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. 3.0 SELECTION OF INDICATOR CHEMICALS <br /> i <br /> An indicator chemical is a potentially site-related chemical with sufficient data to quantify <br /> exposure in a risk assessment (USEPA, 1989). In addition, the indicator chemicals must be <br /> associated with an exposure pathway which is complete (Le , can occur) at the site The <br /> purpose of selecting indicator chemicals is to focus the assessment on those chemicals that <br /> could reasonably be expected to pose a significant risk Criteria for selecting indicator <br /> chemicals generally include the chemical's toxicity, detected concentrations, frequency of <br /> detection, and mobility Consideration of frequency of detection is only applicable if the <br /> nature and extent of the chemical concentrations have been adequately characterized with <br /> a sufficient number of samples <br /> Because data was not available regarding the chemical properties of the aged diesel fuel, <br /> a constituent of the aged diesel fuel was selected for the environmental fate and transport <br /> modeling. Based on the partitioning studies, the straight-chained alkane, tetradecane, was <br /> selected to conservatively represent aged diesel fuel characteristics Tetradecane was the <br /> smallest and most mobile compound of the aged diesel fuel detected in the water of the <br /> partitioning studies Therefore, the use of tetradecane will conservatively overestimate the <br /> migration of the aged diesel fuel at the site. <br /> Diesel fuel as a whole mixture will be used as the indicator chemical for the Santa Fe <br /> Railway Company site The selection of diesel fuel satisfies the criteria for an indicator <br /> chemical previously listed In all the previous investigations, TPH as diesel was detected <br /> (1) at elevated concentrations, and (2) in most of the soil borings <br /> Although benzene was detected during the May 1991 ERM-West soil investigation, it was <br /> not selected as an indicator chemical because (1) it was detected in only one sample, (2) it <br /> has not been detected over time, and 3) the one detect was not associated with the highest <br /> petroleum hydrocarbon levels In addition, the partitioning studies performed on impacted <br /> soil from the site did not reveal the presence of benzene or any volatile organic compounds <br /> in the soil <br /> G WMI7%SANrwFE FNLMmW 2693 3-1 Im <br />