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' Union Ice Company,p y, W eber Street ' Phase III Site Assessment - Report of Findings & Phase IV Additional Remediation Pioposal Page 2 ' 30 GEOLOGY & HYDROLOGY 31 Regional Geology and Hydrology The Union Ice Weber Street site is located within the Cit of Stockton in the e flood plain of the San Joaquin River The site is situated near the center of the Central Valley structural trough, a large, broad, synclinal feature that traverses central California for several hundred miles The site is located about two (2) miles northwest of the Tracy-Stockton fault zone, which cuts across the Central Valley in east/northeast-west/southwest direction from Tracy to Stockton and beyond The fault is buried by Quaternary basin fill sediments and is not known ' to be active 32 Site Geology The Weber Street site is underlain by fluvial deposits of the San Joaquin River Holocene (geologically recent) Age The surface of the site is flat, with an average elevation of about 10 feet above mean sea level The site receives about 20 inches of rain annually, mostly in the Fall and Winter nionths The stratigraphy of the site was exploied in previous phases of the project to a depth of til feet and is characterized by interbedded clay and Silt with sand and gravel units The uppermost unit is a clay/silt, designated Clay A, which ex- tends from the surface down to a depth of 15 to 17 feet This unit is red to brown in color down to 7 to 10 feet, where it changes to a bluish-gray hue The depth of color change is indicative of a historically high water table at the site, the gray color indicating reducing conditions prevailing beneath the water table level, while the brown color indicates oxidizing conditions above the water table Local carbonaceous fragments and lenses of peat are dispersed throughout this unit, which is typical of back swamp deposits of a large river system The Clay A unit is underlain by a fiiie to coarse-grained sand with local gravel units This sand is present from about 15 to 35 feet in depth The clay and silt lenses occur in the lower half of the unit This sand, which is named Sand 1, is classified a poorly sorted to well sorted sand to silty/clayey sand due to its vary- ing grain size This lithology is typical of overbank or waning flood deposits of a large river system Sand 1 is underlain by Clay B, which is a thick, fairly uniform blue-gray clay This clay is 15 to 23 feet thick where explored it is a stiff clay and contains I