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2905 Benjamin Holt <br /> December 1997 <br /> Page Z . <br /> The groundwater assessment included in the addendum indicated that concentrations of MTBE <br /> observed in MW9 may be originating from the ARCO facility immediately south of the site; <br /> while the increasing concentrations observed in MW4,adjacent to the operating Chevron . <br /> tanks,has not been determined. PHS/EHD will require additional monitoring wells cross and <br /> ' down gradient to MW4,so that the extent of the plume'can be estimated. PHS/EHD also <br /> suggests investigation of source of contamination evidenced in MW4, The most recent results <br /> (September 1997) documented MTBE contamination of 2,100 ppb when in February 1997, 16 <br /> ppb MTBE contamination was observed. The sharp increase in MTBE contamination <br /> evidenced first in May 1997 followed the start-up of operations of the new tank configuration. <br /> i <br /> i <br /> The addendum reviewed the feasibility and cost effectiveness of the soil vapor extraction/air j <br /> sparging remediation system which has been installed. The addendum indicated that the soil <br /> vapor extraction system would be operating for approximately one year. It is PHS/EHD's <br /> understanding the system has not been operating due to high groundwater levels. The most <br /> recent quarterly report which PHS/EHD received indicated that the groundwater levels have <br /> decreased,please keep PHS/EHD updated on the status of the vapor extraction system's <br /> operation'. <br /> The addendum also included cleanup goals which reasonably comply with the Central Valley <br /> Regional Water Quality Control Board's Basin Plan. <br /> y <br /> Please submit a work plan by January 15, 1998 which proposes at least two monitoring wells <br /> to investigate the contamination observed in MW4. Quarterly remediation progress reports!, <br /> should be included with the monitoring results. <br /> If you have any questions,please contact me at (209) 468-0337. <br /> Donna Heran, REHS,Director <br /> Environmental Health Division <br /> Mary Meays,Senior REHS Mar�etLagorio,REHS <br /> Site Mitigation Unit Supervisor <br /> c: Pat Anderson,CVRWQCB <br /> c: David Irey,SJC,DA Environmental Prosecution Unit <br /> c: Annabel Mackey,SWRCB-Cleanup Fund <br /> c: Ed Formosa,City of Stockton MUD <br /> c: Paul Supple,ARCO <br /> c: Aaron O'Brien,Geraghty and Miller <br />