Stormwater Management Fact Sheet: Infiltration Basin

Description

An infiltration basin is a shallow impoundment that is designed to infiltrate stormwater into the soil. Infiltration basins are believed to have a high pollutant removal efficiency, and can also help recharge the groundwater, thus restoring low flows to stream systems. Infiltration basins can be problematic at many sites because of stringent soils requirements. In addition, some studies have relatively high failure rates compared with other stormwater treatment practices.

Applicability

Infiltration basins need to be applied very carefully, as their use is often sharply restricted by concerns over groundwater contamination, site feasibility, soils, and clogging at the site.

Regional Applicability
Infiltration basins can be utilized in most regions of the country, with some design modifications in cold and arid climates. In regions of karst topography, these infiltration basins should not be applied due to concerns of sink hole formation and groundwater contamination.

Ultra Urban Areas
Ultra urban areas are densely developed urban areas in which little pervious surface exists. In these areas, few stormwater practices can be easily applied due to space limitations. Infiltration basins can rarely be applied in the ultra urban environment. Two features that can restrict their use are the potential of infiltrated water to interfere with existing infrastructure, and the relatively poor infiltration capacity of most urban soils. In addition, while they consume about the amount of space as infiltration trenches, they need a continuous, relatively flat area. Thus, it is more difficult to fit them into small unusable areas on a site.

Stormwater Hotspots
A stormwater hotspot is an area where land use or activities generate highly contaminated runoff, with concentrations of pollutants in excess of those typically found in stormwater. Infiltration basins should never receive runoff from stormwater hotspots, unless the stormwater has already been fully treated by another stormwater treatment practice. This is due to potential groundwater contamination (see Risk of Groundwater Contamination from Infiltration of Stormwater, Article 104 in the Practice of Watershed Protection).

Stormwater Retrofit
A stormwater retrofit is a stormwater practice (usually structural) put into place after development has occurred, to improve water quality, protect downstream channels, reduce flooding, or meet other watershed restoration objectives. Infiltration basins have limited application as a stormwater retrofit. Their use is restricted by three factors. First, infiltration basins are generally used to treat small sites (less than five acres). Small site practices, such as infiltration basins, generally are a high cost retrofit option (in terms of construction cost and the maintenance burden) since a large number of practices are needed to fully retrofit a watershed. Second, it is often difficult to find areas where soils are appropriate for infiltration in an already urban or suburban environment. Finally, infiltration basins are best applied to small sites, yet need a flat, relatively continuous area. It is often difficult to find sites in a watershed that meet these criteria.

Cold Water (Trout) Streams
Infiltration basins are an excellent option for cold water streams because they encourage infiltration of stormwater and maintain dry weather flow. Because stormwater travels underground to the stream, it has little opportunity to increase in temperature.

Siting and Design Considerations

When designing infiltration basins, designers need to carefully consider both the restrictions on site and design features to improve the long-term performance of the practice (see Figure 1).

Siting Considerations
Infiltration practices need to be located extremely carefully. In particular, designers need to ensure that the soils on the site are appropriate for infiltration, and that designs minimize the potential for groundwater contamination, and long term maintenance problems.

Drainage Area

Infiltration basins have been used as regional facilities, providing both water quality and flood control in some communities. This practice may be feasible if the soils are particularly sandy. In most areas of the country, however, infiltration basins experience high rates of failure when used in this manner. In general, the practice is best applied to relatively small drainage areas (i.e., less than ten acres), and exclusively for groundwater recharge and water quality treatment.

Slope

The bottom of infiltration basins needs to be completely flat to allow infiltration throughout the entire basin bottom.

Soils /Topography

Soils and topography are a strongly limiting factor when locating infiltration practices. Soils must be significantly permeable to ensure that the practice can infiltrate quickly enough to reduce the potential for clogging, and soils that infiltrate too rapidly may not provide sufficient treatment, creating the potential for groundwater contamination. The infiltration rate should range between 0.5 and 3 inches per hour. Detailed soil tests are needed to determine if fragipans, hardpans or other confining layers are present. In addition, the soils should have no greater than 20% clay content, and less than 40% silt/clay content (CWP, 1998). Finally, infiltration basins may not be used in regions of karst topography, due to the potential for sink hole formation, or groundwater contamination.

Groundwater

Designers always need to provide significant separation distance (2' to 5') from the bottom of the infiltration basin and the seasonally high ground water table, to reduce the risk of contamination. Infiltration practices should also be separated by at least 150 feet from adjacent drinking water wells.

