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INTRODUCTION <br />Tailing Dam No. 3 is the smaller, south impoundment at the Henderson tailing <br />facilities. When it was commissioned in 1976 seepage from the dam was intercepted <br />and pumped into the main tailing water pool. The original pump system was replaced in <br />2000 by a series of earthen diversion structures and the gravity-flow No. 3 Dam Seep <br />Water Pipeline, as an Environmental Protection Facility, to increase the effectiveness <br />and reliability of collection. <br />One subsurface seepage pathway at the extreme south end of the dam was not <br />identified nor intercepted by those diversion structures. The seepage emanating from <br />that pathway flows at only about 5 gallons per minute, but contains concentrations of <br />iron and manganese and is acidic with a pH of less than 4.0. Characterization of the <br />seepage 1) reveals that it originates from Dam No. 3, 2) it is likely a "perched" or near- <br />surface condition, and 3) that there is no demonstrable connection between the seepage <br />and the Williams Fork River. Collection and mitigation is relatively simple, however the <br />collected seepage must be delivered a distance of about 1,000 feet at a very flat grade <br />to discharge it into the existing seep water pipe. <br />Steffens & Associates, Inc. prepared a design for the collection of the seepage <br />and for the delivery of the collected waters into the seep water pipeline. This report <br />documents the basis and rationale for that design and describes the details of <br />construction. <br />Henderson Operations, Mill Site 9/7/10 <br />No. 3 Dam Seep Mitigation, Design Report Page 1