South Gate, CA Environmental Rules: Stormwater Management (2026)

South Gate's storm drains discharge ultimately to the Los Angeles River and the Pacific Ocean, and the city is one of more than 80 LA County co-permittees under the Regional MS4 Permit (Order R4-2021-0105) adopted by the Los Angeles Regional Water Quality Control Board on July 23, 2021 (federal NPDES Permit No. CAS004004). The permit prohibits any non-stormwater discharge (NSWD) to the storm drain system other than narrowly listed exceptions (firefighting, potable water line flushing, landscape irrigation runoff if managed). Construction projects 1 acre or larger must also enroll in the statewide Construction General Permit (Order 2022-0057-DWQ) and prepare a SWPPP (Storm Water Pollution Prevention Plan) with erosion and sediment controls. Smaller projects in South Gate must still install BMPs (silt fence, fiber rolls, stabilized construction entrance, covered stockpiles) and prevent track-out per the Regional MS4 Permit's construction-site requirements. New/redevelopment projects meeting the Low Impact Development (LID) thresholds in Part VI.D.6 of Order R4-2021-0105 (typically projects creating or replacing 5,000+ sq ft of impervious area in LA County) must capture the 85th percentile 24-hour storm volume on-site through infiltration, bioretention, or harvest-and-use. Illicit discharges — including washing vehicles into the gutter, pouring paint/concrete/oil into a storm drain, or discharging pool water without dechlorination — are prohibited and tracked through the city's IDDE (Illicit Discharge Detection and Elimination) program. Industrial sites discharging stormwater must separately enroll in the Industrial General Permit (Order 2014-0057-DWQ as amended). State law (Cal. Water Code §13385) sets administrative civil liability of up to $10,000 per day per violation plus $10 per gallon for any unauthorized discharge.