2012-09-06_REVISION - M2008070 (25)

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Design and Operations Plan Western Gravel E & P Waste Disposal Facility • Two feet of unsaturated cover soils will be placed in maximum 12 -inch loose lifts and compacted to a minimum of 90 % of the maximum standard Proctor in accordance with ASTM D -698. As required, moisture will be added or removed from the material in order to achieve required compaction criteria. • A vegetative layer will be placed in order to establish vegetation, absorb water used to sustain vegetation, and prevent erosion. The vegetative layer will be constructed in accordance with Section 5.0. 3.4.9.2 HELP Model Computer hydraulic modeling using the Hydrologic Evaluation of Landfill Performance (HELP) U.S. Environmental Protection Agency (EPA) software of the final cover and double liner system proposed for the cells was performed using HELP Model, version 3.07. The program models the effects of hydrologic processes including precipitation, surface storage, runoff, infiltration, percolation, evapo - transpiration, soil moisture storage, and lateral drainage using a quasi -two dimensional approach. Model input parameters used include anticipated site specific soil parameters, vegetative cover, and evapotranspiration values of the Meeker, Colorado area. Based upon modeling results, significant leachate will not be generated from this facility. Any leachate generated will be a result of precipitation falling on waste, water used for dust control, liquids contained in amended saturated soil/mud, and/or re - circulated leachate. Leachate generation will occur mainly during filling operations since final cover will be designed to shed and/or absorb precipitation. Generated leachate will be managed using the LCRS. HELP Model results are provided in Appendix F. 3.5 SURFACE WATER CONTROL The facility will incorporate surface water channels to divert clean run -on water from the cells, collect surface water run -off from the cells, convey run -off into retention ponds located within the COD, and prevent surface water impacts. Clean run -on water that flows toward the facility from off -site will be managed separate from water that drains from run -off of cell surfaces in order to facilitate compliance with stormwater management regulations. Surface water drainage design details have been prepared for construction of the operational and permanent drainage structures. The operational drainage structures will prevent run -on from entering the active cell areas during waste placement. The permanent drainage structures will collect and contain any run -off from the facility and prevent this flow from leaving the COD. In addition, individual cells are designed in a manner where run -off will not leave the waste cells but be intercepted by a leachate collection system. Therefore, drainage entering the stormwater retention pond will not come into contact with cell waste materials and is not considered leachate. The locations of run -on and run -off control channels and storm water retention ponds are shown on Figure 3. Gravel mining and aggregate processing operations will be conducted concurrently with landfill operations. Since mining will be conducted ahead of landfilling as operations are phased, mining operations stormwater controls will be addressed separate from landfill requirements. However, it should be noted that as a whole, landfill run -on and run -off controls are considered an enhancement and more stringent than required by mining regulations. 3.5.1 Design Criteria In accordance with the Regulations, storm criteria used to design permanent run -off control structures at the facility is based upon a 100 -year, 24 -hour storm event. Storm criteria used to design operational clean water run -on control structures was also based upon a 100 -year, 24 -hour event. These storm events are estimated NWCC, Inc. October 24, 2011 Page 29 of 51