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Ellet® Mr James Barton ' June 14, 2004 Page 3 ' impacted groundwater at the Unocal site and around the Robinhood Irrigation Well In September 2001, four additional wells (MW-8D, MW-9, AS-26 and MW-27) were installed to further define vertical and lateral hydrocarbon-impacted groundwater ' In May 2002, the RIW was replaced The new Delta College irrigation well became operational in July 2002 ' SITE CONCEPTUAL MODEL The initial Site Conceptual Model (SCM) was submitted to the RWQCB in an Environmental Audit, ' Incorporated (EAI) report entitled "Supplemental Investigation Report, Problem Assessment Report, and Work Plan, Cleanup and Abatement Order No 5-00-707, Montgomery Ward Auto Service Center and Former Unocal Site, 5400 and 5606 Pacific Avenue, Stockton, California," ' dated October 13, 2000 in a letter dated February 8, 2002, the RWQCB requested that Unocal and Wards review and update the SCM in future quarterly monitoring and status reports ' Site stratigraphy is generally segregated into four hydro-stratigraphic units The shallowest wells are screened across the hydro-stratigraphic unit designated as Unit A and the deepest wells are screened across the hydro-stratigraphic unit designated as Unit D Although there are reported to be up to four groundwater zones, the water table elevation data and subsurface lithology reported on associated boring logs does not appear to support the existence of more than one distinct hydro-stratigraphic unit The approximate depth intervals corresponding to the hydro-stratigraphic ' units, as defined by EAI are as follows Hydro-Strati graphic Unit Depth Interval feet bgs)-_ ' A 30-50 B 50-70 C 85-90 ' D 98-103 Unit A wells include MW-10A, MW-14A, MW-18, MW-19S, MW-21S, MW-22S, and MW-25S and ' are screened from approximately 30 to 50 feet below grade surface (bgs) Unit B wells include MW-1 through MW-7, MW-9 through MW-17, MW-18D, MW-19D, MW-21 D, MW-22D, MW-25D, AS-26, and MW-27 and are screened from approximately 50 to 70 feet bgs Unit C wells include MW-23S and MW-24S and are screened from approximately 85 to 90 feet bgs Unit D wells include MW-8D, MW-23D, and MW-24D and are screened from approximately 97 to 103 feet bgs Analytical data from wells MW-2, MW-3, MW-11 and MW-12 is assessed quarterly to evaluate overall trends of petroleum hydrocarbon concentrations in groundwater Trend analyses for wells MW-2, MW-3, MW-11 and MW-12 indicates a general decline in hydrocarbon concentration in a groundwater with slightly higher concentrations observed during winter months Hydrocarbons observed in groundwater during the fourth quarter, 2003 were generally lower than the May 2003 testing results The two exceptions are wells MW-11 and MW-12, which indicated higher J 1Projects10694M5098 2931FinaRReports15098 Work Plan doc ' _ Over 30 Years of Excellence rn Enmonmenta!Services