Stormwater infiltration guidance for sites with known or high potential for soil or groundwater contamination on or off-site

Infiltration of stormwater runoff is not recommended at sites with known soil or groundwater contamination or a high potential for contamination. However, infiltration may be feasible if contaminants at the site are below levels of concern or contact with contaminants can be avoided. Making these determinations requires knowledge of the type and location of contamination at a site. This page provides guidance for determining if infiltration is feasible at a site that has known or high potential for soil or groundwater contamination. For sites less likely to have contaminated soils or groundwater or where there is limited information about the likely presence of contaminants, see Screening assessment for contamination at potential stormwater infiltration sites.

Information: This page provides guidance for determining if infiltration is feasible at a site that has known or high potential for soil or groundwater contamination. For sites less likely to have contaminated soils or groundwater or where there is limited information about the likely presence of contaminants, see Screening assessment for contamination at potential stormwater infiltration sites.

Potential concerns with infiltration at sites with contamination or high potential for contamination

A concern with infiltrating stormwater through or into contaminated soils or groundwater is potential liability for a contaminant release. Minnesota statute 115B.03 defines the conditions under which a person may be associated with a release of a hazardous substance, pollutant, or contaminant from a facility. Specifically, "An owner of real property is not a person responsible for the release or threatened release of a hazardous substance from a facility in or on the property unless that person: ... (4) knew or reasonably should have known that a hazardous substance was located in or on the facility at the time right, title, or interest in the property was first acquired by the person and engaged in conduct associating that person with the release; or (5) took action which significantly contributed to the release after that person knew or reasonably should have known that a hazardous substance was located in or on the facility."

Thus, a person or party becomes liable only when a release occurs. In a case where a site investigation reveals the presence of contamination, but no release occurs, liability assurances can be given. These are explained in this guidance document. Specifically, the MPCA may provide No Association Determinations (NAD) or Off-site Source Determinations.

• A NAD is provided when the specific actions proposed by the voluntary party will not associate them with the identified contamination.
• An Off-site Source Determination is provided when groundwater contamination at a property originates from a source located on an adjacent or nearby property.

Each of these determinations is contingent on certain site conditions, which are described in detail in this guidance document.

Contributing to the release of a pollutant in soil or groundwater is also a violation of the Construction Stormwater permit. Permit language is described below.

How do I know if a site has contaminated soil or groundwater?

Sites that fail the screening assessment have one or more features that should be investigated further before proceeding with infiltration. These features include the presence of one or more of the following:

• a monitoring well;
• a soil stockpile
• underground storage tank vents or fill ports
• staining of soils and/or dead vegetation
• unusual odors
• mismanaged drums
• excavations that are not backfilled with clean material
• presence of debris that may indicate presence of structures or activities that could result in contamination

These features do not confirm the presence of contamination. If infiltration is desired, more information on the feature is needed. If sufficient information does not exist to alleviate concerns, soil and/or groundwater sampling should occur.

Sites that are or were entered into an investigation and cleanup program will likely have had soil and/or groundwater sampling. Most of these sites have or had contamination on site. Carefully review documents for the site to determine if the contamination is at levels of concern or if the site was remediated below levels of concern.

What constitutes an appropriate environmental review?

When choosing to pursue infiltration at a site that is contaminated, an appropriate environmental review must be conducted. This consists of, at a minimum, a Phase 1 investigation. A Phase 2 investigation ([1], [2]), which involves sampling, is likely needed at sites with known or high potential for contamination.

Who do I need to contact?

If there is any reason to suspect soil or groundwater may be contaminated at a site, or if the current or recent land use is associated with activities involving potential contaminants, contact the Minnesota Pollution Control Agency.

Construction Stormwater permit language

Permittees are prohibited from constructing infiltration systems where high levels of contaminants in soil or groundwater may be mobilized by the infiltrating stormwater. Permittees must either complete the MPCA's site screening assessment checklist or conduct their own assessment to determine the suitability for infiltration. The assessment must be retained with the SWPPP. See Screening assessment for contamination at potential stormwater infiltration sites for more information and guidance.

For more information on the screening assessment, see the links below.

