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North County Sanitary Land • -2— Module 2—Module 4-Liner Performance Demonstration San Joaquin County 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 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 applicable 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. Section 2.4 Site Geology The Report describes the first 3 feet of soil and %2-foot layer of cemented soil. It should be made clear in the text that this soil has been removed as part of the excavation for the landfill. Section 2.5 Groundwater Monitoring System The Report states that the maximum historical groundwater elevation near proposed Module 4 is—31.56 feet below MSL and the average elevation since the well was installed is—36.59 feet MSL. Please provide a discussion of any production, irrigation, and/or domestic wells in the vicinity. Include a map showing all wells within a one-mile radius of the site. Also,provide a discussion of historical regional high water levels in the Stockton area. This comment is also appropriate for Section 2.6 in Highest Anticipated Groundwater section. The San Joaquin County Flood Control District offices should have historical data referencing the highest groundwater elevations recorded. Section 2.6 Aquifer Characteristics Groundwater Quality: This subsection states there is no indication that groundwater beneath the site has been impacted from land filling activities. The 4th Quarter 2001 report shows that there have been trace hits of VOCs in G-1, the upgradient well, and G-3D, and G-5. The report states that all VOCs detected, other than Freon 113 in G-1, were also found in one or more sample blanks. We realize this information may not have been available when drafting this document,but there has been a history of trace hits of VOCs at this site historically. Please provide a historical summary of such events. Section 3.1 Liner System Properties Overview Paragraph 4 states the Geomembranes are essentially impermeable and can contain gases. Please provide documentation/information supporting this statement. '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.gov/rwgcb5 2 Water Quality Control Plan, Sacramento River and San Joaquin River Basins,Fourth Edition(Basin Plan)