The Idaho Pollutant Discharge Elimination System (IPDES) program, administered by the Department of Environmental Quality under Idaho Code Title 39 Chapter 36, requires construction sites disturbing one or more acres to obtain stormwater coverage. State permits preempt conflicting local discharge standards.
EPA delegated NPDES authority to Idaho in 2018, and the Department of Environmental Quality (DEQ) now administers the IPDES program under Idaho Code 39-3601 et seq. Construction operators that disturb one or more acres, or are part of a common plan disturbing one acre, must obtain coverage under the IPDES Construction General Permit (CGP IDR100000). The CGP requires a Stormwater Pollution Prevention Plan (SWPPP), best management practices for sediment and erosion control, regular inspections by qualified personnel, and post-construction stabilization. Industrial facilities and certain Municipal Separate Storm Sewer System (MS4) operators require separate IPDES permits. Cities may adopt stricter local stormwater ordinances under their MS4 permits but cannot relax IPDES discharge limits.
Discharging without a required IPDES permit can result in DEQ enforcement orders, civil penalties up to $10,000 per day, and federal Clean Water Act liability for repeat violations.
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