Watershed TMDL

Ecotoxicologists view the watershed as the ultimate aquatic ecosystem level. The watershed is defined as the geographical area that drains to a common point. For example, the Mississippi River watershed is the area of the United States and southern Canada that lies between the Rocky and Appalachian mountain chains and channels surface water to the Mississippi delta in Louisiana. Watersheds range in scale from major river systems, that is, the Mississippi, to small single order streams that may drain only a few hectares. Watersheds, in fact, entail not only the aquatic system but also the land that drains into the aquatic system. This framework recognizes the relationships between land-use, geological, ecological, and societal factors that can influence water quality. The land is a major source of pollutants that are carried into the system. Managing land uses to reduce surface flows is one strategy to maintain water quality. This is a daunting task given the often large land areas and varied uses, including agriculture, industry, and residential. It is sometimes difficult to separate water-quality from water-quantity management. Historically, lands, including wetlands, have been managed to drain rapidly. This lack of residence time results in water laden with particulates and dissolved contaminants. Natural systems have a great assimilative capacity if given a minimal contact time. Tiled agricultural lands, impervious surfaces in developed areas, and direct drainage of storm water increase the overall volume of water flowing through systems and increase the range ofminimal and maximal flows. From an ecotoxicological perspective, natural systems are stressed by this variability in flow and accompanying water-quality dynamics.

Implementation ofthe watershed approach to water quality requires methods of data collection relevant to this different way of thinking about the land and water. Since all parts of a watershed are connected both spatially and temporally by the water that runs through it, data need to be collected in a way that reflects the dynamic nature of the system. Data must be collected from different points throughout the system to provide a comprehensive picture of water quality, and at a rate that allows for relationships to be drawn between areas separated geographically, but linked by the flow ofwater. Data regarding water and habitat quality must be collected at a scale relevant to an understanding of a large dynamic system. As water flows through a watershed, its quality must be tracked and the information presented to interested parties in a manner that would allow for the maintenance of water quality and quantity to meet both habitat and drinking water needs. A time-relevant, continuous, water-quality monitoring system is a necessary tool for successful implementation of the watershed paradigm.

Total maximum daily loads (TMDL) is a strategy promulgated by the US EPA in an effort to address water-quality issues at larger system scales. This approach establishes a pollution budget for a segment of receiving water based on identification of impairment(s). If a segment of a receiving system is found to be impaired, a TMDL must be established and the waste load is then allocated to all contributors through the NPDES process. The intent is to identify the total amount of pollution a system can assimilate and retain functionality and then split that total amount among all polluters.

See also: Mesocosm Management; Microcosms.

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