Karst (20.9 and 28.2)

[Kristin Seaman]

For those with karst and development in their MS4, the City of Woodbury is planning to include the following in our comment letter. Have any thoughts/feedback? Depending on what everyone thinks, it’d be great to have a similar message between organizations.

1. Requested Action: Add a definition of “active karst features” to Section 28.2 that distinguishes individual features (e.g., sinkholes, caves, losing streams, springs) from general “active karst terrain.”

Reason: The permits currently use the term “active karst” inconsistently—defining it as terrain in Section 28.2 while applying restrictions to individual features in Section 20.9—without providing a definition of “active karst features.” This lack of clear, consistent terminology creates regulatory uncertainty and makes it difficult for MS4 permittees to determine compliance, particularly in areas where large portions of the MS4 are mapped as karst terrain. Given the wide variability in karst sensitivity across Minnesota, distinguishing between specific features and broader terrain is essential to ensure groundwater protection while avoiding over- or under-regulation.

2. Requested Action: Revise Section 20.9 to clarify whether infiltration prohibitions apply only to defined features, to terrain, or to both—and under what circumstances. When infiltration is prohibited, please consider a uniform 1,000-foot buffer around known karst features, as “up-gradient” and “down-gradient” are difficult to define and apply consistently.

Reason: Section 20.9 does not clearly specify whether infiltration prohibitions apply to individual karst features, to active karst terrain, or to both, resulting in inconsistent interpretation and implementation by MS4s. The current reliance on “up-gradient” and “down-gradient” determinations is difficult to apply consistently without site-specific hydrogeologic analysis, adding uncertainty and cost. Clear applicability criteria—such as a defined buffer distance around known karst features—would provide predictable, implementable standards that better reflect variable karst conditions, protect groundwater resources, and allow infiltration where it can be safely and effectively used.

3. Requested Action: Align the MS4 General Permit and the Construction Stormwater Permit so that terms, prohibitions, and conditions related to karst are consistent.

Reason: Consistency between the MS4 and Construction Stormwater Permits is critical and is, in general, achieved throughout both permits. While both permits include karst-related restrictions on infiltration near active karst features, the Construction Stormwater Permit also includes an explicit requirement to line stormwater basins/ponds in active karst terrain that is not currently mirrored in the draft MS4 Permit.

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