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0 411� <br /> GEOMATRIX <br /> Although the TPHg and TPHd concentrations exceed the ESLs, the concentrations of individual <br /> ' chemicals in TPH were below their respective ESLs TPH measurements, such as TPHd and TPHg, <br /> represent mixtures of chemicals that,because of their highly variable composition,do not have <br /> ' descriptive health criteria The TPH ESLs assume that the entire concentration of TPH has toxicity <br /> strmlar to pyrene(a polycyclic aromatic hydrocarbon), which is a highly conservative assumption <br /> ' since petroleum hydrocarbons are not primarily composed of polycyclic aromatic hydrocarbons <br /> When the concentrations of key individual chemicals in the TPH mixture have been measured,the <br /> toxicity can be described by the aggregate toxicity of key individual chemicals in the mixture <br /> ' (Cal—EPA, 1999)rather than an approximation for the mixture as a whole Because the individual <br /> concentrations of chemicals in the mixture are below their individual ESLs, the TPH mixture should <br /> ' not present a potential risk to human health <br /> ' Although samples were not collected from vadose zone soils less than 10 feet bgs,higher <br /> concentrations are not anticipated in shallow soil because the source of chemicals in.soil was an <br /> underground tank Additional samples will be collected to confirm that petroleum constituents are <br /> ' not present at levels exceeding ESLs in soil less than 10 feet bgs <br /> ' Groundwater analytical results were compared to ESLs developed for the potential for chemicals in <br /> groundwater to volatilize and impact indoor air There are no ESLs for TPHd or TPHg No <br /> ' ethylbenzene has been detected in site groundwater samples The maximum detected concentrations <br /> of benzene,toluene, and xylenes in site groundwater samples were 4 6, 600, and 19,000 p.g/l, <br /> respectively,which are well below their respective ESLs of 530, 500,000,and 150,000µg/1 <br /> ' Therefore,the detected concentrations of petroleum constituents in site groundwater should not <br /> adversely affect human health under the volatilization from groundwater to indoor air exposure <br /> ' pathway <br /> ' 3.0 CORRECTIVE ACTION OBJECTIVE <br /> As discussed in Section 2 5,the small pocket of affected groundwater is not migrating and will not <br /> impact surface water or water supply wells As discussed in Section 2 6,the residual petroleum does <br /> not likely pose a human health risk Therefore,corrective actions are only being considered to <br /> ' address the small pocket of groundwater with TPHd and TPHg at concentrations greater than <br /> dnnkmg water WQOs Although it is extremely unlikely that this small pocket of shallow affected <br /> groundwater will be used for drinking water purposes, RWQCB policy requires that it be considered <br /> ' as such Therefore,the corrective action objective is to reduce TPHd and TPHg concentrations in <br /> groundwater to below WQOs <br /> I 1Doc_Safe19000s199631CAP\CAP txt doc 7 <br />