REP20177 - Laserfiche WebLink

Home Browse Search

SECTIONONE 1 1 Background and Purpose The Henderson Mill tailing impoundment is an upstream method tailing dam compirised of an earthfill starter dam, foundation drain system, tailing impoundment, and a seep water collection system. Seepage from the dam and impoundment is collected by a foundation drain system that discharges to a main seep water collection canal down gradient of the starter dam. 'fhe main seep water collection canal discharges into the north and south seep water collection canals. Water in the north and south seep water collection canals flow to the Ute Park pump station, where it is pumped back to a decant pond. An alluvial channel is present beneath the starter dam. The alluvial channel is approximately 1,100 feet wide, 5 to 40 feet thick, and generally slopes northeasterly. Groundwater flow in the alluvial channel is to the northeast towazds the seep water collection canals; however, mine personnel have identified a groundwater pathway that may elude the current seep writer collection system. Elevated levels of some groundwater constituents have been measured in downgradient monitoring wells and it is believed that these levels may exceed comF~liance standazds in the future unless additional measures are implemented to control groundwater seepage. To control migration of seep water through the alluvial channel, Phelps Dodge retained Woodward-Clyde in 1996 to conduct a geotechnical investigation to evaluate the feasibility of installing a soil-bentonite (S-B) barrier wall (Woodward-Clyde, 1997). The investigation ' concluded that an S-B slurry wall constructed along the north access road and keyed into the underlying Troublesome Formation would serve as a suitable cutoff to the preferential groundwater path created by the alluvial channel. However, construction of the slurry wall ' would be a lazge, costly project and a drain system would be needed to collect groundwater that would be forced to flow up to ground surface along the slurry wall. Based on these factors, Phelps Dodge requested that URS assess the feasibility of installing a series of extraction wells along the north seep water collection canal as an alternative to the slurry wall. The purpose of the extraction wells would be to capture seep water that may be ' eluding the collection system. This Technical Memorandum describes the work perl.'ormed as part of this investigation, assesses the feasibility of the proposed extraction well system, and provides estimated costs for design, installation, and operation of the system. 1 1 ^^Y\ \W INfFlLE11E1R~Piq.1EC1516lMSWJIErOE0.5CN_MlL_FFA.'~S~OJb.O PgO.I_DEW\HENSEDTEd MpIyX1 DCQVl91>16 RA 1-1 1