Design Considerations
Specific designs may vary considerably, depending on site constraints or preferences of the designer or community. There are some features, however, that should be incorporated into most infiltration basin designs. These design features can be divided into five basic categories: pretreatment, treatment, conveyance, maintenance reduction, and landscaping (for more information see Manual Builder Category).

Pretreatment

Pretreatment refers to design features that provide settling of large particles before runoff reaches a management practice, easing the long-term maintenance burden. Pretreatment is exceptionally important for all infiltration practices. In order to ensure that pretreatment mechanisms are effective, designers should incorporate "multiple pretreatment," using practices such as grass swales, sediment basins, and vegetated filter strips in series, prior to the infiltration basin.

Treatment

Treatment features enhance the pollutant removal of an infiltration basin. Designers need to stabilize upland soils to ensure that the basin does not become clogged with sediment. In addition, the basin needs to be sized so that the volume of water to be treated infiltrates through the bottom in a given amount of time. Because infiltration basins are designed in this manner, infiltration basins designed on less permeable soils will be significantly larger than those designed on more permeable soils.

Conveyance

Stormwater needs to be conveyed through stormwater treatment practices safely, and in a way that minimizes erosion. Designers need to be particularly careful in ensuring that channels leading to an infiltration practice are designed to minimize erosion. In general, infiltration basins should be designed to treat only small storms, (i.e., only for water quality). Thus, these practices should be designed "off-line," using a flow separator to divert only small flows to the practice.

Maintenance Reduction

In addition to regular maintenance activities, designers also need to incorporate features into the design phase to ensure that the maintenance burden of a practice is reduced. These designers can make regular maintenance activities easier or reduce the need to perform maintenance. In infiltration basins, designers need to provide access to the basin and its forebay for regular maintenance activities. Where possible provide a means to drain the basin, such as an underdrain, in case the bottom becomes clogged. This feature allows the basin to be drained and accessed for maintenance in the event that the water has ponded in the bottom of the basin , or the soil is saturated.

Landscaping

Landscaping can enhance the aesthetic value of stormwater practices, or improve their function. In infiltration basins, the most important purpose of vegetation is to reduce the tendency of the practice to clog. Upland drainage needs to be properly stabilized with a thick layer of vegetation, particularly immediately following construction. In addition, providing a thick turf at the basin bottom helps encourage infiltration and prevent the formation of rills in the basin bottom.

Design Variations
Some modifications may be needed to ensure the performance of infiltration basins in arid and cold climates.

Arid or Semi-Arid Climates

In arid regions, infiltration practices are often highly recommended because of the need to recharge the groundwater. In arid regions, designers need to emphasize pretreatment even more strongly to ensure that the practice does not clog, because of the high sediment concentrations associated with stormwater runoff in the arid southwest. In addition, the basin bottom may be planted with drought tolerant species and covered with an alternative material such as sand or gravel.

Cold Climates

In extremely cold climates (i.e., regions that experience permafrost), infiltration basins may be an infeasible option. In most cold climates, infiltration basins can be a feasible practice, but there are some challenges to its use. First, the practice may become inoperable during some portions of the year when the surface of the basin becomes frozen. Other design features may also be incorporated to deal with the challenges of cold climates. One such challenge is the volume of runoff associated with the spring snowmelt event. The volume of the infiltration basin may be increased to account for snowmelt.

Another option is the use of a seasonably operated facility (Oberts, 1994)(for more information see Performance of Stormwater Ponds and Wetlands in Winter, Article 71 in The Practice of Watershed Protection). A seasonally operated infiltration/detention basin combines several techniques to improve the performance of infiltration practices in cold climates. Two features, the underdrain system and level control valves, are useful in cold climates. These features are used as follows: In the beginning of the winter season, the level control valve is opened and the soil is drained. As the snow begins to melt in the spring, the underdrain and the level control valves are closed. The snowmelt is infiltrated until the capacity of the soil is reached. Then, the facility acts as a detention facility, providing storage for particles to settle.

Other design features can help to minimize problems associated with winter conditions, particularly concerns that chlorides from road salting may contaminate groundwater. The basin may be disconnected during the winter to ensure that chlorides do not enter the groundwater in areas where this is a problem, or if the basin is used to treat roadside runoff. Designers may also want to reconsider application of infiltration practices on parking lots or roads where deicing is used, unless it is confirmed that the practice will not cause elevated chloride levels in the groundwater. If the basin is used for snow storage, or to treat roadside or parking lot runoff, basin bottom should be planted with salt tolerant vegetation.

