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.
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.
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.
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:
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.
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, which involves sampling, is likely needed at sites with known or high potential for contamination.
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.
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.
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.
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.
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 |
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.
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.
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 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
MPCA's guidance outlines the following process for Phase 1 investigations.
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.
The focus of a Phase II Investigation 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 ground water 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:
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.
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.
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.
Case studies can be found at this link.