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UNOCAL <br /> Page 2 <br /> contamination more often (including current results) than the samples from MWS. The proposed <br /> soil boring should be located further south and closer to MW2. <br /> The work plan states that drilling will stop if a competent(greater than 3 feet thick) confining unit is <br /> encountered. PHS/EHD questions the definition of a confining layer being 3 feet thick and would <br /> comment that a continuous confining layer at the depths to be investigated has never been <br /> documented. <br /> Collection of groundwater samples at a minimum of 10 foot intervals from first encountered water <br /> to 50 feet is acceptable. After that depth samples should be collected at changes in lithology or a <br /> maximum of 10 foot intervals. <br /> Once the lateral and vertical extent of soil and groundwater contamination have been defined, site <br /> specific feasibility studies must be conducted to determine the most cost effective remedial <br /> alternative. Site conditions have changed since the vapor extraction tests in 1992 and 1996. <br /> Vapor extraction and air sparging should be evaluated since they have proven to be effective in <br /> areas of Stockton. Scientific literature and data from other sites in California are not appropriate <br /> for use in evaluating the most cost effective remedial alternative for this site. PHS/EHD <br /> recommends that vapor extraction 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 laboratory <br /> using EPA Method 8020, they should be reanalyzed for MTBE, Di-isopropyl ether, Ethyl tertiary <br /> butyl ether, t-Amyl methyl ether, and tertiary butanol by a fixed-site laboratory using EPA method <br /> 8260A. 1 <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 to <br /> 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 /> I <br /> Margaret Lagorio, Supervising REHS <br /> Site Mitigation Unit <br /> c: CVRWQCB- Mark List <br /> ARCADIS Geraghty & Miller-Dr. Stephen Cullen <br /> Helen McCrary <br /> SWRCB -Jim Munch <br />