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Due to the uncertainty introduced by these two conditions, we recommend drilling a few new <br /> confinnatron borings into the Modesto Formation to determine its present potential to release <br /> hydrocarbons to the gromidwater if the water table should rise from the Riverbank Formation into <br /> the Modesto Fon-nation in the future Figure 16 shows proposed locations for six borings Same of <br /> them are located near former bonngs, and others ale located along the channel talweg beyond the <br /> 1OLatron5 of previous borings Although not shown in the figure, it would probably be advisable to <br /> drill an additional boring east of GT-21 to attempt to locate the eastern limit of soil contamination <br /> within the channel <br /> 6 2 Procedures <br /> I he borings will be dulled through the Modesto Channel into the Modesto aquitard using 8-inch <br /> diameter hollow stem augers Most borings will ther6fore be drilled to a depth of 55-60 feet, <br /> although the depth will decrease eastward (updrp) Samples will be collected at 5-foot intervals <br /> from the top of the channel (approximately 35 feet depth) to the bottom of the hole and described <br /> on the boring log before being preserved in a cooled ice chest Drill cuttings will be added to the <br /> existing soil stockpile and allowed to aerate (if hydrocarbon odors are detected) The total voltuzie <br /> of cuttings will be only a few cubic yards <br /> We propose analyzing all samples from each boring in order to derive an accurate calculation of the <br /> hydrocarbon mass within the total volume of Modesto channel and aqurtard sediment Selected <br /> samples will also be analyzed for gasoline and BTEX which have been detected in the vicinity of <br /> the tormer US F cavity Revised TPH-d rsoconcentratron maps will be prepared from the new data <br /> for each 5-foot depth interval, older data will be incorporated into the maps only it they appear <br /> reasonable Any anomalies or discrepancies between new and old data will be thoroughly analyzed <br /> and dr5cussed in the resulting report <br /> The rscoconc,entratron maps will be used to calculate the hydrocarbon mass in each 5-toot zone, and <br /> the zone maps will be compiled to derive a total mass in place for the Modesto Formation This <br /> mass estimate will theft form the basis for fate and transport calculations using one of the available <br /> computer software programs <br /> If approved drilling would take place late in the 2 n Quarter of 2005 The results of the <br /> investigation would likely be included in the 3rd Quarter 2005 Report <br /> 11 <br />