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Executive Summary <br />San Luis Project Tailing Dam — Initial Detailed Dam Safety Evaluation <br />This report documents the initial detailed dam safety evaluation of the San Luis Tailing Dam, <br />located in Costilla County, near San Luis, Colorado. The 155 ft high, 1900 ft long compacted <br />earth dam and 125 -acre geomembrane -lined impoundment area stores mine tailing from an <br />inactive gold mine. Tailing deposition at the facility has not occurred since the mid — 1990s. <br />The facility currently is used to dispose of water from various sources including groundwater <br />that is pumped from a backfilled mine pit and brine fluid which is a byproduct of on -going water <br />management and treatment at the nearby mine site. These waters are delivered to the tailing <br />facility via a pipeline, and are disposed of by evaporation and evapotranspiration from the <br />surface of the tailing. Water within the impoundment area that is not removed by evaporation <br />and evapotranspiration infiltrates the tailing, is captured on the geomembrane liner and is <br />conveyed via a sloping granular blanket drain and network of drainage pipes under the dam to <br />outfalls at the downstream toe of the dam. The collected water is delivered via a lined channel <br />into a lined collection pond below the dam, and then returned (pumped back) up to the <br />impoundment area. <br />The dam does not have a spillway or outlet works. Storm water is diverted around the perimeter <br />of the impoundment via ditches and berms. The storm water run -on diversion system was <br />originally designed to handle a 100 -year storm event. Most of the watershed is captured and <br />diverted from the south side of the facility along a primary ditch referred to as the South <br />Diversion Ditch. Flows in the South Diversion Ditch are routed over the left (south) abutment of <br />the dam via a concrete and 48 -inch diameter corrugated metal pipe (CMP) structure that is <br />referred to as the Drop Structure. For storms larger (more rare) than a 100 -year event, up to and <br />including a probable maximum flood (PMF) event on the contributing watershed, the design <br />envisioned that the perimeter diversion ditches and berms would spill over and allow excess <br />water to enter the impoundment. There is sufficient volume available behind the dam and on top <br />of the tailing deposits to completely contain the entire PMF storm runoff without overtopping the <br />dam. The primary focuses of this dam safety study were: <br />1. To evaluate the structural stability of the dam. <br />2. To evaluate the current condition of the dam. <br />3. To evaluate the anticipated hydraulic performance of the storm -water diversion systems <br />during major hydrologic events. <br />Based on site inspections conducted in 2013, review of available historic construction and design <br />records for the facility, and additional engineering analyses performed as part of this evaluation, <br />the San Luis Tailing Facility dam is considered to be in good condition and the embankment is <br />determined to be structurally stable in accordance with accepted engineering standards (factors <br />of safety) under static and earthquake loading. The dam also is deemed to be within acceptable <br />ES -1 <br />