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Work Plan for the Former Two 20,000-Gallon USTs Location at Stockton AASF California Army National Guard 1.4 Geology 1.4.1 The site lies within the Sacramento Valley.The Sacramento Valley is composed of unconsoli- dated Quaternary sediments from alluvial and lake deposits.The Sierra Nevada Mountains to the east are a source of the alluvium deposits as a result of erosion.Also contributing to the deposits are the Sacramento Valley Coast Ranges to the west.Both ranges consist of a wide variety of rocks and granite,which are metamorphic(ranging widely in age and composition)and sedimentary.In addi- tion,the Stockton Fault,which trends northeast-southwest, is located within the city limits,as well as,the Midland Fault Zone(trending north), located approximately 25 miles northwest of the site (Versar,Inc.,2004). 1.4.2 Depth to groundwater in the vicinity of the site is estimated to be approximately 37 to 44 feet bgs,and groundwater gradient has been determined to be flat because the average hydraulic ground- water gradient of 0.004 foot/foot exists beneath the site(Versar,Inc.,2004).The groundwater gradient was determined to flow to the north,according to the CA ARNG(Hilyard,2007). 2.0 AREAS OF POTENTIAL ENVIRONMENTAL CONCERN 2.1 The area of potential environmental concern at the AASF, in regard to this WP, is the location of the two former 20,000-gallon USTs. 2.2 Review of the preliminary assessment/site investigation update report indicates that Versar,Inc., has conducted a preliminary assessment and site investigations of the Stockton facility(including the AASF). This WP is a continuation of those investigations,and is intended to determine the extent of contamination at the AASF.Contaminants of potential concern associated with this area include TPHD,TPHG,BTEX,oxygenates,and lead. 3.0 CONCEPTUAL SITE MODEL This subsection presents details of the previous investigations of the area of the two former 20,000- gallon USTs at the Stockton AASF,the nature and extent of petroleum impacts,and migration potential.Currently,no investigations have been completed downgradient(north)of the two former 20,000-gallon USTs location;however,the following information describes the investigations nearby/adjacent of the three former 5,000-gallon USTs and fuel spill area.. These sites lie to the northwest/west and south of the two former 20,000-gallon USTs location,respectively. 3.1 Previous Investigations 3.1.1 After the three 5,000-gallon USTs were removed in 1990 by Wallace-Kuhl,five soil samples were collected from beneath the tanks and shipped to a laboratory for the following analyses: TPHG, TPHD,and total petroleum hydrocarbons as jet fuel(TPHJ). Following the UST removal,additional investigations were completed to further assess and characterize the magnitude of the contamination in soil beneath the former UST locations. The investigations determined the petroleum impacts in soil were limited to the former UST area and warranted the installation of three groundwater MWs, which were installed in 1993 to assess the condition of groundwater beneath the former USTs.The MWs(1 through 3)have been sampled six times to date.Groundwater samples were collected and analyzed for TPHG,TPHD,TPHJ,TPH as oil(TPHG),BTEX,and methyl tertiary butyl ether (MTBE). KAWprocess\00251\CA ARNG\Stockton AASF WP\FINAL\WP text.doc 3 June 2007