Effluent dominated watersheds (EDWs) in the Central Valley, California, represent a large number of watersheds, ranging from agricultural to urban settings with varying degrees of hydrological modification. Appropriate beneficial uses for these watersheds have been extensively debated.
Use attainability analyses (UAAs) or similar studies are needed to change designated uses and to derive scientifically defensible water quality objectives or criteria. Several technical issues in the past have impeded the success of UAAs, making it difficult to update basin plans and state
water quality standards appropriately. Using EPA's watershed ecological risk assessment paradigm as a model, we developed a framework for addressing the monitoring and assessment requirements of a UAA. Key elements of this framework are: (1) identification of assessment indicators and
explicit measures for each use based on definitions of the beneficial use as given in the Basin Plan or other related documents; (2) conceptual models to delineate and weight effects of the six different 40 CFR 131.10(g) factors on attainability of a use within the watershed; (3) reliance
on indicator fish habitat suitability indices and their component factors, as well as qualitative thresholds for other species, to derive minimum thresholds for evaluating use attainability; and (4) a weight of evidence approach to integrate differences observed between minimum habitat and
water quality requirements for a given indicator species and actual conditions in different parts of the watershed. Use of habitat suitability indices, and similar peer-reviewed habitat requirement information, resulted in explicitly dealing with different life stages as appropriate for a
given use and evaluating attainability based on minimum requirements for each life stage. This approach also identified limiting stressors or restrictions for each use that were examined to determine if there are feasible or reasonable options for attainment of the beneficial use. The framework
was developed and tested in two different types of EDWs: a constructed agricultural watershed with livestock and crop uses dominating, and a partially natural and partially modified watershed with a mixture of agricultural and urban influences. Aquatic life beneficial uses addressed in these
studies included warm or cold water aquatic life and their respective habitats, and migration and spawning of anadromous and catadromous species. Data needs for UAAs were more transparent as a result of developing this framework and the framework provides a template with which similar uses
in other EDWs could be evaluated efficiently.
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