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(typically 1 -ft thick) lifts from rotating locations around the perimeter. This thin -lift deposition <br />process was intended to facilitate desiccation and drying of the tailing by evaporation under the <br />typically dry climatic conditions at the site. The under -drain system was incorporated to collect <br />remaining tailing process water expressed under self - weight consolidation of the tailing through <br />bottom drainage. Tailing drain -down fluid that collects on top of the liner is conveyed via a <br />network of pipes to a central drain that penetrates under the dam and discharges into the lined <br />collection pond at the downstream toe of the dam. The reclaimed water is then pumped back up <br />into the impoundment area to be evaporated and cycled back through the impounded tailing. <br />According to available design and construction documents, the foundation for the impoundment <br />liner is compacted, low permeability soil. The liner material is predominantly constructed of a <br />40 -mil very low- density polyethylene (VLDPE). In some areas, 60 -mil high- density <br />polyethylene (HDPE) material was used where high hydraulic head or ultraviolet radiation <br />exposure was expected. A total of 12.6 million square feet of liner was installed in the <br />impoundment. <br />Directly above the geomembrane liner, a 2 -foot thick drainage layer of granular soil was placed, <br />with a network of perforated drainage pipes. The drainage blanket and pipe network convey <br />water collected on top of the liner and drain it under the dam to a double -lined collection pond. <br />Typical recent (2013) flow rates emerging from this drainage collection system and reporting to <br />the collection pond are on the order of 30 -40 gallons per minute (gpm). <br />The collection pond below the dam is double - lined, with a lower liner of 40 -mil VLDPE and an <br />upper liner of 60 -mil HDPE, with a geonet layer in between the two liners. Any leaks through <br />the upper liner are conveyed via the geonet to a secondary recovery sump. Fluids stored in the <br />collection pond are pumped back up to the free water pool located within the impoundment area. <br />The storm -water control facilities are reported to be designed for a 100 -year, 24 -hour design <br />storm event, defined as 2.9 inches of precipitation. The main diversion ditches and berms are <br />located on the south and southeast perimeter of the facility. These diversion features are <br />designed to intercept and divert runoff from the largest catchment areas, which are south of the <br />tailing facility. According to the tailing disposal area design documentation, the main South <br />Diversion Ditch is designed to divert most of the intercepted flows, with a 100 -year peak design <br />discharge capacity of up to 292 cubic feet per second (cfs). These flows are conveyed in the <br />South Diversion Ditch towards the west and over the left abutment of the dam via a drop <br />structure consisting of a concrete inlet, 48 -inch corrugated metal pipe and concrete energy <br />dissipation structure. A second ditch referred to on the drawings as the East Diversion Ditch is <br />designed to convey the remainder of southerly runoff, having an estimated 100 -year peak <br />discharge of approximately 80 cfs, around the east end of the facility and into a saddle that drains <br />northeastward into the Rito Seco creek. Runoff from the much smaller contributing catchment <br />area on the north side of the valley is not diverted, but is intended to drain directly into the <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 3 February 2014 <br />