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of any five foot section of borehole are 18" in length from a 60" column representing only 30% of the <br /> material in the five foot column). In addition, continuous core boreholes are advised for all sites installing <br /> monitor wells as screened intervals which bridge significantly different lithologic units may create <br /> preferential pathways for contaminant migration particularly into deeper portions of beneficial use aquifers. <br /> Comprehensive site definition and well construction detail efforts will likely increase the potential for <br /> Chevron to achieve closure at sites quickly, with greater confidence, potentially reducing long term <br /> liability. Considering these issues, PHS/EHD recommends that Chevron install limited continuous core <br /> boreholes and prepare cross-sections for each scope of subsurface assessment work in order to more <br /> efficiently and effectively guide the site characterization process toward closure. PHS/EHD also <br /> recommends the use of reliable cost-effective technologies such as Cone Penetration Testing (CPT) for <br /> both continuous logging and subsurface sampling at UST sites. <br /> I have conditionally approved the work plan requesting continuous cores, oxygenates analyses and cross <br /> sections. Please contact me at your earliest convenience regarding this memo(209)468-0342. PHS/EHD <br /> will require a written response from Chevron by December 20, 1998. <br /> i <br />