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Skip Navigation LinksDNREC : Division of Water : Information : Surface Water Discharges Information : MS4


Individual NPDES Permit for Municipal Separate Storm Sewer Systems (MS4s)

SamplingUrban stormwater runoff is commonly transported through Municipal Separate Storm Sewer Systems (MS4s), and is often discharged, untreated, into local waterbodies. To prevent harmful pollutants from being washed or dumped into an MS4, operators must obtain a NPDES permit (Phase I or Phase II) and develop a stormwater management program. This permit type is based on population. For additional information, please refer to the EPA’s MS4 Website

  • Phase I, issued in 1990, requires medium and large cities or certain counties with populations of 100,000 or more to obtain NPDES permit coverage for their stormwater discharges.
  • Phase II, issued in 1999, requires regulated small MS4s in urbanized areas, as well as small MS4s outside the Urbanized Areas (UA) that are designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges.

Phase I MS4 Permits

Phase II MS4 Permits

General NPDES Permit for Small Phase II Municipal Separate Storm Sewer Systems (MS4s)

The Phase II Final Rule established under the Federal Clean Water Act requires nationwide coverage of all small MS4s that are located within the boundaries of a Bureau of the Census-defined “urbanized area” (UA) based on the latest decennial Census.  The 2010 Census results expanded the UA area within Delaware which in turn requires DNREC to expand the universe of municipalities requiring NPDES Phase II MS4 permit coverage.  Currently the four Phase II MS4 permits in Delaware are individual permits; however, as our permitted universe continues to expand DNREC has decided to move Phase II MS4 coverage to a General Permit Program. 

Below please find links to our Draft Phase II MS4 General Permit and Draft Phase II MS4 General Permit Fact Sheet.  DNREC-SWDS is soliciting comments/feedback on the below drafts from current MS4 permittees, newly identified permittees, the general public and any other interested parties from now through January 31, 2016.  This is not an official public notice draft, but rather an informal solicitation for comments and feedback.  The DNREC-SWDS will assess all comments received and make any necessary changes to the draft general permit prior to proceeding to official public notice during Spring of 2016.   Also below you will find a Map and Table detailing all 2010 Census defined “urbanized areas” in Delaware and the associated jurisdictions identified as needing NPDES MS4 permit coverage.  We have also included a couple of useful links to EPAs website for additional information on NPDES MS4 Programs.  

The Department is in the process of reviewing comments for the Phase II General Permit.  A response to the comments and any changes to the MS4 Phase II General Permit will be provided online.


On November 5, 2015, DNREC and the Delaware Section of the American Water Resources Association (DEAWRA) held a symposium on MS4 permit requirements and status of Phase II MS4 permitting in Delaware. Presentations can be found here:

Information from the symposium can assist in understanding the Delaware MS4 Program and the Draft Phase II MS4 General Permit. For information on the DEAWRA and future events please visit their website: The Department intends to hold and/or host additional education and outreach events in the future on MS4 topics of interest and the Phase II MS4 General Permit Program. This website will be updated with future training event information as it becomes available.

Draft Phase II MS4 General Permit

If you wish to provide comments on the Draft Phase II MS4 General Permit, please direct comments to Beau Croll via email at or via U.S. Postal Mail to:

DNREC - Surface Water Discharges Section
Attn: MS4 Program
89 Kings Highway
Dover, DE 19901

DE MS4 Permitted Universe

MS4 Useful Links

MS4 Training Opportunity

MS4 Training

  • January 28, 2016 League of Local Governments Presentation


Contact Us

For more information please email Jennifer Roushey.

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