SITE INFORMATION AND CORRESPONDENCE_CASE 2

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VI. DISCUSSIONS AND CONCLUSIONS Results of the investigation indicate the upper 20 feet of site soils, within an area of about 30 — feet by 30 feet centered on the former diesel tank pit, have elevated diesel concentration. This area with apparent diesel contamination, as defined by boring locations B-1 and B-6 through B- 8, encompasses the former diesel tank site and the western portion of the former spray rack. A graphic presentation of the interpreted area of diesel contamination is shown on Figure 5. The levels of diesel contamination in the upper 10 feet of site soils within the affected area are generally high, in the thousands-ppm range. However, the concentration levels decrease significantly with depth to less than 40 ppm at about 20 feet below ground surface (bgs) and "non-detect" at 25 feet bgs. This spatial distribution pattern of diesel concentrations in the site's soils suggests surface residue from truck bed spraying is the primary cause of soil contamination. Since the depth to groundwater table at the subject site is more than 100 feet and no detectable diesel concentrations were found in soil samples from 25 feet bgs or deeper, the likelihood that site groundwater has been impacted by the surficial diesel contamination is negligible. Further groundwater investigation at the study site is not warranted. Even though data from this investigation indicate the extent of diesel contaminated soils is limited, the existence of elevated diesel concentrations in site soils is a potential source for future groundwater impact. To protect the groundwater quality and to prevent further contamination, the diesel impacted soils should be mitigated to eliminate a potential source of future groundwater contamination. The most direct and expeditious method to remediate the site's soils is to excavate and treat/dispose the impacted soils. VII. RECOMMENDATIONS Based on the investigation findings and conclusions formulated, AllWest recommends the following course of actions: 1. Submit a copy of this report to the lead regulatory agency, the San Joaquin County Environmental Health Division, to inform the regulators of the findings and recommendations; 2. Remove the potential source of groundwater impact by remediating the diesel contaminated soils. Prior to soil remediation, a work plan outlining the remedial methods and the target clean-up level should be submitted to the regulatory agencies for concurrence; 3. Conduct soil remediation and perform verification sampling to document the effectiveness of soil remediation; and 6