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FEvaluationContamination January 3, 1984 <br /> Service Station, Manteca, California AGS 38053-1 <br /> l results of the ground-water sample showed levels <br /> of total xylene isomers at 0.63 parts per million which is in <br /> excess of the allowable limit defined by the California <br /> Department of Health services (0.62 ppm) . The tank cavity was <br /> backfilled with the excavated soil at a later date. <br /> GEOLOGY AND HYDROGEOLOGY <br /> The site is located in the San Joaquin Valley, in the southern <br /> Portion of the Great Valley geomorphic province of California. <br /> The valley has been a sedimentary trough since pre-Cretaceous <br /> time. Most of the <br /> Pre-Cretaceous to mid-Tertiary sediments in <br /> this portion of the valley are of marine origin, while the late <br /> Tertiary to Recent sediments consist primarily of continental <br /> deposits. Depositional and erosional sequences were primarily <br /> controlled by fluctuations in sea level. F-reshwater and brackish <br /> water lakes formed in the valley, resulting in the deposition of <br /> lacustrine clay over extensive areas. Higher energy gy environ- <br /> ments, such as rivers and streams, resulted in the deposition of <br /> channel sand and gravel. <br /> The site is underlain by very gently dipping Quaternary fluvial <br /> ---------------- <br /> sediments consisting of sand, silt, and clay. The topographic <br /> v <br /> 4 <br /> `� App/ied �eo�yste�s <br />