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Section Summary and Conclusions 5.1 Introduction - The contaminant plume delineation investigation,consisting of three objectives,as established in Section 1.3,are as follows: 1. Define the current nature and horizontal and vertical extent of the VOC contaminant plume; 2. Determine the effectiveness of the existing extraction system in containing the plume at the landfill boundary;and 3. Estimate the rate and direction of contaminant plume movement in the aquifer. These objectives were established as the information required to understand the hydrogeologic setting at the ARL sufficiently to develop and evaluate corrective action alternatives at the site. These alternatives,which will consist of proposed modifications to the existing pump-and-treat system,will be analyzed during the engineering feasibility study phase of the project. 5.2 Plume elinetion The VOC contaminant plume extends from the northern edge of the landfill perimeter,to approximately 4,000 feet to the northeast of the site as depicted in Figure 3-5. The primary contaminants of concern are PCE and TCE,with concentration levels up to 69 and 48 pg/L, respectively. The highest VOC concentrations are present within a sand layer which extends over an interval of approximately 80- 104 ftbgs. The investigation results indicate that contaminant concentrations decrease at distances from the source with the plume migrating in deeper lithologic units at distance from the site. The plume shape reflects the north to northeast groundwater flow direction with the leading edge of the plume extending east of Austin Road as depicted in Figure 3-5. 5.3 Existing Extraction System Effectiveness The current groundwater extraction system consists of two pumping wells located at the northern edge of the waste fill. Based on the results of the capture zone modeling presented in Section 4, these wells do not provide complete capture of the contaminant plume emanating from underneath the landfill. Plume breakthrough is exhibited both between the two pumping wells and to the east of pumping well EW-1 as depicted in Figures 4-1 through 4-4. Vertical containment of the VOC plume appears to be achieved within the capture zone of the two wells. Additional extraction wells along the northern boundary of the landfill are needed to achieve complete source control. Dresser&McKee y S 010790 5-1 CDM Camp City of Stockton 1906554006 VV..IREPORTS\STocicro RLPLUME.961SEC5.WPD