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n-Lot Infiltration runoff that could be significantly contaminated with sediment and other pollutants, such as runoff from high - potential pollutant loading areas like garages and gas stations. In general, on -lot infiltration systems can be implemented for soil types of loam and coarser. Some authorities discourage infiltration systems at sites where soils have 30 percent or greater clay content, or 40 percent or greater silt content. A soils analysis is helpful in assessing the viability of infiltration systems. More detailed information on procedures for testing infiltration rates can be found in the Infiltration Basins and Infiltration Trenches BMP Sections. If native soils are considered to have a low infiltration capacity, filtration systems should be considered as an alternative to infiltration (see the Filtration Systems BMP Sections). Advantages • Can reduce the volume of runoff from a site, thereby reducing the size and cost of downstream stormwater control facilities. • Can be utilized in retrofit areas where space is limited and where additional runoff control is necessary. • Rainwater gardens can provide an aesthetically pleasing amenity when designed to support perennial flowers in the summer and display vividly colored or patterned shrubs in the winter. • The potential for clogging of rainwater gardens is reduced compared to end -of -pipe infiltration techniques (infiltration basins and trenches) because these systems generally accept runoff only from roofs (roof drain- age contains fewer suspended solids than road runoff) or driveways, lawns and sidewalks. • Can be used at sites where storm sewers are not available. • Can provide groundwater recharge. • Flowering plants and ornamental grasses incorporated into the design of rainwater gardens are attractive to birds and butterflies. Limitations • Only applicable in small drainage areas of a half -acre or less. • Water ponded on lots may take 24 to 48 hours to drain, which may restrict some of the use of the land. • Some maintenance (unclogging soakaway pits, periodically removing sediment from rain barrels and rainwa- ter gardens) is required to ensure the proper functioning of these systems. However, sediment accumulation is an indication that the infiltration techniques are working. This sediment would otherwise have washed downstream to a larger water body. • Not recommended for lots with high sediment loadings or contaminated runoff. • If the infiltration rate of the native soils is low, these systems may not function as desired. • The bottom of these structures (with the exception of rain barrels) should be a minimum of 3 feet above the seasonally high groundwater table to prevent the possibility of groundwater contamination. 3-142 Minnesota Urban Small Sites BMP Manual