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~i revision and description of the environmental setting as it relates to pertinent closure components aze presented in detail in the following sections. 2.0 GENERAL CLOSURE ACTIVITIES 2.1 Introduction Under this alternative, the leach pad closure would require the following activities: • Continue leaching ore while evaporating process solution until it is no longer economical; • Dtaindown the leach pad and dschazge solution to tailings facility if available for evaporation, recirculate it to another heap leach pad as make up solution, land apply it, or evaporate the residual solution on the heap leach pad; • Recontour and compact the surface, spread topsoil and revegetate the leach pad; • Construct a Biopass System with dischazge to a leach field; and, • Monitor short-term to verify that physical stabilization and closure objectives are being met. A[ the comple[ion of metals extraction all process solutions in the leach circuit aze allowed to drairrdown. Draindown solution is collected in the ponds at the facility and subsequently pumped either to other leach pads for make up solution, pumped to a tailings impoundment for evaporation or [o a land applica[ion operation. Once draindown rates stabilize, all remaining seepage is routed into the Biopass System, which is designed to accommodate expected long-term seepage. The reclaimed heaps will contain a residual moisture content after draindown, which is generally immobile. Precipitation and extreme s[orm events can contribu[e moisture to a heap tha[ is discharged in a combination of the following processes: • Evaporation or evapotranspiration; • Uptake by spent ore raising the residual moisture content; • Redistribute via unsaturated flow within the heap if the field capacity moisture content is reached; and, • Discharge from the heap, should the field capacity be exceeded or, if saturated conditions occur at the base of the heap. These processes can be modeled for each heap leach pad using an unsaturated flow code which accounts for pad thickness, laboratory determined moisnrre release characteristics, and site-specific climatic data. Extensive research has been done on the theoretical and field measuremenrs of infiltration and evaporation from covers on mine waste facilities (Wilson, et al, 1993; Swanson, et al, 1994). The unsarurated flow modeling can be designed to act as a quantitative closure evaluation regarding the effectiveness of the selected closure alternatives. Climatic inputs to the system can be modeled using site-specific precipitation and temperature data in conjunction with the synthetic weather generator (WGEN) included as pan of the Hydrologic Evaluation of Landfill Performance (HELP) (USEPA, 1984) model. The WGEN model is used to simulate daily precipitation, solar radiation and [emperarure for a one year period. These values are used as input to the unsarurated flow model VS2DT (Lapalla, 1987) to approximate climatic conditions duough out the simulation period. The unsaturated flow characteristics of both the topsoil and spent ore materials must be determined empirically in the laboratory or estimated using information gathered during similar investigations at other heap leach facilities. The laboratory characterization includes es[imating saturated hydraulic conduc- -3-