Municipal Separate Storm Sewer Systems

Stormwater runoff from urban and industrial areas can contain harmful pollutants. To help keep these pollutants from being washed or dumped into surface waters, operators of municipal separate storm sewer systems (known as MS4s) must get a permit and develop a stormwater management program.

Pollutants found in stormwater runoff include nutrients, heavy metals, pesticides, herbicides and synthetic organic compounds such as fuels, waste oils, solvents, lubricants and grease. These can have damaging effects on both human health and aquatic ecosystems if not treated.

The MS4 program operates under the Delaware Regulations Governing the Control of Water Pollution (7 DE Admin. Code 7201) and the Federal Clean Water Act.

Municipal Stormwater NPDES Permits

The national program of NPDES MS4 Individual and General Permits was first introduced into the EPA’s NPDES program in 1990.

It tackled larger municipal governments with Phase I permits.

Phase II permits, for smaller communities in Census-designated Urbanized Areas and some non-governmental stormwater systems, came in 1999.

Since then, DNREC has worked with the Delaware’s public entities to ensure the compliance of both the Phase I and Phase II permits. The Department has also introduced a more encompassing Phase II General Permit.

Learn More: Managing Stormwater in Delaware

Phase I Permits in Delaware

The Phase I Individual Permit provides coverage to Principal Permittees New Castle County (NCC) and Delaware’s Department of Transportation (DelDOT), including co-permittees the City of Delaware City, the City of New Castle, the City of Wilmington, the Town of Elsmere and the Town of Newport.

NCC and DelDOT Phase I MS4 Permit (2013)

NCC and DelDOT Phase I MS4 Permit Fact Sheet (2013)

Phase II Permits in Delaware

The Phase II General Permit provides coverage to multiple permittees. The new cycle of the Phase II General Permit was broken into two tiers; Tier I and Tier II.

First drafts of the Phase II General Permits were released in 2020 for public comment and review. Based on comments received, new drafts were released for review and comment in 2022.

Tier I

The Tier I General Permit provides coverage to those who currently work under an Individual Phase II MS4 permit — the City of Newark/University of Delaware, the Town of Middletown, the City of Dover and DelDOT in Kent County.

The Phase II Tier I general permit will become effective on Jan. 1, 2024. The proposed permit, and a companion fact sheet, was offered for a 30 day public review on July 5, 2022. The new general permit was signed on Sept. 28, 2023, following a detailed review of all public comments.

Phase II Tier I MS4 General Permit (2024)

Phase II Tier I Fact Sheet (2024)

Summary of Public Comments and Responses (Appendix A)

Tier II

The Tier II General Permit provides coverage to those who seek a new MS4 permit. That currently includes the Town of Smyrna, the Town of Camden, the Town of Laurel and the City of Seaford. It would also cover any other communities brought into the MS4 program during the term of the permit.

public hearing on the revised draft of the proposed NPDES MS4 Phase II Tier II General Permit was held on Jan. 25, 2023. The public comment period was open through Feb. 23, 2023.


National Municipal Stormwater Alliance/EPA MS4 Resource Guide

EPA NPDES Stormwater Discharges from Municipal Sources

EPA NPDES Urbanized Area Maps (Phase II)

EPA NPDES eReporting Tool (NeT) MS4 Support Portal

DelDOT Stormwater Page

University of Delaware Stormwater

Penn State Extension – What is an MS4?