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Work Plan: Subsurface Investigation and Page 1 Remedial Excavation of Hydrocarbon-affected Soil Proposed Cintra Park Phase II Development, Tract 2705, Tracy, California June 17, 1997 i CTE Job No. 10-2276 INTRODUCTION AND BACKGR UND INFORMATION This workplan generally follows the Tri-Regional Boa d Staff Recommendations for Preliminary Evaluation and Investigation of Underground Tank Sites. The proposed scope of work includes the assessment and remediation by excavation of hydrocarbon-affected soil from the Proposed Cintra Park Phase II Development, Tract 2705, Tracy, California. Two underground fuel storage tanks (USTs) were excavated and removed from the site on June 2, 1997. Hydrocarbon-affected soil was apparently observed during removal of the US s. The two, steel USTs, of approximately 300- and 800-gallons capacity, were reportedly used t store leaded and unleaded gasoline from the late 1960s. until approximately 1992. fi At the northwestern corner of the intersection of West I lyron and Coral Hollow Roads the site was formerly occupied by a working farm. Figures 1 and 2 are index and site maps. t J t Land use in the site's vicinity generally consists of single-family residences and agricultural space. However, this area is currently undergoing rapid buildout and conversion from agricultural to residential uses. Two soil samples were collected., at depths of approximately 8 feet below grade (fbg) from the open excavations below the former UST locations. The collected soil samples were analyzed for: Total Petroleum Hydrocarbons-Gas (TPH-G) and benzene, toluene, ethylbenzene, and total xylenes (BTEX). TPH-G concentrations were 8,100 and 8,200 mg/kg; benzene was detected at the laboratory detection limits used, toluene concen ations were 380,000 and 110,000 µg/kg, ethylbenzene concentrations were 140,000 and 230,000 g/kg,and total xylenes concentrations were 790,000 and 1,200,000 µg/kg. Soil displaced during the excavation and removal of the tanks was replaced back into the UST excavations. The site, at an elevation of approximately 40 feet abovemean sea level, is on Recent alluvial fan deposits of the Great Valley Sequence (Rogers, 1966). These generally eastward-facing fans are composed of sediments derived from the Coast Range (approximately 5 miles west of the site). Wind directions vary but generally can be expected to be from the west. i According to a geotechnical investigation by Hallenb k& Associates (1995) the site is underlain by approximately nine to 11 feet of medium stiff to stiff, brown, silty and sandy clays. Soils underlying these materials (to the total depth exploired of 19 fbg) consisted generally of soft to medium stiff sandy clays and loose to medium dense, clayey sands and silts and silty sands. 1 h a W APROJECTS51023051ENRP01.WPD 4