Stormwater runoff from construction sites can carry sediment, concrete washout, chemicals, and debris directly into local waterways if it isn’t properly controlled. While most contractors understand the rules around discharging into rivers and streams, many don’t realize that municipal storm drains—known as MS4s—also fall under strict federal permitting requirements.
In urban and suburban areas, MS4s handle the majority of stormwater movement. These systems discharge stormwater directly into creeks, rivers, and lakes without treatment, so any pollutants entering an MS4 immediately threaten public water quality.
If your construction site discharges into an MS4, you must follow NPDES permitting requirements and implement specific stormwater controls to stay in compliance.
This guide explains what MS4s are, why they require special attention during construction, and how contractors can meet federal and local stormwater regulations.
What Are MS4s in Stormwater Management?
MS4s, short for “Municipal Separate Storm Sewer System,” regulate any municipal-owned separate sewer system that is not connected to a sewage treatment facility–hence the word “separate.”
These drainage systems are typically designed to release built-up stormwater and discharge it directly into public water systems.
The main goal of MS4s is to regulate the amount of stormwater runoff without causing flooding or overloading treatment facilities.
The most common examples of MS4s include:
- Retention basins
- Drainage pipes that run underground
- Ditches
- Swales
Since municipal facilities do not treat MS4s, it is very important that only stormwater, not “illicit discharge,” flows through MS4s.
Why MS4 Regulations Are Stricter in Urban Areas
Cities and suburbs have significantly more paved surfaces than rural areas. Because MS4s drain directly into surface waters, urban construction projects must use stronger BMPs, more frequent inspections, and tighter erosion-control practices to prevent pollutants from entering the stormwater system.
Concrete, asphalt, and rooftops prevent water from soaking into the soil, causing stormwater to move faster and collect more pollutants as it flows.
Urban runoff often contains:
- Oil and grease from vehicles
- Trash and debris
- Road salts
- Construction sediment
- Fertilizers and pesticides
- Metals from brake dust and building materials
Municipalities face penalties if MS4 pollution exceeds allowable limits, which is why they enforce strict rules on construction sites. Contractors who understand these risks—and design BMPs with urban runoff in mind—reduce violations and project delays.
Acquiring an NPDES for MS4 Discharge
Under Phase II of the Clean Water Act, all construction sites that plan to discharge into municipal separate sewage systems are required to obtain an NPDES permit.
This permit establishes allowable standards and best practices to prevent sediment, debris, or toxins from being discharged from a construction site directly into public waters.
A Notice of Intent (NOI) will be required to obtain a Construction General Permit if you plan to disturb one or more acres of land within an area that discharges into an MS4.
Post-Construction Requirements for Managing MS4s
Under NPDES permitting regulations, each municipality is required to develop a post-construction stormwater management plan (PCSM) protecting MS4s from illicit discharges.
In turn, this could alter regulations between different municipalities of what you are allowed to discharge within MS4s and what you are not.
Researching local codes and contacting municipal authorities to see what additional requirements you will need to follow will be important in acquiring proper local permitting.
Common Mistakes Contractors Make When Working Near MS4s
Even experienced contractors run into compliance issues when working in MS4 jurisdictions. The most frequent mistakes include:
- Not protecting storm drain inlets: Uncovered or poorly protected inlets can immediately expose the MS4 to sediment and debris.
- Improper stockpile management: Uncovered soil piles near an inlet can wash directly into the stormwater system during rainfall.
- Neglecting secondary containment: Fuel, chemicals, and paints can leak into storm drains if not properly contained.
- Assuming all BMPs apply everywhere: MS4s often require municipality-specific controls beyond the CGP—missing these can lead to violations.
- Failing to update the SWPPP: If site conditions shift, the SWPPP must be revised to reflect new drainage patterns, slopes, and BMP needs.
Avoiding these mistakes keeps projects on schedule and protects contractors from costly enforcement actions.
Best Management Practices for Protecting MS4s
Construction managers and local municipalities can work together to create a list of best management practices to protect MS4s. Some common examples of positive BMPs include:
- Infiltration practices: Permeable pavements, rain gardens, and grass swales.
- Filtration practices: Green roofs, vegetated filter strips, wet swales, and sand filters.
- Retention measures: Dry retention basins and wet ponds.
- Onsite treatment and harvesting: Internal filtration systems and recycling stormwater runoff.
Working with municipal authorities, as well as third-party environmental consultants, to develop a proper SWPPP and post-construction stormwater plan will ensure your regulatory compliance.
FAQs
What is the purpose of an MS4 program?
The primary purpose of an MS4 program is to manage stormwater within urban areas to prevent pollution and protect water quality. MS4 programs aim to control the discharge of stormwater into local water bodies, ensuring that it meets environmental standards and regulations.
By implementing MS4 programs, municipalities can reduce the harmful effects of stormwater runoff, such as flooding and the contamination of rivers, lakes, and oceans.
Who needs to comply with MS4 regulations?
Municipalities, often referred to as “permittees,” are the entities required to comply with MS4 regulations. These municipalities are typically located in urbanized areas and must obtain permits from the Environmental Protection Agency (EPA) or their state environmental agencies.
The permits outline the specific requirements and responsibilities for managing stormwater within their jurisdictions. However, construction sites and all dischargers will need to follow municipal rules to ensure proper compliance.
How are MS4 permits obtained?
MS4 permits are obtained through a regulatory process governed by the EPA and their state environmental agencies. To obtain a permit, municipalities must submit an application that outlines their stormwater management plan.
This plan details how the municipality intends to address the six minimum control measures required for MS4 compliance. After review and approval, the permit is issued, and the municipality is responsible for implementing and adhering to the permit’s requirements.
What are the penalties for MS4 non-compliance?
Non-compliance with MS4 regulations can lead to significant penalties. Penalties may include fines, legal action, and, in severe cases, the revocation of the MS4 permit.
Municipalities failing to meet the minimum control measures and other compliance requirements can face costly consequences. Additionally, non-compliance can harm the local environment, potentially leading to environmental damage and public health concerns.
How can communities benefit from MS4 compliance?
Communities can benefit from MS4 compliance in several ways. Firstly, compliance ensures improved water quality, reducing the risk of contamination in local water bodies, which directly benefits residents.
Secondly, effective stormwater management also leads to decreased flooding and erosion, making communities safer and more resilient. Compliance can also enhance a community’s reputation, attracting businesses and investors.
Furthermore, it fosters a sense of environmental responsibility and can lead to community involvement in sustainability efforts. Overall, MS4 compliance helps create healthier, more prosperous, and sustainable communities.
What happens if illicit discharge enters an MS4?
The municipality must report and remediate it, often at the contractor’s expense. This can lead to significant fines and mandatory corrective actions.
Does every municipality follow the same MS4 rules?
No. Each MS4 has its own ordinance, design standards, and BMP requirements, so contractors should verify local regulations early.
Are post-construction stormwater controls required?
Yes. Long-term controls like basins, filter strips, and infiltration practices are often mandatory to manage runoff permanently.