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Beacon Station No 641 April 22, 1999 <br /> Stockton California Work Plan <br /> s <br /> In June 1996 three (two 10,000-gallon and one 12,000-gallon capacity), double-walled, <br /> fiberglass-clad, steel USTs were installed in the former USTs cavity During the <br /> replacement of the USTs, ten 18-inch diameter conductor casings were installed in the <br /> former UST cavity These conductor casings were installed to facilitate future assessment <br /> and/or confirmation drilling through the backfilled UST cavity Tank Closure Letter Re ort, <br /> Horizon, August 2, 1996) <br /> Previous subsurface assessment work has been performed at the site since the UST <br /> replacements In July 1996, Horizon drilled soil borings B-1 and B-2 Upon completion of <br /> drilling, a groundwater monitoring well (MW-1) and two vapor extraction wells (VW-I and <br /> VW-2) were installed in soil boring B-2, and two nested soil vapor extraction wells (VW-3 <br /> and VW-4) were installed in soil boring B-1 Groundwater was encountered during drilling <br /> at a depth of approximately 64 feet bsg <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 Re ort ofl <br /> 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 remediation design criteria for the site (Results o <br /> Remedial Testrn , Horizon, September 28, 1998) <br /> Since October 1997, the groundwater flow direction has primarily fluctuated between the <br /> northeast and northwest Horizon has been reporting the groundwater monitoring since the <br /> first quarter of 1998 Quarterly monitoring performed on December 30, 1998 indicated the <br /> depth-to-water beneath the site to be approximately 44 feet bsg and the groundwater flow <br /> direction to be towards the northeast (Table 1) <br /> 1641-14 wp 3 HORIZON ENVIRONMENTAL INC <br />