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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 (µg/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-1 have ranged from a high of 196,000 pg/]L 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), 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 Monitoring data indicates that the lateral extent of DIPE impact is limited to the Earthgrains property boundaries 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 As shown in Table 1 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 14 Remedial Action On April 25, 2001, PSC initiated operation of an air sparging and soil venting remediation system to address subsurface petroleum hydrocarbon impact As described in PSC's August 31, 2000 Remedial Action Work Plan, sparging is performed by supplying an approximate volume of 5 standard cubic feet per minute (scfm) of air to well SP-1 Venting is performed by withdrawing an approximate volume of 25 to 35 scfm of soil gas from well VW-1 Soil gas is treated initially through a primary 1,000 pound granular activated carbon (GAC) adsorber and then through a secondary 160 pound GAC adsorber Permits to operate the remediation system and emit treated air have been obtained from the San Joaquin Air Pollution Control District (SJAPCD) 6 January 2002 Ground,Aaler Monitoring Report The Larthgralns Baking Company 2651 South Airport Way Stockton California Prepared for IIIF E\RTI{GRAINS BAKING COMPANY Prepared by Philip Services Corp H1ENV\62400132\121382LG1SwcktonlStnd.ton10102mon doc February 15 2002