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SECTIONT' EE Technical Approach • o Determine site-specific chermcal concentrations that have acceptable levels of risk (cleanup levels) Each of these study processes is discussed in further detail in the following sections 3.x.9 Prepare a Conceptual Site Model The conceptual site model identifies potential chemical sources, release mechanisms, transport media, routes of chemical migration through the environment, exposure media, and known or potential human receptors Thus model provides a framework for problem defirution, identifies data gaps, identifies media that may need to be sampled, and helps identify appropriate remedial technologies, if warranted A site model includes both detailed graphic representations of plausible pathways and integrated site models compiling possible sources of media contamination and exposure points Identified potential pathways are screened to determine those that require further site-specific numencal analysis, including additional sampling if necessary The conceptual site model facilitates early agreement among parties to apply a particular risk assessment process to site issues Figure 3-1 is an example of a proposed conceptual site model for Shop 10 3.9.2 Data Evaluation and Selection of Chemicals of Concern Data summarized action of this report will be reviewed for use in the risk assessment Data validation of those data used to estimate potential human health risks will be according to EPA's "Guidance for Data Useability in Risk Assessment (U S EPA, 1990) The analytical data should be applicable to each exposure pathway identified in the conceptual site model Tlus information is critical to the effective design and planning of a risk assessment that will meet risk assessment objective Data will be suitably summarized and appropriate statistical parameters will be calculated (such as mean, standard deviation, and 95 percent upper confidence limit[UCL]) The 95 percent UCL on the arithmetic mean (95 percent UCL) for each chemical can be used for the exposure point concentration The 95 percent UCL concentration will be calculated using the H statistic, assuming a normal distnbution of the data Typically, mean concentrations can be used to evaluate typical or expected exposures The 95 percent UCL value (or maximum value if lower) can be used to provide a conservative "upper hmit" estimate of the mean concentration for use in evaluating the "Reasonable Maximum Exposure," or RME ® T 118871979009NA DOGI7,AUG4m 73oOHNA%SNA 3-2