Improving the Tmdl Process Using Watershed Risk Assessment Principles
Watershed ecological risk assessment (WERA) evaluates potential causal relationships between multiple sources and stressors and impacts on valued ecosystem components. This has many similarities tothe placed-based analuses that are undertaken to develop total maximum daily loads (TMDLs), in which linkages are established between stressors, sources, and water quality standards, including support of designated uses. TMDLs focus on achieving water quality standards associated with attainment of designed uses. In attempting to attain the water quality standard, many TMLDs focus on the stressor of concern rather than the ecological endpoint or indicators of the designed use that the standard is meant to protect. The WERA process, at least in theory, examines effects of most likely stressors, as well as their probable sources in the watershed, to prioritize management options that will most likely reslut in meeting environmental goals or uses. The scientific underpinnings of TMDL development and the associated stakeholder process can be improved using several features common in the WERA framework including: development and use of comprehensive conceptual models in the Problem Identification step of T?MDLs; use of a transparent process for selecting indicators, measures, or targets in the Numeric Targets step of TMDLs based on assessment endpoints derived from the management goal or designed use under consideration; analysis of co-occurring stre4ssors likely to cause beneficial use impairment based on the conceptual model; use of explicit uncertainty analyses in the Linkage Analysis step of TMDL development; and frequent stakeholder interactions throughout the process. WERA principles are currently nost applicable to those TMDLs in which there is no numeric standard and therefore, indications and targets need to be developed, such as many nutrient, sediment, and biology TMDLs. WERA methods can also be useful in determining TMDL targets in situations where simply targeting the water quality standard may reattain the numeric criterion but not the broader designed use. Better incorporation of problem formulation principles fron WERA into the TMDL development process would be especially useful in improving the scientific credibility and usefulness of TMDLs.