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I 6.12 Chapter 6: Infiltration Basins J I Depth to'Seasonally High Water Table A minimum of two to four feet of clearance is needed between the floor of the basin and the seasonally high "'ater table. This depth can be readily determined from soil borings taken during wet weather. High water tables often present a majo:t: obstacle to the Use of infiltration basins, since basins are usually located in depressions at the low end of a watershed where local water tables are located near the the ground surface. I 1 Proximity to Wells and Foundations Basins should be located at least 100 feet away from drinking water wells to minimize the possibility of groundwater contamination) and should be situated at least 10 feet down-gradient and 100 feet up-gradient from building foundations to avoid potential seepage problems. I Maximum Depth of Reservoir I To insure that the basin completely drains within 72 hourst it may be necessary to limit the depth of the basin if underlying soils have relatively low exfiltration rates. Recommended depth limits for basins are shown for various soil textures in Table 6.2. I Watershed Size I Md WRA (1983b) suggests that basins can be applied to sites ranging from 5 . to 50 acres in size. Other BMPs J such as extended detention ponds and wet ponds J are better candidates on larger sites as they are more capable of handling sustained baseflow. .. Table 6,2: Soil Limitations For Infiltration Basins I SCS SOIL' GROUP MAXIMUM DEPTH OF' STORAGE (inches) 48 hrs 72 hrs I MINIMUM INFIL- SOIL TRATION RATE TYPE (fc--inches/hr) Sand 8,27 Loamy Sand 2,41 Sandy Loam 1.02 Loam 0,52 Silt Loam 0.27 I A 397 595 A 116 174 1 B 49 73 I B 25 37 C 13 19 I Sandy Clay Loams, Clay Loams, Silty Clay Loams, Sandy Clay, Silty Clay,an~ Clay Soils are not included as these soil types are all NOT FEASIBLE for infiltration basins, 1 Maximum Depth in the Basin that can drain completely within 48 or 72 hours after a storm, given the soil infiltra~ion rate. I