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a • Foothill Sanitary Landfill -3— Module 3— Module LF-2-Liner Performance Demonstration San Joaquin County Liner Performance Demonstration The County proposes an engineered alternative single composite liner that should be designed to meet or exceed performance standards of Title 27 as required by the Central Valley Regional Board. The proposed design includes, from bottom to top, a geosynthetic clay layer(GCL) over a prepared subgrade; 60-mil HDPE; gravel leachate collection and removal system(LCRS)ranging from 0.75 feet thick above the HDPE and one-foot thick in areas where leachate collection pipes will be placed in trenches; geosynthetic filter; and a two-foot operations layer of sand. Additional protection will be provided in leachate sump areas. Based on our review,we have determined that there are some areas of concern that must be addressed by the County prior to our acceptance of the liner performance demonstration report as follows: Section 1.2 Proposed Liner System In addition to the containment system presented,the installation of a secondary composite liner system below the leachate removal sump area, equipped with a monitoring and removal system(LCRS), is proposed. Please provide more details on the design and operation of this system. Include information on how the LCRS will be maintained/cleaned out and how it will be tested on an annual basis. Section 1.3 Liner Performance Modeling Results The constituents benzene, 1,2-DCA, methylene chloride,MTBE, vinyl chloride, barium and chloride were selected as modeling parameters because they represent constituents found in the leachate at the nearby North County landfill. The basis for choosing these constituents was their threat to human health given the high ratio between the constituent's CA Primary MCL and maximum concentration detected in leachate. However, other factors should be considered for choosing constituents, including those constituents found in groundwater at the Foothill landfill, high human toxicity,high contaminant solubility and transport properties, and whether they are representative of the various chemical groups associated with municipal solid waste landfills. We recommend that metals also be considered in the modeling effort as potential pollutants, such as hexavalent chromium and arsenic, due to their solubility and/or mobility in groundwater. The Report should include a readable,printed table of historical data from the Foothill Landfill and North County, as applicable, for the Board to review in support of the chosen COCs. Bold the highest value of each COC monitored to date. Provide a comparison of the modeled numerical values with background concentrations or established Water Quality Protection Standards, as applicable, for inorganic constituents. Organic constituents should be compared to the method detection limit for each constituent and the water quality objective' relative to the most applicable beneficial uses (i.e., chemical constituents, toxicity, tastes and odors)2 of groundwater. 'A Compilation of Water Quality Goals,California Environmental Protection Agency,Regional Water Quality Control Board-Central Valley Region,August 2000,with updates. This document and the latest updates may be found at www.swrcb.ca. og v/rwgcb5 2 Water Quality Control Plan,Sacramento River and San Joaquin River Basins,Fourth Edition(Basin Plan)