Maryland’s NPDES Municipal Separate Storm Sewer System (MS4) Permits

​Department of the Environment Issues Stormwater Permits for Large Maryland Jurisdictions, Advances Climate Resiliency and Equity​

​The Maryland Department of the Environment (MDE) has issued a series of municipal stormwater permits to advance Chesapeake Bay restoration while reducing flooding and making communities more resilient to the effects of climate change.​

Maryland’s Municipal Stormwater Permits

When it rains, stormwater washes excess nutrients, chemicals, and dirt from impervious surfaces, such as buildings, roads, and parking lots into local waters and the Chesapeake Bay. Excess stormwater runoff can also flood local communities, scour sediment from waterways, and degrade the health of stream systems.
MDE’s MS4 permits continue the state’s robust work to reduce nutrient and sediment pollution from stormwater as part of a larger effort that requires all sectors, such as sewage treatment plants and septic systems, in Maryland and the surrounding region to protect and restore the Chesapeake Bay.
The MS4 permits are a result of extensive stakeholder engagement. Maintenance of existing infrastructure is essential and the new permits require local jurisdictions to maintain the previous stormwater pollution reduction efforts while also requiring additional  stormwater pollution reduction to restore waterways.  The additional restoration requirements are to reduce the impacts of  impervious surface areas that have little through stormwater treatment with green infrastructure and other techniques. These new permits meet Maryland’sChesapeake Bay commitments and also increase accountability, enhance public education and include innovative and cost-effective monitoring options.
In the next five years the new permits add another 19,000 acres to the 35,000 impervious acres restored under prior permits. This restoration encourages climate resiliency and green infrastructure and at the same time continues to advance innovations such as pay-for-performance contracting, public-private partnerships, and new technologies. The permits also include new incentives for climate resiliency and green and blue infrastructure projects.

Maryland’s 11 phase I urban jurisdictions have established themselves as national leaders in reducing stormwater pollution by collectively investing over $1 billion in clean water infrastructure. Since July 2019, MDE’s Water Quality Finance Administration has guaranteed $117.8 million in low interest loans to counties and local governments for stormwater restoration projects, and another $257 million in low interest loans are pending for planned projects. During the prior MS4 permit term, which started in 2014, the Chesapeake Bay Trust awarded an additional $36.5 million in grants to stormwater programs for restoration projects, which leveraged another $27.7 million in matching funds. Maryland continues to push for additional federal funding for local stormwater projects – especially for those that increase climate resiliency in underserved communities and for those that help sustain our Chesapeake Bay restoration efforts for decades to come.

Advancing Stormwater Resiliency in Maryland

Urban flooding is a growing issue in Maryland. The increasing number of extreme rainfall events that produce intense precipitation will continue to lead to more urban flooding events unless steps are taken to mitigate their impacts. The Maryland General Assembly recognized the need to address urban flooding with the passing of Senate Bill 227. The Bill tasks the Maryland Department of the Environment (Department) with developing plans to evaluate current flooding risks and update regulations to improve urban stormwater flood management. The "Advancing Stormwater Resiliency in Maryland" report provides a roadmap towards modernizing stormwater management in Maryland.​

Where Do We Start?

The Department is taking action through collection of data sets, development of tools and web applications, targeting funding opportunities, developing outreach and education and updating stormwater regulations. Step one is performing a statewide comprehensive urban flood management assessment. With the help of local floodplain and stormwater managers, the Department will identify and characterize Maryland’s urban flooding events and report the current state of local stormwater and flood management programs.​ For more information on this effort, please visit the Advancing Stormwater Resiliency in Maryland ​page here​.

Notice of Tentative Determination to Issue Stormwater Permit to MDOT SHA (April 19, 2024)


The Maryland Department of the Environment, Water and Science Administration (Department) has reached a tentative determination to issue a National Pollutant Discharge Elimination System permit to the Maryland Department of Transportation State Highway Administration (MDOT SHA) for Discharges from Large Municipal Separate Storm Sewer Systems (Discharge Permit No. 24-DP-3313, NPDES No. MD0068276.  The Department has drafted a permit designed to comply with the United States Environmental Protection Agency’s (EPA) regulations and to control stormwater pollutant discharges from the MDOT SHA’s storm drain system.  The permit is issued for five years.

Under the conditions of the permit, MDOT SHA is required to possess the legal authority to control storm drain system pollutants, maintain its storm sewer system, monitor stormwater discharges, and develop and implement comprehensive management programs.  The permit also sets new requirements for impervious area treatment, property management and maintenance, and options for comprehensive water quality monitoring.  MDOT SHA is also required to develop and implement plans to address wasteload allocations established under EPA approved total maximum daily loads.  Penalties for failure to comply with the terms of the permit are provided.

The draft ​permit and fact sheet can be viewed here:

MS4 Guidance Includes:​

The Department used the following documents to help assess the physical and financial capacity of MDOT SHA to meet restoration goals proposed in the permit.

