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Ctry of Stockton A68104 02A June 6, 1996 Page 8 • Mass of remaining toluene (1 08 X 105) * (13 X 10-6) = 14 x 10-1 lbs '� • Mass of remaining ethylbenzene (1 08 X 105) * (1 0 X 10-6) = 1 1 x 10"1 lbs • Mass of remaining xylenes (1 08 X 105) * (8 5 X 10-6) = 9 2 x 10-1 lbs I • Mass of remaining TPH-G (1 08 X 10) * (39 X 10 6) = 4 2 lbs • Volume of remaining TPH-G (gasoline ;= 6 lbs per gallon [gall) 4 2 lbs * _Lgal = 0 7 gals 6 lbs The above listed mass and volume estimates are considered worst case Since it is most likely the concentrations of PPCs in soil actually attenuate as distance from the source increases, it is likely that the actual mass and volume of PPCs remaining in the soil are less than the estimated amounts 4.2 Groundwater Monitoring Well Destruction Monitoring well MW-1 will be destroyed consistent with California Well Standards (California Department of Water Resources, 1991) The material which was installed in the original borehole (i e , well casing, filter pack, and annular seal) will be removed by "overdrilling" The drilling will be performed with a Central Mine Equipment CME-75, mobile drill rig equipped with an 11-5/8-inch, hollow-stem auger Drilling operations will be performed consistent with Twining's standard operating procedures a5 outlined in Appendix A After completing the drilling, the borehole will then be backfilled with a Portland cement slurry (i e , neat cement) The slurry will be gravity placed through the annulus in the hollow-stem augers in one continuous operation from the bottom to the top of the borehole When the annulus is filled with the slurry, auger flights will be removed (no more than 10 feet at a time) from the borehole, allowing the slurry to uniformly fill the borehole to the surface Material removed from the borehole by overdrilling will be disposed of as outlined in the following section