Unincorporated San Mateo County is a permittee under the San Francisco Bay Area Municipal Regional Stormwater Permit (MRP 3.0, Order R2-2022-0018) which imposes NPDES Phase I stormwater requirements. Projects creating/replacing 2,500+ sq ft of impervious surface trigger C.3 post-construction stormwater treatment requirements (LID, biofiltration). Illicit discharges to storm drains are prohibited. Construction sites disturbing 1+ acres require SWPPPs and monthly wet-season inspections.
The County operates under the San Francisco Bay Regional Water Quality Control Board's MRP (Municipal Regional Stormwater NPDES Permit), which regulates municipal discharges from the County's MS4 (municipal separate storm sewer system). Provision C.3 (New Development and Redevelopment) applies to 'Regulated Projects' creating/replacing 2,500 sq ft or more of impervious surface — these projects must implement Low-Impact Development (LID) site design, source control, and stormwater treatment. Treatment options include bioretention (rain gardens), flow-through planters, pervious pavement, and infiltration trenches. Provision C.6 (Construction Site Controls) requires Erosion & Sediment Control Plans and monthly wet-season (Oct 1-Apr 30) inspections of construction sites 1+ acre. Illicit discharges of pollutants (paint, oil, soap, chemicals, gray water, pool chlorine) to storm drains are prohibited under Provision C.5 and County Code. Residents and contractors can report illicit discharges to the County stormwater hotline. All new development in unincorporated County must follow the SMC Green Infrastructure Plan's LID approach.
Illicit discharge to storm drain: Clean Water Act civil penalties up to $56,460/day plus County Code $500-$10,000 per incident. C.3 non-compliance: permit denial, required retrofit, $1,000-$25,000. SWPPP violation: stop-work order plus daily $250-$2,500. Sediment entering waterway: RWQCB enforcement up to $25,000/day.
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