MS4 stands for Municipal Separate Storm Water Sewer System. This refers to drainage systems that are not treated in a sewage treatment plant (nearly all stormwater systems in the western U.S. (including the MSD). The Environmental Protection Agency (EPA) sets standards for stormwater runoff to reduce the discharge of pollutants from MS4s into surface waters such as rivers and streams. To maintain MS4 Permit Compliance, the justification must create and follow through with a plan to adhere to stormwater quality standards. Compliance includes litter prevention and cleanup, public education and involvement, regular water testing, code enforcement, and other pollution monitoring and prevention activities. The MSD is currently in compliance with all MS4 permit requirements.