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i~ <br />' Technical Revision TR-18 was applied for and subsequently granted by the Division for the <br />partial backfilling of the West Pit to allow free drainage of meteoric water. This partial pit <br />' backfilling consisted of overburden and waste rock materials generated from the West Pit area <br />composed primarily of pre-Cambrian and Santa Fe Formation materials. Completed <br />' backfilling operations returned the final pit surface to an elevation of approximately 8,600 <br />feet-msl. <br />' Waste rock from the West Pit was composed of gravels and cobbles from the Santa Fe <br />Formation, pink gneiss from the pre-Cambrian, biotite gneiss and biotite-antphibole gneiss <br />' which underlied the pre-Cambrian. Waste rock excavated from the pit during early <br />development was transported to the South and D Waste Rock Disposal Areas and as mining <br />' progressed with depth the material was transported and temporarily stored tat Waste Rock <br />Disposal Area B for eventual backfill into the pit. As mining was completed in the deeper <br />' West Pit area and progressed into the Pink Gneiss area, waste rock was transported and <br />backfilled into the deeper portions of the West Pit. Following completion of mining of the <br />Pink Gneiss azea (post-operations) the temporarily stored waste rock at Waste Rock Disposal <br />Area B was rehandled and deposited in the Pink Gneiss and deeper West Pit areas to allow for <br />' the free draining reclamation surface. As required by TR-18, a lift of Santa Fe Formation <br />material was placed within the 5 feet surrounding 8,770 feet-msl to provide alkalinity <br />buffering at the final predicted water table elevation. A total of approximately 5.8 million <br />tons of waste rock and overburden material was backfilled to the West Pit. <br />' With concurrent backfilling operations during the later phase of mining and completion of <br />backfilling during reclamation, the West Pit has been filled with waste rpck consisting <br />primarily of pre-Cambrian and Santa Fe Formation materials. The backfill was then capped <br />with topsoil and graded to direct surface water runoff through an erosion control structure that <br />' met best management practices (BMP). <br />' The backfilling of the West Pit altered the ground water flow hydraulics through the West Pit, <br />so that it is different both from the pre-mining and mining periods. Where there were two <br />distinct localized aquifers prior to mining, there is now one aquifer in the West Pit composed <br />' of backfilled rock of Santa Fe and pre-Cambrian origins. Therefore, not orally are the two <br />aquifers co-mingled, but the porosity and permeability through which ground water moves <br />' have also changed. These effects in pit geology and hydrogeology are shown In Figure 4. <br />6 <br /> <br />