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M " - {,y-� ;_:. -•f ..r.�r,ry - �,�" -„1 *r .'.#-'� ?' "arch .:'.;t,�� . ;.;,t �.�,".tea+_ .-� _.t?;c:•t b',+�.a, _ ?. «_. t+!...� AZ R-� SSB 91-1038 Thrifty Oil Company Siouan No.172 page 2 7647 Pacific Avcnuc Stockton.California After surveying each well head, the ground-water gradient and flow direction were evaluated, using depth-to-ground-water measurements collected in June,1991- The field and analytical data collected during this investigation were compiled and are presented in this report. 2.0 BACKGROUND INPURNWMON Background information required to obtain an understanding of the project is presented in the following sections. 2.1 site Location The site is located at the southwest corner of the intersection of Pacific Avenue and Rivara Road in Stockton, California. Elevation at the site is approximately 13 feet above sea level. The site location is shown on the Site Vicinity Map (Plate P-1). 2.2 Site Description The site consists of a gasoline retail facility,three pump islands under a canopy,and paved traffic and parking areas. Gasoline is stored on-site in four underground storage tanks. Four ground- water monitoring wells were installed during the course of this investigation. The locations of the site facilities,including the monitoring well locations,are shown on the Site Plan (Plate P-2). 2.3 Reyionnl Genlo issettlor. Stockton is located at the border of the Sacramento and San Joaquin Valleys in the central portion of California or the center of the central valley geomorphic province of California (valley), The valley in the area of Stockton is underlain by unconsolidated Holocene continentally derived young alluvium. The younger alluviuni is underlain by unconsolidated older alluvium of Pleistocene and Holocene age. Pliocene to Pleistocene deposits of continentally derived sand, silt, clays, and poorly sorted gravels underlie older alluvial deposits. Regionally extensive clay bodies referred to as A through E clays are not believed to exist below the site. Other confining layers of less than regional extent may underlie the site. Marine sedimentary SSB ENVIRONMENTAL.CONSULTANTS,INC.