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Ellet® 1 0 Mr James L Barton ' January 26, 2005 Page 3 concentrations of redox indicators in an aerobic aquifer and were based in part on the results from wells MW-7, MW-14, MW-27, MW-31 and MW-32 which are located outside the plume ' boundary Groundwater at MW-2, MW-4, MWA 1 and MW-8D appears to be strongly reduced (methanogenic or sulfate reducing) Groundwater at wells MW-16, MW-31 and MW-32 appears ' to be less reduced (nitrate or iron reducing) Groundwater at MW-7, MW-14, and MW-27 is the least reduced (aerobic or nitrate reducing) ' Based on an average of background concentrations of redox indicators from wells MW-7, MW-14, MW-27, MW-31 and MW-32, located outside the plume boundary, groundwater immediately upgradient of the site is not anaerobic The analytical data supports the conclusion ' that biodegradation is occurring The biodegradation process depletes oxygen from the groundwater creating the anaerobic conditions reported in wells MW-2, MW-4, MW-7, MW-8D, MW-11, MW-14, MW-16, MW-31 and MW-32 BTEX biodegrades fastest under aerobic ' conditions Therefore, biodegradation rates will be slower in strongly reduced wells than in the less reduced wells • The ferrous iron analytical data was reported as non-detect Possible reasons for this condition, may be that there is no iron in the aquifer or it may be that the Fe(II) oxidized before it reached the lab ENSR will take steps to confirm Fe(II) concentrations during the next sampling event Field measurements of dissolved oxygen (D O ) were typically an order of ' magnitude greater than lab measurements, except for MWA 1 These elevated rates observed in the field but not in the laboratory could indicate that the sparging system is aerating the groundwater but that the oxygen is being rapidly depleted by biodegradation of the contaminants This would explain the extremely low levels of D O measured at the laboratory Site Conceptual Model Update In a letter dated February 8, 2002, the RWQCB requested completion of an updated SCM for this site The initial SCM was submitted to the RWQCB in an Environmental Audit Incorporated ' (EAI) report titled "Supplemental Investigation Report, Problem Assessment Report, and Work Plan, Cleanup and Abatement Order No 5-00-707, Montgomery Ward Auto Service Center and Forn7er Unocal Site, 5400 and 5606 Pacific Avenue, Stockton, California," dated October 13, 2000 This report provides an updated SCM and Fourth Quarter 2004 Monitoring and Remediation Status Report for the subject site Site stratigraphy is segregated into four hydro-stratigraphic units that generally correspond to screened intervals of site wells The shallowest wells are screened in the hydro-stratigraphic unit designated as Unit A and the deepest wells are screened across hydro-stratigraphic Unit D A cross-section location map, cross section A-A', cross section B-B', and cross section C-C' are included in Attachment E Fourth quarter 2004 analytical results for TPHg, benzene, and • methyl tertiary butyl ether (MTBE) are summarized on the respective cross-sections ' i IProlects10694015098 2931WorkinglReporls15098 4th Qtr 04-QMR doe celebrating 35 Years of Excellence in Environmental Services