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URS [RFCE V ED <br /> JAN 0 7 2Da4 <br /> ENVIRONMENT HEALTH <br /> Mr Mike Infurna PERMIT/SERVICES page 5 <br /> San Joaquin County Environmental Health Department <br /> ARCO 6020 <br /> September 29, 2003 <br /> Groundwater Analytical Data <br /> Analytical data from groundwater samples collected from the newly installed monitoring wells reported that <br /> TPHg and BTEX were not present above reporting limits in the two new wells (MW-11 and MW-12) <br /> However, MtBE was reported at 7 8 micrograms per liter(µglL) in deep well MW-13D It is possible that <br /> this concentration could be the result of residual contamination from drilling through shallow,impacted soil <br /> and groundwater when the well was installed Additional quarterly groundwater samples will be collected <br /> from all site wells to verify the detection of MtBE in MW-13D and to monitor the groundwater impacts at <br /> MW-6 Groundwater analytical data are presented in Tables 4 and 5 A summary of groundwater analytical <br /> data for the third quarter 2003 sampling event is presented in Figure 5 Laboratory analytical reports and chain <br /> of custody documentation are included in Attachment G <br /> Groundwater elevation and contaminant concentration trends for monitoring well MW-6 are presented as <br /> Figure 6 Although concentrations of MtBE of 200 gg/L remain in MW-6, the linear trend of MtBE <br /> concentrations indicates that dissolved MtBE concentrations are declining at a significant rate <br /> • REMEDIAL ACTION SUMMARY <br /> Impacted soils were over-excavated from beneath the dispenser islands and USTs during facility upgrade <br /> activities conducted in April 1999 Soil samples collected after over-excavation was completed indicated that <br /> impacted soils remained in these areas No additional soil or groundwater remediation has been conducted at <br /> the site <br /> MASS OF HYDROCARBONS REMAINING IN SOIL AND GROUNDWATER <br /> Mass calculations for TPHg and MtBE in soil and groundwater are presented in Attachment I and summarized <br /> below Analytical data indicate that the mass of benzene released at the site is very low <br /> Mass Remaining in Soil <br /> Assuming an area of impacted soil of approximately 900 square feet remains beneath the dispenser islands <br /> and UST complex,and that the interval of soil contamination extends between 6 feet bgs and the water table, <br /> which is at approximately 21 feet, the volume of impacted soil is approximately 13,500 cubic feet Using <br /> average historical soil analytical data collected during the UST upgrade(Appendix B),the average TPHg and <br /> MtBE concentrations were assumed to be 220 parts per million(ppm) and 2 8 ppm, respectively Based on <br /> these assumptions, the estimated mass of TPHg and MtBE remaining in soil are 72 7 pounds and 0 57 <br /> pounds, respectively <br /> K 1WprocessU5 BP ARCM0602012003 Assessment16020 Assessment&SCM 2003 doe <br />