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subsequently drilled soil borings 90-1 and 90-2, and installed wells MW-6 and MW-7 in February 1990, then MW-8 in September 1990 Philip (formerly ' Burlington Environmental) installed wells MW-9 through MW-13 in March 1994, and piezometers PZ-1 and PZ-2 and air sparge well SP-1 in September 1994 1 Philip performed a soil venting and air sparging pilot test in October and November of 1994 to evaluate the effectiveness of this remediation technology Soil venting tests were performed using well MW-1, and air sparging tests were performed using well SP-1 Results of the tests, contained in Philip's February 1, 1995 Final Remediation Plan, indicated that soil venting and air sparging would be effective in remediating hydrocarbon impact In June 1995, Philip proceeded with the installation of air sparging wells SP-2 through SP-6 However, implementation of the sparge and vent remediation was ' postponed due to petroleum hydrocarbon concentration reductions that were observed to be naturally occurring at the site ' In May 1999, Philip performed the latest subsurface investigation using cone penetrometer (CPT) technology Three CPT borings (CPT-1 through CPT-3) were driven, and soil and groundwater samples were collected for analysis This CPT ' investigation indicated that the lateral extent of impacted groundwater is largely contained within Earthgram's property boundaries Results from this investigation were reported in Philip's May 1999 Subsurface Investigation and Groundwater Monitoring Report 1.3 Historic Groundwater Conditions iAssessment of petroleum hydrocarbon impact in groundwater has been performed at this site since July 1987 Historic groundwater monitoring indicates that decreases of petroleum hydrocarbon concentrations have occurred due to natural degradation Benzene concentrations in samples collected from well MW-1, located near the original source of the petroleum release, have ranged from a high ' of 6,800 micrograms per liter(Rg/L) in July 1987 to a low of 170 µg/L in October 1998 (see Table 4) Total petroleum hydrocarbons as gasoline (TPH-G) concentrations in samples from MW-I have ranged from a high of 196,000 pg/L 1 in April 1993 to a low of 15,000 µg/L measured in August 1999 Current petroleum hydrocarbons concentrations are discussed in Section 3 of this report ' Oxygenate compounds methyl tert butyl. ether(MTBE), ethyl tert butyl ether (ETBE), tert butyl alcohol (TBA), and tert amyl methyl ether (TAME)have never been detected in site groundwater samples (see Table 5), prior to this round of monitoring (see Section 3 3) The oxygenate compound di-isopropyl ether (DIPE) has been measured at concentrations less than 16 µg/L, prior to this monitoring 4 November 1999 Groundwater Monitoring Report The Earthgrams Company 2651 South Airport Way Stockton,CaliforniaPrepared for THE EARTHGRAINS COMPANY Prepared by Philip Services Corp January 17,2000