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• • <br />SCOPE OF WORK: 1 -DAM WELLFIELD FOR GROUNDWATER <br />CAPTURE PROJECT <br />PROJECT BACKGROUND <br />Seepage from the Henderson Mill tailings dam is collected by a foundation drain system that <br />discharges to a main seep water collection canal downgradient of the starter dam. The main seep <br />water collection canal discharges into the north and south seep water collection canals. Water in <br />the north and south seep water collection canals flow to the Ute Park pump station, where it is <br />pumped back to a decant pond. <br />An alluvial channel is present beneath the starter dam. The alluvial channel is approximatel:' <br />1,100 feet wide, 5 to 40 feet thick, and generally slopes northeasterly. Groundwater flow in tho <br />alluvial channel is to the northeast towards the seep water collection canals; however, mill <br />personnel have identified a groundwater pathway that may elude the current seep wale- <br />collection system. Elevated levels of some groundwater constituents have been measured in <br />downgradient monitoring wells and it is believed that these levels may exceed complianc <br />standards in the future unless additional measures are implemented to control groundwater <br />seepage. <br />URS conducted a feasibility study of the groundwater extraction well system in the Fall of 2000 <br />(URS, 2001). The investigation consisted of installing an extraction well and two observatio <br />wells, conducting aquifer tests, and preparing a groundwater flow model for the site. Results of <br />the investigation indicate that it would be feasible to capture tailings - affected groundwater <br />seepage by installing and operating an extraction well system. Groundwater flog modelin g <br />suggested that an extraction well system consisting of approximately 17 wells, spaced an averag <br />of 50 feet apart, pumping at an average rate of 5 gallons per minute (gpm) would be needed t <br />capture a majority of groundwater flowing through the alluvium. <br />SCOPE OF WORK <br />The extraction well system design and construction management project will be conducted i <br />phases. Phase 1 will consist of initial design of the extraction well and discharge <br />collection/conveyance system along with construction of a portion of the system. The portion c f <br />the system constructed as part of Phase l will be operated for a period of time sufficient t <br />evaluate its effectiveness. <br />