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North County Sanitary Landfi -4— Module 4_Module 4-Liner Performance Demonstration San Joaquin County Maximum Contaminant Levels for the inorganic constituents chloride and barium to demonstrate that the liner design will prevent degradation of groundwater. However,the results of the performance demonstration modeling need to be compared with background concentrations of the constituents of concern. Section 4.3.2—Leakage Zone For a one centimeter hole a 10-foot by 10-foot polygon was used. Please provide an explanation of why this size polygon was chosen. To be more conservative and possibly more realistic with regards to preferred pathways,we recommend that a 1-foot by 1-foot polygon be modeled. The vadose zone depth of 82 feet was used in the modeling to simulate the most conservative conditions at the landfill. This was based on data since the landfill was monitored. If a larger data set were used based on regional groundwater elevations prior to land filling at North County,the vadose zone depth would be thinner and a more conservative value would be computed. Therefore, historic elevation of the groundwater(before the landfill) should be evaluated and determined if it is possible, the water will rebound to these levels. Based on the determination the model should be run on the highest anticipated elevation. Section 4.3.4—Model Simulation Period We are concerned that additional infiltration will be occurring through the modeled 12 inch interim layer cover (HELP model) over the life of the entire landfill(multiple modules). Most of the rainfall in the valley comes in large storm events rather than slow rainfall over time. These storms erode the cover and expose the waste to infiltration on the steep side-slopes. We propose that the interim cover be thicker than 12 inches in order to minimize infiltration throughout the lifetime of the facility. This cover material may be excavated upon final waste disposal to reach final elevation/capacity. We expect that additional soil cover(i.e., 2, 3, or 4 feet)may aid in the settlement/compaction of the waste over time and possibly provide additional landfill space at the end of the landfill's life. Alternative interim cover materials may be acceptable. Title 27, 20705 (b) Minimize Percolation states that, "Interim cover over wastes discharged to a landfill shall be designed and constructed to minimize percolation of liquids through wastes." Please address the issue of a thicker layer for interim/intermediate cover during the inactive life of the modules as part of the HELP simulation. Please justify the minimum 12"cover considering that heaviest leachate flows are before the final cover, and long length of time before this unit is closed. Maybe a thicker interim cover could be used to "minimize percolation of liquids"per Title 27 and then removed when new waste will be placed on top. Section 4.4.1 Sensitivity Analysis The performance demonstration report compares the modeled attenuated constituent concentrations to drinking water Maximum Contaminant Levels for the inorganic constituents chloride and barium to demonstrate that the liner design will prevent degradation of groundwater. However, the results of the performance demonstration modeling need to be compared with background concentrations of the