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Additional Site Assessment and Sod Vapor Extraction Test Second Quarter 2002 Groundwater Monitoring Data 2 7-Eleven, Inc, Store 21756,853 E Yosemite Avenue, Manteca California August 5,2002 I ' On December 20, 1999, three groundwater monitoring wells (MW-1 to MW-3)were Installed to depths of 25 feet bg, and screened from 10 to 25 feet bg (IT Corporation, February 2000) Soil samples were collected from each borehole from depths of 5 feet bg to total depth Soil benzene and TPH-G were not detected Soil MTBE was found only in one location (GP-4) at a concentration of 0 167 mg/kg Initial groundwater analytical data indicated dissolved concentrations of benzene (up to 0 32 ug/L), TPH-G (up to 55 ug/L), MTBE (up to 230 ug/L), and TBA(3,710 ug/L) I At the request of the SJCPHS, a workplan for additional site assessment and feasibility testing (IT Corporation, October 2001) was submitted for review and approved I 30 GEOLOGY AND HYDROGEOLOGY The shallow stratigraphy in the study area consists of Quarternary alluvium Materials underlying the site (Appendix A) consist largely of fine-to medium-grained sand to maximum exploration depths of 40 feet bg, interbedded from about 13 to 24 feet bg with clayey silt and silty clay Depth to groundwater is encountered during drilling at about 16 to 19 feet bg During the current sampling ' event, equilibrated depth to groundwater ranged from 17 12 to 20 57 feet below top of casing (TOC) Historic groundwater flow is to the northwest at a hydraulic gradient of about 0 002 foot per foot 40 SENSITIVE RECEPTORS ' A well search for public water supply wells located within one mile of the site (IT Corporation, 1999) indicates that the nearest well is an active municipal well located 1,987 feet downgradient of the site The well is reported to be 155 feet deep, with top of screen at approximately 65 feet bg The site is therefore ' considered to be of relatively low risk to sensitive receptors 50 ASSESSMENT WORK 51 Perm ittinglSite-Specific Health and Safety Plan ' Permits for the groundwater monitoring wells were obtained from the appropriate agencies The existing site-specific Health and Safety Plan was updated as applicable for the recent scope of work The Health ' and Safety Plan was initially prepared for the site as required by the Occupational Safety and Health Administration (OSHA) Standard"Hazardous Waste Operations and Emergency Response" guidelines (29 CFR 1910 120) The document was reviewed and signed by all Shaw personnel and direct-bill subcontractors prior to commencement of work at the site ' 21756SAR doc