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RM Associates 701-0102 ' C:5:00 ouDdof TPH and 18 ounds of benzene were recovered from more than 6 million gallons pg p o extracted groundwater Although the amount of recovered product in groundwater is nearly 10 times more than the estimated pre-remediation amount, a plausible explanation is that the major part of difference represents dissolution of TPHg from the impacted soil (particularly in the "smear zone" during the earlier stages of the remediation A mass balance calculation for TPHg and benzene in ' both soil and groundwater is presented as Item 3 in Appendix D Only about 1,100 lbs of total petroleum hydrocarbons of the 2,000 pounds originally estimated were apparently recovered in the aqueous and vapor granular activated carbon canisters It is probable that the difference might be accounted for by the amount believed to be destroyed underground by ' biodegradation processes enhanced by the soil vapor extraction (drawing fresh oxygenated air through the impacted soil) ' The most conclusive indication that remediation of the soil at the site has been achieved, is the continuing decline in TPHg and benzene concentrations in groundwater (see Figures 5 to 8) following the soil vapor/groundwater extraction system shut-down in February 2000 4.0 IMPACT OF NO FURTHER ACTIVE REMEDIATION ' 4.1 Health and Safety The maximum concentration of benzene detected in monitoring well BEI-MW3, during the six monitoring events (over more than four years) that followed the remediation system shutdown, ' averaged less than 100 micrograms per liter (µg11) with the most recent benzene concentration detected at only 9 3 µg/l Although the commercial building at the site is only about 30 feet from the source, groundwater samples collected from monitoring wells BEI-MW2, SRK-MW2, and SRK- ' MW3,which are located near the budding and include the area between the building and the source, have been completely free of any detectable concentrations of petroleum hydrocarbons for nearly six years There is no perceivable threat to health and safety as a result of any residual hydrocarbons in the soil at this site 42 Impact to Surface Water ' There are no identifiable pathways near the site whereby the small remaining concentrations of petroleum hydrocarbons at the site could potentially threaten surface waters ' 43 Sensitive Receptor Survey the sensitive receptor survey is based on field survey and doLuinCnt ieview conducted by ATC Associates Inc , upgraded by a review of reports provided to the SJCEHD by Condor Earth Technologies, Inc (Condor) in their January 16, 2003 groundwater monitoring report for a site at 419 South Main Street, approximately 900 feet south of the subject (151 South Main Street) Condor's survey included data received by the California Department of Water Resources (DWR) and contact with the City of Manteca All identified wells and schools within the RMA study area ' Page 7 of 11