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These formations, composed of gravels, sands, silts, and :lays, have a total thickness of approximately 1 , 000 feet. Good quality groundwater is present in these units, under generally unconfined aquifer conditions, although minor confined aquifers exist. A previous study (DWR 1955 ) had indicated that there was a "barrier effect" separating the poor-quality chlorided waters on the west from the good-quality waters flowing into the Stockton area from { Sierran .sources to the east. Results of a later investigation (DWR 1967) indicate that no such barri•_r exists; that in fact, j water of poor quality is migrating 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 hydraulic parameters by lihich groundwater is controlled. There are' no. lithologic, structural, or apparent consistent hydraulic A discontinuities that would stop the incursion of water in an easterly direction from th�b Delta. 2.2.2 Hydrogeoloqy eolo 1:ive,monitoring wells have been installed at the site during previous investigations. The initial. depth- to' roundwater was recorded between 47 and 50 feet (GTI .1987)_ The water level in the wells`has'dropped since the wells were installed, so much so that. MW4 and .MWS: were dry during the summer and fall of. 1989. On. 14 February .1990, the static depth to water was 56.5-57.5 feet, and all `five: of: the wells contained water. Figure 5 i a geologic cross-section drawn for the site from the —well logs. The site is located on interbedded sands, silts, and clays, heterogeneous formations that are. both laterally and ver= f. tically discontinuous fluvial deposits. The principal aquifers at the site are sand lenses with.. some silt and clay, and clay f lenses with sand and silt. Each has a variable thickness ranging M I M-1 from 1 to :20 feet. 1 e35/7,1050/2-.90/tx 16 t