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i <br /> 4. <br /> Leaking Underground Field Tank manual (LOFT) was performed based on a water <br /> table at 90 feet, the site falls into the medium leaching potential category. In this <br /> category, the maximum allowable TPH(diesel) levels which can be left in place without <br /> threatening groundwater resources is 1,000 ppm. San Joaquin County Department of <br /> Health Services; however, used the historical high water level of 50 feet to perform <br /> this analysis. Using the historical high groundwater, the site falls into the high potential <br /> for leaching category within which 100 ppm TPH as diesel is the maximum <br /> recommended concentration to be left in place. <br /> Due to the risk to groundwater, AGI recommended remedial action. Excavation <br /> followed by bioremediation of impacted soil on-site was-proposed as the most effective <br /> approach. A remedial action plan submitted by AGI was approved by PHS/EHD. In <br /> accordance with the requirements of PHS/EHD, all soil impacted with a TPH(diesel) in <br /> excess of 100 ppm was to be removed. Based on the Site Assessment findings, an <br /> excavation.to at least 30 feet was required to meet this goal. <br /> 3.0 SITE GEOLOGY AND HYDROGEOLOGY <br /> Escalon is located along the eastern margin of the Great Valley of California. Recent <br /> alluvial sediments derived from the primarily granitic terrain of the Sierra Nevada <br /> Range blanket the area in the shallow subsurface. The borings completed at <br /> Derickson Trucking penetrated to a maximum depth of 40 feet. The sedimentary <br /> section encountered was uniform and consisted of a sequence of silty sands and silts <br /> i from the surface to approximately 25 feet. These sands were yellow-brown to reddish <br /> brown in color, generally fine to very fine grained, with intervals of medium to coarse <br /> grained material. A high silt content was characteristic of this unit. At 30 feet, a very <br /> stiff clay and clayey sand was encountered which was underlain by another silt <br /> interval. Between 37 and 40 feet, a­loose, quartzose, medium to very coarse-grained <br /> sand with common pebbles was present. <br /> Groundwater was not encountered in any of the borings. A domestic water supply well <br /> is located on the property next to the truck washing area. The current water level in <br /> this well is reported to be approximately 90 feet below grade. According to Mr. Michael <br /> Collins of the San Joaquin County DHSIEHD, the water table in the Escalon area was <br /> at approximately 50 feet in 1986. <br /> 4.0 EXCAVATION <br /> The excavation was conducted by Crisp Construction under supervision from AGI <br /> personnel during a four day period between July 8-13, 1993.' During the four days <br /> impacted soils were removed and placed on the rim of the excavated site where they <br /> were later transported to the east end of the property and placed in a treatment cell. <br /> 4 of 9 <br />