What is a Municipal Separate Storm Sewer System (MS4)?

Municipal storm water sewer systems (MS4s) that are located within the boundaries of a Census Bureau defined "urbanized area" are regulated under EPA's Phase II Storm Water Rule. This requires MS4s to develop a storm water management program that will reduce the amount of pollutants carries by storm water during storms events to water bodies tot he "maximum extend practicable". The goal of the program is to improve water quality and recreation use of waterways. MS4 storm water programs have six elements called Minimum Control Measures (MCM) that when implemented together, are expected to result in a reduction of pollutants discharged into water bodies.