Every five years, the Maine Department of Environmental Protection (MDEP) issues a state-wide General Permit to all communities designated as small Municipal Separate Storm Sewer Systems (MS4s). The MS4 designation is based on decennial U.S. census data for "Urbanized Areas," which are generally defined as densely developed census tracts or blocks with populations of at least 50,000. The 2013-18 General Permit identifies thirty MS4 communities in Maine and requires them all to implement Minimum Control Measures (MCMs) to reduce pollution from stormwater runoff. Windham's Urbanized Area, seen in the image above, consists primarily of the North Windham shopping centers as well as destinct urban area within the town boundaries.