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-2- ,. _ for treatanent. The surface diversion ditch around the bank was designed • ~ by Leonard Rice & Associates. The ditches are to 6e ,grasps 1i-ned to • .. minimize erosion. In areas where water velocities may cause erosion of -,, ~; ~ •grass-lined ditches, at least 12 inches of 6-inch diam:ter riprap is to ~ ~ •. be,place.• . ,. ~, We understand that the area for construction of the bank was•pre- viousy used by the County as a gravel borrow area. We believe that • ground .water occurs at great depths below this site. ielatively imper- ~'' meable shale and sandstone was observed in cut slopes below the planned disposal site. Any leachate would tend to perch on these relatively impervious materials. The near-surface soils consist of relatively • .' permeable sands and gravel with some clays. By reshaping the surface of~. •the embankment area and compacting the on-site soils to high density '. 'along with provision of a collector drain, we believe t:he risk of infil- tration of leachate from the disposal site into the ground water system •~ will be very. low. The waste bank will be constructed in an area where most surface runoff ,~ will be directed away from the bank. Any water which could infiltrate. the pile would. be the result of moisture which falls on the bank it- self. Processing waste will consist of clay, sandstone, shale and some coal. The waste is deposited in piles and periodically leveled and compacted. The rate of production is relatively small when compared with the capacity of conventional earth moving equipment for compac- tion. Because of the method of construction, we believe the permea- ~. bility of the pile will vary through wide extremes. Provided the pile is compacted to the recommended density, the overall permeability will ', ~ ,probably range from semi-pervious to semi-impervious. 'Considering the " local arid climate, we doubt that a fully saturated condition will •.. ~~develop in the pile and, therefore, do not recommend an extensive inter- • nal drain system. ' ;It is possible that some water could collect at isolated locations. For ,`' -.~„ :.this possibility,'we recommend shaping and sloping the foundation soils ' ~~ • •.of the pile to drain to a toe drain. Provided the foundation soils are j' • ;-.compacted to the recommended density, any moisture which enters the pile ; .should flow towards the drain. The toe drainage system should be con- ', l' strutted below the toe of the waste bank to reduce the likelihood of ..surface runoff directly entering the drain. The system 'is to consist ofd ~,' perforated plastic pipe encased in free-draining gravel. The gravel is ~~ to be wrapped in a filter fabric to prevent clogging. Very little topsoil is present within the disposal area. The original surface soils were probably removed by the county. Two small ponds are .;'located within the limits of the pile. The ponds have been created by • ~ directing surface runoff into the proposed. disposal site. .These ponds should be removed as part of the initial site development. In confor- •, mance with 4.10.4(1), all topsoil and organic matter should be removed '. 'prior•.to the placement of waste. Two topsoil stockpiles from .mine .'.7.._ , .,