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EON 1 1 1999 RI -� , ENVIRONMENTAL RESOLUTIONS, INC.r ,, I u ' January 5, 1999 ERI 227904 L01 Mr Bob Boust Unocal DBG/AMG 2121 N California Boulevard Suite 250 Walnut Creek, California 94596 Subject Hydrogeologic Investigation at Former Unocal Service Station #5098, 5606 Pacific ' Avenue, Stockton, California Mr Boust At the request of Union Oil Company of California (Unocal), Environmental Resolutions, Inc (ERI) conducted a hydrogeologic investigation at the subject site ERT performed the hydrogeologic investigation to evaluate the groundwater flow direction and gradient during pumping and non- pumping periods of the adjacent San Joaquin Delta College (Delta) agricultural well and provide an estimate of the radius of the cone of depression caused by pumping the well Work was performed in accordance with ERI's work plan dated August 24, 1998 (Work Plan for Hydrogeologic Investigation at Former Unocal Service Station #5098) ' On September 21, 1998, ERI installed single channel, 15 psi, pressure transducers/data loggers in observation wells MW3, MW4, MW11, MW15 and MW17 to record changes in groundwater elevation ' in one-minute intervals The transducers recorded data from approximately 12 00 p m on September 21, 1998 to approximately 12 00 p in on September 25, 1998 According to Delta College personnel, the Delta College well is pumped from 7 00 p in to 6 00 a m at approximately 606 gallons per minute (gpm) Based on data collected in the field, ERI created ten potentiometric maps representing both pumping and non-pumping periods of the Delta College well (Plates 1 through 10) During non- pumping periods the groundwater flow gradient ranged from 0 0057 to 0 0088 During pumping ' periods the groundwater flow gradient ranged from 0 0100 to 0 0160 Throughout the investigation the groundwater flow direction was to be interpreted generally toward the Delta College well All wells momtoied during this investigation exhibited a decrease in water elevation during the pumping period of the Delta College well Based on the initial depth to water (DTW) reading, the change in groundwater elevation in well MW 17 (closest to the Delta well) during pumping periods ranged from - 0 715 to -1 829 feet Select hydrogeologic investigation groundwater data from each well are presented in Table 1 Using all recorded data for each well, graphs l through 5 represent plots of change in groundwater elevation (feet H20) versus time (minutes) The complete set of data recorded by the pressure transducers for the duration of this investigation is provided in Attachment A 74 Digital Drive, Suite 6, Novato, California 94949 415-382-9105 (FAX 415-382-1856) Lake Forest , Novato • Seattle