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Infiltration Basins The design storm for an infiltration basin is typically a frequent, small storm such as the 1-year event. This provides treatment for the "first flush" of stormwater runoff. Infiltration basins provide total peak discharge, runoff volume and water quality control for all storm events equal to or less than the design storm. This infiltra- tion reduces the volume of' runoff, removes many pollutants and provides stream baseflow and groundwater recharge. Infiltration basins have limited capabilities for controlling peak discharge for storms greater than the design storm. Because infiltration basins will not significantly affect peak discharges of runoff, they are best used in conjunction with other BMPs; downstream detention is often still needed to meet peak - runoff -rate requirements. Dissolved pollutants are effectively controlled for storm events Tess than the design storm, but these substances may not be removed from the runoff water as it infiltrates, and some of them could move to the groundwater. For this reason, the impact of infiltrated runoff on the groundwater should be considered, although in most cases, the magnitude of this impact is unknown. Chloride from road salt is an example of a soluble material that will not be removed during the infiltration process. Currently, there is much disagreement as to whether chlorides do indeed pose a significant threat to groundwater. A general guideline for groundwater protection is to design infiltration basins with the bottom of the basin a minimum of 3 feet above the seasonally high groundwater table. This is consistent with the MPCA's guidelines for septic systems (MPCA, 2000). If the water table is too close to the ground surface, infiltration practices should not be used. Figure 1 provides a schematic of a typical infiltration basin. Figure 2 shows an infiltration basin with pretreat- ment in the form of a settling pond. Advantages • Reduces the volume of runoff from a drainage area • Can be very effective for removing fine sediment, trace metals, nutrients, bacteria, and oxygen- demanding substances (organics) • Reduces downstream flooding and protects streambank integrity • Reduces the size and cost of downstream stormwater control facilities and/or storm drain systems by infiltrat- ing stormwater in upland areas • Provides groundwater recharge and baseflow in nearby streams • Reduces local flooding • Appropriate for small sites (2 acres or Tess) Limitations • Potentially high failure rates due to improper siting, design and lack of maintenance, especially if pretreat- ment is not incorporated into the design • Depending on soil conditions and groundwater depth, a risk of groundwater contamination may exist • Not appropriate for treating significant loads of sediment and other pollutants due to the potential for clogging 3-156 Minnesota Urban Small Sites BMP Manual