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UNOCAL <br /> Page 2 <br /> The results of the soil samples collected from boring UL revealed soil contamination <br /> from 15.5 feet to 45.5 feet bsg. The results of soil samples collected from soil boring UN <br /> revealed non detectable soil contamination to 40 feet bsg. The results of soil samples <br /> from borings U3 and UM show higher levels of contamination at 46.5 and 45.5 feet bsg <br /> respectively, than at 36.5 and 30.5 feet bsg respectively. The proposed soil boring <br /> should be located further south between U3 and UM. <br /> The work plan states that drilling will stop if a competent (greater than 3 feet thick) <br /> confining unit is encountered. PHS/EHD questions the definition of a confining layer <br /> being 3 feet thick and would comment that a continuous confining layer at the depths to <br /> be investigated has never been documented. <br /> Collection of groundwater samples at a minimum of 10 foot intervals from first <br /> encountered water to 50 feet is acceptable. After that depth samples should be <br /> collected at changes in lithology or a maximum of 10 foot intervals. <br /> Once the lateral and vertical extent of soil and groundwater contamination have been <br /> defined, site specific feasibility studies must be conducted to determine the most cost <br /> effective remedial alternative. Vapor extraction and air sparging should be evaluated <br /> since they have proven to be effective in areas of Stockton. Scientific literature and data <br /> from other sites in California are not appropriate for use in evaluating the most cost <br /> effective remedial alternative for this site. PHS/EHD recommends that vapor extraction <br /> and air sparge wells be installed during this phase of work. <br /> If Methyl tertiary butyl ether(MTBE) is detected in any samples analyzed by the mobile <br /> laboratory using EPA Method 8020, they should be reanalyzed for MTBE, Di-isopropyl <br /> ether, Ethyl tertiary butyl ether, t-Amyl methyl ether, and tertiary butanol by a fixed-site <br /> laboratory using EPA method 8260A. <br /> The size of the hollow stem auger to be used for drilling should be specified. <br /> An addendum to the work plan, incorporating the above comments should be submitted <br /> to PHS/EHD by July 27, 1998. <br /> If you have any questions contact me at (209) 468-3449. <br /> Donna Heran, REHS, Director <br /> Environmental Health Division <br /> Margaret Lagorio, Supervising REHS <br /> Site Mitigation Unit <br /> c: CVRWQCB - Mark List <br /> ARCADIS Geraghty & Miller-Dr. Stephen Cullen <br /> UNOCAL - Bob Boust <br /> SWRCB - Lori Casias <br />