Location: Washington, United States
Service Offering: Engineering & Design; Environmental, Health & Safety Management
Market Sector: Mining
Client Type: Private Sector
Status: In Progress
Located near Lake Chelan in north-central Washington State, the Holden Mine was one of the largest operating copper mines in the US developed and operated by the Howe Sound Company between 1937 and 1957. Major features of the mine area cover about 125 acres and include a former Mill Building that produced over 90,000 tonnes of copper, as well as by-product zinc, silver and gold during its life. Nearly 62 miles of underground tunnels had been excavated and 7.6 million tonnes of mill tailings placed on US National Forest lands near Railroad Creek. In 1960, the Howe Sound Company transferred the patented land and unpatented mining claims to the Lutheran Bible Institute that later became the home of Holden Village Inc., a not-for-profit religious retreat that resides in the former miners’ town and hosts approximately 5,000 to 6,000 visitors each year.
The Site is one of the larger cleanup sites in the State of Washington, extending from the former mine operations along an approximately 10-mile reach of the Railroad Creek Watershed to Lake Chelan. Contamination from the mining operation results primarily from metals and depressed pH.
The overall cleanup strategy is to prevent releases of hazardous substances to surface water by containment, collection, and treatment of impacted groundwater, as well as by preventing the future release of tailings into the creek by stabilizing the slopes to withstand the maximum creditable earthquake. Drainage from the mine will be collected and treated to remove hazardous substances, then discharged into Railroad Creek. MWH provided a multi-disciplinary integrated system of mine closure components including infrastructure improvements, surface water and sediment management, landform and cover design, slope stability improvements, groundwater collection and treatment, demolition, and a restoration approach to re-establish vegetation consistent with that of the surrounding forest.
Based on MWH’s on-site pilot scale water treatment plant, the full scale 2,200 gpm mine water treatment employs a high density sludge processes that utilize hydrated lime to neutralize acidity and induce precipitation of metal oxyhydroxides. A 4,700 foot long fully penetrating slag cement bentonite barrier wall and groundwater collection was designed and installed to capture contaminated ground water. Jet grouting was used to stabilize tailings slopes to eliminate the need of a costly buttress. Railroad Creek in the project area was restored to promote fish habitat.
The client has received the American Exploration & Mining Association (AEMA) Environmental Excellence award.
See December 14th, 2015 Press Release.