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1.3 Historic Groundwater Conditions Assessment 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 partly occurred due to natural degradation Recent groundwater monitoring indicates that PSC's remedial efforts are ' having a substantial affect on further reducing petroleum hydrocarbon concentrations Benzene concentrations in groundwater samples collected from well MW-1 have ranged from a high of 6,800 micrograms per liter (µg/L) in July 1987 to a low of 18 µg/L in April 2002 (see Table 4) Total petroleum hydrocarbons as gasoline (TPH-G) concentrations in samples from MW-1 have ranged from a high of 196,000 pg/L in April 1993 to a low of ' 1,100 pg/L measured in April 2002 Current petroleum hydrocarbon concentrations are further discussed in Section 3 2 of this report Oxygenate compounds methyl-tert-butyl ether (MTBE), ethyl-tert-butyl ether (ETBE), and tert- amyl-methyl ether (TAME) have never been detected in site groundwater samples (see Table 5) The oxygenate compound di-isopropyl ' ether(DIPE) had historically been measured at concentrations of 140 µg/L or less in samples collected from wells near the source of the release The compound tert-butanol (TBA) was detected in one groundwater sample collected from MW-1 on November 8, 1999, and again in PZ-1 samples collected on October 18, 2001 and April 20, 2001, but has never been detected in any other groundwater sample collected at the site Current oxygenate ' concentrations are further discussed in Section 3 3 of this report As shown in Table I and Table 2, recent depths to groundwater average about ' 40 feet below ground surface (bgs) The lowest depth to groundwater, observed during our historic site monitoring, was about 60 feet bgs in December 1992 The EHD suggests that groundwater was at a depth of about ' 80 feet in 1978 Groundwater has consistently been evaluated to flow in an east-northeast direction at approximately N70E across the site Current groundwater elevation information is summarized in Section 3 1 of this report 1.4 Remedial Action On April 25, 2001, PSC initiated operation of an air sparging and soil venting remediation system to address subsurface petroleum hydrocarbon impact Air sparging had initially been performed in well SP-1 However, FSC suspected that air sparging at the SP-1 location was not resulting in an adequate decrease 6 April 2002 Groundwater Monitoring And Air Sparging Well installation Report The Eartligralns Baking Company 2651 South Airport Way Stockton,California Prepared for THE EARTHGRAINS BAKING COMPANY Prepared by Philip Services Corp ' H 1ENV1624001325121382EGLSia kton\Sicekton10462mon clot May 21,2002