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r <br /> UNOCAL <br /> Page 2 <br /> ' contamination more often(including current results) than the samples from MW5 The proposed <br /> sod bonng 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 PHSIEHD 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 PHSIEHD <br /> recommends that vapor extraction and air sparge wells be installed during this phase of work. <br /> ' If Methyl ternary butyl ether(MTBE)is detected in any samples analyzed by the mobile laboratory <br /> using EPA Method 8024, 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 <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 /> PHSIEHD by July 27, 1998 <br /> ' If you have any questions contact me at(209)468-3449 <br /> Donna Heran, RENS, Director <br /> Environmental Health Division <br /> ' Margaret Lagono, Supervising REHS <br /> Site Mitigation Unit <br /> C PVRWQCB-Mark List <br /> ✓ARCADIS Geraghty&Miller-Dr Stephen Cullen <br /> Helen McCrary <br /> ' SWRCB-Jim Munch <br />