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09/02 '00 WED 13:23 FAX 19Y594UUVUs rxmnnal Fri <br /> PAGE 3 <br /> (1,2-DCA);1,35-trimethylbenzene(1,3,5-TMB);and 1,2,4-' <br /> ! trimethylbenzene (1,2,4-TMB).7 <br /> ti All of these constituents are commonly associated with weathered' <br /> gasoline,and all of these constituents were considered constituents of <br /> potential concern for the purposes of the HRA. <br /> Soil and ground water data for the subject properties included 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 /> • The toxicity of mixtures may be highly variable,particularly in.',the case <br /> of weathered materials such as those present at these sites. <br /> • Little information is available to characterize the toxicity of mixtures. <br /> • The toxicity of mixtures is largely related.to the presence'of specific <br /> constituents. <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 /> IDENTTFICATTON 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 /> 1 properties. <br /> Identification of these compounds was based on information provided in the <br /> Closure Report(ERM,1999)regarding the results of ground water samp 17ing <br /> conducted after oxygen regenerating compounds were iintroduced into the <br /> monitoring system to enhance natural degradation processes. <br /> II <br /> j <br />