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' T31 t 6 E ?Ramer REGAL 60.3 2-10-89 lA INTRODUC'll'lON EXXON,Inc. has subleased Regal Service Station 603 from Wiekland Oil Company. Wickland Oil is upgrading the station with a double contained product storage and dispensing system. During the removal of the old underground storage tanks,gasoline range hydrocarbons were found contaminating the native soil beneath the tanks. 1.1 LOCATION The station is located at 6425 Pacific Avenue and Douglas Road,Stockton, California. The site is approximately 12 feet above mean sea level (MSL)in Section 21; T2N; R6H; M1DB&M, and is parcel 31 as found in assessors book 97,page 41 (see figures 1,2,3 and 4). 1„2 GIONAL GF-MORPHOLOGY AND HYDROGEOIOGY The site lies within the northestem edge of the San Joaquin Valley which is the southern half of the Great Central Valley geomorphic province of California. The Coastal Range province is the western boundary and the Sierra Nevada province is the eastern boundary of the Cheat Cel Valley. All three of the above mentioned provinces trend in A northwesterly direction as well as the mapr faults which are associated with the province boundaries. I to Central Valley p.ovince represents the low position basin separating the Sierra Nevada and Coastal Range mountain masses. Erasion of t+he mountain masses produce sediments which are carried into the valley and deposited as sequences of clay,0t,sand and gravel formations, predosronautly unconsolidated. The valley is continually subsiding,allowing the deposited sediments to build-up and become thicker,with little effect upon the surface elevations. The final result is a thick sequence of sediments (gravels, sands,silts and clays)along with the associated folds and faults, see figure 5. Clay layers,are found throughout the sedimentary section, These clay layers drastically slow the downward migration recharge of ground water to the underlying formations. The recitarge is by downdip movement from the outcrop areas to the east along the foothills of the Sierra Nevada Mountains, by slow leakage from the above clay layers, and by more rapid movement vertically beyond the limits of the clay layers. i.