The Development of Watershed Scale TMDLS

Since 1996, some 24,000 TMDLs have been completed by the states and EPA. The states continue to update their 303(d) lists every two years, working to prioritize and schedule development of additional TMDLs. However, to address the ever growing number of impaired waterbodies requiring TMDLs, techniques are still needed to achieve a more nationally consistent and efficient approach for developing and implementing TMDLs. Grouping multiple impaired segments into a single, larger watershed TMDL development effort could provide added efficiencies to the overall TMDL development process. Watershed based TMDLs can reduce effort per TMDL required for a variety of tasks typically undertaken in the TMDL process. Allocation analysis in a watershed TMDL may ensure that water quality impacts from source reductions upstream to downstream adequately account for the effects of pollutant fate and transport mechanisms. In this way, impacts to the entire watershed from all sources can be evaluated. This integrated method of analysis is also consistent with the technical principles of several additional initiatives being supported by EPA (e. g., watershed planning and watershed based NPDES permitting). In the near future, EPA will produce a Practical Handbook for TMDL Practitioners to support the development of watershed TMDLs, and will sponsor a series of "hands on" training workshops for those involved.