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Monla Brothers Service Station 12 July 16, 1992 T k 2 sin 4e—g oundwater--monitoring—wed—located—down—gradient--from—•the--loca onof t mer—gam ne tank Site Geology The subject property is located east of the San Joaquin delta near the northern end of the San Joaquin Valley This area consists of a thick sequence of fine to coarse sediments Regionally, the property is located in the San Joaquin sub-basin of the ' geomorphic province known as the Great Valley This province is characterized by a northwest-trending asymmetric structural trough bounded on the east by the Sierra Nevada Province and on the west by the Coast Range Province The Great Valley is composed of a thick sequence (60,000 feet) of sedimentary rocks ranging in age from Jurassic (205 - 138 million years ago) to Recent (10,000 years ago) and overlies a ' metamorphic and igneous basement complex (Bailey, et al, 1964) Jurassic, Cretaceous (138 - 63 million years ago) and Tertiary (63 - 2 million years ago) rocks are predominantly of marine origin (Knoxville Formation, Shasta Series, Chico Group) Tertiary sediments are almost entirely clastic, and were largely derived from ' the eroding Sierra Nevada Tectonism beginning in the upper Miocene (24 - 5 million years ago) and continuing to the present has resulted in considerable erosion of the valley edges and has deposited sediments continental in origin The Pliocene (5 - 2 million years ago), Pleistocene (2 million - 10,000 years ago) and Recent rocks consist mainly of semiconsolidated to unconsolidated claystones, sandstones and conglomerates, and contain most of the groundwater resources in the area Subsurface material encountered was alluvium to the total depth of each boring in ' the previous assessment The alluvium was characterized by low to moderate permeability, loose, light brown, very silty, very fine- to medium- grained sand (SM/ML) to a depth of 13 feet, overlying moderate to highly permeable, loose, dark brown, silty, fine- to medium- grained sand (SM) to a depth of approximately 27 feet Underlying this is a zone of consolidated, slight to moderate plasticity, medium dense, ' low permeability, gray brown, silty, organic clay (OH/ML) to a depth of 53 feet This clay zone was underlain by consolidated, slight to moderate plasticity, medium dense, low permeability, light brown, trace very fine sand, minor silt, clay (CH) to a depth EB-8lo1-3 9207-EO37 WP