This submission was based on instructions and a spreadsheet provided by the Department. These instructions may be found here: ​

Hard copies of the draft permit may also be procured at a cost of 36¢ per page. Written requests for copies should be directed to Mr. Stewart Comstock, Maryland Department of the Environment, Water and Science Administration, Stormwater, Dam Safety, and Flood Management Program, 1800 Washington Blvd., STE 440, Baltimore, Maryland 21230-1708. Additional information on stormwater management in Maryland can also be found on the Department's website or by calling the Stormwater, Dam Safety and Flood Management Program at 410-537-3543 or 1-800-633-6101.

In 2022, revisions to Maryland’s law require applicants who are applying for a permit that requires public notice to use the Department’s Environmental Justice (EJ) Screening Tool to develop an EJ Score for census tracts in the applicant’s permit area.  The Department is required to review the submitted EJ scores and census tracts to verify the applicant’s information.  MDOT SHA submitted this information, and the Department verified the scores by comparing the reported scores to the EJ Screening Tool.  More information on these EJ scores and EJ at the Department may be found here:​​
The Department will hold a public hearing concerning this tentative determination from 11:00 AM to 1:00 PM on June 4, 2024 at the Department’s Aeris and Aqua Conference Rooms at 1800 Washington Blvd., Baltimore, MD 21230.  Any hearing-impaired person may request an interpreter at the hearing by contacting the Office of Fair Practices at 410-537-3964 at least ten working days prior to the scheduled hearing date.  TTY users should contact the Maryland Relay Service at 1-800-201-7165.​
Written comments should be directed to Mr. Stewart Comstock, Maryland Department of the Environment, Water and Science Administration, Stormwater, Dam Safety, and Flood Management Program, 1800 Washington Blvd., STE 440, Baltimore, Maryland 21230-1708, or Stewart.Comstock@Maryland .gov.  Written comments concerning this tentative determination will be accepted through July 18, 2024.  This comment period already incorporates the additional 60 day extension period provided in Environment Article §1-606(d)(1)(ii). The Department will respond to all pertinent comments during the Final Determination process. Once the Final Determination is issued, the public will have 30 days to request a judicial review of the permit.

NPDES Phase I Permits


​You can view background information on each jurisdiction by clicking on the municipality's name in the chart below. Initial permits, current permits, annual reports, and links to the agency responsible for administering each local permit are also accessible by clicking on the respective links below.

​Initial Permit
​Current Permit
​MS4 Annual Reports​ ​Local Agency Responsible for Annual Reporting
Large Jurisdictions

Anne Arundel County12/02/1993
Dept. of Public Works / Watershed Protection and Restoration
Baltimore City 11/17/1993
Dept. of Public Works
Baltimore County
Dept. of Environmental Protection & Sustainability
Montgomery County
Dept. of Environmental Protection
Prince George’s County
Dept. of Environment​

​Medium Jurisdictions
Carroll County
05/30/1995​​​​ 12/30/2022 Reports
Lan​d Use, Planning &
Charles County05/01/199712/30/2022ReportsPlanning & Growth Management
Frederick County 11/15/199412/30/2022​ReportsDept. of Stormwater
Harford County05/17/1994
12/30/2022Reports ​Dept. of Public Works
Howard County04/17/1995
12/30/2022 Reports
​​Dept. of Public Works / Stormwater Management Division​​

State Storm Drain Systems

State Highway Administration
Office of Environmental Design

Summary of Local Requirements
National Pollutant Discharge Elimination System (NPDES) municipal stormwater permits require the jurisdictions to develop comprehensive programs to reduce storm drain system pollution to the maximum extent practicable (MEP). Typical NPDES stormwater permit conditions require jurisdictions to: 
  • Legal Authority: provide certification from appropriate legal counsel that adequate authority exists to control discharges from the municipal storm drain system
  • Source Identification: map storm drain pipes and best management practices (BMPs), land use, impervious cover, and watershed restoration projects in geographical information system (GIS) format
  • Management Programs: implement erosion and sediment control, stormwater management, illicit connection detection and elimination, and public education and outreach programs
  • Watershed Assessment: evaluate all urban watersheds thoroughly regarding water quality and develop goals and action plans for restoration,
  • Restoration Projects: restore a specific amount of uncontrolled impervious surfaces based on watershed assessments during each five-year permit cycle
  • Assessment of Control: document work toward meeting watershed restoration goals, including total maximum daily loads (TMDLs)*, using chemical, biological, and physical monitoring
  • Program Funding: provide an annual fiscal analysis of the capital, operation, and maintenance expenditures necessary to comply with permit conditions
More information on TMDLs may be found on MDE's webpage here: Introduction to TMDLs

Additional Information for Previously Issued Permits


Response to Comments

Phase I Large MS4 Permit Tentative Determination Comments​

The Maryland Department of the Environment, Water and Science Administration (the Department) reached a final determination to issue a National Pollutant Discharge Elimination System (NPDES) permit to Anne Arundel County (Discharge Permit No. 20-DP-3316, NPDES Permit No. MD0068306), Baltimore City (Discharge Permit No. 20-DP-3315, NPDES Permit No. MD0068292), Baltimore County (Discharge Permit No. 20-DP-3317, NPDES Permit No. MD0068314), and Montgomery County (Discharge Permit No. 20-DP-3320, NPDES Permit No. MD0068349) for Discharges from Large Municipal Separate Storm Sewer Systems (MS4) on November 5, 2021. The Department met with various stakeholders over the past several years regarding these permits, held public hearings to accept testimony on each permit through online webinars in November 2020, and accepted public comments through January 21, 2021. The written comments received by the Department and the transcripts of the public hearings are posted below. 

