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PAGE 3 <br /> f � <br /> + (1,2-DCA); 1,3,5-trimethylbenzene (1,3,5-TMB); and 1;2,4- <br /> trimethylbenzene (1,2,4-TMB).2 <br /> All of these constituents are commonly associated with weathered <br /> gasoline, and all of these constituents were considered constittfents of <br /> potential concern for the purposes of the HRA. <br /> + Soil and ground water data for the subject properties inc <br /> luded,'analysis of <br /> total petroleum hydrocarbons (TPH). However, both ASTM (1996) and <br /> USEPA (1989) recommend that evaluation of chemical mixtures should <br /> focus on specific chemical constituents (e.g., benzene) that may be present, <br /> rather than on an evaluation of the chemical mixture. This <br /> recommendation is based on the following: !' <br /> E • The toxicity of mixtures may be highly variable, particularly in the case <br /> of weathered materials such as those present at these sites. <br />� <br /> • Little information is available to characterize the toxicI!ity of'mixtures. <br /> f • The toxicity of mixtures is largely related to the presence of specific <br /> constituents. <br /> 1 <br /> 11 <br /> For these reasons, this HRA focused on an evaluation of the specific <br />' constituents present (e.g., benzene). However, with regard to the TPH <br /> detections, it is worth noting that weathering appears to Have resulted in <br />€ the loss of the more volatile (and typically more toxic) fractions, as <br /> reflected in the most recent sampling event. Furthermore, as will be <br /> discussed in the following section, the opportunity for any exposure to <br /> constituents of potential concern is limited both by land Use and by the <br /> infrequent occurrence of residual compounds. <br /> IDENTIFICATION OF REALISTIC EXPOSURE PATHWAYS <br /> The following subsections describe land use information for the subject <br /> properties, discuss potential exposure pathways for soil and ground <br /> water, and summarize the realistic exposure pathways for the subject <br /> properties. <br /> 2 Identification of these compounds was based on information provided in the <br /> l <br /> Closure Report(ERM,1999)regarding the results of ground water sampling <br /> conducted after oxygen regenerating compounds were introduced into the <br /> monitoring system to enhance natural degradation processes.' <br />