SITE INFORMATION AND CORRESPONDENCE

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Theresa Lanctot I )� 2651 S. Airport Way Page 2 of 5 1 Stockton, California Q, • Potential exposure pathways for impacted soil are generally improbable or incomplete and risks to associated potential receptors are limited; and • Groundwater modeling indicates that benzene concentrations will not exceed the maximum concentration limit (MCL) in groundwater beyond the facility boundaries. Based on these findings, interpretations, and conclusions, PSC recommends consideration of case closure for the site. EHD comments on these findings and interpretations as follows: • The first six items bulleted above raise no concern to EHD; • EHD intuitively agrees that excavation removed the most intensely impacted soil, but it has not been shown that most of the impacted soil or that the major portion of the contaminant mass has been removed, as is discussed below; • EHD does not concur with PSC's position that the vertical extent of impacted groundwater has been demonstrated; therefore, EHD cannot concur with the absolute values of the dissolved contaminant mass reduction; EHD does agree that the general magnitude of mass reduction has probably occurred; • PSC did not include the natural attenuation parameter data in the report for EHD to review; however, based on PSC's description of the data trends, EHD is inclined to believe that the natural attenuation process is in operation on the site; please present the natural attenuation data in the next report; • EHD agrees with the exposure pathway analysis for impacted soil; and • EHD will delay evaluation of the groundwater monitoring data until the additional data requested below have been obtained and integrated in to the model. PSC has shown some trends favorable for a site closure consideration, including: a general decline in dissolved contaminant concentration; a decline of dissolved contaminant mass in shallow ground water; a decline of recoverable hydrocarbon mass in the vadose zone addressed by the soil vapor extraction system; and an estimate of the vadose zone mass extracted to date. However, EHD cannot support a recommendation for site closure at this time as an estimate of the sorbed vadose zone contaminant mass has not been prepared; the vertical extent of impacted groundwater has not been demonstrated; and there is insufficient data to eliminate several contaminants of potential concern. These items are discussed below, and EHD's recommendations are presented to address these issues and to move toward site closure. Sorbed Vadose Zone Contaminant Mass One critical deficiency of the current site model is a lack of characterization of the plume of impacted soil in the unsaturated (vadose) zone, that is, the soil above groundwater. An estimate of the current vadose zone contaminant mass is needed to evaluate the future risk for impacting groundwater. An estimate of the original and current contaminant mass in soil has not been prepared, and indeed would be difficult to prepare, as little data have been acquired directly in the area of contamination in the vadose zone soil. No borings have been advanced through the former UST pits, under the former dispenser location, or immediately adjacent to a sump located approximately 10 feet north of the former UST location and 5 feet north of the former dispenser location. The sump provides a potential contaminant migration route into the subsurface that should be investigated. Soil samples collected from the boring for monitoring well MW -1,