• Access the MPCA's site screening assessment checklist
• Access the guidance: Screening assessment for contamination at potential stormwater infiltration sites
• Access example screening assessments

What is a high level of contamination

High levels of contamination are determined by comparing soil or groundwater concentrations at or adjacent to a site with risk criteria. The risk criteria include drinking water criteria (Health Risk Limits, Health Based Values, or Maximum Contaminant Levels) and soil criteria (Soil Leaching Values). Note that in addition to having high contaminant levels at a site, there must be the potential to mobilize the contaminants.

For more information on these risk criteria and determining if high contaminant levels exist at a site, see the following sections.

Overview of risk based criteria

• HRL: A health risk limit (HRL) is the concentration of a groundwater contaminant, or a mixture of contaminants, that can be consumed with little or no risk to health and which has been promulgated under rule. A HRL is expressed as a concentration in micrograms per liter. 2015 HRLs are listed on a Minnesota Department of Health website.
• HBV: A health-based value (HBV) is the concentration of a groundwater contaminant that can be consumed daily with little or no risk to health. HBVs are derived using the same algorithm as HRLs; however, they have not been promulgated as rules, have not undergone peer review, and may be based on less data and/or subject to greater uncertainty than HRLs. There are several reasons why a chemical might have an HBV rather than a HRL: the chemical may not have been found in groundwater until after the HRL rules were promulgated, toxicological data may not have been available until after rulemaking, or the level of uncertainty in the value may be greater than accepted in the rules. HBVs are re-evaluated when the HRL rules are updated. An HBV is expressed as a concentration in micrograms per liter. 2015 HBVs are listed on a Minnesota Department of Health website.
• MCL: Maximum Contaminant Levels (MCLs) are standards set by the United States Environmental Protection Agency (EPA) for drinking water quality. An MCL is the legal threshold limit on the amount of a substance that is allowed in public water systems under the Safe Drinking Water Act. MCLs are listed on EPA's website.
• RAA: Risk Assessment Advice (RAA) for water is technical guidance concerning exposures and risks to human health. RAA guidance is not likely to be promulgated, either because of a high level of uncertainty about the numeric guidance; because the methods used to develop the guidance are different than the methods in rule; or because the resulting guidance is qualitative rather than a water value. RAA water values can be applied at the discretion of risk assessors and risk managers in the same way that HBVs and HRLs are applied. MDH also develops RAA on a case-by-case basis for specific conditions or specific sites. It is not appropriate to apply this site-specific RAA to other sites without consulting with MDH.
• SLV: Soil Leaching Values (SLVs) are compound‐specific, risk‐based screening criteria developed by the MPCA to evaluate risks posed to groundwater by soil leaching. The SLV is an estimation of the unsaturated soil contaminant concentration for a given compound above which may resulting groundwater contamination in excess of chemical-­specific drinking water criteria. The MPCA developed a spreadsheet to calculate SLVs.

The Minnesota Pollution Control Agency developed guidance for the risk criteria described above.

Example determinations of risk-based criteria are presented in the next section and are included in the Examples section further down on this page.

Example determinations of risk-based criteria

Example 1 - groundwater criteria (HRL, HBV, MCL)

Summary of risk assessment for example 1
Chemical	Concentration (ug/L)	HRL	HBV	MCL	Endpoint	Exceed	Notes
Arsenic	8			10	skin damage/cancer	No
Benzene	2.5	2		5	cancer	Yes	Compare the site concentration to the HRL, which is the lowest risk value. Site concentration exceeds the HRL.
Benzo[a]pyrene	<0.10		0.06	0.2	cancer	Unknown	HBV is the lowest risk value. Detection limit exceeded HBV and therefore cannot determine risk
Cadmium	0.3	0.5		5	chronic - renal	No	Compare the site concentration to the HRL, which is the lowest risk value.
Chloroform	26		20		chronic - multiple	Yes	Site concentration exceeds the HBV
Methyl tert-butyl ether (MTBE)	100	1	1	1	cancer	Yes
1MTBE does not have a HRL, HBV, or MCL but has Risk Assessment Advice (RAA). A RAA is typically used to assess risk when other criteria are not available.

The table to the right compares groundwater concentrations at a site to risk criteria. Note that the table lists all applicable risk criteria. When conducting the risk assessment, use the lowest value. For example, the HRL for benzene (2 ug/L) is less than the MCL (5 ug/L) and the HRL should be compared to the site concentration. In this case, the groundwater concentration of 2.5 ug/L exceeds the HRL and the groundwater is considered to be contaminated at a level of concern. When examining the MDH table for HRLs and HBVs (see link), note that there may be multiple values for a chemical. Again, use the lowest value for comparing to site concentrations.

