MS4 stands for Municipal Separate Storm Sewer System. It refers to a publicly owned system of conveyances, such as pipes, ditches, gutters, and other infrastructure, that collects and carries stormwater runoff from urban areas. These systems are “separate” from sanitary sewers, meaning they are not connected to a sewage treatment plant.
Pollutant transport in a Municipal Separate Storm Sewer System (MS4) involves stormwater runoff from various urban sources, like roads and lawns, picking up pollutants such as oil, pesticides, dirt, and bacteria. This contaminated runoff enters the MS4’s pipes and ditches, which are designed to quickly transport it, untreated, directly into local waterways, harming aquatic life and water quality. Human activities, from careless fertilizer use to illicit discharges, are primary drivers of the pollution that enters these systems.
Municipalities and other entities such as universities and prisons that meet certain criteria must obtain NPDES permit coverage for discharges of stormwater from their municipal separate storm sewer systems (MS4s). In Pennsylvania, there is one Large MS4, one Medium MS4, and 1,059 Small MS4s. Additional Small MS4s may be designated by DEP when determined to be appropriate using DEP’s designation criteria.
MS4s must apply for NPDES permit coverage or a waiver if they are located in an census-designated Urban Area that meets EPA’s automatic designation criteria for small MS4s, or if they are designated as needing a permit by DEP.
For NPDES permits issued after 2017, MS4s that discharge to surface waters impaired for certain pollutants or that discharge to waters in the Chesapeake Bay watershed were required to develop Pollutant Reduction Plans (PRPs) or TMDL Plans. Hemlock Township is one of those Municipalities.
MS4 permittees must develop, implement, and enforce a Stormwater Management Program (SWMP) designed to reduce the discharge of pollutants from the MS4 to the maximum extent practicable, to protect water quality, and to satisfy the appropriate water quality requirements of the Clean Water Act and Pennsylvania Clean Streams Law. A requirement of Minimum Control Measure (MCM) #1 of the MS4 permit is that the Township provide public education and outreach on stormwater Impacts to our residents.
The downloads below provide more information about MS4 activities in Hemlock Township:
