Stormwater infiltration is the process by which rainfall and stormwater runoff flows into and through the subsurface soil. Stormwater infiltration occurs when rainfall lands on pervious surfaces, when runoff flows across pervious surfaces, and when runoff is collected and directed to a stormwater infiltration Best Management Practice (BMP).
Current stormwater management policies encourage, when appropriate, maximizing the infiltration of stormwater to reduce the volume of runoff discharging to surface waters. In addition to reducing runoff volume, stormwater infiltration helps reduce stormwater pollutant loading to surface waters. Many factors influence the rate and volume of stormwater infiltration, including soil and rainfall characteristics. Once stormwater infiltrates into the soil, it has the potential to enter the groundwater, become part of the subsurface flow, or be taken up by vegetation.
For the purposes of this section, infiltration BMPs are considered BMPs without underdrains.
Stormwater runoff is considered to be any water that runs off pervious and impervious surfaces after a rainfall or snowmelt event, with a greater percentage running off from impervious surfaces. In urban areas, runoff can constitute approximately 30 to 55 percent of the water budget. In comparison, runoff may constitute as little as 10 percent in forested or rural areas (FISRWG, 1998).
The increase in impervious surfaces can disrupt the natural water cycle and alter the surrounding environment via the decrease of groundwater recharge and the increase of water directly flowing to surface waters. As a likely consequence, this can lead to a reduction in the baseflow of streams, a decrease in the elevation of the groundwater table, and transport of sediment and pollutant loads into surface waters. A more detailed explanation of stormwater runoff and its effects on the environment can be found here.
Stormwater infiltration in urban areas can be enhanced by the disconnection of impervious surfaces and by the improvement of pervious surfaces such as turf. Impervious surface disconnection is the direction or redirection of stormwater runoff from impervious surfaces (e.g., sidewalks, parking lots, rooftops, etc.) onto pervious surfaces. An example is routing discharge from a roof drain onto a lawn. Redirection of impervious surface runoff to pervious areas promotes infiltration and reduces overall site runoff. The reduction in site runoff from impervious surface disconnection can vary considerably depending on many factors including the size of the contributing drainage area, size of the area receiving routed runoff, and infiltration capacity of the soils, vegetation in the area receiving the additional stormwater, slope and site grading and other site conditions. Stormwater infiltration can also be enhanced by improvement of pervious surfaces, such as amending soil with compost to increase the water holding capacity of the soil. The section on turf in this manual provides a discussion of impervious surface disconnection and improvement of turf.
In new developments, stormwater infiltration is often intended to mimic the natural hydrologic cycle. Stormwater infiltration in redevelopment aims to mitigate changes in the urban water cycle brought about by increases in impervious surfaces caused by the urbanization. Infiltration can achieve the following objectives.
Typically, stormwater from disconnected impervious surfaces is routed to a stormwater BMP. Infiltration BMPs achieve the above objectives by capturing, retaining, and infiltrating stormwater. Infiltration BMPs include
Small and medium sized infiltration BMPs are often located at the beginning of a stormwater treatment train, while larger BMPs are often placed at the end of the train. These BMPs can also be installed as off-line treatment systems. Infiltration BMPs are most suitable in sites with permeable soils and sufficient separation from the seasonally high groundwater table, bedrock, and polluted sites. While infiltration practices can remove some types of physical and chemical pollutants, careful consideration should be given to implementing stormwater infiltration practices in potential stormwater hotspots (PSH) or at sites with known pollution issues such as Brownfields. For a discussion of constraints on use of stormwater infiltration practices, see Constraints on the stormwater infiltration page.
Several interrelated factors that control the infiltration of stormwater into the soil/subsurface are described below. It is important to clarify the terms soil, subsurface, and engineered media. In general, the soil or subsurface is the native material at a site. These terms may be used interchangeably when discussing a BMP, although subsoil typically refers to soil at depth. Engineered media is a material created for a site to enhance infiltration, plant growth, and pollutant removal. Engineered media range from coarse-textured aggregate beneath permeable pavement systems to high organic materials used in bioretention systems. When an engineered media is used, the soil or subsurface refers to the material underneath the engineered media.
The following general limitations should be recognized when considering infiltration BMPs.
For infiltration prohibitions under the Construction Stormwater General permit, see When infiltration is not authorized under a stormwater permit.
As stormwater travels across the land surface into infiltration BMPs, it can pick up various water quality pollutants and deliver them to the subsurface. There is concern that these pollutants have the ability to travel through the vadose zone and ultimately impact the groundwater. Whether or not this will occur depends on the type and amount of pollutant present, the volume of infiltration, the type of infiltration BMP, and subsurface conditions.
Detailed information on this topic can be found on the page addressing pollutant fate and transport in stormwater infiltration.