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r <br />!■ <br />ft <br />W <br />ij. <br />rK< <br />V'r*' <br />it'ft' <br />i: <br />.«■•■.i <br />X <br />i.i <br />92-47.03 <br />toxicological effects (toxicity), and the weight of evidence <br />supporting the validity of data and applicability to human <br />receptors. <br />Generally, those metal contaminants which are regulated in <br />drinking water or hazardous waste regulations, have been studied <br />and their toxicity documented to support the establishment of <br />regulatory limits such as the Ambient Water Quality Criteria <br />(AWQC), Maximum Contaminant Levels (MCLs), or action levels. <br />These regulatory concentrations are established on the basis of <br />toxicity research data mainly on animals and include generous <br />safety factors for human exposure. For example, an MCL may be <br />set at a fraction of the lowest concentration determined through <br />toxicity studies, to exhibit no observable effects. <br />Dose-Response Evaluation <br />A major component of the risk assessment process is the <br />relationship between the dose of a compound, (amount to whicii a- <br />individual or population is exposed) and the potential for <br />adverse health effects resulting from exposure to that dose. <br />Dose-response relationships provide a means by which pc'irtial <br />public health impacts are evaluated. <br />Contaminants identified in the three roadway materials for <br />which there are established regulatory limits and toxicity data <br />supporting potential health and environmental effects are <br />included in this assessment. These contaminants of concern <br />include the eight SWDA and Resource Conservation and Recovery Act <br />(RCRA) metals as well as 2378-tetrachlorodizeno-p-dioxin <br />(dioxin). <br />It should be noted that some of the contaminants of concern <br />were not detected in a roadway material. In these instances, <br />concentrations equal to one-half of the analytical detection <br />limit were used in modeling and exposure calculations. This <br />conservative procedure is in conformance with U.S. EPA risk <br />assessment guidance and recommended criteria (U.S. EPA, 1989). <br />A brief summary of the methodology used to establish these <br />parameters follows. This summary includes a discussion of the <br />implications and limitations of the dose-response relationships, <br />and the derived regulatory criteria. <br />• gontaminant Levels (MCLs) - MCLs are enforceable <br />standards promulgated under the Safe Drinking Water Act and <br />are designed for the protection of human health. MCLs are <br />based or. laboratory or epidemiological studies and apply to <br />drinking water supplies consumed by a minimum of 25 persons.