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Section 303(d) of the Federal Clean Water Act requires states to develop a Total Maximum Daily Load (TMDL) allocation plan for water bodies determined to have limited water quality. A TMDL allocation plan documents the amount of a pollutant that a waterbody can assimilate without exceeding a state's water quality standards, and allocates that amount as loads to point and nonpoint sources. TMDLs are defined in 40 Code of Federal Regulations (CFR) Part 130 as the sum of the individual waste load allocations for point sources and load allocations for nonpoint sources, including a margin of safety and natural background conditions.

Water quality standards, which are intended to provide protection for designated uses, form the basis of TMDL targets. In cases where numeric water quality standards are not available, narrative standards are used to develop TMDLs. These narrative standards cannot be allocated; hence, there is a need to link the narrative standards to pollutant loads generated from the watershed.

The PLOAD/WISE Model has been designed to link the watershed pollutant loads to narrative standards, i.e., biological indices. Specific biological standards based on biological indices such as Index of Biological Integrity (IBI), Fish Score, and Invertebrate Community Index (ICI) are used as measurement tools and are linked to various pollutant loads from the watershed. The model blends two components: PLOAD—a GIS application developed for pollutant load modeling, and WISE—a tool developed for statistical evaluation of pollutant loads versus biological indices. The model allows the user to view various scenarios based on desired goals and make technically sound decisions for developing narrative TMDLs.

Document Type: Research Article


Publication date: 2000-01-01

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