Stormwater Infiltration and soil/groundwater contamination: A guide to the Construction Stormwater Permit requirements

Example of a site that has contaminated soil but where infiltration can occur by locating the infiltration system away from the contaminated soil and placing an impermeable surface over the contaminated soil.

The construction stormwater permit may prohibit infiltration practices on properties where existing contamination in groundwater or soil is present. The permit requires permittees to analyze their specific site to determine if infiltration is appropriate. This page explains the assessment required in order to demonstrate compliance with the permit at sites with potential or known soil and/or groundwater contamination.

Information: A question that has been asked is whether a Phase 1 investigation can substitute for completing this screening assessment. A properly conducted Phase 1 investigation satisfies the permit requirement and may be used in place of this screening assessment. Phase 1 investigations and guidance are described in detail on MPCA's website.

Permit language (Section 16.15)

"This permit prohibits permittees from constructing infiltration systems where infiltrating stormwater may mobilize high levels of contaminants in soil or groundwater. Permittees must either complete the MPCA's contamination screening checklist or conduct their own assessment to determine the suitability for infiltration. Permittees must retain the checklist or assessment with the SWPPP."

What does this mean?

The MPCA does not expect soil or groundwater testing for all construction projects. Permittees should use the screening assessment tool below. The screening assessment can be completed with a site visit and some basic historic knowledge of the property. If the screening tool indicates that contamination may be mobilized by infiltration the permittee should consider designing a different stormwater management system such as a lined filtration system or a wet sedimentation basin. If infiltration is still desired, permittees can investigate further using the guidance to determine the appropriateness of infiltration. Many contaminated properties can still utilize infiltration by siting the stormwater management system properly or determining that the contamination present will not be mobilized.

The permit does not specifically require the MPCA's assessment. Other sources of information can be used instead of, or in addition to, the assessment. For example, information on contaminated properties that have been remediated through any of the MPCA's programs can be found at What's in My neighborhood. The guidance page below contains additional information on conducting a property review. The construction permit requires permittees to make a reasonable determination using these tools, that contamination will not be mobilized and to document the findings in the stormwater pollution prevention plan for the project.

MPCA's screening assessment tool

• Access the MPCA's site screening assessment checklist: This is an Excel file
• Access the guidance: Screening assessment for contamination at potential stormwater infiltration sites: This page provides guidance for using MPCA's site screening checklist