SITE INFORMATION AND CORRESPONDENCE

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• N • f Mr. Ben Hall - 2- 24 July 2003 Musco Family Olive Company The Addendum adequately addresses Items 1, 2, 3, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 21, and 22. Item 4 requests a schedule to replace monitoring wells that have gone dry (MW-5, MW-6, and MW-10). The Addendum indicates the dewatered condition in these wells may result from the seasonal effect of a perched water system, and replacement of these wells is premature. Completion of the work proposed above should provide the necessary data to determine which of these wells, if any, should be replaced. Item 5 requests additional information regarding the installation of three wells to be drilled near MW-1, MW-2 and MW-6. The Addendum indicates that exploratory soil borings, not groundwater monitoring wells,-would be completed near these wells within three weeks following workplan approval. This work should-be re-evaluated in light of the proposal described in the enclosed. Item 6 regards how the drilling method used(air rotary) may have failed to identify a water table aquifer in the northern area near MW-7, MW-8, and MW-12. The Addendum recognizes that a hollow stem auger would have been a better drilling method to assess shallow perched intervals,but defends the use of the air rotary method in this situation based on experience. The Addendum concludes that further investigation of the northern area is not necessary or justified. It reports that data obtained to date do not suggest the presence of a continuous shallow perched zone at the site and that a registered geophysicist reviewed"the most recent geophysical data" and determined that it"tends to support discontinuous perched conditions at the site in the uplands areas." The Addendum indicates that Musco will submit a report on this geophysical data review in the hydrogeologic investigation's final report. However, to facilitate staff s review of data related to the site's stratigraphy and to expedite staff's concurrence of the investigation's scope of work, Musco should submit.this information as soon as possible. As explained in the enclosed staff memorandum, further investigation in this area is necessary to more adequately define the site's hydrogeologic framework. Item 7 requests an explanation of the basis for selecting samples for isotope analysis. The Addendum indicates that groundwater samples would be collected from select wells based on their location and screened depths and on the anomalous groundwater chemistry from MW-2. It further explains that the basis for selection would be to provide a background water quality comparison of wells completed in different zones of the site and of the discharge. It indicates that, at a minimum, samples for this analysis would be collected of the discharge and of groundwater sampled from MW-1, MW-2, MW-3, and MW-8. While this proposal appears reasonable, it may be appropriate at some future date to expand this analysis to include additional groundwater monitoring wells constructed as a result of implementing the work recommended in the enclosed. Item 11 requests additional information on the proposed soil drilling work. The Addendum proposes to drill three soil borings with a hollow stem auger drill rig equipped with the capability to switch over to an air rotary drilling system if drilling becomes difficult. One boring would be drilled near MW-1 (80 feet), the second near MW-6 (160 feet), and the third near the central portion of the site's southwest corner(165 feet). Soil samples would be collected during the drilling work for lithologic classification and the results correlated to the geophysical data. The Addendum also proposes to drill at least two shallow (40-50 feet) soil borings north and south of MW-2 to better define the occurrence of low resistivity pockets, as recommended in the geophysical report described above. The Addendum does not propose to geophysically log the soil borings. This proposed work should be re-evaluated in light of the recommended work described in the enclosed.