SITE INFORMATION AND CORRESPONDENCE_2

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Nov-29-01 12 :OOP P.04 Michael Infurna Page 2 November 29,2001 131420.A1.09 pressure is great enough to keep the borehole from collapsing,and most importantly, prevent formation water from entering the borehole.Therefore,this physical property, coupled with the filter cake,greatly reduces the possibility of cross-contamination during well installation. • A competent clay aquitard approximately 40 feet thick is present between the impacted water bearing zone and the deeper water-bearing zone.The presence of this aquitard,a properly placed grout seal,and minimal vertical gradients will prevent downward migration of 1,2-DCA after the monitoring well is installed.Vertical gradients,based on existing well pairs MW-DIS-7S/7D,and MW-DIS-8S/8D,indicate that vertical gradients in this area of Stockton are minimal.Further,this well will not be used for pumping,and all groundwater samples will be collected using low-flow sampling techniques, therefore,vertical gradients will not be induced by this well. Monitoring well MW-DIS-11D will be installed using procedures described in the December 2000 monitoring well work plan with the following exceptions: • All drilling operations will be conducted using a direct mud rotary system.Drilling fluid will be mixed using a sodium bentonite viscosfier/gellant such as Baroid QUICK-GEL or similar products.The onsite geologist will be responsible for monitoring and maintaining the optimum drilling fluid properties for well installation. • Drill 7.25-inch diameter boring to approximately 155 feet. • Install 10 feet of 0.020-inch slot Schedule 40 PVC well screen from approximately 145 to 155 feet bgs,and Schedule 40,2-inch OD PVC blank casing to the ground surface.A Screw-on well cap will be place don the bottom of the well. • Sand pack will be installed through a 1-inch tremie pipe. A sand size will be chosen such that no more than 10 percent of the sand passes through the screen slots(such as Lone Star No.2/12). The sand pack depth will be sounded at regular intervals to ensure a solid pack with no bridging.The sand pack will extend approximately 2 feet above the top of the screen. We are scheduled to install this well the week of December 10,2001. Installation of the well with a conductor casing will take at least 5 full days.The extremely low probability of causing cross-contamination to the lower water-bearing zone does not warrant the difficulty and the associated high costs of installing this well with a conductor casing at this location on a freeway onramp.