SITE INFORMATION AND CORRESPONDENCE

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Fire Station#9 - 2 - 6 July 1998 Stockton, San Joaquin County Groundwater was encountered at approximately 48 feet bgs during the execution of this work in SP-1. Prior to backfilling, a"temporary, 1-inch diameter polyvinyl-chloride casing was inserted into the probe hole." The bottom 10 foot section of the casing was cut with 0.01-inch slots. One groundwater sample was collected and analyzed for BTEX, MTBE and TPHd. The results of the groundwater analysis indicated TPHd at a concentration of 410 ug/L, and toluene at 0.7 µg/L in groundwaier.No other A analytes were detected above the LDL. The presence of TPHd and toluene in the groundwater sample, indicates that contamination from the site has impacted groundwater. Conclusions: The boring log for the deep soil boring, SP-1, indicates the presence of clay from approximately 6 feet to 47.5 feet below round surface. Either the 3 0-3 5 foot thick sequence of clay app Y g separating the indicated zone of contamination from groundwater, is not an effective barrier to inhibit vertical mi ration of contaminants, or there is a greater mass of contamination in tkle subsurface than has been indicated by the limited laboratory data. ' The presence of TPHd contamination in groundwater, without an associated soil smear zone may indicate an additional on-site or off-site source of contamination, and further support the argument that the site characterization is incomplete. The groundwater contamination is anomalous when compared to the zone of soil contamination. The potential groundwater impact and subsequent impact to sensitive receptors has not been determined, and cannot be determined based on the provided information. The groundwater flow direction and gradient below the site have not been determined, and the vertical and horizontal extent of contamination in soil and groundwater is poorly characterized, or not characterized at all.No mass balance has been performed to evaluate the mass of contamination that is being left in place, nor has a volumetric estimate been completed to determine the distribution of the contaminated subsurface. A regional well survey was not completed to determine the potential receptors. With the limited soil documentation and the groundwater impact that has been presented, it appears that there is not sufficient site characterization to support site closure. f t During phone conversations on 29 June and 1 July 1998, with Ms. Margaret Lagorio of San Joaquin County, she outlined the County's position on the site. She agreed with my evaluation of the characterization, indicating that the information-supplied,is-of.-poor-quality:7�LHowever;she believes-that° the concentrations at the site do not constitute a problem because diesel fuel; and low concentrations of toluene, ethylbenzene and xylenes are the only indicated contaminants. Additionally, she indicated that no water supply wells exist in the area, and that the potential for impact is low. With the incorporation of the County's comments, and the condition that sample results are representative, Regional Board staff ' does not dispute the No Further Action Required request.