SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr. Ken Gillies • Gillies Trucking 3931 Newton Road, Stockton Page 3 of 5 EHD concurs with the conclusion that diesel fuel contamination is the more prominent concern on the site with concentrations commonly 5 to 10 times that of gasoline components where the two occur together. Dissolved gasoline components have been detected in ground water samples collected from monitoring wells GT-1, GT-2 and GT-3, screened in the lower First Riverbank Sand, and in GT-10, screened in the Modesto Aquifer and Modesto Aquitard. Dissolved diesel concentrations ranged from 1.2 to 390 times the dissolved gasoline concentrations when both have been detected. If, as UEC concludes, contaminants migrated downward through the Modesto Channel until encountering impermeable silt and clay underlying the channel, then characterization of that channel is of great importance as the channel may have then been a preferred lateral fluid migration route through the presumably coarse, permeable bed load channel fill. UEC believes that few remedial options are available to address the site, citing soil vapor extraction and natural attenuation through biological activity and that the relatively low health risks posed by the release do not warrant the costs of remediation. To evaluate these statements, a feasibility study should be performed, cost estimates prepared, and the health risk and/or hazards posed by the release should be calculated. These actions require delineation of the impacted media and an estimate of the contaminant mass. UEC states that active remediation of impacted ground water would be difficult to implement as most of the more intensely impacted ground water is within the Modesto Channel and is of limited volume. As noted above, by identifying a channel structure, UEC has also identified an important lateral fluid migration pathway. Channels tend to be linear or curvilinear structures of limited lateral extent, but of potentially great axial extent. Greater volumes of impacted ground water may exist in the downgradient portion of the Modesto Channel if in the saturated zone. This demonstrates the importance of further assessment of impacted ground water within the channel that presumably migrates along the paleochannel axis in the paleochannel downgradient direction, which may have been missed with the current monitoring well layout. Also, if the channel cut through the Modesto Aquitard, which is shown locally to be less than 5 feet thick on UEC's Cross Section A-A', then fluids migrating along the channel bottom would have a vertical conduit to the underlying upper Riverbank sand interval. UEC's statement that the hydrogeology of the site has been well understood since as early as October 1996 and that subsequent work has merely confirmed the interpretation does not appear to be so. UEC states in First Quarter Report Gillies Trucking, dated 01 June 1999, "Additionally, we must conclude that groundwater gradient and flow direction are