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Understanding Municipal Separate Storm Sewer Systems (MS4s)

A Municipal Separate Storm Sewer System, or MS4, is a conveyance system designed to gather rainwater and snowmelt runoff, transporting it away from urbanized areas. The key objective is to prevent the accumulation of excess water on streets, parks, and other public spaces that could lead to flooding or other related hazards.


What is an MS4?


As the name suggests, an MS4 is a municipally-owned structure that encompasses a broad array of infrastructures such as streets, curbs, gutters, ditches, man-made channels, and storm drains. It is worth noting that MS4s are separate from sanitary sewers, which manage domestic sewage and industrial wastewater. This separation is important, as the runoff collected by MS4s is often discharged untreated into local water bodies, including rivers, streams, and lakes.





Role of an MS4


The principal role of an MS4 is to effectively manage stormwater runoff, mitigating the potential risks of urban flooding. During periods of heavy rainfall or snowmelt, the runoff can rapidly overwhelm urban areas, causing significant water damage and even threatening public safety.


Further, MS4s play an essential role in controlling the discharge of pollutants into water bodies. As the stormwater traverses over urban surfaces, it can pick up various contaminants such as motor oil, pesticides, litter, and other pollutants, which can potentially degrade the water quality of the receiving bodies.


MS4 Requirements and Operation


Under the Clean Water Act, MS4s are required to develop and implement a stormwater management program to minimize the discharge of pollutants. This is primarily overseen by the Environmental Protection Agency (EPA), but the responsibility can be delegated to state-level environmental agencies, depending on the region.


To meet these requirements, MS4 operators usually have to develop six minimum control measures:

  1. Public Education and Outreach: Increase public awareness and understanding of stormwater issues.

  2. Public Involvement and Participation: Engage the community in stormwater management programs.

  3. Illicit Discharge Detection and Elimination: Identify and remove unauthorized discharges into the stormwater system.

  4. Construction Site Runoff Control: Implement strategies to prevent pollutants from construction sites from entering the stormwater system.

  5. Post-Construction Runoff Control: Manage stormwater runoff from new developments and redevelopment areas.

  6. Pollution Prevention/Good Housekeeping: Establish a program to prevent and reduce pollutant runoff from municipal operations.

MS4s work through a series of drains, pipes, and open channels designed to quickly transport runoff from urban areas to nearby water bodies. The system essentially relies on gravity, with water flowing from higher to lower areas, and is usually supplemented with pumping stations in low-lying areas.


MS4s and the Private Sector


Regulating the private sector is another crucial aspect of MS4 management. Since many potential sources of stormwater pollution, such as construction sites or industrial facilities, are privately owned, the operators of MS4s have to ensure that these entities comply with local, state, and federal stormwater regulations.


This might involve monitoring and enforcing the implementation of Best Management Practices (BMPs) at these sites, or requiring the private sector to obtain stormwater permits for certain activities that may potentially contaminate the runoff.





Relationship with the State and the EPA


The relationship between MS4 operators, the state, and the EPA is central to effective stormwater management. The EPA sets the overarching regulations and standards for stormwater management across the nation, under the authority of the Clean Water Act.

In many cases, the EPA delegates the authority to regulate and enforce stormwater permits to state environmental agencies. The state can also provide funding, guidance, and technical assistance to MS4 operators. It’s important to note that while states can set more stringent requirements, they cannot make them less rigorous than the EPA’s guidelines.

MS4 operators, in turn, are responsible for local implementation of these regulations. They are required to periodically report to the state and the EPA about their stormwater management activities, demonstrating their compliance with the regulations and their progress in minimizing the discharge of pollutants.


In conclusion, MS4s play a pivotal role in managing urban stormwater, minimizing pollution, and reducing the risk of flooding. They function in compliance with regulations set by the EPA and state environmental agencies and also regulate the private sector to prevent the contamination of runoff. The effectiveness of MS4s is instrumental in safeguarding our urban environments and preserving the quality of our water resources.


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