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Field Tank manual (LOFT) was performed. based on a water table at 90 feet, the site falls into the medium leaching potential category. In this category, the maximum allowable TPH(diesel) levels which can be left in place without threatening groundwater resources is 1,000 ppm. San Joaquin County Department of Health Services, however, preferred the use -of the historical high water level of 50 feet to preform this analysis. Using this criteria, the site falls into the high category under which only 100 ppm maximum be left in place. Due to the risk to groundwater, AGI recommended remedial action. Excavation followed by bioremediation of impacted soil on-site was proposed as the most effective approach. A remedial action plan submitted by AGI was approved by PHSIEHD. In accordance with the requirements of RHS/EHD, all soil impacted with a TPH(diesel) in excess of 100 ppm was to be removed. Based on the Site Assessment findings, an excavation to at least 30 feet was required to meet this goal. 3.0 SITE GEOLOGY AND HYDROGEOLOGY Escalon is located along the eastern margin of the Great Valley, of California. Recent -.alluvial sediments derived fromthe primarily granitic terrain of the Sierra Nevada Range blanket the area in the shallow subsurface. The borings completed at Derickson Trucking penetrated to a maximum depth of 40 feet. The sedimentary section encountered was uniform and consisted of a sequence of silty sands and silts from the surface to approximately 25 feet. These sands were yellow-brown to reddish brown in color, 'generally fine to very fine grained, with intervals of medium to coarse grained material. A high silt content was characteristic of this unit. At 30 feet, a very stiff clay and clayey sand was encountered which -was underlain by another silt interval. Between 37 and 40 feet, a loose, quartzose, medium to very coarse-grained sand with common pebbles was present. Groundwater was not encountered in any of the borings. A domestic water supply well is located on the property next to the truck washing area. The current water leve#-in this well is reported to be approximately 90 feet below grade. According to Mr. Michael Collins of the San Joaquin County.DHS/EHD, the water table-in the Escalon area was at approximately 50-feet in 1986. 4.0 EXCAVATION The excavation was conducted by Crisp Construction under supervision from AGI personnel during a four day period between July 8-13, 1993. During the four days impacted soils were removed and placed.on the rim of the excavated site were they were later transported to.the 'east end-of the property and placed in a treatment cell. Once the walls and bottom of the excavation were deemed clean, the walls and base, with the exception of the south wall of the cavity, were sampled in the presence of the Sari Joaquin Environmental-Health:inspector, 'Michael, Collins. The .south-wall was sampled approximately two •hours later as access was achieved. The excavation was :_.. :..., 4:6f 7