Considering benzo[a]pyrene, the site concentration is less than 0.10 ug/L. This is lower than the MCL of 0.2 ug/L. The HRL is 0.06 ug/L. Because we can't determine if the concentration of benzo[a]pyrene is less than the HRL, we should resample and achieve lower detection limits.

Methyl tert-butyl ether (MTBE) does not have a HRL, HBV, or MCL. Instead, it has a Risk Assessment Advice value of 20 ug/L. The RAA should be used the same as other risk criteria, but the MPCA should be consulted before applying the value. In this example, the site concentration of 100 ug/L exceeds the RAA of 20 ug/L.

The concentration of chloroform (26 ug/L) exceeds the HBV of 20 ug/L. Although HBVs are not promulgated in rule, they should be used at a site to assess potential risk to groundwater.

In determining the MCL at a site, be aware that the concentrations given at EPA's website are in milligrams per liter (mg/L). Multiply by 1000 to convert to micrograms per liter (ug/L).

Infiltration at this site is not recommended because concentrations of some contaminants exceed risk criteria. However, detailed modeling or additional investigation may be employed to determine if appropriate locations for infiltration exist on the site.

Summary of risk assessment for example 2
Well	Concentration of benzene (ug/L)	HRL	HBV	MCL	Endpoint	Exceed	Well location relative to BMP
1	5.8	2		5	cancer	Yes	Up-gradient
2	3.5	2		5	cancer	Yes	Up-gradient
3	1.1	2		5	cancer	No	Down-gradient
4	0.6	2		5	cancer	No	Down-gradient
5	<0.1	2		5	cancer	No	Down-gradient

Example 2 - groundwater criteria (HRL, HBV, MCL)

In this example, the concentration of benzene exceeds risk criteria in two wells. However, both of these wells are located up-gradient of the proposed infiltration practice. Down-gradient of the proposed practice, concentrations are below risk criteria. A mounding analysis should be conducted to determine if a mound resulting from infiltration will extend up-gradient into groundwater having concentrations of benzene that exceed risk criteria. If the mound does not extend into the contaminated groundwater, infiltration is acceptable at the site.

Example 3 - soil leaching criteria (HRL, HBV, MCL)

Summary of risk assessment for example 3
Chemical	Concentration (mg/kg)	SLV at water table	Exceed	SLV with 10 foot separation	Exceed	Notes
Arsenic	12	5.4	Yes	17.5	No
Chromium VI	38	36	Yes	109	No
Copper	1006	700	Yes	2110	No
Pentachlorophenol	253	230	Yes	685	No
Benzo[a]pyrene	1.5	1.4	Yes	4.23	No	B(a)P equivalents

An infiltration practice proposed at a former wood treating site shows the concentrations in the table to the right. The default Soil Leaching Values (SLVs) assume the contaminated soils occur at the water table and the contaminated layer is 15 feet thick. Default SLVs are exceeded for all chemicals. Infiltration would not be recommended for a default condition. However, soil borings indicate the contaminated layer is 5 feet thick and there is 10 feet of separation between the bottom of the contaminated soils and the water table. Under these conditions, infiltration is acceptable because the SLVs are not exceeded for any chemical.

Investigating the site

Sites with know contamination or a high potential for contamination must undergo a Phase 1 investigation. Phase 2 investigations may be necessary after completion of the Phase 1 investigation. These are described below.

Phase 1 Investigation

Phase 1 investigations and guidance are described in detail on MPCA's website. A brief overview, summary of objectives, and contents of the guidance page are provided below.

The Phase I Investigation is considered by the Minnesota Pollution Control Agency (MPCA) staff to be the initial investigative stage in conducting an environmental assessment. The Phase I Investigation should be completed before preparing a soils and ground water investigation work plan (Phase II Investigation Work Plan). It is equally important that the information included in the Phase I Investigation Report reflect current conditions at the site. If you completed the Screening assessment for contamination at potential stormwater infiltration sites, you may wish to proceed directly to a Phase 2 investigation since the screening assessment is similar to a Phase 1 investigation.

