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-SHELL OIL COMPANY -3- 3 June 1992 <br /> STOCKTON FACILITY <br /> Shell states that the vertical extent of contamination has not been <br /> defined because petroleum product constituents are less dense than <br /> water, and therefore, float. Shell 's statement would be correct if <br /> there was no vertical gradient to influence the ground water flow. The <br /> presence of a vertical gradient is likely because there are extensive <br /> ground water depressions east of the facility, the closest one being <br /> three miles away. At the ARCO facility, which is approximately 3,500 <br /> feet southeast of Shell , contamination was found in monitoring wells <br /> screened at 65 to 75 feet below ground surface. Detection of benzene <br /> at this depth at the ARCO facility was surprising because the first <br /> water bearing zone consists of silts and clays underlain by a thin <br /> layer of sand which is underlain by an aquitard. The benzene was found <br /> below the aquitard. Based on contamination found at the ARCO facility, <br /> it is likely that contamination also exists beyond the first water <br /> bearing zone at Shell . <br /> Shell also states that another reason for not installing deep <br /> monitoring wells is to avoid cross contamination. The latter could <br /> easily be prevented through the use of conductor casing, borehole <br /> liners, and/or specialized drilling equipment. <br /> • Comment 5: See Comment 4. <br /> Shell also requests extension of the work plan submission to 30 June 1992. The work <br /> plan will address off-site migration and the extent of free phase petroleum product <br /> on the eastern side of the property. I think this request is reasonable. <br /> • <br />