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E' Beacon Station No 641 June 15, 2000 <br /> Stockton,California Results of Additional Subsurface Investigation <br /> triple-completion groundwater monitoring well (NIW-1) and vapor extraction well (VW-1 <br /> and VW-2) <br /> In September 1997, Horizon drilled eight soil borings and installed groundwater monitoring <br /> wells MW-2 through MW-5, vapor extraction wells VE-5 through VE-8, and sparge well <br /> SW-1 (Report [of] Additional Subsurface Investigation, Horizon, December 8, 1997) <br /> In May 1998, Horizon conducted a vapor extraction test (VET), a combined VET and air <br /> sparging test (VET/AST), and an aquifer pumping test The purpose of these tests was to <br /> evaluate remedial alternatives and design criteria for a remediation system (Results of <br /> Remedial Testing, Horizon, September 28, 1998) Results of remedial testing indicated the <br /> soil vapor extraction will effectively remove hydrocarbons from the site sandy soils and air <br /> sparging in conjunction with vapor extraction would be effective in remediating hydrocarbons <br /> in groundwater Additionally, it was indicated that groundwater extraction may only be <br /> effective in controlling migration of gasoline-impacted groundwater from a limited area and <br /> not cost-effective for treatment of hydrocarbons <br /> Since October 1997, the groundwater flow direction has fluctuated from northeast to <br /> northwest Horizon has performed groundwater monitoring and reporting since the first <br /> quarter of 1998 Quarterly monitoring performed on March 1, 2000 indicated the depth to <br /> groundwater was approximately 42 feet bsg and flowed northeast under a gradient of <br /> approximately 0 001 (Quarters Groundwater Monitoring Report Horizon, May 2, 2000) <br /> ADDITIONAL SUBSURFACE INVESTIGATION <br /> The existing site-specific Health and Safety Plan was updated for the proposed fieldwork <br /> Well permits were obtained from the SJCEHD (Appendix A) Prior to mobilization, <br /> Underground Service Alert (USA) was notified to identify and mark the locations of <br /> underground utilities <br /> • <br /> 641AddSbsrfcIvest5-00 doc 3 HORIZON ENVIRONMENTAL INC. <br /> 164114 <br />