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SECTIONTHREE T®chnlcat Approach In some cases, apparently anomalous results are calculated for average or 95 percent UCL concentrations because of the methods used in accordance with agency guidance In some cases, very high UCL concentrations are calculated, especially for small data sets with a high degree of variance When the calculated 95 percent UCL of the mean concentration is higher than the reported maximum concentration for a given medium,the maximum reported concentration will be used in estimating risks for RME scenarios In some cases, the calculated average concentration is higher than the reported maximum concentration This may occur when a compound has a limited number of detected values, and one-half of an elevated detection limit is substituted for non-detected results In these cases, the maximum reported concentration will be used for characterizing average exposure scenarios Risk assessment for sites contaminated with petroleum products is complicated by a general lack of information relevant to exposure to and toxicity of petroleum mixtures (especially TPH) Benzene, ethylbenzene, toluene, and xylenes are frequently used to represent TPH in the environment BTEX are typically quantified separately from TPH and were detected at Shop 14 The 95 percent UCL for BTEX m soil will be compared to Region IX Preliminary Remediation Goals (PRGs) and the State of California modified PRGs (1995) This comparison will indicate if levels in soil may pose a potential threat to human health and the environment 3.9.3 Identify Exposure Pathways Exposure pathways, as identified in the conceptual site model, define the migration route that chemicals may take from sources to exposure points The exposure pathway analysis evaluates all environmental transport media soil, groundwater, surface water/sediment, and air Potential groundwater exposure points will be selected at varying distances from the site to estimate potential worse-case concentrations in these wells The identification of exposure pathways is dependent on the completed conceptual site model and the chemicals of concern for each specific source evaluated at a site An exposure pathway consists of four necessary elements o A source and mechanism of chemical release to the environment o An environmental transport medium for the released chemical o A point of potential human contact with the contaminated medium o A human exposure route at the point of exposure ® T%1097MM00eNAWTOCKTOMREPORT D=7-ALX;-W973009ftn4SNa 3-3