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water in the Rito Seco farther downstream at the next available water quality <br />monitoring location RS-2. <br />Attached to this response document is a figure showing a geologic map of the area <br />encompassing the West Pit facilities at the San Luis Project. The map was <br />reproduced from a Colorado School of Mines Ph.D. dissertation that was <br />completed in 1997 by Robert Benson in the Departrnent of Geology. The <br />geologic map serves as the primary basis for the observation that historical water <br />quality data measured at the upgradient surface water monitoring station RS-1 <br />would not reasonably reflect the naturally occurring hydrologic and geologic <br />processes that ultimately result in ambient water quality conditions in the reach of <br />the Rito Seco that could be influenced by the dischazge of waters associated with <br />water treatment activities in the West Pit. <br />As shown in the figure, the primary geologic formation in the vicinity of the RS-1 <br />location is the Precambrian bedrock. As a result, the geomorphologic chazacter of <br />the Rito Seco at the surface water station RS-1 is best described as a bedrock <br />controlled reach with only a thin veneer of unconsolidated alluvial material <br />overlying the Precambrian bedrock channel. In addition, the valley sidewalls are <br />steep resulting in a more V-shaped valley that is chazacteristic of river channels <br />that have formed in weathering resistant, competent bedrock. <br />In contrast to the geologic and geomorphologic conditions present at surface <br />water station RS-1, the region of the Rito Seco in the vicinity of the West Pit is in <br />a completely different geomorphologic regimen that is more chazacteristic of <br />alluvial channels. In the vicinity of the West Pit, and at the outfall of the water <br />treatment facility, the Rito Seco is deeply incised into unconsolidated alluvial <br />material that is comprised of weathered remnants of the underlying Santa Fe <br />Formation and Precambrian bedrock. As such, the naturally occurring hydrologic <br />and geochemical processes influencing the baseline water quality of the Rito <br />Seco, for purposes of establishing system-based effluent standazds, aze more <br />appropriately evaluated using historical water quality data obtained from the <br />Ba!(le ,Mountain Resources, lnc. HSI GeoTrans <br />17 July 11, 1000 <br />