This paper presents the liner performance demonstration performed for a Subtitle D alternative composite liner proposed for a 153-acre (61.9-ha) expansion at the Bakersfield Metropolitan (Bena) Sanitary Landfill in Bakersfield, California. The Bena Sanitary Landfill is operated by the Kern County Waste Management Department (KCWMD). This paper summarizes the regulatory background, the site characterization, and computer modeling using the U.S. EPA's Multimedia Exposure Assessment (MULTIMED) model. This paper also presents the results of the liner performance demonstration, that are required by regulatory agencies as a condition for final approval of the liner system. In April 2001, the Central Valley California Regional Water Quality Control Board (RWQCB) put all landfill owners on notice that henceforth all applications for landfill construction would require a demonstration that the proposed liner system meet state groundwater protection standards, even if the prescriptive Federal Subtitle D liner system was employed. The performance demonstration was to employ site-specific geology, hydrogeology, waste, and leachate characteristics. The work presented in this paper describes this performance demonstration for the Subtitle D alternative composite liner system proposed for the Bena Sanitary Landfill. Site characterization included evaluation of geologic and hydrogeologic site conditions and waste and leachate characteristics to derive properties and input values for MULTIMED modeling, including porosity, aquifer thickness, groundwater gradient, saturated and unsaturated zone hydraulic conductivity, waste characterization, and leachate chemistry. The leachate chemistry evaluation identified the landfill-related constituents that posed a threat to groundwater quality (i.e., compounds with low concentration limits, high concentrations in leachate, and/or high mobility) for use in the MULTIMED modeling. These evaluations were incorporated into the MULTIMED model to perform a site-specific analysis of the subsurface transport of landfill-related constituents to the point of compliance. Conservative input parameters for the MULTIMED computer modeling were selected in order to produce "worst-case" concentration levels at the point of compliance. These "worst-case" results indicate that the calculated dilution-attenuation factor (DAF) from MULTIMED is approximately 448 times higher than the DAF value of 100 recommended by the U.S. EPA for Subtitle D landfills. Based on these MULTMED results, it was demonstrated that the alternative composite liner proposed for the Bena Sanitary Landfill is capable of preventing degradation of waters of the state as a result of waste discharges to the landfill.
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