MS4s must currently comply with the minimum control measure for illicit discharge detection and elimination (IDDE) in their current stormwater NPDES permits.

• Adopt a local ordinance to prohibit illicit discharges to the storm drain system
• Develop a storm drain map, including all outfalls 36 inches in diameter or larger
• Provide IDDE education and outreach to public employees, businesses and the general public
• Use visual indicators to screen outfalls for the presence of illicit discharges
• Implement appropriate enforcement procedures to fix illicit discharge after they are discovered

Source:  Chesapeake Stormwater Network