Laserfiche WebLink
Human Health Risk Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br /> Tracy, California <br /> In April, 1990, Alton Geoscience drilled 48 exploratory soil borings in a grid pattern to <br /> characterize hydrocarbon concentrations in situ, In July and August, 1990, approximately <br /> 6,100 cubic yards of soil was excavated to a depth of approximately 10 feet below grade (fbg). <br /> To the greatest extent possible, hydrocarbon-affected soil was removed and properly disposed <br /> of at the appropriate Class II and III facilities (Alton Geoscience, 1903a). <br /> Between September 1990 and April 1993, Alton Geoscience installed nine ground water <br /> ' monitoring wells. Ground water monitoring and sampling began on a regular basis in October <br /> 1990 (Alton Geoscience, 1993a). <br /> ' 1.2.1 Regional Setting <br /> ' The site exists in the Central Valley of California, which is approximately 400 miles long and <br /> averages roughly 50 miles wide. Geologically, the valley is a large symmetrical trough that is <br /> ' bounded by granitic, metamorphic, and marine sedimentary rocks of pre-Tertiary age. The <br /> trough has been filled with as much as 30,000 feet of sediment in the San Joaquin Valley to the <br /> south and as much as 60,000 feet of sediment in the Sacramento Valley to the north. These <br /> sediments range in age from Jurassic to Holocene and include both marine and continental rock <br /> and sedimentary deposits. <br /> The scope of this discussion includes only the uppermost deposits within the Central Valley. <br /> ' Continental rocks and deposits of Tertiary and Quaternary age compose a number of formations <br /> and, in total, constitute the major aquifer of the Central Valley. Lithologically, these deposits <br /> ' are heterogeneous mixtures of poorly sorted clay, silt, sand, and gravel, and in places, some <br /> beds of claystone, siltstone, sandstone, and conglomerate. Continental deposits of Quaternary <br /> age crop out primarily along the major rivers and streams of the valley, as well as other low- <br /> lying areas. The deposits include river and flood-basin deposits, and sand dunes. River <br /> deposits, including channel and flood-plain deposits, are considered to be the most permeable <br />' deposits in the valley. However, the river deposits are generally not tapped by ground water <br /> wells. Flood-basin deposits consist largely of fine-grained beds that restrict the vertical <br />' movement of water. <br /> 3U-0136-1I <br /> �R 1-2 <br /> 1 <br />