SITE INFORMATION AND CORRESPONDENCE

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rr Shelby Lathrop (ConocoPhillips) and Don Pratt(ARCO) 1502 N. EI Dorado Street and 16 E. Harding Way, Stockton Page 3 of 5 The C zone wells appear to be screened near the lower extent of impacted ground water on the CP site, the highest concentrations for analytes of concern detected during the October 2006 monitoring event were 100 micrograms per liter (pg/1) TPH-g (MW-16), 0.94 ,ug/I benzene (MW-17), 0.6 ,ug/I total xylenes (MW-16), 1.4 pg/I MTBE (MW-16) and 1.5 Ng/1 1,2-DCA (MW-16). Soil data: 16 E. Harding Way — Soil samples from DMW1-50', EW1-47.5', DMW3 to 50' bgs, and MW4-40' exhibit significant gasoline component impact and indicate a significant potential for impact to the B' zone ground water. MtBE was detected by EPA Method 8260 in soil samples at concentrations as high as 13 milligrams per kilogram (mg/kg) in sample DMW-3 @ 50 feet), 291 mg/kg (EW-1 @ 275) and 684 mg/kg (MW-4 @ 25 feet bsg). These data confirm the presence of MtBE in the release on the ARCO site and demonstrate a high potential for MtBE impact to groundwater in the A and B' zones. 1502 N. EI Dorado Street—At the time of the UST system removal, some shallow soil samples (3 to 4 feet bsg) from the dispenser and product line areas had MtBE concentrations as high as 4.3 mg/kg, as analyzed by EPA Method 8260B. MtBE was detected in some boring soil samples collected in early 1999 by DHS LUFT Method; results that were not checked by EPA Method 8260. Soil samples collected in July 1999, and later, that had MtBE identified by the DHS LUFT Method were checked by EPA Method 8260. The latter soil samples, some with MtBE detections as high as 33.8 mg/kg by the DHS LUFT Method (MW-6 @ 60 feet bsg), were found to have less than method detection limits by EPA Method 8260 (less than 0.4 mg/kg). These results indicate that the initial deeper soil sample MtBE detections may have been false positives; therefore there is no definitive evidence of MtBE in the release on the CP site likely to directly impact B' zone and deeper groundwater; however, TPH-g and BTEX data from soil samples from GP2-50', GP3-60', MW-6, MW-9, and MW-10 demonstrated significant gasoline component impact which would have a significant potential for contaminant impact to groundwater in the unmonitored B' zone. Groundwater data: The EHD plotted and contoured the maximum MtBE concentrations in the CP and ARCO A zone wells and did the same with the date of the maximum dissolved MtBE impact in each monitoring well. The result for both maps can be interpreted to show a northeastward progression of the maximum MtBE impact with time originating from the ARCO site. A zone wells MW-3 and MW-4 on the CP site were impacted as soon as the wells were installed, probably due to the MtBE impact to soil in the former dispenser area. The available data for the B zone on the CP site indicates that the maximum concentrations for most CP B zone wells occurred recently in 2006, some with MtBE concentrations potentially still increasing. The data for the three ARCO B zone wells