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Weston Solutions, Inc. <br /> Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California <br /> GEOLOGY AND HYDROGEOLOGY <br /> Occasional pockets of fresh water are found in the Delta deposits, but generally speaking the <br /> formation contains poor quality water. <br /> Laguna Formation. The Laguna Formation is Plio-Pleistocene in age and consists of <br /> discontinuous lenses of stream-laid sand and silt with lesser amounts of clay and gravel. There <br /> are no regionally significant fine-grained intervals that could cause water pressure conditions, <br /> although the heterogeneous nature of the sediments causes local confinement. From the <br /> Mokelumne River area, the formation thickens from approximately 400 feet to approximately <br /> 1,000 feet in the Stockton area. Groundwater occurs under unconfined to locally semiconfined <br /> conditions within this unit. Occasional minor perched water zones are encountered in this <br /> formation,particularly in the Mokelumne River area. <br /> Mehrten Formation. This formation is late Miocene to Pliocene in age and is composed of <br /> moderately to well indurated andesitic sand to sandstone interbedded with conglomerate, <br /> tuffaceous siltstone, and claystone. The Mehrten Formation is approximately 400 feet thick in <br /> eastern surface outcrops to over 600 feet thick in the subsurface near Stockton. It is reported <br /> to be 1,300 feet thick at McDonald Island. The top of the Mehrten Formation occurs at depths <br /> of approximately 800 to 1,000 feet in the Stockton area. Regional studies indicate that Mehrten <br /> Formation sands commonly yield on the order of 1,000 gallons per minute from wells. The <br /> formation appears to be semiconfined at least locally in the Stockton area, due to the inferred <br /> extensive fine-grained beds in its upper part. <br /> 3-4 February 2021 <br />