The objectives of the Phase I Investigation are

• to establish a minimal level of inquiry to be conducted during the Phase I Investigation by providing a broad base of historical sources of information;
• to determine whether the property has been the site of a release or threatened release of a hazardous substance, pollutant or contaminant under the definition of the Minnesota Superfund law; and
• to determine the types of additional inquiry which should be included in the Phase II Investigation Work Plan.

MPCA's guidance outlines the following process for Phase 1 investigations.

• Retain a consultant
• Define the purpose of the investigation
• Provide a property description, including location, physical and hydrgeological setting, and previous investigations.
• Gather historical information about the property, including ownership and activities conducted on the property.
• Determine the property features, including presence of tanks, dumps, septic systems, storage containers, utilities, wells, sumps, lagoons, catchment basins and retention ponds. Examine environmental records on property deed.
• Locate and examine property maps and photos, including aerial photos.

Guidance on where to find the above information can be found on MPCA's website or on the Screening assessment for contamination at potential stormwater infiltration sites. In some cases, a Phase 1 investigation may indicate no or very low potential for contaminants to be mobilized by infiltrating stormwater and a Phase 2 investigation will not be necessary. However, in most cases, a Phase 2 investigation, including soil and/or groundwater sampling, will be required to make a final determination.

Phase 2 Investigation

The focus of a Phase II Investigation ([3], [4]) is to determine if potential sources of contamination identified during a Phase I Investigation are causing a release or threatened release of hazardous substances, contaminants or pollutants to the soil, surface water and groundwater on the property. The Phase II Investigation primarily entails gathering site specific data on soils, surface water and ground water. A Phase II Investigation Work Plan should be developed and contain the following:

• describe how the potential sources of contamination identified in the Phase I Investigation will be investigated to determine if there has been a release of hazardous substances;
• determine the magnitude of the sources and determine the magnitude, extent and receptors of any releases or threatened releases;
• gather site information so MPCA staff can evaluate cleanup levels and determine whether a cleanup is necessary;
• gather site information to select a remedial technology, if cleanup is necessary;
• provide the MPCA staff an opportunity to comment on the planned field work in order to reduce the possibility that additional investigative work will be required by the MPCA staff; and
• establish a minimum level of inquiry for a Phase II Investigation.

As with a Phase 1 investigation, a consultant should be retained for a Phase 2 investigation. To conduct a complete Phase II Investigation, sampling and chemical and physical analyses of the site soil and ground water is required. The site investigation includes the following components.

• determining where to sample;
• determining what type of samples to collect (e.g. soil, groundwater, vapor)
• determining what to sample for;
• determining the number of samples;
• developing sampling protocol, including sampling methods, quality control, and laboratory analysis to be performed;
• developing procedures for managing waste (e.g. contaminated samples);
• a site safety and contingency plan;

Guidance for Phase 2 investigations

MPCA guidance provides information on collecting soil, water, and vapor samples. This guidance also discusses samples collected from soil, groundwater, surface water, air, and other features (e.g. tanks, storage containers) located at the site. Additional information exists at the following links.

• MPCA links
  • Site characterization and sampling documents - General, comprehensive document for site remediation
  • Underground Storage Tank and Petroleum Remediation Quality Assurance Program Plan - Petroleum guidance
  • Spatial data collection at petroleum release sites - Petroleum guidance
  • Excavation of petroleum-contaminated soil and tank removal sampling - Petroleum guidance
  • Assessment of Petroleum Contamination at Closed Sites - Petroleum guidance
  • Soil and groundwater assessments performed during site investigations - Petroleum guidance
  • Vapor Intrusion Assessments Performed During Site Investigations - Petroleum guidance
  • Assessment of Natural Attenuation at Petroleum Release Sites - Petroleum guidance
  • Soil sample collection and analysis procedures (c-prp4-04) - Petroleum guidance
  • Groundwater sample collection and analysis procedures (c-prp4-05) - Petroleum guidance
  • Guidance Document #11 Phase II Investigation Work Plan - Brownfield guidance
• Other sites - note these guidance documents may include procedures not applicable to investigations in Minnesota
  • Guidance for Cleaning Up Groundwater, Soil and Air at Corrective Action Facilities - U.S. EPA
  • Remediation Technologies for Cleaning Up Contaminated Sites - U.S. EPA
  • Arizona site investigation manual - see chapters 5, 6, and 7
  • Guidance for Remediation of Petroleum Contaminated Sites - State of Washington; see Chapters 4, 5, 6, and 7

If investigation activities cause a release of a hazardous substance or a release is identified that has not been previously reported, that release must be reported to the MPCA through the Minnesota Duty Officer, Division of Emergency Management. The notification must be made within 24 hours. Contacting the Duty Officer will fulfill obligations for the notification requirements under Minn. Stat. § 115.061. The number for the Duty Officer is (651) 649-5451 (Metro) and 1-800-422-0798 (Greater Minnesota). TDD numbers are (651) 297-5353 and 1-800-627-3529.

