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Mr. Michael Carroll -2- 30 November 2011 obtainable at optimum moisture content using methods that are in accordance with accepted civil engineering practice. A lesser thickness may be allowed for Units if the RWQCB finds that differential settlement of waste, and ultimate land use will not affect the structural integrity of the final cover." Therefore, in order for our Board to find that a minimum one foot foundation layer will not affect the structural integrity of the final cover, the design engineer must provide written justification why a minimum one foot foundation layer still meets or exceeds the requirements of Title 27 Section 21090(a)(1). 3. CHSL Addendum to the Final Closure Plan (FCP Addendum) Section 8.1 states that the closure cap extension total area is 1.6 acres and that the existing closed area is 43 acres. The percentage of increase in acreage due to the extension is 1.6/43 or 3.72%. However, .the Addendum to the Postclosure Maintenance Plan (PMP Addendum) Section 11.1.0 states that "Revegetation cost for the closure cap extension project is based on 12% percent proportional increase in the closure cap area". Concerns: The justification to increase the costs by 12% to operate and maintain the extended closure cap area is incorrect. Table 1 of PMP Addendum states that the annual maintenance cost for the extension is $2,672 based on a 12% proportional increase in closure cap area. 4. The FCP Addendum Section 2.2 Reason for the Closure Cap Extension states that "The County suspected that if the closure cap was insufficient below the drainage ditch, then concentration of water at the drainage ditch, and percolation of that water through the refuse below, could impact the shallow aquifer." The Discharger acknowledges that percolation of storm water occurs at the drainage ditch along the eastern access road at CHSL. Approximately twenty (20) acres drain to the drainage ditch (Detail 4 of Sheet 51). The drainage ditch discharges to Storm Water Retention Pond 1 (Sheet 2). 27 CCR § 20365 Precipitation and Drainage Controls states that for a Class III Municipal Solid Waste (MSW) landfill the capacity of the precipitation and drainage facilities shall be designed to accommodate a 1-in-100 year 24-hour storm event. The National Oceanic and Atmospheric Administration (NOAA) Atlas 14, Volume 6, version 2 states that for Tracy, California (Latitude: 37.6675, Longitude: -121.4573) and for an average recurrence interval of 100 years and a duration of 24-hours one can expect 2.81 inches of precipitation on average with 90% confidence that precipitation will range between 2.29 and 3.53 inches. Thus, with a 95% confidence that the design of the precipitation and drainage facilities is adequate, the facility should be designed for 3.53 inches of precipitation occurring during a 24-hour period. Detail 4 of Sheet 5 shows that GCL will be placed in a subgrade v-ditch a minimum of one (1) foot above the refuse. The placement of the GCL will greatly reduce percolation of water to the refuse below. Above the GCL adjacent to the eastern access road will be the surface drainage ditch lined with erosion control matting. The drainage ditch will be hydroseeded to reduce erosion by reducing and stabilizing flow velocities in the drainage ditch. The precipitation runoff from approximately 20 acres will be channeled along the All references to Sheets and Details shown on Drawings prepared by Shaw Environmental dated 30 September 2011.