SITE INFORMATION AND CORRESPONDENCE

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Shelby Lathrop (ConocoPhillips)and Don Pratt(ARCO) 1502 N. EI Dorado Street and 16 E. Harding Way, Stockton Page 4 of 5 show MtBE concentrations that appear to have maximized and were declining when the wells were first monitored in 2002; a possible exception being the singular concentration at 9,400 Ng/I in DMW-3 in January 2005. As the MtBE concentrations in that well are usually less than 5 ug/I, the EHD suspects the January 2005 data is not reliable. There are no B' zone data. The contaminant in the CP site A zone wells appears to primarily be MtBE, there is little BTEX present and the TPH-g concentrations tend to be similar to the MtBE concentrations. 1,2-DCA is present in some A zone wells. MtBE appears to have first impacted the CP site B zone wells roughly in the same time period as the A zone wells were impacted, but the B zone wells commonly were also impacted by TPH-g and BTEX one to three years before the MtBE impact was discernable. The TPH-g and BTEX impact to groundwater in the CP site B zone wells years prior to their being impacted by MtBE and the lack of TPH-g and BTEX in the CP site A zone wells can be interpreted to indicate that the B zone contaminants migrated laterally onto the site from an up-gradient source. The MtBE impact long after the groundwater was impacted by TPH-g and BTEX may indicate several stages of contaminant release; the latter one(s) with MTBE formulated gasoline. The lack of confirmed MtBE in CP site boring soil samples collected from depths more than 4 feet bsg may indicate that the dissolved MtBE in A zone wells MW-1 and MW-2 and B zone wells migrated laterally onto the site. The EHD is concerned about the increasing MTBE concentrations in the CP well MW-5 and with the unmonitored B' zone; therefore ConocoPhillips and ARCO are directed to submit work plans th the EHD by 01 August 2007 to characterize the B' zone in up-gradient, cross-gradient and down-gradient directions. The Work Plan for Feasibility Testing (WP), dated November 29, 2006, prepared for CP by SECOR compares soil excavation, dual phase extraction (DPE), chemical oxidation, and natural attenuation as remedial alternatives for the CP site. Based on the depth to first ground water (-30' bgs) and the depth to the most intensely impacted soil (-60' bgs), excavation is deemed ineffective for the site, a conclusion that has EHD concurrence. Due to the same factors and the relatively high permeability of the soil, the potential for successful application of DPE is also in doubt. Natural attenuation is the SECOR-recommended technology to apply to the site; the EHD is unlikely to approve such an approach as it may require many years to achieve background conditions. To grant a "no further action required" status for a site with residual contamination, the contaminant concentration trends must exhibit a continual and predicable decline and reach background conditions within a reasonable, predicable time frame. SECOR did not provide an estimate of the time required for such achievement.