Municipal Stormwater Program
Municipal Stormwater Program
This domain tests the NPDES municipal separate storm sewer system (MS4) program. You need to know which MS4s are regulated, the six minimum control measures (MCMs), how IDDE and construction/post-construction programs work, and how permits, annual reports, and TMDL wasteload allocations fit together.
Core rules
- An MS4 is a conveyance or system of conveyances (roads with drainage, catch basins, curbs, gutters, ditches, man-made channels, pipes) owned by a state or local government, designed for collecting or conveying stormwater, and not a combined sewer. Phase I covers large/medium MS4s (generally populations of 100,000 or more) and designated MS4s. Phase II covers regulated small MS4s in 2020 urbanized areas and those designated by the permitting authority; population alone does not decide.
- Phase II permits require six MCMs: public education and outreach; public participation/involvement; illicit discharge detection and elimination (IDDE); construction site runoff control; post-construction stormwater management; and pollution prevention/good housekeeping.
- IDDE requires an ordinance or other mechanism, a storm sewer map, outfall screening, tracing, removal of illicit connections, and enforcement. Non-stormwater discharges such as sewage, process wastewater, and commercial car wash wastewater are prohibited; limited exemptions (e.g., firefighting, water line flushing) still require controls.
- The construction program must review SWPPPs, inspect sites disturbing 1 acre or more, and enforce locally. The post-construction program must require permanent BMPs, long-term O&M, and hydrologic controls for new development and redevelopment.
- Permittees submit an NOI, implement a stormwater management program, set measurable goals, and report annually. Records are typically kept at least 3 years. Where a TMDL exists, the permit must implement the wasteload allocation through BMPs and monitoring.
Worked example
A town of 48,000 people lies inside a 2020 urbanized area. It owns streets, catch basins, and ditches that discharge to a creek. The town is therefore a regulated small MS4 and needs Phase II permit coverage. It must develop a stormwater management program with all six MCMs, adopt an IDDE ordinance, map its outfalls, screen priority outfalls, and inspect construction sites of 1 acre or more. If the creek has a TSS TMDL, the town must show how its program implements the MS4 wasteload allocation, not just claim that it sweeps streets.
Traps
Treating MS4s as combined sewers; assuming a population below 100,000 is automatically exempt; implementing only some MCMs; writing an IDDE ordinance without screening or enforcement; approving construction plans without SWPPP review; issuing post-construction approvals without O&M; ignoring TMDL wasteload allocations; missing annual reporting.