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i I ARCADIS GERAGHTY&MILLER 3. SITE HYDROLOGY GEOLOGY AND HYDROGEOLOGY ' 3.1 REGIONAL HYDROLOGIC SETTING AND HISTORY Stockton hes in the northernortion of California's San Joaquin Valle within the San p q Y ' Joaquin River Hydrologic Region The San Joaquin River drains the entire region which is bordered on the west by the Sacramento-San Joaquin Delta and on the East by the Sierra ' Nevada Mountain range The regional groundwater basin consists of sediments that have eroded from the surrounding mountains and washed into the valley by streams Marginal to the Sacramento-San Joaqum Delta, the Stockton area is only several feet ' above sea level and surrounding water bodies experience tidal flows from the Pacific Ocean The Site is 115 feet above sea level The Stockton area has experienced tidal flows from the Pacific Ocean The Stockton area has also experienced heavy groundwater pumping for agriculture and urban use since initial settlement(Department of Water Resources, 1994) This ' heavy pumping led to a significant decline in groundwater elevation centered at Stockton during the latter half of the century The Stockton groundwater depression was highlighted in GrouncAvaterMomtoiing(L Everett, 1980) (Figure 3-1) The heavy pumping in concert with the delta margin location have led to a saline groundwater front that intrudes from the west and runs north-south beneath Stockton Sine 1971, the San Joaquin County Flood Control and Water Conservation District has monitored ' groundwater levels and groundwater quality along this front and encouraged better management practices The comparison of Fall 1997 Flood Control & Water Conservation ' District Groundwater Report, to a 1970 groundwater map, shows that the groundwater at the center of the depression has risen approximately 30 feet and that the large depression centered ' on Stockton no longer exists The report, however, also shows that the saline front (based on 300 ppm chloride) has generally moved f wlier inland (east) than in 1980 throughout the Stockton area(Figure 3-2) 9