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~ 1 1.0 INTRODUCTION This document presents the design for construction of a hydraulic barrier between the West Pit and the Rito Seco, at the San Luis Project, located near San Luis, Colorado. The Battle Mountain Resources, Inc. (BMRI) Technical Revision titled "TR-26 Water Ytanagement in ' the West Pit Area, San Luis Project, Costilla County, Colorado" (BMRI, 1999), Section 4.4.4.2 presented a brief evaluation of the potential for the application of a~~hysical barrier ' (e.g., slurry wall or sheet piling) at the San Luis Project. Further consideration has been given regarding the potential use of a hydraulic barrier [o control groundwater flows in the ' vicinity of the West Pit. BMRI proposes to install a slurry wall along the eastern and southeastern boundary of the West Pit to control the inflow of Rito Seco alluvial groundwater and the outflow of groundwater from the backfilled West Pit. This document presents a preliminary engineering design for installation of the slurry wall. BMRI requ:sted Shepherd Miller, Inc. to prepaze the preliminary design and analyses. The potential exists to use a slurry wall as a barrier to minimize: (1) the flow of Rito Seco ' alluvial groundwater into the backfill from the east side of the West Pit and (2) the outflow of groundwater from the West Pit along the southern boundary of the backfill. In application, ' the slurry wall would span a continuous length from the azea of M-30 (on the fast side of the West Pit) to the west of M-20 (on the southern boundary of the West Pit) (Figure 1). ' Installation of the slurry wall would consist of excavating a 2 to 3 foot wide trench to a depth equal to the thickness of the alluvium. A slurry mixture of soil and bentonite clay would then ' be placed in the excavated trench forming a hydrologic barrier with a hydraulic conductivity ' of approximately 1 x 10-6 cm/s or less. Bul71e hfountain Resources. inc. P9/¢¢36: LSlunyM'ufllDengvrpwWor t ~'hepherd Miller, /nc. November 5, 1999