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' GEORGE DEUKMEJIAN,Governor <br /> STATE OF CALIFORNIA <br /> CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD-- <br /> CENTRAL VALLEY REGION <br /> 3443 ROUTIER ROAD <br /> SACRAMENTO,CA 95827-3098 <br /> 3 August 1989 ` <br /> Ai— <br /> Ms. Diane M. Hinson <br /> San Joaquin County Public Health Services <br /> Environmental Health Division <br /> P.O. Box 2009 <br /> Stockton, CA 95201 <br /> SOUTHLAND CORPORATION, 9001 THORNTON ROAD, STOCKTON; JUNE 1989 REVISED WORK <br /> PLAN FOR ADDITIONAL SITE ASSESSMENT <br /> We have reviewed the work plan for additional site assessment, prepared by <br /> Groundwater Technology, Inc. , for the Southland Corporation (Southland) , 7- <br /> Eleven store at 9110 Thornton Road in Stockton. The proposal discusses <br /> installation of two additional ground water monitoring wells and two <br /> monitoring well clusters to investigate the possibility of an upgradient <br /> source of ground water contamination and to better define the vertical extent <br /> of ground water contamination. <br /> We have the following comments and recommendations on the proposed scope of <br /> work: <br /> 1. The practice of installing 2-inch diameter monitoring wells at this site <br /> should be discontinued. All future monitoring wells installed at this <br /> site should be constructed of nominal 4-inch diameter (or larger) casing <br /> and well screen to permit adequate well development and provide <br /> representative ground water samples. <br /> 2. The proposed well cluster to be located in the vicinity of MW-1 and MW-4 <br /> is in an area of known contamination. Therefore, the proposal to drill <br /> to a depth of 70 feet with hollow stem auger should not be approved. <br /> Such a well installation would compromise the integrity of samples from <br /> the other depth-discrete wells in the cluster. The proposed shallow <br /> monitoring well would be screened from a depth of approximately 45 feet <br /> to 60 feet below ground surface. This would leave only a ten foot <br /> stratigraphic interval between the bottom of the shallow monitoring well <br /> screen and the 70 foot deep aquitard for the screened intervals of the <br /> two depth discrete wells. <br /> Because of the difficulty of isolating specific hydrologic units, it <br /> does not seem that the proposed well cluster would be capable of <br /> providing depth discreet samples. Samples obtained from these wells <br /> would be of limited value. <br />