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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 sod 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 sod 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 PHSIEHD 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 sod 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 PHSIEHD recommends that vapor extraction <br /> and air sparge wells be installed dunng 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 PHSIEHD 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 /> it lArz�o0 <br />' ,ono <br /> Margaret La Supervising REHS <br /> 9 9 9 <br /> Site Mitigation Unit <br />' c VRWQCB - Mark List <br /> ARCADIS Geraghty & Miller-Dr Stephen Cullen <br /> UNOCAL- Bob Boust <br /> SWRCB - Lon Casias <br />