MS4

MS4 stands for Municipal Separate Storm Sewer System. In the United States, an MS4 is a publicly owned or operated drainage system that collects stormwater runoff through separate pipes and channels, distinct from sanitary sewers. Urban runoff can carry pollutants such as oil, sediment, nutrients, and debris, which are discharged into nearby streams, rivers, or coastal waters when stormwater exits the system. MS4s are regulated under the Clean Water Act to reduce such discharges into water bodies.

Regulatory framework: Federal and state agencies issue NPDES permits for MS4s. Permits require municipalities to implement a Stormwater Management Program describing how runoff will be controlled. Phase I permits apply to larger cities and facilities; Phase II broadens coverage to many smaller urban areas. Core requirements typically include six minimum control measures: public education and outreach, public involvement, illicit discharge detection and elimination, construction site runoff control, post-construction runoff control, and pollution prevention and good housekeeping for municipal operations. Permits may also require mapping, inspections, monitoring, and annual reporting.

Implementation and scope: Eligible entities include cities, towns, counties, and other public agencies that operate an MS4. Programs involve inventorying the drainage network, identifying outfalls, training staff, conducting inspections, and enforcing violators. Effective management relies on coordination with land-use planning, stormwater best practices, and watershed protection goals.

Impact and challenges: MS4 programs aim to reduce pollutant loads in urban runoff to protect water quality and aquatic habitat. Challenges include funding, data collection, and adapting to changing regulatory requirements as development and climate conditions evolve.