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1.0 INTRODUCTION <br />1.1 Project Background <br />The San Luis Project is an inactive open -pit gold mine located in Costilla County in south central <br />Colorado, approximately 3.5 miles northeast of the town of San Luis (Figure 1.1). The mine <br />facilities included two adjacent open pits, a milling facility, a lined tailing facility, a pipeline <br />from the milling facility to the tailing facility, a water treatment plant, and six waste rock <br />disposal areas. Active mining, milling and tailing disposal occurred between 1991 and 1996. <br />The mine currently is inactive and the pits have been backfilled. However, management of <br />groundwater collected at the mine site is on-going. The tailing facility continues to be used for <br />storage and evaporation of groundwater and brine from the water treatment process which are <br />delivered via pipeline from the mine facilities. <br />The San Luis Project tailing impoundment is situated in an ephemeral drainage on the west - <br />facing foothills of the Sangre de Cristo Mountains. The facility comprises the following main <br />features: <br />• A 155 -feet high, 1,900 -feet long compacted embankment dam constructed on a north <br />south alignment across the valley. <br />• A single-geomembrane-lined, approximately 125 -acre tailing disposal area, with sub <br />drain water collection system. <br />• A storm -water run-on interception and diversion system comprising perimeter ditches <br />and berms partially surrounding the tailing impoundment area and sized to collect and <br />convey storm water up to a 100 -year, 24-hour event from the upland contributing <br />drainage areas. A large portion of the intercepted run-on is diverted over the left <br />abutment of the dam via a concrete drop structure and 48 -inch diameter corrugated metal <br />pipe (CMP). <br />Figure 1.1 Site Location Map <br />San Luis Project — South Diversion Ditch Drop Structure — Final Design Report <br />