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- --_-_MEMORANDUM - <br /> Richmond, California <br /> October 13 , 1993 <br /> I <br /> Former Chevron Service <br /> Station No. 9-1918 <br /> 45 East Harding Way <br /> Stockton,, CA <br /> Ms. B.L. Hunter, <br /> The following comments describe the results and equations of an <br /> analytical solution designed to estimate the downgradient <br /> concentrations of dissolved benzene at former Chevron Service <br /> Station No. 9-1918, in Stockton, California. <br /> Goal - Estimate the downgradient extent of dissolved benzene (10 <br /> ppb line) from MW-11. Monitor well MW-11 is located on the <br /> downgradient site perimeter and has consistently contained <br /> detectable concentrations of dissolved benzene, since the initial <br /> sampling of the well in May 1992 . <br /> The initial benzene source concentration used in the model for MW- <br /> 11 is 1500 ppb, reflecting the maximum dissolved benzene <br /> concentration observed in the well, May 19, 1992 . <br /> Observations <br /> 1) The site is the location of a former Chevron Service Station, <br /> which operated from 1953 to February 1988. There are presently <br /> eleven groundwater monitor wells located on-site (MW-1 to -11) , and <br /> two well off-site (MW-12 and -13) (see attached Site <br /> Map/Potentiometric Surface Map - Figure 3) . <br /> 2) The site is underlain by an unconsolidated sedimentary sequence <br /> dominated by clayey silt, silty clay and sandy clay, with minor <br /> fine sand layers. <br /> Depth to groundwater during the most recent monitoring event (July <br /> 23 , 1993) was 50-51 feet below ground level (BGL) . There has been <br /> an observed decline in depth to groundwater of approximately 15 <br /> feet since initial monitoring began on-site in November 1988 . All <br /> wells, except MW-9 to -13 , have been dry since mid-1989 . MW-2 has <br /> 1 <br />