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]W -16. <br /> i <br /> ERI <br /> ENVIRONMENTAL RESOLUTIONS, INC. <br /> P-7 November 12, 1998 <br /> NOV 1 2 1998 ERI 227903 W01 <br /> Mr Bob Boust ENVIRD'�M`NTAL HEALTH r <br /> Unocal - DBG/AMG PEPWIT / SLHVIC ES <br /> 2121 North California Boulevard, Suite 250 <br /> Walnut Creek, California 94596 <br /> Subject Work Plan for Supplemental Environmental Investigation at Former Unocal Service <br /> Station 5098, 5606 Pacific Avenue, Stockton, California <br /> Mr Boust <br /> At the request of Union Oil Company of California (Unocal), and pursuant to correspondence from San <br /> Joaquin County Public Health Services, Environmental Health Division (the County), Environmental <br /> Resolutions, Inc (ERI) is submitting this Work Plan for the subject site The County requests for <br /> additional site work were presented in a letter dated February 26, 1997 (Attachment A) This Work <br /> . Plan presents the scope of work to evaluate petroleum hydrocarbon concentrations in groundwater in <br /> the vicinity of the site The site is located on the northeastern corner of the intersection of Pacific <br /> Avenue and Robinhood Drive in Stockton, California, as shown on the Site Vicinin Map (Plate 1) <br /> The locations of former underground storage tanks (USTs), dispenser islands, and other selected site <br /> features as well as pertinent properties in the vicinity of the site are shown on the Map of Area (Plate <br /> 2) Properties in the vicinity of the site are generally occupied by commercial developments <br /> SITE CONDITIONS <br /> Based upon environmental work completed at the site to date, sediments beneath the site typically <br /> consist of interbedded heterogeneous fine-grained deposits of clay, clayey silt, and sand ERI's <br /> interpretation of lithology beneath the site is shown on geologic cross-sections A-A' (Plate 3) and B-B' <br /> (Plate 4) Cross section locations are provided on Plate 2 <br /> The groundwater surface in the vicinity of the site has fluctuated from approximately 61 to 41 feet <br /> below grade in existing wells with a general rising trend over the past several years Based on ERI's <br /> review of existing boring logs, the strata beneath the site are in hydraulic equilibrium and comprise one <br /> water-bearing zone ERI has attached a groundwater flow direction and gradient rose diagram <br /> depicting groundwater flow directions obtained during monitoring and sampling events (Plate 5) The <br /> zone of groundwater fluctuation is shown on the cross-sections The groundwater flow direction <br /> appears to be predominantly to the east/southeast during the winter months with a hydraulic gradient <br /> ranging from approximately 0 002 to 0 007 and predominantly to the south during summer months with <br /> a hydraulic gradient ranging from approximately 0 006 to 0 015 _ <br /> 74 Digital Drive, Suite G, Novato, California 94949 415-382-9105 (FAX 415-382-1858) <br /> Lake Forest • Novato • Seattle <br />