Benthic Community Impairments and TMDLs: The Development of a Methodology to Address a Narrative Standard and Multiple Stressors

Aquatic life impairments make up a large proportion of the 303(d) listed water bodies nationwide. In Virginia, approximately 35 percent of the impaired streams are listed on the 303(d) list because of violations of the General Standard (9 VAC 25-260-20).The Commonwealth of Virginia and EPA, working with Tetra Tech, Inc., has developed a Total Maximum Daily Load (TMDL) methodology to address streams with impaired benthic communities. The first such TMDLs are being developed for Cooks Creek and Blacks Run in Rockingham County. These streams are located in Virginia's Shenandoah Valley and together drain an area with extensive urban and agricultural development. Blacks Run drains much of the City of Harrisonburg and is a major tributary to Cooks Creek, which flows into the North River.The TMDL process establishes a link between the impairment and benthic community stressors. A reference watershed approach was used for the identification of benthic community stressors and appropriate TMDL endpoints. The reference watershed selection process is based upon a comparative analysis of key watershed attributes and the results of the Virginia Ridge and Valley Multimetric Bioassessment Index (VRVMBI) that was developed specifically for this project using metric discrimination analyses. The stressor identification process involved the review of water and sediment quality data, toxicity test results, and physical processes (i.e. hydromodification, riparian buffer disturbance, etc.) to determine the primary causes of impairment. Impaired and reference watersheds were modeled to determine the conditions necessary to support a healthy benthic community. This technical approach provides guidance on the development of future TMDLs in Virginia and other states that do not have established numeric criteria for benthic community stressors.