SITE INFORMATION AND CORRESPONDENCE

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Page 1 of 1 Nuel Henderson [EH] From: Nuel Henderson [EH] Sent: Thursday, November 04, 2010 10:00 AM To: 'Kirk Larson' Cc: Vicki McCartney [EH]; 'Jim Barton'; 'Tim Cuellar' Subject: 2402 Pacific Avenue Stockton, Claim Number 757 Kirk, Your recent 5-year review recommendation for this site was closure consideration. The main remaining issue with the site is some deeper water contamination that appears to be persistent. Groundwater in shallow wells screened variably between 20 and 50 feet bsg are nearly all nondetect at this time, although a down-gradient well did contain low TPHg and TEX concentrations recently. Two wells in an intermediate depth zone screened variably between 40 and 70 feet bsg are impacted with moderate concentrations of TPHg and TEX. Three wells in a multi-chamber casing screened in deeper zones (95-100 feet, 125-130 feet and 145-150 feet bsg)are all impacted by low concentrations of TPHg and BTEX. I suspect that because the concentrations of the analytes in the three wells are nearly the same and track together over time that we are seeing some type of communication between the three or between each well and an unrecognized impacted zone. The presence of benzene suggests the contaminants are from depths greater than 70 feet bsg. The lateral extent of the deeper impacted groundwater has not been fully delineated and there is a municipal well approximately 645 feet down-gradient of the site and there are no intervening data points between the site and the municipal well. The impacted groundwater in the shallower intermediate zone well does have some down- gradient coverage by MW3 and possibly MW-4, the deeper intermediate zone well does not have down-gradient coverage. Contaminant concentrations in soil samples appear to decline to the total depth sampled and analyzed, and the concentrations in deeper samples may represent cross contamination from shallower impacted groundwater. The vertical extent of impacted soil I adequately delineated. The more impacted intermediate zone wells are in the former UST area, and intuitively, may well represent the most intensely impacted soil and groundwater. It would appear that the contaminant mass, although not calculated for the deeper zones, would not be sufficient to impact the municipal well. The EHD will issue a directive letter to the responsible party to prepare and submit a site closure summary report that provides a professional evaluation of the risks posed by the deeper impacted soil and groundwater and a fate and transport model. The shallow zone soil and groundwater issues appear to be adequately addressed; confirmation soil boring did not encounter impacted soil, monitoring well samples, with the recent exception of MW-4, have gone to nondetect, and soil gas samples were nondetect. Nuel Henderson 11/4/2010