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1 <br /> western-most gasoline tank had previously failed a tank integrity test, which <br /> • was performed in December 1986 The eastern-most gasoline tank had <br /> previously been abandoned in place, prior to 1986, by removing the fuel and <br /> filling the tank with sand <br /> In January 1987, Environmental Science and Engineering, Inc (ESE) drilled <br /> five soil borings in the vicinity of the diesel and gasoline USTs located near <br /> the southwest corner of the main building (see Figure 2) Analytical results of <br /> soil samples collected from these borings are summarized in Section 1 5 <br /> In July and September 1987, ESE installed three groundwater monitoring <br /> wells (MW-1 through MW-3) In January 1988, Anderson Consulting Group <br /> (ACG) installed wells MW-4 and MW-5 ACG later drilled soil borings 90-1 <br /> and 90-2, and installed wells MW-6 and MW-7 in February 1990, then MW-8 <br /> in September 1990 Philip (formerly Burlington Environmental) installed <br /> wells MW-9 through MW-13 in March 1994 <br /> Philip performed a soil venting and air sparging pilot test in October and <br /> November of 1994 to evaluate the effectiveness of this remediation <br /> technology Soil venting tests were performed using well MW-I, and air <br /> sparging tests were performed using well SPA, which had previously been <br /> installed for this purpose Results of the tests, contained in Philip's February <br /> 1, 1995 Final Remediation Plan, indicated that soil venting and air sparging <br /> would be effective in remediating hydrocarbon impact <br /> In June 1995, Philip proceeded with the installation of air sparging wells SP-2 <br /> through SP-6 However, implementation of the sparge and vent remediation <br /> was postponed due to petroleum hydrocarbon concentration reductions that <br /> were observed to be naturally occurring at the site <br /> 1.3 Historic Groundwater Conditions <br /> Assessment of petroleum hydrocarbon impact in groundwater has been <br /> performed at this site since July 1987 Historic groundwater monitoring <br /> indicates that significant decreases of petroleum hydrocarbon concentrations <br /> have occurred due to natural degradation Benzene concentrations in samples <br /> collected from well MWA, located near the original source of the petroleum <br /> 1 release, have decreased from 6,800 micrograms per liter (µg/L) in July 1987 <br /> to 170 µg/L in October 1998 Total petroleum hydrocarbons as gasoline <br /> (TPH-G) concentrations in samples from MW-1 have decreased from <br /> 196,000 µg/L in April 1993 to 16,000 [tg/L in October 1998 <br /> May 1999 Subsurface Investigation and Groundwater Monitoring Report <br /> The Earthgrams Company Facility <br /> 2651 South Airport Way <br /> Stockton,California <br /> Prepared for THE EARTFIGRAINS COMPANY Prepared by Philip Services Corp <br /> E 4121382112]382EG1t2599710599REP DOC June 22, 1999 <br /> 2 <br />