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Western Slope Carbon, Inc. Page 14 December 15, 1981 I -I ditches should empty into a sedimentation basin with a capacity of at least 4000 cubic feet, excluding a minimum one-foot freeboard above design spillway I flow surface. The sedimentation basin embankment should be constructed in accordance with the grading criteria presented above and should have an emer- gency spillway which is protected from erosion by sack-Crete, rip-rap, or other means. (For construction details on the diversion measures and ditches, refer to Colorado Standard Plans, January, 1980, Standards Nos. M-107-A and M-203-C). Upon completion of all fill activity in the Pit No. 6 area, a final diversion structure should be constructed around the perimeter of the fill to carry sur- face runoff away from the fill surface. It should be designed similar to the temporary diversion ditches but, in addition, should be planted with erosion resistant, water tolerant vegetation to reduce flow velocities, and prevent erosion of the channel and reduce potential sediment generation. The final ] fill surface should be graded to drain to the diversion ditch and should be no steeper than 20:1. The fill surface should be planted with erosion re- f sistant vegetation. Under no circumstances should runoff from the fill surface be directed over the waste retention embankment unless specific erosion pro- ~ tection, such as sack-Crete or rip-rap is provided. LEACHATE GENERATION POTENTIAL AND CONTROL Movement of the coal process waste leachate, if generated, outside of the Pit No. 6 area, could result in contamination of groundwaters in the North Fork ] Gunnison River Valley. As such, leachate control or containment methods are required at the disposal site. Since approximately 32,800 cubic yards of l processed waste have already been placed in the Pit No. 6 area without a 1 leachate control system, movement of leachate out of the pit area is possible. However, our boring in the coal process waste indicates that leachate has not been generated to date. The waste arrives on-site in a moist, but unsaturated condition. Thus, for leachate to be generated, the waste must first be sat- urated either by direct precipitation and infiltration, or by groundwater which J rises up into the fill. We consider the latter extremely unlikely for the following reasons: 1. The site groundwater basin and proposed fill area are essentially identical.