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_ 5 VY r\ l 1 Y� 2. HYDROGEOLOGY w: i < 2. 1 REGIONAL HYDROGEOLOGY } The geological formations in the Stockton area can be classified in two general groups: semiconsolidated saline-water-bearing units and overlying unconsolidated fresh-water--bearing units. The semiconsolidated rock units are composed of materials of the Mehrten and the Valley Spring formations. These formations are of volcanic origin and contain andesitic and rhyolitic breccia, tuff, and ash. The ground pater within these two formations is saline; it is separated from the overlying fresh-water-bearing units by an impermeable bed within the upper Mehrten Formation. i The unconsolidated units consist of alluvial sediments of the Victor Formation, Arroyo Seco Gravel, and Laguna Formation. These formations, composed of gravels, sands, . silts, and clays, t have a total thickness of approximately 1 ,000 feet. Good quality �? ground water is present in these units, under generally uncon- fined aquifer conditions; although winor confined aquifers exist. r A previous study (DWR 1955 ) had indicated that a "barrier effect" #' existed between the poor-quality chloride waters on the west and the good-quality waters flowing into the Stockton area from Sierran sources to the east. Results of a later investigation 3 (DWR 1967 ) indicate that no such barrier exists: that in fact, - ' water of poor quality is migrati-ag into the Stockton area .from the west, at a rate governed by the laws of fluid mechanics, the constant geological parameters, and the variable current i = hydraulic parameters by which ground water is controlled. There are no lithologic, structural, or Fpparent consistent hydraulic discontinuities- which would stop the incursion of water in an easterly direction from the Delta: € 2.2 SITE HYDROGEOLOGY . . ' Fe monitoring were installed and depth to ground water was recorded at between 47 and 50 feet (GTI 1986, 1987 ) . A "F piezometric map prepared in June 1987 (Figure 3 ) indicated that the groundwater gradient at this site is toward the northeast r (GTI .1987 ) . .'.The wells have been gauged weekly by EA since 30 October. 1987, and the data support thu, earlier work by GTI. A fairly consistent and shallow gradient exists to the northeast (Table 0 One anomaly in this weekly-gauging 'series occurred ' - - during the" 11 .January »nitoring; the data suggest that the i` gradient was to the sot?:thwest. A gradient shift :of 1800 is unlikely, given. the consistency of previous and subsequent data supporting -a .gradient to the northeast. Monitoring. well 2. s exhibited a 0.57 foot decrease in depth from 30 December 1987 to . E 11 January 1988. This is mo::e than a six inch. difference in just 12 days, and. this one well greatly influenced the gradient 1 direction. . 12