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.

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 the 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

Public Comment Periods

A public notice regarding the proposed NPDES MS4 Phase II Tier II General Permit was published on June 8, 2022. DNREC invites public comment on the proposal through 4:30 p.m. on July 8, 2022.

The public comment period for the proposed NPDES MS4 Phase II Tier I General Permit is open July 6 through Aug. 5, 2022.

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.

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 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.

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


National Municipal Stormwater Alliance/EPA MS4 Resource Guide

EPA NPDES Stormwater Discharges from Municipal Sources

EPA NPDES Urbanized Area Maps (Phase II)

DelDOT Stormwater Page

University of Delaware Stormwater

Penn State Extension – What is an MS4?