NPDES stormwater compliance means meeting the permit requirements of the National Pollutant Discharge Elimination System, the EPA program under the Clean Water Act that regulates stormwater discharges from industrial facilities, construction sites, and municipalities. It requires obtaining a permit, developing a SWPPP, conducting regular inspections, and submitting monitoring reports to the permitting authority.

A Stormwater Pollution Prevention Plan (SWPPP) is a site-specific written plan identifying pollution sources and the best management practices (BMPs) that prevent them from entering stormwater discharges. SWPPPs are required for industrial facilities under the MSGP and for construction sites disturbing one acre or more under the Construction General Permit. The plan must be kept on site, updated when conditions change, and available to regulators during inspections.

Most industrial facilities discharging stormwater to surface waters must obtain coverage under EPA’s Multi-Sector General Permit (MSGP), which covers 30 industrial sectors. The MSGP requires a SWPPP, routine inspections, benchmark monitoring, discharge monitoring reports (DMRs), and corrective action when benchmarks are exceeded. Facilities in NPDES-authorized states may operate under equivalent state permits with additional requirements.

Clean Water Act violations carry a maximum civil penalty of $64,618 per day, per violation under 2025 EPA inflation-adjusted maximums (40 CFR Part 19). Penalties depend on violation severity, compliance history, and good-faith remediation efforts. Missing a single benchmark monitoring sample, failing to update a SWPPP after a site change, or skipping a required inspection can each constitute a separate, per-day violation.

The Construction General Permit (CGP) authorizes stormwater discharges from construction activities disturbing one or more acres. Operators file a Notice of Intent (NOI), prepare and implement a site-specific SWPPP, install and maintain BMPs, conduct regular inspections, and file a Notice of Termination (NOT) when the site is stabilized. States with NPDES authority issue equivalent state CGPs that may carry stricter or different requirements.

An MS4 (Municipal Separate Storm Sewer System) permit is an NPDES permit issued to municipalities and public entities that operate storm sewer systems discharging to U.S. waters. MS4 permits require six minimum control measures: public education, public involvement, illicit discharge detection and elimination (IDDE), construction site runoff control, post-construction stormwater management, and pollution prevention for municipal operations. Phase I covers large municipalities; Phase II extends to smaller communities.

Best management practices (BMPs) are structural or non-structural measures that prevent or reduce pollutant discharge in stormwater runoff. Structural BMPs include sediment basins, silt fencing, bioretention cells, rock check dams, and outlet protection structures. Non-structural BMPs include employee training, good housekeeping procedures, spill prevention protocols, and routine inspections. NPDES permits require that BMPs be selected, installed, maintained, and documented in the SWPPP.