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CJ <br />ARCADISGERAGHTY&MILLER <br /> <br />Technical Support <br />Document for the <br />CDPS Permit <br />Application <br />' 13) Water Balance: Figure 3 provides a Conceptual Hydrologic Model of [he West Pi[ area and the Rito <br />Seco along two sections through the West Pit. Figure 4 is a schematic showing the volumes of water <br />' moving through the West Pit under current conditions, under initial pumping conditions required to <br />reverse the groundwater gradient to remove the seeps, and under long term pumping conditions <br />required to maintain the gradient away from the Rito Seco. Section 6.1.1 of TR-26 describes the <br />' pumping conditions required to reverse the hydrologic gradient away from the Rito Seco. This permit <br />application concerns the treatment and discharge of the groundwater pumped as part of the overall <br />water management program. The volume of water that is proposed for removal from the West Pit in <br />' order to initially reverse the gradient is estimated to be 400 gallons per minute (0.89 cfs). This water <br />will be treated and then returned to the Rito Seco at an outfall location near Rito Seco monitoring <br />station RS-3A (outfall 0001). This permit application also concems the overall management of the <br />' seepage front as described in more detail in Section 6.0 of TR-26. <br />Pre-mining surface water flow volumes in the Rito Seco show very few gains or losses along the Flow <br />' regime from upstream monitoring station RS-1 to downstream monitoring station RS-5. Average <br />annual flow along the segment from RS-1 to RS-5 ranged from 5.8 to 6.1 cfs (TR-26, Section 3.1 ). <br />During and after mining activities, though not associated with the mining, a significant change in the <br />Flow regime has taken place. This is the result of beaver dams that have been established from below <br />RS-5 to well above the West Pit location, nearly to RS-l . The addition of beaver ponds has made it <br />much more difficult to accurately gauge flow conditions in [he reach from RS-I to RS-5, as the Flow <br />channel changes and as dams are modified or washed out. However, it is clear that the saturated <br />volume of Rito Seco alluvium has increased significantly as the dams increase the width of the stream. <br />This allows for increased baseflow, increased evaporation from ponds, and increased evapotranspiration <br />due to the riparian environment created. <br />This has resulted in a decrease in surface flow. The following data show stream Flows (CFS) measured <br />from RS-1 to RS-5 during low Flow periods in January and February 1999. <br />Date RS-I RS-2 RS-2A RS-3A RS-5 <br />(/13/99 1.92 1.81 1.55 1.96 2.00 <br />' 1/28/99 1.60 2.20 1.80 1.98 2.10 <br />2/23/99 1.85 1.83 1.39 1.98 <br />' These flaw rates are generally less than half of the Flow rates measured a[ [he same locations during [he <br />pre-mining conditions during January and February (TR-26, Section 3.1.3). <br />' As a result of the beaver dams and the increased saturated alluvium, there is an increased likelihood of <br />interaction between the Rito Seco and the alluvial groundwater. <br /> <br /> <br />4/10 <br /> <br />