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7.1.1 Data Evaluation The highest detected concentration and the 95 percent UCL concentrations of chemicals detected in each medium were complied for use in the risk assessment. It was assumed that a chemical not detected in a given sample was actually present at one-half of its detection limit if that chemical was present in any sample in a given media. This assumption is probably conservative and may lead to overestimation of risk, particularly for those chemicals reported with low frequency and low concentrations. The highest detected concentration was used in evaluation of RME exposures, an assumption which, compared to the 95 percent UCL may lead to overestimation of risk. 7.2 CHEMICAL FATE AND TRANSPORT MODELING UNCERTAINTIES The models used to estimate the fate and transport of chemicals for this risk assessment are consistent with those recommended by EPA in its Superfund Exposure Assessment Manual (EPA 1988). Because of uncertainties in modeling, EPA-recommended models and input parameters are conservative and produce exposure concentrations that should not underestimate the health risks. Assumptions of the modeling are discussed in Section 3.0. Modeling assumptions were conservative and were designed to be protective of groundwater and human health. Conservative assumptions include no biodegradation Y in soil or groundwater, highest detected concentrations and 95 percent UCL concentrations, high porosity in groundwater, high infiltration for residential scenarios, and seasonal high groundwater used for RME child resident scenario. 7.3 EXPOSURE ASSESSMENT UNCERTAINTIES Risk assessments require assumptions in order to assess potential human exposure. This risk assessment includes assumptions about general characteristics and potential patterns of human exposure of the population in the vicinity of the site which were based upon: • Limited site-specific information S:1LDC\YELL0.1?FT May 4,1995 7-3