SITE INFORMATION AND CORRESPONDENCE

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j Theresa Lanctot �� 2651 S. Airport Way, Stocron, California Page 3 of 5 approximately 10 feet from the most impacted excavation floor sample, were collected from the saturated zone, with the possible exception of the soil sample collected at 35 feet bsg. PSC infers that the bulk of impacted soil has been removed through overexcavation performed by Environmental Science and Engineering, Inc. (ESE) in 1987 and through the soil venting/air sparging remedial action performed on the site. This inference appears to be drawn from contaminant concentration data thought to have been acquired by ESE from five pre -UST - removal borings advanced in February 1987 along the south side and the west and east ends of the two western -most USTs; however EHD believes this data to be from soil samples collected from the floor of the overexcavation. Boring logs and analytical data from the five 1987 borings are not available to EHD. The sparse data available from the UST areas are not sufficient to calculate the adsorbed hydrocarbon mass in the vadose zone, making it difficult to evaluate the hazard it poses to groundwater. Although PSC's inferences may be correct, EHD concludes that the inferences cannot be demonstrated at this time. Vertical Delineation of Impacted Ground Water A second deficiency of the model is lack of vertical delineation of impacted groundwater, a requirement for closure consideration. Of concern are the vapor concentration readings acquired from the deeper soil samples from the MW -1 boring. Only one soil sample contained detectable concentrations of TPH-g; however, the possibility exists that contaminants may have penetrated deeper into the subsurface than suspected in the immediate gasoline UST area or downgradient of the former UST area, and that groundwater at greater depth is being impacted by remnant adsorbed hydrocarbons. Data from the cone penetrometer test (CPT) borings, the closest being approximately 270 feet from the former gasoline UST pit, rather than demonstrating the vertical extent of impacted groundwater, show that deeper groundwater is impacted and needs to be characterized, starting in the suspected source area. Additional Potential Contaminants of Concern During this review of the site data, EHD did not find data to demonstrate that the lead scavengers 1,2-dichloroethane (1,2 -DCA) and ethylene dibromide (EDB) have been assessed. The Central Valley Regional Water Quality Control Board (CVWQCB) is not giving closure consideration to currently active sites without these chemicals and fuel oxygenates being assessed. To address the issues noted above, EHD directs you to • Reinitiate quarterly groundwater monitoring until directed otherwise by this office; • Complete the vertical assessment of impacted groundwater; • Collect data and prepare an estimate of the sorbed contaminant mass in the vadose zone; and • Assess for 1,2 -DCA and EDB. To meet these directives, EHD recommends: • Preparation of a contaminant/hydrogeological site conceptual model (SCM) as described below;