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r <br /> concentrations increase At the present time, the water depth is in the middle range (in the lower <br /> portion of the Modesto aquitard), and concentrations in the lst Riverbank are correspondingly in the <br /> middle portion of their historical range <br /> The relative uniformity of diesel concentrations in all wells, even those that are close to the former <br /> UST facility, implies that there is no high concentration plume core within the Riverbank <br /> Formation Rattier, the isoconcentration maps suggest that the small mass of diesel fuel that has <br /> migrated downward into the Riverbank during past low-water years has dispersed laterally and not <br /> formed a "hotspot" that is amenable to or deserving of active remediation Over time the diesel in <br /> these zones, which is only a few hundred parts per billion above the detection limit (at most) will <br /> attenuate on its own due to lateral diffusion and natural biodegradation The data support the <br /> conclusion that further assessment or remediation of the Riverbank Formation is unnecessary <br /> t <br /> 5.0 RECOMMENDATIONS <br /> In the Fourth Quarter 2004 Report, we recommended moving the location of GT-19 northward <br /> approximately 100 feet because diesel was detection in GT-13B In the First Quarter 2005 Report <br /> we recommended deferring that recommendation because diesel was not detected in GT-13B This <br /> s <br /> well was again free of hydrocarbons in the second quarter, and we are now more confident that our <br /> original (mid-2004) hypothesis regarding the extent of contamination in the 3td Riverbank was <br /> correct It does not appear to extend west of GT-13, and we therefore do not recommend drilling <br /> • GT-19 <br /> The work plan that was contained within the First Quarter 2005 Report was approved by'San <br /> Joaquin County EHD and drilling will take place in early August 2005 Several borings will be <br /> drilled through the Modesto Formation to collect soil samples for laboratory analysis, and the <br /> results will be compared to samples that were collected during previous phases of drilling to assess <br /> any changes that may have taken place in the past several years When the results of the laboratory <br /> data have been received, we recommend preparing a revised estimate of the remaining hydrocarbon <br /> mass in the Modesto Formation and using that estimate to model the transport of the adsorbed mass <br /> from the unsaturated zone to the saturated zone in future years, should the water table again rise <br /> above 50 feet and come in contact with the remaining fuel in the Modesto Formation The fate and <br /> transport model and mass balance calculations will be included in the Third Quarter 2005 Report <br /> If the results justify our present conclusions, we then recommend preparing a Site Closure Report in <br /> conjunction with the Fourth Quarter 2005 Report <br /> N <br /> 5 <br /> I <br />