19119_Authority_April_2026

34 The Authority │ April For over a decade, Pennsylvania and Maryland have followed different regulatory pathways for stormwater management and water quality improvements, including the Chesapeake Bay Total Maximum Daily Load (TMDL) implementation. Pennsylvania’s PAG-13 Draft General Permit historically focused on Pollutant Reduction Plans (PRPs) for impaired waters, while Maryland pioneered the Impervious Area Restoration model which quantified water treated toward impervious area credits received. The newest iterations of these permits represent a historic "convergence." Pennsylvania’s draft PAG-13 is moving away from purely load-based pollutant tracking toward a Volume Management model that closely mirrors Maryland’s long-standing emphasis on impervious area restoration. Both states will now be utilizing metrics based on impervious area treatment as the primary methodology for meeting the water quality improvement goals. Why the Change? Beginning as early as the 2010s, Maryland adopted the Impervious Area Restoration methodology in the form of Impervious Acre Credit. This approach “ recognizes the impervious cover surrogate as an appropriate, clear, measurable, and enforceable metric to address water quality-based effluent limits 1 ”) . It is widely recognized that stormwater runoff is a catalyst for nutrient and sediment loading to local water bodies through both overland flow and accelerated soil and channel erosion. Greater volumes of stormwater runoff are driven by increases in impervious areas. Given that the runoff volume is the fundamental driver of the erosive processes and flooding, implementation of effective volume management strategies is essential. The Pennsylvania Department of Environmental Protection (DEP) echoes this perspective, contending that the overarching goals of protecting and restoring Commonwealth surface waters are best met by addressing stormwater runoff directly. According to DEP, “ Runoff is the carrier of pollutants and contributes to accelerated in-stream erosion and public safety concerns such as flooding” (VMP Instructions 2 ) . Consequently, managing or reducing runoff volume simultaneously diminishes pollutant loading and mitigates downstream flooding risks. By providing robust volume management for stormwater runoff, Best Management Practices (BMPs) and Stormwater Control Measures (SCMs) can effectively slow the velocity of runoff, filter contaminants, and promote infiltration and biological treatment of pollutants before they enter waterways. Utilizing a volume-based approach provides a straightforward and standardized method for quantifying water quality restoration goals. Permit Similarities and Specifics: Volume vs Impervious Area Although Maryland calculates water quality restoration goals and progress using Impervious Acre Credits, the ultimate goal is still runoff volume reduction. An impervious acre credit is achieved when the volume from 1-inch of runoff over the impervious area to a stormwater management facility is captured and treated (Accounting for SW WLA and IA Treated 3 ) . This is the same methodology that is being proposed in the Draft General PAG-13 permit. The key difference is that PAG-13 tracks the volume of 1-inch of runoff over the impervious area in cubic feet, instead of tracking the impervious area itself. The PAG-13 permit defines an acre of impervious area as “treated” when practices are put in place to reduce or manage the 1-inch runoff volume over these areas. In both cases, the focus is on addressing stormwater runoff in drainage areas with high concentrations of untreated impervious surfaces.