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Ms. Gail Wiggett Project 20C42-001.026 <br /> June 25, 1998 <br /> Page 3 <br /> — The grout was continuously placed to 5 feet bgs as determined from the mark <br /> on the tremie pipe and tagged firmly at 10 feet bgs.Therefore, grout placement <br /> was to 5 feet bgs. When the second batch of grout was placed, the tremie pipe <br /> was set at 15 feet bgs,providing a 5-foot overlap with the level fim-dy tagged <br /> for the fust batch. This overlap provided continuity in the grout seal. In <br /> addition, the second batch of grout was placed within the 2-hour window for <br /> grout setup. <br /> • The well casing should have been removed prior to tremie pipe removal. <br /> — The tremie pipe was removed first, followed by the removal of well casing. <br /> Approximately 44 feet of casing were pulled. When the wellhead was removed, <br /> an additional 3 feet of casing were removed, for a total of 47 feet. The casing <br /> between 45 and 55 feet bgs was extensively corroded with numerous holes <br /> present. These holes acted as perforations to allow the grout to extend to the <br /> formation walls. After the casing was removed, a firm tag at 10 feet bgs <br /> confirmed the grout level. The order of tremie pipe and casing removal did not <br /> adversely affect the integrity of the grout seal. <br /> • The well destruction fell into the category of Section 13.17.6 of the PHS/EHD <br /> Well Standards, which requires complete removal of the well casing, screen, and <br /> filter pack. <br /> — Removal of the well casing and absence of filter pack in the interval to 55 feet <br /> is consistent with the intent of Section 13.17.6. <br /> — Review of subsurface data collected during previous CPT and drilling <br /> programs identified a low-permeability, silty clay unit located between <br /> approximately 37 and 47 feet bgs. This unit correlates with the B-C aquitard by <br /> Harding Lawson Associates(HLA)during subsurface assessment at the ARCO <br /> Products Company Distribution Terminal. HLA reported laboratory <br /> permeabilities for soil samples collected from the B-C aquitard ranging from <br /> 1.43E-6 to 7.61E-8. The grout seal encompasses this interval. <br /> — The presence of the low-permeability unit(B-C aquitard) and the grout seal <br /> across this interval provide a barrier inhibiting future vertical migration of the <br /> dissolved petroleum hydrocarbons present in the first- and second-encountered <br /> water-bearing zones into deeper zones along the well bore. Thus, the source <br /> areas represented by these shallow water-bearing zones and no longer in <br /> communication with deeper aquifer units. <br /> SACN:.\PJW4DllC420012.6DE-98Vbs s:t <br />