Evaluation of Alternate Approaches to Deriving Exposure Point Concentrations for Ecological Risk Assessments. J. H. Samuelian, W. R. Alsop, and P. O. Gwinn, AMEC Earth & Environmental
The determination of the representative soil concentration of a chemical contaminant (Exposure Point Concentration; EPC) is integral to the evaluation of potential ecological or human health risks at chemical waste sites. In 1992, EPA recommended the use of the upper 95th confidence limit (95UCL) of the mean concentration for deterministic risk assessments. The premise behind the use of a conservative upper estimate of the mean was to account for the uncertainty in the estimation of the "true" mean chemical concentration at a site due to the number of samples collected, sampling scheme, areal coverage, and identification of potential source areas. Since this recommendation was released, several authors have summarized alternative approaches to EPC calculation, although only a few have been used routinely. Although biased sampling is often required to determine chemical concentrations that may be from source areas or known conveyances of site chemicals, such sampling schemes are not always applicable to ecological risk assessments. To demonstrate the influence of sampling schemes on EPC calculations, chemical results derived from a grid-spaced sampling scheme will be used to represent the population of soil data, which is then subsampled using a biased sampling scheme such as could be employed as part of a field investigation. Several current approaches (e.g., spatial bootstrap, random walk, floating domain averaging) to estimate EPC values will then be compared against EPA default methods. Their utility for ecological risk assessments will be evaluated and discussed.
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