INTRODUCTION
The U.S. Environmental Protection Agency (EPA) requires that stormwater be controlled to the maximum extent practicable through the use of stormwater control measures. These regulations are enforced by state and local gov­ernments, who often require the use of SCMs to comply with total maximum daily loads (TMDLs). State and local governments also provide guidance on SCM design and assign load reductions, efficiency ratios, and percent removals for various pollutants. However, the load reduction and percent removal methods that are often used to regulate development do
Establishing Target Effluent Concentrations
for Stormwater Control Measures
Stormwater control measures (SCMs), also known as best management practices (BMPs), are required structural devices that mitigate the increased runoff volumes, flow rates, and pollut­ant concentrations that are caused by urbanization. Previous fact sheets in the Urban Water­ways series have discussed the design, construction, maintenance, and research findings on SCMs in North Carolina, including: AGW-588-05 (Bioretention Performance, Design, Con­struction and Maintenance), AG-588-06 (Permeable Pavement, Green Roofs, and Cisterns: Stormwater Practices for LID), AGW-588-12W (Stormwater Wetland Design Update), and AGW-588-20W (Level Spreader Update: Design, Construction, and Maintenance), among many others. This bulletin presents a method for interpreting SCM function by comparing SCM effluent concentrations against ambient pollutant concentrations of receiving waters. Ambient water quality concentrations have been correlated with benthic macroinvertebrate ratings, an accepted measure of stream health. In this way, first-generation target effluent concentrations for stormwater control measures were established. This bulletin provides data on SCM effluent concentrations from past research studies in the Mid-Atlantic and offers SCM selection guid­ance based upon target effluent concentrations.
What Are Total Maximum Daily Loads (TMDL)?
A TMDL is a maximum mass of pollutant allowed to discharge from a watershed on a daily basis. For example, the North Carolina Department of Environment and Natural Resources (NCDENR) currently limits post-development loads of total phosphorus (TP) to 0.4 lb/ac/yr and total nitrogen to 4.0 lb/ac/yr in the Tar-Pamlico River Basin (NCDENR 2007). SCMs are needed to reduce post-devel­opment loads to these thresholds.