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Stalntec September 11, 2009 Page 2 of 6 Reference: Work Plan for Site Closure, Caltrans Shop 10 Maintenance Facility + Stockton,California property boundaries of the Site (URS, Additional Groundwater Study, March 2000). A Human Health Risk Assessment (HHRA) was completed in 1999 and identified one water well located approximately 1,000 feet south of the Site that is screened at 500 ft bgs. Based on discussion'with the local water distributor,. no hydraulic communication between the shallow aquifer and the deeper aquifer has been observed in the area (URS, Human Health Risk Assessment and Groundwater Impact Analysis, February 8, 1999). The California Water Service Company,the primary provider of water to residents in`Stockton reported no other wells in the area of the Site. Remedial Action Summary A Corrective Action Pian (CAP), and Soil Vapor Extraction (SVE) pilot test were completed in 1996 and-pians for an Air Sparge and Soil Vapor Extraction Treatment System (AS/SVETS)`were submitted to Caltrans in 2002. An ASISVETS was installed by Stantec in June through October 2005 based on plans prepared by Brown and Caldwell consultants and approved by Caltrans in June 2003. The system consisted of two vapor extraction wells (SVE-1 and SVE-2)installed at a depth of approximately 40 ft bgs and four air sparge wells (ASW-1 thru ASW-4)installed to depths of approximately 72 and 96 ft bgs. The treatment compound is supplied with three-phase power from ari adjacent pole-transformer and consists of the following: 1) King Buck Technology Model MMC-5A, trailer-m6'6nted catalytic oxidizer with 100- standard cubic feet per minute (scfm), 5 horsepower(hp) SVE blowe , and 12 kilowatt electric heater; 2) 7.5 hp Ingersoll-Rand rotary screw 28 scfm compressor; 3) 385-gallon poly condensate and knockout water collection tank;4) 200 amp distribution panel; and 5)miscellaneous valves,'piping, and gauges. :F } '.System operation began on December 1,2005, and continued throu6h this-,year, when the AS/SVETS was shut down on June 25, 2009, due to low levels at system influent. Radius of influence of vapor extraction wells was approximated at 80 ft during the 1996 SVE pilot test and confirmed to be near 70 ft during operation of the AS/SVETS in 2009. The-system-removed-approximately-28;150 pounds-of hydrocarbons L during operation. SITE GEOLOGY AND HYDROGEOLOGY The Site is located within the San Joaquin Valley, approximately 4.8 km (3 miles) east of the San Joaquin River. The Site is located within the Tracy (Stockton) Fault Zone's Generally, surlicial soils are comprised of Quaternary-aged unconsolidated sediments that were deposited during flood stages of major streams in areas between natural stream levees and alluvial fans(IT Corporation, Remedial Action Plan, March 11, 2002). � ,I A review of the soil survey indicates that the surficial soils are comprised of,Landlow adobe clay alluvial deposits within low-lying areas. These soils are dark, grayish,.brown, fine-textured clays resting on a substratum with a thin layer of lime-cemented hardpan,.exhibiting low erosion and permeability. Based on soil borings, soil beneath the Site consists of a fill material overlying alluvial deposits. Generally, the fill is a sandy gravel to a silty sand with lenses of sandy silt and sandy clay encountered at depths between 3 and 15 ft bgs. The alluvium is interbedded sandy, clayey silts, and clayey sand with minor lenses of silty clays encountered at depths between 15 and60 ft figs. Silty sand, to poorly graded sand is encountered at depths between 60 and 75 ft bgs and sandy, clayey sifts and clayey sand extends beyond 90 ft bgs. Review of the Central Valley Regional Water Quality Control Board (RWQCB) Basin Plan indicates that the Site is located in the Duck-Littlejohns Hydrologic Area of the North Valley Floor Hydrologic Unit of the San lAPro1ectslCALTRANS1Shop 10120091ClosureWosure work plan fnl.doc