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I4.6 Area Hydrology and Hydrogeology • According to BC, the San Joaquin River is the major surface drainage within the Stockton area of the San Joaquin Valley The river flows approximately 40 miles from its point of origin, in the central San Joaquin Valley to Suisun Bay Groundwater in the valley flows west, from the Sierra Nevada Mountain range, and east from the Coastal Ranges, towards the center of the valley, then north towards the Central Valley Delta area The general groundwater flow direction in the Stockton area is reportedly to the northwest, towards the Stockton Divertion Channel, and exists at approximately 50 to 100 feet bgs I Shallow groundwater is considered unconfined and has been designated as a beneficial water use unit Groundwater is used for farming and ranching (irrigation of crops and watering of farm animals), municipal and industrial water supply, and as a drinking water supply for the general public ' The San Joaquin River is located approximately 4 miles southwest of the site However, the nearest body of surface water, a water transportation and irrigation canal, is located approximately %Z mile northeast of I the site Within the last 10 years groundwater beneath the site has risen from approximately 80 to 50 feet bgs Based on the groundwater monitoring event conducted by ATC on November 20, 2002, the calculated groundwater hydraulic gradient and flow direction indicates a north-northeastern flow direction ' at a gradient of 0 003 ft/ft beneath the site 5 0 SCOPE OF WORK The additional site assessment activities performed during this phase of work included on and off-site groundwater monitor and remediation well installations The scope of work initiated at the site was in ,. accordance with the Work Plan for Additional Assessment, dated December 21, 2001, prepared by ATC The ATC work plan was approved by Mr Harlin Knoll of the San Joaquin County Public Health Services, Ireceived by ATC on May 22, 2002 A copy of this correspondence is included in Appendix A In accordance with the approved work plan, ATC performed the following work 1 Installed five groundwater monitor wells for the purpose of delineating the lateral extent of dissolved phase fuel hydrocarbons in groundwater beneath the site and site vicinity Groundwater monitor wells MW-13 through MW-15 were installed as replacement monitor wells in the proximity of MW- ' 6 through MW-8, respectively, who's well screens have all been submerged due to an increase in groundwater surface elevations in the area Groundwater monitor well MW-16 was installed as an off-site assessment well on the adjacent property to the west Groundwater monitor well MW-17 was installed as an off-site assessment well on the property east of the site across Auto Avenue The groundwater monitor wells were installed to an approximate depth of 65 feet and screened from Iapproximately 40 to 65 feet bgs 2 Installed one groundwater extraction well (EX-1) on-site for the purposes of conducting a 24-Hour constant rate discharge test The groundwater extraction well was installed to an approximate depth of 70 feet, and screened from approximately 40 to 69 5 feet i EZ 100966ASAR ipt ATG Associates Inc 11San DiegolEZ Serve11009661Reports1100966SAR001 rpt ATC Project No 43 25827 0026 I 5