Fact Sheets and MEP Submissions​

You may v​iew the fact sheets, responses to comments, and maximum extent practicable (MEP) submissions​​ for recently issued MS4 permits by clicking on the links below.


Fact Sheet​​

​MEP Submission​
Large Jurisdictions
Anne Arundel County

Fact Sheet

201​9 MEP Submission
2020 Revised MEP Submission​
Baltimore County​

Fact Sheet

2019 MEP Submission
2020 Revised MEP Submission​​

Baltimore City

Fact Sheet

2019 MEP Submission
2020 Revised MEP Submission

Montgomery County

Fact Sheet

2019 MEP Submission
2020 Revised MEP Submission

Prince George’s County

Fact Sheet

2019 MEP Submission
2020 MEP Submission
Medium Jurisdictions

Carroll County

Fact Sheet

2021 MEP Submission

Charles County

Fact Sheet

2021 MEP Submission
Frederick County

Fact Sheet

2021 MEP Submission

Harford County

Fact Sheet

July 2021 MEP Submission
September 2021 MEP Submission

Ho​ward County

Fact Sheet

July 2021 MEP Submission
September 2021 MEP Submission

The Department used the MEP submissions to help assess the physical and financial capacity of Phase I permittees to meet proposed restoration goals. ​The MEP submissions were based on the following instructions and spreadsheets that were sent to each Phase I permittee:

Prince George's County Consent Decree

The Maryland Department of the Environment and the Prince George’s County Department of Environmental Protection entered into a court-sanctioned consent decree on December 1, 2021 resolving issues with the county’s performance pursuant to the NPDES MS4 permit issued January 2, 2014. The five-year permit expired on January 1, 2019.  

The settlement can be viewed here:

The consent decree imposes a $475,000 penalty for failure to complete all of the restoration work required by the 2014 permit. The penalty can be satisfied through the construction of one or more Department-approved Supplemental Environmental Projects by December 31, 2024. In addition, the decree requires that the restoration work remaining under the terms of the 2014 MS4 permit be completed in the same timeframe. The settlement calls for additional penalties for failure to meet interim goals.  

Prince George’s County initiated efforts to address urban stormwater runoff well before the Chesapeake Bay Total Maximum Daily Load (TMDL) was established in 2010 and has been diligent in its efforts. This consent decree requires the county to complete the impervious surface restoration work that remains to be performed notwithstanding the expiration of the 2014 permit.

"We appreciate Prince George’s County's commitment to improved environmental performance under its state permit and collaboration on the consent decree, including provisions on penalties, schedules, and supplemental environmental projects”, said Secretary Ben Grumbles. “Preventing stormwater pollution can be challenging but it's important for local water quality and the Chesapeake Bay and the state will continue to enforce its laws and provide assistance to counties and communities."

Additional Resources


Guidance for Maryland's NPDES Stormwater Permits

NPDES municipal separate storm sewer system permits in Maryland require the restoration of a certain percent of a jurisdiction's impervious surface area (e.g., 20%) that has little or no stormwater management. How to calculate impervious surface requirements and treatment credits has generated numerous questions. This document standardizes procedures for reporting of traditional, new, and alternative best management practices (BMPs) and the impervious area each controls. The guidance can be found below.

The Department has revised Appendix B of the Accounting for Stormwater Wasteload Allocations and Impervious Acres Treated (MDE, November 2021). The revision corrects a technical error and provides additional clarification related to the calculation of total nitrogen load reductions for measuring progress toward meeting total maximum daily load (TMDL) goals. The revision ensures consistency with the guidance and the reporting criteria found in the Department’s Urban Stormwater Management Credit Calculator for TN, TP, TSS (MDE, April 28, 2022) and the TMDL Implementation Progress and Planning (TIPP) Tool (MDE, April 6, 2022).

Other Guidance Documents:

Since 2014, MDE has distributed several memoranda that provide additional guidance concerning BMPs and impervious area crediting.  Please note that these memoranda are intended to clarify, supplement, or replace the information found in the Accounting for Stormwater Wasteload Allocations and Impervious Acres Treated (MDE, 2014).  When there are conflicts between these memoranda and the original 2014 guidance, the newest shall supersede. These documents may be found here:
Additionally, in March 2018, MDE sent a letter to OptiRTC, Incorporated that established requirements and interim credits for the use of continuous monitoring and adaptive control (CMAC) systems for restoration.  While this letter is addressed to OptiRTC, the conditions and restrictions apply to CMAC systems in general, includiong those marketed by others.  A copy of this letter is available here:

 Contact Info

Questions about NPDES municipal permits may be directed to the Sediment, Stormwater, and Dam Safety Program at 410-537-3543 or emailed to Mr. Raymond Bahr at ​​​

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