Stormwater infiltration is defined as the process by which water enters the soil and recharges streams, lakes, rivers, and underground aquifers. Stormwater infiltration is a fundamental component of the water cycle and is quickly becoming the centerpiece of stormwater management strategies across the United States. Stormwater infiltration is an effective means of managing runoff because it allows practitioners to address both water quality and water quantity concerns.
The amount of rainfall that infiltrates into the ground is largely dependent on the soil type, land use, level of soil saturation, and a number of other variables. Sandy soils tend to have higher infiltration rates than finer silt and clay soils because there are larger voids between soil particles allowing water to pass through. Saturated soils typically have lower infiltration rates than relatively dry soils.
Stormwater is commonly infiltrated by directing runoff to a variety of engineered best management practices (BMPs) designed to optimize infiltration rates and water quality while minimizing ongoing maintenance needs. Common infiltration BMPs include trenches, basins, bioretention cells, rain gardens, and a growing list of underground BMPs such as perforated pipe, bottomless chambers, and vaults.
Infiltration standards vary somewhat from one agency to the next, but a number of common variables exist. Below are a few points of interest regarding stormwater infiltration:
1) Many agencies establish an allowable range of infiltration rates.
2) If water infiltrates too slowly, the facility may never fully drain, but if water infiltrates too quickly, it may not be properly treated and create a contamination risk.
3) Infiltration rates are assessed by conducting tests at the proposed BMP location.
4) Most agencies discourage infiltrating when rates fall outside a specified range (0.5 to 5.0 inches per hour is common).
5) Agencies generally specify a minimum separation between the infiltration facility and the groundwater and/or bedrock. Most agencies call for 2 to 3 feet of separation to allow adequate time for pollutants to be filtered by the soil.