SITE INFORMATION AND CORRESPONDENCE

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page 2, 1112 Harding Way former dispenser island area from 20-90 feet bsg, though no analytical data has been submitted to SJC/EHD for soil samples collected deeper than 65 feet bsg. Review of soil analytical data submitted to SJC/EHD indicates that soil contamination was defined at less than 50 feet bsg near the former tank pit by boring SB-7. In the Addendum, ATC proposes screen interval depths of 20-40 feet bsg for the SVE wells and 60-62.5 feet bsg for the AS well. The proposed location of these wells is onsite between MW-2 and MW-3. A sand interval of varying thickness, located between 45-56 feet bsg, was reported on all boring logs except MW-3 and cone penetrometer boring CPT-1. The only boring logs completed at this site to a depth greater than 56 feet bsg are CPT-1, completed to 100 feet bsg, and SB-7 completed to 66 feet bsg. Both of these logs indicate the lithology at 60 feet bsg is clay, and that intervals labeled clay, stiff fine grained, clayey silt or silty clay extend from approximately 55-80 feet bsg. Groundwater contamination has been reported in all monitoring wells at this site. The highest concentrations are consistently reported in wells MW-2 and MW-5. One grab groundwater sample collected from CPT-1 at a depth of 78-80 feet bsg was non-detect for all constituents analyzed. This could indicate the vertical extent of the groundwater contamination is defined; a comprehensive site conceptual model is needed to further evaluate if this is a supportable conclusion. SJC/EHD does not approve the submitted CAP or Addendum at this time. Documented soil contamination at this site is concentrated from 10-35 feet bsg in the source area, and from 30-40 feet bsg up-to cross-gradient from the source area (MW-2, MW-5 and MW-6). Groundwater contamination exists across the site, and offsite in the up- and cross- gradient directions. SJC/EHD is concerned that air sparged into the clays at 60 feet bsg may not be effectively captured by the SVE wells screened to 40 feet bsg, which is also in an interval characterized by silts and clays that is above the sand interval at 55 feet bsg. Please proceed with preparation of a Work Plan to define the lateral extent of the groundwater contamination at this site. The Work Plan is due for submittal no later than October 20, 2006. A report of findings is due for submittal no later than 6 weeks following completion of the fieldwork, and must include a Site Conceptual Model (SCM). Include in the SCM current, detailed, scaled cross-sections, and calculations of contaminant mass remaining in both soil and groundwater at this site. If you have any questions please call Lori Duncan at (209) 468-0337. Donna Heran, REHS, Director Environmental Health Department Lori Duncan, Senior REHS Nuel C. Henderson, Jr., PG LOP/Site Mitigation Unit IV LOP/Site Mitigation Unit IV cc: James Barton, PG, CVRWQCB Jeanne Homsey, PG, ATC