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<br />do not reflect consideration of economic impaCts or the technological feasibility of meeting the <br />chemical concentrations in ambient water. <br /> <br />Under CW A section 304(a), EP A has developed to date, aquatic life criteria guidance for 31 <br />chemicals and human health criteria guidance for 100 chemicals. For the most part, States and <br />Tribes have found such EP A criteria guidance useful in setting standards to protect designated <br />uses. Since 1980, most States and Tribes have adopted at least some of the criteria guidance <br />published by EP A pursuant to CW A section 304(a). <br /> <br />States and Tribes may establish numeric criteria using CW A section 304(a) criteria guidance, <br />section 304(a) criteria guidance modified to reflect site-specific conditions, or other scientifically <br />defensible methods. 40 CFR 131.11(b)(I). <br /> <br />Numeric criteria provide a proven effective basis for implementation of the CW A. Numeric <br />criteria often form the basis for NPDES water quality-based permit limits for point source <br />dischargers and for TMDLs for whole water bodies. Numeric criteria can also be useful in <br />assessing and managing nonpoint source pollution problems. <br /> <br />. <br /> <br />. <br /> <br />. <br />