Recent changes to EPA stormwater regulations have expanded which facilities qualify as small municipal separate storm sewer systems (MS4s), and that shift may directly affect your bioretention facility. If you own or manage a rain garden, bioretention cell, or similar stormwater feature, understanding these new requirements is essential for staying compliant and avoiding costly enforcement actions.
Bioretention & MS4 Compliance: At a Glance
| Topic | Details |
| Regulatory Change | EPA redefined “urban areas” as locations with 50,000+ people (July 12, 2023) |
| Program Affected | MS4 (Municipal Separate Storm Sewer System) |
| Who May Be Impacted | Bioretention facility owners newly classified under MS4 |
| Primary Requirement | SMW Site Development Plan |
| Plan Components | Storm Drain Plan, Flood Plain Checklist, Geotechnical Investigations, Drainage Area Maps |
| Available Inspections | VSMP, MDE, DOEE |
What Changed for Stormwater Management?
On July 12, 2023, the Environmental Protection Agency (EPA) issued a final rule that clarified that urban areas are ‘locations with 50,000 or more people’. This clarification has implications for what can be classified as a small municipal separate storm system (MS4), meaning that stormwater management facilities may now be required to implement stricter guidelines, including bioretention facilities.
If your facility is located in an area that now meets this updated urban threshold, you may be newly classified under the MS4 program, triggering a new set of compliance obligations for your bioretention system.
What the MS4 Classification Means for Your Bioretention Facility
If your facility now falls under the MS4 program, your bioretention facility might be subject to additional stormwater management rules. If you own a rain garden to collect runoff from nearby impervious surfaces, you may fall under the new MS4 guidelines. You may need to provide an SMW Site Development plan for your site and ensure that your facility meets the required guidelines. This includes conducting textural, chemical, pH and organic analyses for plants and topsoil in your bioretention system.
If you are an existing or new stormwater management facility under the MS4 program, you might need an SMW Site Development Plan. This plan includes critical items that are needed to meet storm management approval, including:
- A Storm Drain Plan checklist
- A Flood Plain Checklist
- Geotechnical investigations
- Drainage area maps
An SMW Site Development Plan is more than a permit formality—it’s a comprehensive documentation package that demonstrates your facility is properly designed, sited, and capable of managing stormwater in compliance with federal and state standards. Missing components can delay permit approval or trigger regulatory review.
Bioretention Facility Compliance Requirements
Once classified under MS4 oversight, your bioretention facility must meet specific technical standards across four primary areas. Here’s a quick-reference breakdown before we go into detail:
| Requirement Area | Key Standards |
| Sand Filter Testing | Watertight with no leakage for a minimum of 8 hours; all overflows and channels tested |
| Manholes | OSHA-compliant; steps drilled into wall or cast in place below each manhole |
| Planting Soil | Free of stones, stumps, or roots over 2 inches; uniform mix; no noxious weeds or contaminants |
| Plant Installation | Shredded hardwood mulch aged 6+ months; proper planting distances; thoroughly watered post-installation |
Below is a more detailed look at what each area entails:
Sand Filter Testing Specifications
Underground sand filters must be watertight and have no leakage for a period of eight hours. All overflows and channels must also be tested to ensure the facility is watertight and equally distributes flows.
Manholes
Manholes must be provided to underground channels and must comply with OSHA guidelines. Steps and height clearance guidelines must be adhered to, and steps must be drilled into the wall or cast in place below each manhole.
Planting Soil
Planting soil must be free from stones, stumps, or roots over two inches. The soil must be a uniform mix with no contaminants that might impact vegetative growth. No noxious weeds can be present in the soil.
Plant Installation
Shredded hardwood mulch must be used in plant installation and be well-aged for over 6 months. Trees must be set at the appropriate planting distance, and the plant bed must be thoroughly watered after installation.
These requirements extend beyond initial setup; routine maintenance and consistent documentation are essential to demonstrate ongoing compliance with your MS4 permit obligations.
How Muller, Inc. Supports Bioretention Compliance
Muller, Inc. is the region’s leading stormwater maintenance specialist, with a qualified team of stormwater-certified professionals who can meet the new guidelines for your bioretention facility. Whether you’re building new, redeveloping an existing system, or simply need to confirm you’re within current federal, state, and local guidelines, we offer the full range of support needed to stay compliant:
- Routine maintenance and repair of underground overflows, planting soil, and plant installations
- Certified VSMP, MDE, and DOEE inspections
- Detailed compliance documentation to submit to regulatory authorities
- Erosion inspection and repair, including silt fencing and slope stabilization
When issues are identified, our team can carry out any corrective recommendations as part of our services, so you’re not left coordinating multiple vendors to close compliance gaps.
Manage Stormwater Confidently with Muller, Inc.
The amendments to federal, state, and local laws regarding stormwater management may impact whether your bioretention facility falls under stormwater management guidelines. Staying ahead of compliance requirements protects your facility from enforcement action, and helps safeguard the local waterways your system is designed to protect.
Muller’s certified stormwater team is ready to assess your bioretention facility, identify compliance gaps, and carry out the maintenance and documentation work needed to keep you in good standing with federal, state, and local regulators.
Contact Muller, Inc. today to schedule your bioretention compliance assessment.
Frequently Asked Questions
How often should a bioretention facility be inspected under MS4? Inspection frequency depends on your permit conditions and applicable state guidelines, but most MS4 permits require at least annual inspections, with additional checks recommended following significant storm events.
What happens if my bioretention facility fails an MS4 inspection? A failed inspection typically results in a notice of deficiency requiring corrective action within a specified timeframe. Repeated non-compliance can escalate to permit violations and potential fines from the regulating authority.
Does the 2023 EPA rule apply to privately owned stormwater facilities? Yes, if your privately owned facility is located in a newly designated urban area and discharges to a regulated MS4 system, the program requirements apply regardless of ownership type.
Do I need an NPDES permit for my bioretention facility? If your facility falls under the MS4 program, you will likely need to operate under an NPDES permit or contribute to a larger MS4 permit held by your municipality. A certified stormwater professional can help clarify your specific permit obligations.
