ARCHIVED REPORTS XR0012277 - Laserfiche WebLink

Home Browse Search

Report. Groundwater-quality Monitoring—March 29-31,2006, 7500 West Eleventh Street, Tracy, CA Page 5 1.4 Geology and Hydrogeology The subject property and the surrounding area are situated on level terrain on the distal, �r northern slope of an alluvial fan. The shallow underlying alluvial sediments are of Quaternary to Recent age. l"' The site and the immediately adjacent property along the south side of West Eleventh Street and the west side of Chrisman Road have been extensively excavated and backfilled during prior filling station construction and remodeling, utility installation, and highway expansion. } Beneath the paving and fill, the soils are composed of alluvial materials consisting of interbedded clays, silts and sands. These materials have been deposited in a complex lenticular form composed of relatively low permeability clays and silty clays inter- bedded with permeable silts and sands. The sizes of individual permeable lenses vary"' from relatively large features having considerable areal extent to small, localized lenses of limited extent and thickness. In some instances, these lenses merge into each other to form semi-continuous permeable strata within the less permeable clayey material. In the neighborhood of the 7500 West Eleventh Street site it is estimated that these alluvial materials are some 100 ft. thick. The stratigraphy described above is typical of the %P alluvial fan upon which Tracy and the surrounding area are situated. --- Beneath the alluvial sediments are the poorly sorted clays, silts sands and gravels of the Tulare Formation that were primarily derived from the Inner Coast Range hills that rise along the west side of the San Joaquin Valley. The Tulare Formation is separated into two members, the Upper Tulare Formation and the Lower Tulare Formation. Both „ members of the Formation are, on the regional scale, moderately to highly permeable and yield moderate to large quantities of water to wells. The Upper Tulare Formation is separated from the Lower Tulare Formation by the low-permeability, lacustrine Corcoran Clay, which acts as a confining bed within the regional groundwater basin. At the subject site, the top of the Corcoran Clay is estimated to be at a depth of approximately 230 ft. beneath the ground surface and to be some 100 ft. thick. The total thickness of the 4” underlying Lower Tulare Formation is not well documented; however, estimates suggest that it ranges in thickness from 300 ft. to greater than 1,400 ft. �.• Figures 4-9 are hydrostratigraphic longitudinal and cross-sections drawn along section lines A-A' through F-F', the locations of which are shown on Figure 2. To reduce the complexity of the stratigraphy of the Navarra Site so that it is tractable to practical interpretation, the sections shown on Figures 4-9 were developed by dividing the sediments into general classes based on their hydraulic conductivity and cation exchange capacity, which are the soil characteristics of primary relevance to contaminant transport analysis. In the cross-sections, the permeable gravels, sands and silts have been segregated from the relatively less permeable clays and silty clays. ' The depth to the natural groundwater table beneath the site varies seasonally between 7 and i 1 ft. Regionally, the general direction of groundwater flow is to the north toward the sic