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Section 1 Introduction 1 .1 Purpose This report presents the results of Camp Dresser&McKee,Inc.'s(CDM's)groundwater contaminant plume characterization investigation at the Austin Road Landfill.The objective of the report is to document the field activities associated with the groundwater investigation and to present the understanding of the hydrogeologic setting at the site based on the data collected. The major components of the investigation include the following activities: (i) Installation of boreholes and lithologic logging of the holes; (ii) Groundwater and soil sampling and analysis; (iii) Performance of an aquifer pump test on the existing groundwater extraction system;and (iv) Capture zone modeling to determine the effectiveness of the existing groundwater corrective action measure. The purpose of the groundwater investigation was to delineate the vertical and horizontal extent of the contaminated groundwater plume downgradient of the Austin Road Landfill. Defining the plume boundary,in conjunction with the evaluation of the effectiveness of the existing groundwater extraction system,will provide the basis for developing groundwater remediation system alternatives during the engineering feasibility study phase of the project. These alternatives will consist of modifications to the existing pump-and-treat system. 1.2 Site Description and History The Austin Road Landfill is a 299-acre municipal solid waste disposal facility located on the southeast limits of the City of Stockton(City). The landfill is located on Austin Road on the southern boundary of the California Youth Authority Center as depicted in Figure 1-1. The facility has been owned by the City since it began operations in 1958. The landfill currently consists of a fourteen-well groundwater monitoring well network as depicted in Figure 1-2. The initial system consisted of four wells,MW-1 through MW4,installed in the mid-1980x. Based upon work performed for the Solid Waste Assessment Test(SWAT)conducted in late 1988,volatile organic compounds(VOCs)were detected in monitoring wells MW-1 and MW4. A verification monitoring program designed to determine the horizontal and vertical extent of contamination was initiated in late 1989,as directed by the Central Valley Regional Water Quality Control Board (CVRWQCB). This investigation resulted in the conclusion that VOC contamination existed downgradient of the site at monitoring wells MW-10 and MW-11. Development of a corrective action program was initiated in accordance with state regulatory requirements to remediate the offsite groundwater contamination. The measures implemented included the installation of two groundwater extraction wells,EW-1 and EW-2,located at the perimeter of the waste fill, and construction of downgradient monitoring wells MW-12,MW-13,and CDM Camp Dresser&McKee 1-1 CS W:\,REPORTMTOCKTOMARIPIUME.98\SECI.WPD OZO -' City of Stockton 199IS6 0855.0006