2008-09-17_REVISION - M1980244 (28)

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acidic environment impeding the growth will be required. Please provide the amount of • lime per ton of overburden that will be utilized to ensure the success of the cover. Response: The use of the term "clay cap" in our ECOSA presentation was not meant to infer zero- infiltration or a specified permeability of the cover. As described in the MLE Project Application, the ECOSA cover fulfils tzvo of the objectives of the ECOSA design: reduction of infiltration and minimization of air entry (MLE Project Application, Appendix 6, p.1). The hydraulic purpose of the ECOSA cover is to control infiltration to the overburden to an amount that is comparable with the infiltration that currently enters the ground surface in the same location, so that any flow through the overburden will be accepted into the subsurface, rather than possibly traveling along the base of the overburden to flow at the base of the facility. To achieve this hydraulic objective, CC&V has proposed to remove existing clayey material from the footprint of the ECOSA, and create a layered cover comprising (accepting Dr. Van Zyl's recommendations) a 6" base layer of clayey bedding, a 12" compacted layer of high plasticity material screened from the existing clayey material, an 18" clayey material removed from the footprint of the ECOSA, and topped with a 6" layer of growth medium. No permeability requirements Were-proposed in this design because the cover material will be constructed on the mostly sloping surface of the overburden, and will be subject to freeze/thaw cycles, penetration by plant roots, and subsidence of the underlying end-dumped overburden material; thus, the intent of the ECOSA cover is not to achieve the same level of impermeability as the soil line fill for the VLF. • For this reason the design callsfor the 071) limiting element to be constructed of screened, fl g f sceened, compacted plastic material, with its moisture and plasticity protected by the overlying clayey material and growth medium. Dr. Van Zyl concurred in his review that the cover would meet the claimed permeability, and stated in discussion with the designer (Adrian Broum) that in his opinion the long-term permeability of the material could not be maintained at a value lower than the design value, regardless of method of placement. It is computed that the permeability of this cover material alone would limit the infiltration rate on the pile to approximately 5.0 inches per year for approximately half the year (MLE Project Application, Appendix 6, Attachment 1, p. 2), and would limit the total infiltration to approximately 2.5 inches per year (for the other half of the year), due to the effects of freezing and periods when the surface is dry. This infiltration rate is approximately the same as the infiltration that currently is computed to infiltrate to the diatreme which is 5-7 inches per year, depending on assumptions (MLE Project Application, Appendix 1, Attachment 1). Actual infiltration to the ECOSA would likely be less, due to higher runoff from the steeper slopes of the reclaimed ECOSA, evapotranspiration from the reclaimed surface in the summer and fall, and freezing of the surface in the winter and spring. The requirements of the ECOSA cover are conceptually and technically different from the requirements of the clay liner of the VLF. The VLF liner is designed to collect water to act as • additional protection of the environment, and to monitor the integrity of the overlying synthetic primary liner. This duty requires that the clay liner be essentially leak-proof, for this additional environmental protection. 1R