin systems infiltrate a portion of the runoff from a rain event (usually the first flush or up to the first inch) while the remaining runoff bypasses the infiltration basin. The design infiltration volume can be calculated in many ways. Ultimately, the magnitude of the design infiltration volume depends on local authori- ties' practices and requirements. Off -Line Placement The purpose of the basin is to temporarily store surface runoff for a specific design frequency storm and allow it to infiltrate through the bottom and sides of the basin. A flow splitter or weir is usually used to divert runoff into an off -line infiltration basin. Infiltration basins provide total peak discharge, runoff volume and water quality control for all storm events equal to or less than the design storm. Storm events greater than the design storm simply continue down the larger conveyance system, bypassing the infiltration basin. Pretreatment Infiltration basins are susceptible to high failure rates due to clogging from sediments, and therefore require pretreatment of stormwater in order to remove as much of the suspended solids from the runoff as possible before it enters the basin. Pretreatment, such as grit chambers, swales with check dams, filter strips, or sediment fore - bays/traps should be a fundamental component of any BMP system relying on infiltration. Even when infiltrating rooftop runoff, it is a practical decision to implement some form of pretreatment to remove sediments, leaf litter, and debris. This pretreatment will help to ensure the proper functioning of the infiltrating facility and allow for longer periods between maintenance. When designed properly, pretreatment devices may remove some 25 -30% of sediment loads. Figure 2 shows and infiltration basin with pretreatment in the form of a settling pond. 3 -160 Minnesota Urban Small Sites BMP Manual