MS4 Permit Updates: What You Need to Know

November 5, 2025krista@marketingbykam.comNews

The Pennsylvania Department of Environmental Protection (PADEP) recently released guidance outlining draft updates to the Municipal Separate Storm Sewer System (MS4) program. These proposed changes aim to shift the focus from pollutant reduction to volume management — impacting how municipalities plan, fund, and document stormwater management activities.

Key Updates at a Glance

Volume Management Plans (VMPs) will replace Pollutant Reduction Plans (PRPs) and Total Maximum Daily Load (TMDL) plans.
- Goal: achieve treatment of 88% of impervious area within 50 years.
- A new Maximum Extent Practicable (MEP) Calculator will support feasibility and cost analyses.
- Certain stormwater practices, such as streambank stabilization and street sweeping, may see reduced or no credit.
Best Management Practices (BMPs) will now be referred to as Stormwater Control Measures (SCMs).
The model stormwater ordinance will be updated and must be adopted by permittees, introducing several notable changes, including:
- Allowance for residential vehicle washing
- New conveyance design standards (up to 100-yr storm +20%)
- Homeowner inspection recordkeeping requirements (after every precipitation event for 5 years)
- Higher rainfall rates for stormwater calculations
Minimum Control Measures (MCMs) remain largely consistent, with a few new requirements:
- Public outreach will include environmental justice areas
- Quicker response to illicit discharge complaints (within 5 days)
- Documented MOUs (Memorandums of Understanding) with Conservation Districts if relying on statewide construction stormwater programs

Budget & Timing Considerations

Municipalities should plan for the development and funding of new VMPs and potential updates to impervious area mapping.

The draft permit was published January 18, 2025; public comments were due March 19, 2025.

Target submission dates (subject to change):

NOI: September 30, 2026
VMP: September 30, 2028

Additional Notes

Review the model ordinance with your solicitor to avoid conflicts with other municipal statutes.
Retrofitting existing basins or BMPs on municipal property may be a practical way to meet new requirements.
If PRP/TMDL reductions are not yet complete, municipalities must finish them before applying for renewal under the new regulations.

Need guidance navigating these changes?

Carroll Engineering’s Municipal Services team can assist with compliance planning, VMP development, and funding strategies to help your community stay ahead of upcoming MS4 requirements.

Questions? Get in touch with our team directly with inquiries or to see how we can support your municipality through this transition: