A municipal separate storm sewer system, commonly called an MS4, is a conveyance or system of conveyances, such as roads with drainage systems, municipal streets, catch basins, curbs, gutters, constructed channels or storm drains, owned or operated by a governmental entity that discharges to waters of the state. Municipalities that need to obtain an MS4 permit are classified as either a "Phase I" or "Phase II" MS4. Phase I MS4s cover areas with populations greater than 100,000 (large and medium) while regulated Phase II (small) MS4s serve populations less than 100,000 and are located within a Census Bureau designated "urbanized area" (UA). Federal regulations also provide the EPA and the states the discretion to require other MS4s outside of UAs to apply for a permit.
On November 30, 2018, DEQ issued a NPDES general permit for discharges from small MS4s. This permit is Oregon’s first MS4 Phase II general permit issued after EPA’s revision to the Phase II Stormwater Rule in 2016.
The permit became effective on March 1, 2019.
MS4 Phase I NPDES permits
Clean Water Services
Watershed-based NPDES Permit: covers four wastewater treatment facilities operated by Clean Water Services; regulates the discharge of treated domestic, commercial and industrial wastewater from the four facilities to the Tualatin River; authorizes discharge of stormwater from the urban areas of the Tualatin River watershed to the Tualatin River and its tributaries.
Clackamas County Group
Clackmas County Group (Clackamas County DTD, City of Gladstone, City of Johnson City, City of Lake Oswego, City of Milwaukie, City of Oregon City, City of West Linn, City of Wilsonville, Oak Lodge Sanitary District, CCSD #1, City of Happy Valley, SWMACC and City of Rivergrove)
Stormwater Management Plans
Portland Group (City of Portland and Port of Portland)
Multnomah County, Gresham Group, City of Salem, City of Eugene
Gresham Group (Cities of Gresham and Fairview)
City of Salem
City of Eugene
Oregon Department of Transportation
MS4 Phase II NPDES permits
Willamette Basin mercury monitoring
Willamette Basin Mercury Monitoring Memo
This memo applies to Multnomah County, Gresham, Eugene, Salem and Portland MS4 permits. Mercury data was needed from MS4 permittees in the Willamette Basin to complete Phase 2 of the Willamette Mercury Total Maximum Daily Load. DEQ incorporated this data collection into monitoring requirements in Schedule B of the renewed permits..
For general information about NPDES MS4 Stormwater Discharge Permits, email MS4Stormwater@deq.state.or.us, or call 503-229-5347 or 503-229-6826