Doña Ana County, NM Environmental Rules: Stormwater Management (2026)

§ 1342: because Doña Ana County operates a municipal separate storm sewer system (MS4), it holds an EPA general permit issued in 2007 for small MS4s in New Mexico, and that permit obligates the county to prohibit non-stormwater discharges into the MS4 and to require construction site operators to control erosion, sediment and waste. Section 350-605G translates that federal obligation into design math for developers. Under G(17), total surface runoff from the proposed improved site into existing or future developments shall not exceed the runoff from the area of land prior to development, which the design engineer can meet with retention or detention ponds, swales, drainage channels or pumping facilities.

Under G(19)(a), every drainage structure in a site's improvement plans must be designed to carry the one-hundred-year stormwater runoff, and every drainage pond must hold 125% of that one-hundred-year storm volume, with all plans and details subject to county review and approval. Ponds that do not drain within 72 hours need an approved operation and maintenance plan, and any pond deeper than three feet or with slopes steeper than 4:1 must be fenced. Construction operators disturbing soil must also file a Notice of Intent at least 14 days before work starts, maintain a stormwater pollution prevention plan (SWPPP) with monthly BMP inspections, and file a Notice of Termination within 30 days of achieving final stabilization; the County Engineer can deny a SWPPP, or any related building, grading, or subdivision permit, if it fails to control erosion and sediment. As with the rest of the Unified Development Code, these standards bind unincorporated Doña Ana County only.