Laserfiche WebLink
Mining and Reclamation Permit Amend.-M-1988-112 <br /> Groundwater Management Plan Battle Mountain Resources,Inc. <br /> October 1996 although pit dewatering continued until December 1996. Backfilling of the <br /> West Pit occurred from 1995 through 1997. BMRI (1999a) estimated that approximately <br /> 5.8 million tons of material was backfilled into the pit after mining. Backfilling was <br /> accomplished by layering sequences of spoils from the Precambrian gneiss and Santa Fe <br /> Formation (Fm) into the open pit. The backfill material has hydraulic conductivity <br /> several orders of magnitude higher than the geologic units that existed in place prior to <br /> mining. <br /> The backfilled West Pit is oblong, oriented northwest to southeast, and covers an area of <br /> approximately 27.1 acres (1.2 million ft2) that is roughly 2,000 feet long and 600 feet <br /> wide. The maximum thickness of the backfill material is over 140 feet. <br /> BMRI (1999a) calculated the total volume of backfill to be approximately 94.7 million ft3 <br /> with a total pore space volume of 28.4 million ft3 (assuming a porosity of 30 percent). <br /> BMRI further estimated that the saturated portion of backfill (assuming an average depth <br /> to water of 12 feet) was approximately 24.2 million ft3 (or 181 million gallons). The <br /> backfilled surface elevation of the West Pit ranges from approximately 8,625 feet above <br /> mean sea level (ft amsl) on the northwest end to approximately 8,600 ft amsl on the <br /> southeast end, using the Colorado State Plane South (CSPS) National Geodetic Vertical <br /> Datum 1929 (NGVD 29) vertical projection. <br /> Once mine dewatering ceased, groundwater began to saturate the backfill material within <br /> the West Pit. The potentiometric surface within the West Pit rapidly established a new <br /> equilibrium at an elevation of approximately 8,595 ft amsl (CSPS NGVD29). The <br /> potentiometric surface is relatively flat across the entire West Pit in response to the <br /> extremely high hydraulic conductivity of the backfill material relative to the surrounding <br /> hydrologic units. By October 1998, seeps were observed along the north bank of the Rito <br /> Seco, directly south of the West Pit. The occurrence of the seeps was attributed to <br /> discharge of groundwater from the West Pit. The response to the observed seeps was the <br /> implementation of the GWMP as prescribed in Technical Revision 26 (TR-26) to the <br /> Colorado Mined Land Reclamation Permit M88-112 (BMRI, 1999a). <br /> 3.0 SITE GEOLOGY <br /> The geology in the immediate area of the site has been extensively evaluated. Geologic <br /> investigations were conducted as part of the historic mining operations and site <br /> environmental investigations have included installation of a large number of monitoring <br /> wells and subsurface borings used to evaluate the ongoing groundwater remediation. The <br /> site geology has been described in detail in the original mine permit application (BMRI, <br /> 1988), permit technical revisions TR-18 and TR-26 (BMRI, 1994 and 1999a, <br /> respectively) and site investigations (BMRI 1999b, and Water Management Consultants, <br /> Inc., 1994). A summary of the mine site geology is provided below. <br /> The stratigraphic sequence in the vicinity of the San Luis Mine consists of, from oldest to <br /> youngest, the Precambrian age gneiss-granite complex, Tertiary age igneous rocks, <br /> Tertiary age Santa Fe Fm, and Quaternary Rito Seco alluvium. After mining, the West Pit <br /> April 2025 2 Engineering Analytics,Inc. <br />