Discussion on Karst

[Lori Haak]

Following the joint MCSC/Metro Water Resources Group meeting on 2/5, Drew Chirpich provided this message from Emily Resseger at MWMO. It has some information that might be helpful to the MS4s with karst.

From: Emily Resseger <eresseger@mwmo.org>
Sent: Wednesday, February 4, 2026 2:23 PM
To: Nancy Stowe <NStowe@mwmo.org>
Subject: MWMO karst comments on the MS4 permit pre-public release draft

Hello-

As attendees of the MWMO Karst Workshop in December, we wanted to share comments that MWMO is planning to submit on the MPCA pre-public notice MS4 general permit draft related to karst (comments due February 13, see https://www.pca.state.mn.us/business-with-us/ms4-general-permit).

While MWMO will be submitting comments on other portions of the draft permit beyond those involving karst, our karst specific comments reflect the recommendations in the “MWMO Karst Infiltration and Suitability Workshop Summary.pdf” document shared with this group by Nathan Campeau on 1/23/26, and are as follows:

MWMO is especially interested in language in the MS4 general permit (and construction stormwater permit) related to karst, due to potential significant limits on infiltration within our watershed boundaries in active karst regions. It is our experience that stormwater regulators are interpreting permit language around karst conservatively, out of concern for the possibility of unknown karst features in active karst regions.

Our primary request related to karst is that the MPCA convene a workgroup to research and perform engagement to add further guidance and regulations around infiltration in active karst regions. We have had conversations over the past year with scientists, regulators, and other stormwater managers that have made it clear that there is more nuance to the risks associated with infiltration in karst areas than permit regulations indicate. Topics for the workgroup might include: karst definitions, differentiation between bedrock units that are more or less likely to have karst features, additional limitations to infiltration in active karst regions, and providing guidance on infiltration practice design to minimize risk in active karst regions. Outcomes from the workgroup should be incorporated into both the MS4 general permit, Phase I MS4 permits, and the construction stormwater permit.

We also have two specific recommendations for changes to the MS4 general permit:

• Provide a clear definition in the permit for “active karst features”, separate from “active karst”. There is currently no definition for “active karst features” in either permit, and “active karst features” is being interpreted by regulators to mean the same thing as “active karst”. A karst features dataset is available from the DNR (https://gisdata.mn.gov/dataset/geos-karst-feature-inventory-pts) which could be utilized, but we think it would be appropriate for the workgroup requested above to establish a clear definition.

• Simplify guidance on the distance from active karst features where infiltration is prohibited to a single uniform distance around a feature. Determining if a given location is up-gradient or down-gradient from a karst feature is not straightforward and has led to confusion in application. We recommend the workgroup requested above decide on an appropriate uniform distance around a karst feature that provides adequate protection.

If you are interested and planning to submit your own comments, we encourage you to review the workshop summary document sent out on 1/23/26 and our comments above and use and adapt as you see fit. We would love to see a strong statement made on how karst is referenced in this permit by several stakeholders to hopefully encourage MPCA to convene a karst workgroup as requested.

Thank you for your engagement with us around karst over the past year!

Emily Resseger, PE
Monitoring, Assessment, and Research Program Manager
She / Her / Hers*
(612) 746-4980 direct
(612) 746-4970 office
Mississippi Watershed Management Organization
2522 Marshall Street NE
Minneapolis, Minnesota 55418-3329
http://www.mwmo.org

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