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II. Cameo Refuse Disposal Area 1 CRDA-1 also is located on the west/northwest side of Interstate 70 across the highway from the main mine facilities area, but is on the south side of the canyon access road across from CRDA-2. Its MSHA identification number is 1211-CO-09-03012-01. Construction of the pile began in 1980. The disturbed area associated with the pile, access road, ditches, and pond is about 38 acres. Three piezometers (CRDA-01 through 03) originally were installed in CRDA-1 in 1984, and a fourth (CRDA-01A) was installed in 1991. CRDA-1's use was suspended in 1988 due to stability concerns related to internal water levels as evidenced by periodic outslope seepage. Those concerns prompted installation of 30 additional piezometers in CRDA-1 that are monitored quarterly. Volume 1 of the mine plan states that piezometrc monitoring and stability analyses showed the pile is stable as designed despite the presence of " a complicated series of isolated and interconnected permeable zones' ` `"within the pile, some of which carry water. Use of CRDA-1 resumed in 1994 and continues. Approved design parameters include: 2h:1v maximum slope steepness (s 50%, or 26° 34'); constructed in layers 2 feet thick or less compacted to 90% of maximum dry density; benches 10' wide; and lifts constructed on 30' intervals above the first lift, which has a maximum height of 50 feet. The approved depth of non-toxic, non- combustible cover material is 18 inches based on field trials. Bench inslopes are to be graded at 2% to 5%. Benches are to be graded to the side collection ditches at 2% to 5% as well. The top of the fill also is to be graded at 2% to 5%, but not to drain over the outslope. Bench and outslope completion are as described above for CRDA-2. Importantly (and applicable to the other piles as well), Volume 1 of the mine plan notes that "The time schedule for covering a bench may be delayed another year if the bench is actively dewatering through seeps.° Drainage controls for the CRDA-1 area include two collection ditches and two culverts under access roads. The northwest and southeast collection ditches (Exhibit 20, cross sections G-G and H-H) were designed with trapezoidal cross-sections to handle runoff from 100-year, 24-hour precipitation events. Northwest collection ditch design criteria include: Top width, 4.6'; 2' bottom width; total depth of 1.3', including .3' freeboard; 1h:1v inside slopes; and 23.3 cfs capacity. Southeast collection ditch design criteria include: Top width, 5.6'; bottom width, 2'; total depth 1.8'; 1h:1v inside slopes; and capacity of 34.1 cfs. A section of the northwest collection ditch located on the north and northeast ends of the refuse pile is designed as a combined V-ditch and berm (Exhibit 20, cross section E-E). The design criteria for this section of ditch are: ditch depth, 2.2'; top width of ditch, 8.8'; inside ditch slopes of 2h:1 v; top width of berm, 10'; bottom berm width, 15'; and height of berm, 2.5'. Exhibits 9A and 9B call for one 30" diameter CMP culvert under an access road near topsoil storage area 5 and a second 30" CMP under access road 6 at the road fork north of the Public Service of Colorado ash disposal area. 10