Tmdl Development from the "Bottom Up" – Part IV: Connecting to Storm Water Management Programs

Abstract

A challenge facing many local jurisdictions across the country is finding ways to connect Storm Water Management Programs (SWMPs) required by NPDES permits with TMDLs intended to address water quality problems. A “bottom up” approach towards TMDL development is one way to establish a meaningful, value-added framework which links water quality concerns to proposed solutions. Information on Best Management Practices (BMPs) related to both source control and delivery reduction methods can be incorporated into TMDL allocations. Management measure information considered in TMDL allocations can then complement benchmarks that guide implementation efforts. Basic hydrology presented in the form of duration curves supports a “bottom up” approach and offers opportunities for enhanced pollutant source targeting. Duration curves add value to the TMDL process by characterizing water quality concerns in terms of flow conditions, linking these concerns to key watershed processes, prioritizing source assessment efforts, and identifying potential solutions. Federal regulations identify a set of minimum control measures to be included in SWMPs for municipal separate storm sewer systems (MS4), which are intended to reduce pollutant loads to the “maximum extent practicable” (MEP) and often directly support or advance TMDL water quality objectives. A number of these control measures are typically most effective in achieving intended benefits under specific flow conditions, depending on watershed characteristics. For example, water quality concerns experienced during low flow conditions in urban watersheds might involve eliminating illicit connections under an MS4 storm water program. Similarly, sediment problems observed during mid-range and moist conditions might be best addressed through construction site BMPs. Finally, water quality concerns associated with stream bank erosion under high flow conditions might involve post development BMPs intended to address channel protection. The natural linkage between basic hydrology and storm water makes the duration curve framework a potentially useful tool in targeting appropriate BMPs and in guiding implementation of SWMPs. Because duration curves are also used to support TMDL development, the framework is wellsuited for connecting water quality-based control efforts with SWMPs.