A discussion of the U.S. EPA methodology for determining Water Quality Standards (WQS)

Abstract

Based on material published by the U.S. Environmental Protection Agency (U.S. EPA) in the Federal Register for 19 November 1991, many state environmental agencies have proposed and/or adopted revisions to their State Water Quality Standards (WQS) for organic and inorganic chemicals in fresh and marine waters (see, for example, State of Connecticut, Department of Environmental Protection, Bureau of Water Management, (1992), memorandum to Interested Parties concerning the Water Quality Standards Hearing Report). Generally, many states simply republish the U.S. EPA's proposed Water Quality Criteria (WQC) as the State's proposed WQS. Many of the state WQS and federal WQC values--especially those for organic compounds regulated as human or animal carcinogens--are much more stringent than the values now in effect because the U.S. EPA's new methodology (i) for estimating exposure point concentrations, exposure doses, carcinogenic potency, and incremental lifetime cancer risk and (ii) for setting the target acceptable risk combine a series of conservative assumptions into an equally conservative set of results. In the Federal Register proposal, the U.S. EPA failed to honor its standard risk assessment methodology in that (i) it failed to perform a quantitative or even qualitative uncertainty analysis and (ii) it failed to analyze the overall degree of conservatism in the results. The U.S. EPA suggested that the analysis is suitably conservative for the average exposed adult, but it failed to consider various phenomena that make the proposed WQC far more conservative than acknowledged or intended. To focus on a central problem of manageable size, this article dissects the method by which the U.S. EPA calculates proposed WQC for organic chemicals regulated as human or animal carcinogens. Because the results for most such chemicals are driven by the pathway for the human ingestion of fish which have bioconcentrated the chemicals from the water column (as opposed to the pathway for direct ingestion of water by humans), this article focuses exclusively on the fish-to-human pathway. These considerations form the basis of general quality assurance criteria and standards.