19119_Authority_April_2026

municipalauthorities.org │ 35 R egulatory C onvergence : L essons from M aryland ’ s MS4 S uccess and the D raft G eneral PAG-13 By Brittany Ayers Moyer, PE (PA and MD), Project Manager, Water Resources, RK&K Restoration Goals The restoration goals for both the Maryland and Pennsylvania Phase II General MS4 permits are based on the total impervious area within the urban area in an MS4 jurisdictional boundary. According to the U.S. Census Bureau, 4 an urban area is a densely settled core of census blocks and surrounding territory. The PAG-13 permit specifies that the impervious area must consider the regulated area as determined by the 2010 Census, plus any additional impervious area from the 2020 Census. Maryland permits must also be updated for the newest Census. The water quality restoration goals for both permits are based upon impervious area and 1 inch of runoff, but the restoration requirements differ slightly. In Maryland, the next Phase II General MS4 permit is expected to propose the restoration or treatment of 10% of the untreated impervious area within the urban area within the next 5-year permit term, on top of a previously treated 20% from previous permits. In Pennsylvania, the draft PAG-13 proposes treatment of 88% of the untreated impervious area in the urban area over a period of 50 years. The PAG-13 permit introduced an “MEP Calculator” spreadsheet to calculate the Volume Management Objective (VMO) for the first 5-year permit term from 2026-2031. The VMO is then broken down into an annual objective. In both permits, additional “credit” in the form of either impervious acre credit or volume managed is available for any stormwater management facilities that treat more than 1 inch of runoff. Maryland allows up to 3 inches of runoff treated, while Pennsylvania allows up to the 2-year 24-hour rainfall depth of 2.4 inches. To meet the impervious area restoration requirement and/or the VMO, permittees must plan, prioritize, and implement a set of projects in the given timeline. These projects should include stormwater management facilities or alternative practices such as disconnection of impervious surfaces, floodplain restoration, permeable pavement, impervious area removal, riparian buffer establishment/enhancement, and more. The Maryland permit utilizes a Restoration Activity Schedule (RAS) to track the estimated impervious area credit that will be achieved by each planned, designed, or implemented project. The PAG-13 permit emphasizes use of the Volume Management Plan (VMP), which must be prepared by each permittee to achieve the VMO. The VMP is a list of SCMs including ID, type, and predicted total volume management. The VMP must include preliminary design plans (30-50% designs) and operation and maintenance plans. The goal with both the RAS and VMP is to demonstrate a plan for full achievement of restoration requirements over the permit term. Differences There are a few key differences between requirements in Maryland and Pennsylvania. The PAG-13 gives permittees the ability to reduce their VMO using a feasibility index, which reduces requirements based on financial and socioeconomic factors including poverty rate, unemployment rate, and long-term affordability indicator. The PAG-13 permit also offers credits toward the VMO for any permittees that collaborate with one another to implement a VMP. There is an additional 10% credit if MS4s become co-permittees. SCMs on the VMP can be located anywhere within the boundaries of collaborating municipalities. While Maryland allows for cost and credit sharing across jurisdictions, this collaboration does not reduce the restoration requirements of either jurisdiction. This is a key piece Continued on page 49.