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USEPA REGION VIII -MAY 5, 2000 <br />The following responses to comments by the U.S. EPA aze indexed to the attached copy <br />of the original letter from the EPA by the roman numeral as indicated on the attached <br />copy of the letter. <br />Comment: <br />I-Water Oualiri Data in the Antideeradation Analvsis: <br />Response: <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 Department 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 character 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-l, 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 />Barlle Mounioin Resources, Inc. HS! GeoTraru <br />1 July Z!, 2000 <br />