Stormwater infiltration at permitted industrial stormwater facilities

When feasible, infiltration of stormwater runoff is a preferred option for managing stormwater. Infiltration keeps water near its point of origin, helps reduce downstream flooding, may help replenish groundwater supplies, and can be an effective treatment practice. This page provides a discussion of infiltration at industrial stormwater sites, including BMP options and infiltration constraints.

When should I consider infiltration?

If an industrial site fails benchmark monitoring, treatment may be required. If site conditions are appropriate for infiltration, it is usually the preferred option for managing stormwater runoff. The following site conditions are conducive to infiltration.

• Hydrologic Soil Group (HSG) soils A, B, and C. For more information, see this table.
• A minimum distance of 3 feet from the bottom of the infiltration practice to bedrock. For more information, see the section on shallow soil and shallow bedrock.
• A minimum distance of 3 feet from the bottom of the infiltration practice to the seasonal high water table. For more information, see the section on shallow groundwater.
• Absence of active karst, which is defined as areas underlain by carbonate bedrock with less than 50 feet of sediment cover. For more information, see the section on karst.
• Soils that do not have excessively high infiltration rates. Soils with excessive infiltration rates do not adequately attenuate pollutants in the stormwater runoff.

In addition to the above physical site conditions, pollutants in the stormwater runoff must also be considered. The following general conditions apply for pollutant retention in infiltration BMPs.

• Metals are typically highly attenuated by infiltration practices.
• Sediment is highly attenuated by infiltration practices.
• Most organic pollutants are moderately or highly attenuated in infiltration practices that have engineered media, such as bioinfiltration practices, tree trenches and tree boxes, and swales that are designed to infiltrate runoff. Underground infiltration practices may be less effective for attenuating organic pollutants, though this is an area of active research.
• Pathogens are likely to be highly attenuated in most infiltration practices, although research on movement of pathogens in soil is limited. Underground infiltration systems may be less effective in attenuating pathogens.
• Chloride will not be attenuated by infiltration practices.
• Ammonia and organic nitrogen are moderately attenuated by infiltration practices. Some of this nitrogen may be transformed into nitrate,
• Nitrate is poorly attenuated by infiltration practices.

Some pollutant sources should not be infiltrated or may be prohibited from infiltration. These are discussed in greater detail in the section on infiltration prohibitions.

Selecting infiltration BMPs

There are several options for infiltrating stormwater runoff. BMP options include the following:

• infiltration basins,
• infiltration trenches (includes dry wells),
• underground infiltration systems,
• bioinfiltration,
• permeable pavements,
• tree trenches and tree boxes, and
• dry swale with check dams.

These BMPs are discussed in greater detail in [https://stormwater.pca.state.mn.us/index.php?title=BMPs_for_stormwater_infiltration this section of the manual. This page includes links to pages discussing design, construction, operation and maintenance, and cost for these BMPs.

Additional information about these BMPs can be found in the following tables.

• Stormwater infiltration BMPs - overview
• Stormwater infiltration BMPs – selection considerations
• Stormwater infiltration BMPs - treatment properties.

Other options for infiltrating stormwater include discharging stormwater to vegetated (e.g. turf) areas, including capturing stormwater and using it for irrigation. Water applied to these vegetated areas is applied under conditions that allow the water to infiltrate. For more information, see the sections on turf and stormwater and rainwater harvest and use.

Constraints and prohibitions on infiltration

There are several constraints and prohibitions on infiltration in the industrial stormwater permit. These are summarized below.

• Industrial stormwater ponds and infiltration devices, located in areas where high levels of contaminants (as defined by the Agency) exist in the soil or in the shallow aquifer, must not contribute to contaminant(s) spreading to a greater extent or rate. At any contamination site, as determined by the Agency, a site analysis shall be conducted by a qualified professional (e.g.

professional hydrogeologist, engineer, etc.) and a report filed with the SWPPP. If industrial stormwater ponds and infiltration devices are found to be a contributor to contaminant increase or movement, the Permittee shall submit a plan to the Agency that describes how the Permittee will be reducing contaminants, redesigning, relocating, or eliminating the industrial stormwater ponds and infiltration devices, as needed, to eliminate the contribution to contaminant problems. The plan shall be submitted to the Agency within one year of the Permittee’s authorization to discharge under this permit or if discovered after application, within one year of discovery. The plan shall be implemented as soon as approval is granted by the Agency. The plan may utilize the results of, but does not reduce or eliminate more stringent requirements that may be imposed by other Agency regulatory programs. If agreement with the Agency on an acceptable plan cannot be reached, the Permittee must seek an individual NPDES/SDS permit.

• Industrial stormwater ponds and infiltration devices shall not be used in any high risk karst area unless a professional geotechnical evaluation is conducted by a qualified professional (e.g.

professional hydrogeologist, engineer, etc.) to ensure that the industrial stormwater pond or infiltration device does not present a significant risk to groundwater. Generally accepted practices are described in the applicable portions of the Minnesota Stormwater Manual. If the industrial stormwater ponds and infiltration devices present a risk, appropriate measures, such as sealing or removal of the industrial stormwater ponds or infiltration devices, shall be taken to minimize or eliminate the risk. Evaluations shall be documented with the SWPPP.

• The construction of a new infiltration device is prohibited in
  • areas that receive discharges from vehicle fueling and maintenance activity.
  • areas with less than three (3) feet of separation distance from the bottom of the infiltration device to the elevation of the seasonally saturated soils or the top of bedrock.
  • areas of predominately Hydrologic Soil Group D (clay) soils unless allowed by a local unit of government with a current MS4 permit.
  • areas where soil infiltration rates are more than 8.3 inches per hour or as allowed by a local unit of government with a current MS4 permit.
• Industrial stormwater ponds and infiltration devices in vulnerable wellhead protection areas must be coordinated with local drinking water authorities and shall be designed to not adversely affect drinking water supplies. The Permittee shall contact the appropriate local drinking water authorities and document coordination efforts with the SWPPP.
• Permittees with any infiltration device defined as a USEPA “Class V injection well” shall contact the USEPA Region V to determine the need to register as a “Class V injection well.” Refer to the USEPA Underground Injection Well Program for the definitions and complete registration process. Contacts and USEPA response shall be documented with the SWPPP.
• For the following 5 sectors, on or after April 5, 2010, the Permittee is not authorized to construct new infiltration devices, expand infiltration activities or practices that result in infiltration, or expand volume of infiltration.
  • Sector A (SIC 2491) - wood preserving.
  • Sector K - hazardous waste treatment, storage, or disposal facilities
  • Sector M - automobile salvage yards
  • Sector N - scrap waste materials
  • Sector S - air transportation with deicing