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LC- A S S O C I A T E S l N C . (ROI) of approximately 34 feet. Approximately 2.64,'pounds of total petroleum hydrocarbons quantified as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes (BTEX) were removed during the vapor extraction pilot test. Soil vapor extraction appeared to`�be a viable method for remediating TPHg and BTEX impacted soil at the site. Field activities and analytical results are detailed in ATC's Remedial Investigation/Feasibility Study, dated November 19, 2002: ' HYDROG.EOLOGY The site is located withiri the Great Central Valley geomorphic province. Ground water in the Stockton area is present in Recent alluvium. Ground water enters the Great Central Valley via surface and subsurface flow and by direct percolation of.]precipitation, -stream flow, and applied water. Replenishment of,the groundwater occurs by percolation of :precipitation, stream flow, and applied water occurs in the forebay areas where permeable sediments are exposed at ground surface. ' `i According to the United States Geological Survey(USES) 7.5-Minute Series Topographic Map of the Stockton ATS West, California Quadrangle, the site is located at an approximate elevation of 24 feet above mean sea level. qs SSS The regional surface topographic gradient in the vicinity of the site is towards the west. The ground water flow direction beneath the site is generally towards the south and the gradient has historically been very flat. Since 1998, depth to groundwater beneath the site has varied from approximately 45 to 55 feet bgs. Historical ground water data are included in Table 1. r � The estimated extent of petroleum impacted soil is shown on Figure 3. A boring advanced at the site on kbo r December 13, 2001, to 100 feet bgs utilizing CPT methods indicated variable layers of clayey silt and silty clay with some organic material were encountered fromthe surface .to approximately 18 feet bgs. Soil from approximately 18 to 33 feet bgs generally of consisted jof silty sand and sandy silt. Soil from approximately 33 L feet to 40 feet bgs consisted of clayey silt and silty clay. Soil from approximately 50 feet to 55 feet bgs s consisted of clay. Soil from approximately 55 feet to"57 feet bgs consisted of very stiff fine-grained material. Soil from approximately{57 to 73 feet bgs generally of consisted of silty sand and sandy silt. —s col"d`1 6r--5 3 f L ` ` d STRATIGRAPHY s Stockton, California is located in the San Joaquin Valley,the southern extension of the California Great Central k Valley (Figure 1). The- Great Central Valley is a deep alluvial plain extending nearly 500 miles from the Siskiyou Range of the Cascade Mountains in the north to the Tehachapi. Range to the south. At Stockton, the San Joaquin Valley is approximately 40 miles wide. The western Valley boundary consists of the low, rolling foothills of the California Coast Range Mountains (maximum elevation due west is less than 3,000 feet) and the eastern boundary consists of the more rugged foothills of the Sierra Nevada (maximum elevation due east is over 10,000 feet). i { The Great Central Valley contains a thick sequence of sediment, which, in places,reaches a depth of tem miles. t These sediments range in geologic age from Jurassic(205 million years before present)to Recent(present time) { and include both marine and continental deposits. The site's immediate subsurface geology consists of Pleistocene and Recent alluvial deposits. These alluvial deposits consist of heterogeneous sequences of sand and gravel originating from active stream channels, and silt and clay originating from overbank and marsh depositional environments. Diesel Performance January 5,2004 ATC Project Number 5462.577 Page 2