Example Phase 2 investigations

• Phase II Investigation Report - Trunk Highway 7 and Louisiana Avenue Interchange , Saint Louis Park, Minnesota - This Phase 2 investigation report, completed by Short Elliott Hendrickson Inc., is over 1300 pages in length. However, a review of the project summary and table of contents provide an indication of factors that can be considered in a Phase 2 investigation.
• Phase II Investigation Work Plan, Sites of Concern 1-3 and 6-8, UMore Mining Area, Dakota County, Minnesota - completed by Barr Engineering
• Initial Phase II - Exterior Investigation Report - Ford Motor Company site, completed by Arcadis
• PHASE II ENVIRONMENTAL SITE ASSESSMENT FORMER U-HAUL PROPERTY (Site 4) 3890 NICHOLS ROAD , EAGAN, MINNESOTA - completed by Stantec
• PHASE II INVESTIGATION REPORT - Former Wheeling Corrugating Company, 340 27th Avenue Northeast, Minneapolis, Minnesota 55418 - completed by URS Corporation

What do I do if my site is contaminated?

If there is any reason to suspect soil or groundwater may be contaminated at a site, or if the current or recent land use is associated with activities involving potential contaminants, contact the Minnesota Pollution Control Agency.

If contamination on a site is confirmed, typically through a Phase 2 investigation, the following options exist.

• select a non-infiltration stormwater BMP that will not mobilize contaminants
• identify areas on the site where infiltration is feasible
• remediate the site to acceptable contaminant concentrations

Each of these is discussed below.

Choosing a non-infiltration practice

The Construction Stormwater permit states "Permittees must design and construct a permanent stormwater treatment system to treat the water quality volume if the project's ultimate development replaces vegetation and/or other pervious surfaces creating a net increase of one (1) or more acres of cumulative impervious surface. Permittees must calculate the water quality volume as one (1) inch times the net increase of impervious surfaces created by the project. Permittees must first consider volume reduction practices on-site (e.g., infiltration or other) when designing the permanent stormwater treatment system. If this permit prohibits infiltration as described in item 16.14 through item 16.21, permittees may consider a wet sedimentation basin, filtration basin or regional pond. This permit does not consider wet sedimentation basins and filtration systems to be volume reduction practices."

Non-infiltration practices include filtration practices, wet sedimentation basins, and regional ponds. Filtration practices include biofiltration, media filters, and swales.

Some infiltration occurs below all these practices. The amount of infiltration is a function of

• the type of practice selected,
• presence or absence of a liner, and
• position of the underdrain in filtration practices.

Sedimentation practices (e.g. constructed ponds, constructed wetlands) typically have low infiltration or leakage rates through the bottom of the practice. Even when constructed in permeable soils, these practices tend to self-seal over time as sediment accumulates at the bottom of the practice. Annual leakage rates through the bottom of constructed sedimentation practices are shown below.

• conductivity = 10^-4cm/s = 1262 in/yr (105 feet)
• conductivity = 10^-5cm/s = 126 in/yr (10.5 feet)
• conductivity = 10^-6cm/s = 12.6 in/yr
• conductivity = 10^-7cm/s = 1.3 in/yr

Case studies

Case studies can be found at this link.

Related pages

• Portal for Stormwater infiltration and contaminated soils and groundwater
• Contaminated sites and stormwater infiltration
• Stormwater Infiltration and soil/groundwater contamination: A guide to the Construction Stormwater Permit requirements
• Screening assessment for contamination at potential stormwater infiltration sites
• Stormwater infiltration guidance for sites with known or high potential for soil or groundwater contamination on or off-site
• Links to Minnesota Pollution Control Agency cleanup and remediation information
• Case studies for stormwater infiltration at contaminated sites
• Links to information and guidance on stormwater infiltration at contaminated sites
• References for stormwater infiltration at contaminated sites