Municipal Separate Storm Sewer Systems (MS4)
Polluted stormwater runoff is commonly transported through Municipal Separate Storm Sewer Systems (MS4s), from which it 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 and develop a stormwater management program.
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. There are approximately 750 Phase I MS4s.
Phase II, issued in 1999, requires regulated small MS4s in urbanized areas,as well as small MS4s outside the urbanized areas that are designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges. There are approximately 6,700 Phase II MS4s.
Generally, Phase I MS4s are covered by individual permits and Phase II MS4s are covered by a general permit. Each regulated MS4 is required to develop and implement a stormwater management program (SWMP) to reduce the contamination of stormwater runoff and prohibit illicit discharges.
WHAT IS AN MS4?
An MS4 is a conveyance or system of conveyances that is:
• Owned by a state, city, town, village, or other public entity that discharges to waters of the U.S.;
• Designed or used to collect or convey stormwater (including storm drains, pipes, ditches, etc.);
• Not a combined sewer; and
• Not part of a Publicly Owned Treatment Works (sewage treatment plant).
During Phase I, NDOT was defined as the co-permittee on large MS4 Permits for Clark and Washoe Counties. In the wake of Phase II, NDOT was faced with co-permittee responsibilities for all regulated small MS4 areas, as well as the individual permit for the Lake Tahoe basin. NDOT requested a single statewide NPDES permit from NDEP to fulfill the large and small MS4 requirements, as well as the Lake Tahoe permit. In February 2004, NDOT was issued the NPDES MS4 Permit NV0023329 by NDEP. In September 2004, NDOT received a letter from NDEP confirming acceptance of NDOT’s request to cancel the discharge permit NV0023205 for NDOT discharges within the Lake Tahoe basin. NDEP reissued MS4 Permit NV0023329 to NDOT in 2010.
NDOT’s MS4 Permit covers “state and interstate highways and their right-of-ways within the jurisdictional boundary of the Nevada Department of Transportation (“NDOT” or “Permittee”) served by, or otherwise contributing to discharges into receiving Waters of the United States from municipal separate storm sewer systems (“MS4s”) owned or operated by NDOT.”