Feedback Requested – New Stormwater Permit for Construction

Although the current construction permit is not scheduled to expire until August of 2018, the MPCA intends on re-issuing the permit as soon as fall of 2017. The expiration date coincides with the expiration of the Municipal Separate Storm Sewer System (MS4) Permit and the MPCA is concerned about having multiple general permits expiring on the same schedule due to limited staff resources. Additionally the MPCA will not be proposing major changes in the draft permit.

Although no major permit changes will be proposed, several minor changes identified by external stakeholders and MPCA staff are needed. The MPCA is seeking early input on some of these proposed changes in the permit. This is not the official public comment period and the MPCA will not be providing formal responses to all of the input received. The MPCA will use the information gathered from this early input process when writing the proposed requirements in the draft permit. The MPCA expects to public notice the draft permit and receive formal public comments sometime in the late summer or fall of 2017.

Issue #1

The permit requires that stormwater runoff from new impervious surfaces be treated prior to discharge and, in general, requires permittees to consider infiltration or other volume reduction practices first as a means to accomplish this. However, there are numerous site conditions in which infiltration is undesirable, such as the presence of contaminated soils or groundwater. The permit language should define such conditions as clearly as possible. This proposed language change for the prohibition would direct permittees to design a different and more desirable type of stormwater treatment system in these locations such as a wet sedimentation basin or a filtration system.

Current CSW language (Part III.D.1.j.iv):
  1. Infiltration is prohibited when the infiltration system will be constructed in: Areas where high levels of contaminants in soil or groundwater will be mobilized by the infiltrating stormwater.
Proposed CSW permit language:
Infiltration is prohibited in:
  1. Areas where infiltrated stormwater could enter contaminated groundwater with levels over Health Risk Levels (HRL's), Health Based Values (HBV's) or Maximum Contaminate Levels (MCL's) unless appropriate environmental assessment is performed to demonstrate that infiltration will not have a negative impact on groundwater.
  2. Areas where infiltrated stormwater could come into contact with contaminated soils above Soil Leaching Values (SLV’s) unless an appropriate environmental assessment is performed to demonstrate that infiltration will not have a negative impact on groundwater.

This proposed change in language is not intended to change the agency’s policy on this subject but rather clarifies the language “high levels” as used in the current permit. The MPCA does not expect soil or groundwater testing at every site but rather intends for permittees to continue to use the screening assessment tool in the stormwater manual to determine if contamination might be present. In addition, Permittees can look up past remediation sites using the “What’s in my Neighborhood” tool on the MPCA website to access information on those properties.

Screening Assement Tool:

What’s in my neighborhood:

Issue # 2 Temporary stabilization of ditches

The current permit requires stabilization of any ditch or swale within 200 feet of the point of discharge within 24 hours after connecting to a surface water or property edge. This part of the requirement is prosed to remain the same. The current permit also specifically states that some less effective stabilization methods such as mulch cannot be used in ditches or swales for stabilization and permittee(s) must rely on more robust practices such as erosion control blankets. The MPCA has heard from numerous stakeholders that Best Management Practices such as disc anchored mulch may be adequate under certain limited conditions such as areas with little or no slope and installing blanket in all locations is cost prohibitive and not necessary.

Current CSW language (Part IV.B.4):
Applying mulch, hydromulch, tackifier, polyacrylamide or similar erosion prevention practices is not acceptable stabilization in any part of a temporary or permanent drainage ditch or swale.
Proposed CSW permit language:
Applying mulch, hydromulch, tackifier, polyacrylamide or similar erosion prevention practices is not acceptable stabilization within the normal wetted perimeter of a temporary or permanent drainage ditch or swale section that has a continuous slope of greater than 2%.

This change allows permittees more flexibility for the type of stabilization chosen for a ditch bottom. The permit still requires that all BMP's chosen “must be selected, installed, and maintained in an appropriate and functional manner that is in accordance with relevant manufacturer specifications and accepted engineering practices”. Additionally, as with all BMP requirements in the permit, If the selected BMP is found to be inadequate at minimizing erosion from ditches or swales, another more effective BMP must be utilized.


The MPCA is seeking input on these proposed changes. Please submit your comments and suggestions below. Providing your name and contact information is appreciated but not necessary.

Construction Stormwater at the MPCA.

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