Limitations

While infiltration basins can be useful practices, they have several major limitations. Infiltration basins are not generally not an attractive practice, particularly when they clog. If they clog, soils become saturated, and the basin can be a source of mosquitoes. In addition, infiltration basins may not be feasible because of concerns over groundwater contamination and sufficient soil infiltration. Finally, maintenance of infiltration practices can be burdensome, as they have the highest rate of failure of any stormwater treatment practice.

Maintenance Considerations

Regular maintenance is critical to the successful operation of infiltration basins. Historically, infiltration basins have had a poor track record. In one study conducted in Prince George's County, Maryland (Galli, 1992), all of the infiltration basins investigated clogged within two years. This trend may not be the same in soils with high infiltration rates, however. A study of twenty-three infiltration basins in the Pacific Northwest showed somewhat better long-term performance in an area with highly permeable soils (Schueler, 2000). In this study, some infiltration basins continued to fail after 10 years (for more information, see Longevity of Infiltration Basins Assessed in Puget Sound, Article 102 in The Practice of Watershed Protection).

Effectiveness

Structural management practices can be used to achieve four broad resource protection goals. These include: Flood Control, Channel Protection, Groundwater Recharge, and Pollutant Removal (see Manual Builder Category for more information). Infiltration basins can provide groundwater recharge and pollutant removal.

Groundwater Recharge
Infiltration basins recharge the groundwater because runoff is treated for water quality by filtering through the soil and discharging to groundwater.

Pollutant Removal
Very little data are available regarding the pollutant removal associated with infiltration basins. It is generally assumed that they have very high pollutant removal, because none of the stormwater entering the practice remains on the surface. Schueler (1987) estimated pollutant removal for based on data from land disposal of wastewater. The average pollutant removal, assuming the infiltration basin is sized to treat the runoff from a one inch storm, is as follows:

These removal efficiencies assume that the infiltration basin is well designed and maintained. The information in the Siting and Design Considerations and Maintenance sections represent the best available information on how to properly design these practices. Also consult the design references below.

Cost Considerations

Infiltration basins are relatively cost-effective practices, because little infrastructure is needed when constructing them One study estimated the total construction cost at about $2 per cubic foot (adjusted for inflation) of storage for a 0.25-acre basin (SWRPC, 1991). Infiltration basins typically consume about 2% to 3% of the site draining to them, which is relatively small. Maintenance costs are estimated at approximately 5% to 10% of construction costs.

One cost concern associated with infiltration practices is the maintenance burden and longevity. If improperly maintained, infiltration basins have a high failure rate (see Maintenance). Thus, it may be necessary to replace the basin after a relatively short period of time.

References

Center for Watershed Protection (CWP), Environmental Quality Resources and Loiederman Associates. 1997. Maryland Stormwater Design Manual. Draft Prepared for: Maryland Department of the Environment. Baltimore, MD.� http://www.mde.state.md.us/environment/wma/stormwatermanual/mdswmanual.html

Center for Watershed Protection (CWP). 1997. Stormwater BMP Design Supplement for Cold Climates. Prepared for: US EPA Office of Wetlands, Oceans and Watersheds. Washington, DC.

Ferguson, B.K., 1994. Stormwater Infiltration. CRC Press. Ann Arbor, MI.

Galli, J. 1992. Analysis of Urban BMP Performance and Longevity in Prince George's County, Maryland. Metropolitan Washington Council of Governments. Washington, DC.

Oberts, G. 1994. Performance of Stormwater Ponds and Wetlands in Winter. Article 71 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, MD.

Schueler, T. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs. Metropolitan Washington Council of Governments. Washington, DC.

Schueler, T. 2000. Longevity of Infiltration Basins Assessed in Puget Sound. Article 102 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, MD.

Southeastern Wisconsin Regional Planning Commission (SWRPC). 1991. Costs of Urban Nonpoint Source Water Pollution Control Measures. Waukesha, WI.

US EPA. 1993. Office of Water. Guidance to Specify Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA-840-B-92-002. Washington, DC.

Watershed Management Institute (WMI). 1997. Operation, Maintenance, and Management of Stormwater Management Systems. Prepared for: US EPA Office of Water. Washington, DC.