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1.4 INTRODUCTION This report has been prepared by Upgradient Environmental Consultants (Upgradient) for Apache ' Plastics L P (Apache) to document the monitoring and remedial activities that have taken place at its facility since March 1996 The work described here has been performed under contract 96004 2.4 BACKGROUND 2.1 Location and Setting The Apache Plastics manufacturing plant is located in a mixed commercial/residential neighborhood in east-central Stockton(Figure 1) Topography in the area is flat, at a surface elevation of 22 feet above sea level The site is underlain by discontinuous beds of medium to fine-grained alluvial sediment 1 ranging in grain size from sand to silt Finer-grained beds of silty clay are interbedded at various depths (0-15 feet, 35-40 feet, and below 65 feet) ' 2.2 Soil and Groundwater Contamination Gasoline was detected in the soil following the removal of a 1000-gallon underground storage tank ' (UST) from the building in August 1988 (Figure 2) Subsequent investigative work by Western Geologic Resources (WGR) and Remediation Services, Inc (RSI) indicated that total petroleum hydrocarbon concentrations as high as 13,000 mg/kg were present at a depth of 40 feet, and concentrations exceeding 1,000 mg/kg extend more than'25 feet in all directions from the UST excavation Contaminated groundwater was encountered in the first monitoring well that was installed (MW-1, subsequently destroyed during road construction) and six additional groundwater monitoring wells were drilled and sampled Gasoline was detected in some of these during sampling several years ago, but drought conditions in recent years have lowered the water table and the wells went dry in 1991 Annual rainfall returned to normal levels in 1994, and groundwater has been present in most of the wells since early 1995 ' 2.3 Sod Remediation To remediate the soil contamination beneath the site, Apache elected to use in-situ vapor extraction ' after a 1992 feasibility test performed by RSI showed this method to be effective at the site RSI installed two wells in a side-by-side configuration within the former tank excavation (Figure 2) Both wells are completed with blank 2-inch-diameter PVC casing that is open-at the bottom end to allow vapor inflow The shallow well is bottomed at 35 feet, and the deep well is bottomed at 60 feet The wells are marnfolded at the surface to an internal combustion engine destruction unit manufactured by VR Systems of Anaheim, California 2 1 y