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divisi-o n of <br /> NYIRONMENTAL SOLUTIONS ^ EmcoA <br /> October 12, 1998 <br /> Project 20805-152 008 <br /> Ms Margaret Lagorio <br /> San Joaquin County Public Health Services <br /> Environmental Health Department <br /> 304 East Weber, Third Floor <br /> Stockton, California 95201 <br /> Re Hydrocarbon Mass Evaluation, ARCO Service Station No 6100, 25775 South <br /> Patterson Pass Road, Tracy, California <br /> Dear Ms Lagorio <br /> On behalf of ARCO Products Company (ARCO), Pinnacle Environmental Solutions, a <br /> division of EMCON (Pinnacle) evaluated the information collected during site <br /> investigations conducted at ARCO Service Station No 6100, 25775 South Patterson Pass <br /> Road, Tracy, California (see Figure 1) The purpose of this evaluation is to determine the <br /> suitability of site closure consistent with The San Joaquin County Public Health Services, <br /> Environmental Health Division (PHS/EHD low risk criteria) The PHS/EHD requested <br /> evaluation of contaminant mass and lithologic distribution during a February 18, 1998 <br /> meeting held at the PHS/EHD offices and confirmed in a February 28, 1998 letter <br /> The evaluation consisted of estimating the total petroleum hydrocarbon as gasoline mass <br /> (TPHG) and total mass of benzene, toluene, ethylbenzene and total xylenes (BTEX) in soil <br /> using analytical results and assessing lithologic controls on the distribution of petroleum <br /> hydrocarbon impacts using boring log data obtained from previous site investigations <br /> Data used in this evaluation came from reports prepared for ARCO and submitted to the <br /> PHS/EHD Tabular summaries of historical analytical results are located in Appendix A <br /> and the mass calculation sheets are presented in Appendix B <br /> The lateral and vertical extent of impacts to site soils have been defined and the majority <br /> of the hydrocarbon mass lies between 20 and 60 feet bgs Groundwater was not <br /> encountered during site assessment activities that explored subsurface conditions to depths <br /> of approximately 118 feet bgs Depth to water information in the site area, based on logs <br /> of the on-site domestic well suggest ro wateris�encoi ntUpd_a.LaRpmx!mately_396 feet <br /> bgs This evaluation suggests that case closure is appropriate under the low risk criteria <br /> Supporting evidence is presented in the following sections <br /> SA(7C 1LSG12NDQTR--1\61OOMASS DOC 451mtt 1 <br /> 3939 Cambridge Road Suite 220 Cameron Park California 95682 (530)676 6884 (530)676 6885 Fax <br />