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000 GEOMATRIX Mr.Robert D,Mihalovich Chevron Environmental Management Company October 4,2001 Page 4 If shallow groundwater underlying the site is a source of drinking water, the theoretical excess cancer risk and noncancer hazard index using the maximum concentrations are 3x 10"6 and 0.4, respectively. Using the 95%UCL as the representative exposure point concentrations, the excess cancer risk and noncancer hazard index are 2x10"6 and 0.08,respectively. If shallow groundwater underlying the site is not a source of drinking water, the excess cancer risk and noncancer hazard index using the maximum concentrations are 1x10'6 and 0.4, respectively. Under the same scenario,the excess cancer risk and noncancer hazard index using the 95% UCL as the representative exposure point concentrations are 2x10"7 and 0.06, respectively. Under the assumption that groundwater is source of drinking water,the results of the.screen- ing risk evaluation indicate that ingestion ofbenzene in shallow groundwater is the predomi- nant exposure pathway and chemical contributing to the total risk. Benzene was detected in 1 of 6 shallow grab groundwater samples at 0.6 µg/L which is below the California maximum contaminant level (MCL)of 1 ug/L for drinking water. As presented in Table 2,benzene historically has not been detected above the laboratory reporting limit of 0.5 gg/L. The procedures used in this screening evaluation result in conditional estimates of risk that incorporate assumptions concerning chemical toxicity and human exposure and unavoidably uncertainties. Because this is a screening evaluation,the types of assumptions incorporated into this evaluation were selected.in a manner that purposely biases the process toward health con- servatism. Consequently,it is important that the magnitude of uncertainties and biases that are incorporated should be considered when interpreting these screening risk results. One of the uncertainties associated with the estimates of health risks from groundwater expo- sures is whether the estimated exposure point concentrations are realistic and representative of long-term levels for the potentially complete pathways evaluated. Estimation of the exposure point concentration is an important part of the exposure assessment. The potential exposures from shallow groundwater were calculated based on the assumption that the current chemical concentrations and site conditions would remain constant throughout the exposure period, For this screening evaluation,the concentrations of benzene currently observed in shallow ground- water are assumed to be the same as the levels that potential receptors could be exposed to over the next 30 years. No decrease in concentrations as a result of natural attenuation or dilution from chemical transport was considered. Consequently,the estimated risks associated with shallow groundwater exposures are likely over-estimated. In addition,the use of the maximum or the 95%UCL to represent the concentration tenri presents a conservative and health pro- tective estimate. This degree of conservatism can overestimate the actual risks. In summary, the estimated noncancer hazard indexes and theoretical excess cancer risks for all scenarios are below or at the lower end of the range generally considered acceptable by regulatory agencies (i.e.,a hazard index of 1 for noncarcinogens and a cancer risk of 1x10'6 to 1x10-4 for carcinogens). Because these risk estimates are based on RMB parameters and contain a number of simplifying assumptions,they are likely an overestimate of actual risks. i rte..