Stormwater Terminology Cheat Sheet
Stormwater runoff is a leading cause of water quality degradation in the Charles, Mystic, and Neponset Rivers in Greater Boston. On October 31, 2024, the EPA released a long-awaited draft permit aimed at significantly reducing stormwater pollution from large properties with extensive paved surfaces. The draft permit, which includes an implementation period of at least 11 years, has sparked considerable discussion. Below are some key terms to understand as you review the draft permit:
CII-GP (Commercial, Industrial, and Institutional General Permit) A type of permit regulating stormwater discharges from commercial, industrial, and institutional properties to reduce water pollution.
CWA (Clean Water Act) A landmark federal law enacted in 1972 to restore and protect the quality of U.S. waters by regulating discharges of pollutants and supporting water quality standards.
Impervious Area Surfaces like roads, parking lots, and rooftops that don’t allow water to soak into the ground, increasing stormwater runoff and potential water pollution.
MS4 (Municipal Separate Storm Sewer System) For developed areas the MS4 permit regulates stormwater as part of the NPDES program. Under this program municipalities are responsible for pollution entering the natural environment via their stormwater pipes.
NPDES (National Pollutant Discharge Elimination System) A key permitting program under the Clean Water Act (CWA) that controls the discharge of pollutants from point sources, such as pipes from wastewater treatment plants. The NPDES framework requires facilities to obtain permits and follow strict pollution control measures.
Phosphorus A nutrient found in fertilizers, detergents, and natural materials that can cause harmful algal blooms when too much enters lakes, rivers, or bays.
RDA (Residual Designation Authority) A regulatory tool under the Clean Water Act that authorizes allows the EPA to require permits for stormwater discharges from sources not previously covered, if they are found to significantly contribute to water pollution.
Stormwater Runoff Rainwater or melting snow that flows over land and impervious surfaces, picking up pollutants like oil, chemicals, and debris before entering waterways.
TMDL (Total Maximum Daily Load) The maximum amount of a pollutant a body of water can receive while still meeting water quality standards. TMDLs guide pollution reduction efforts.
Watershed An area of land where all the water, including rain and streams, drains into a single body of water.
