Ms Helena Lidhage, Mr Charles Masala, Mr Michael Dabiri
The Premier Gold Mine, located near Stewart, British Columbia, ceased mining operations in 1998. Since then, closure activities have included stabilization and revegetation of waste rock dumps, treatment of discharge from the underground mine, and upgrades to the Tailings Storage Facility (TSF). This paper focuses on implementation of a long-term water management strategy for the TSF and related components, in a wet environmental, receiving an average of 2,200 mm of annual precipitation. Additional consideration is given to treatment of underground mine discharges and management of the resulting water treatment sludge. Site-specific challenges include ML/ARD from contemporary and historic mining activities on the site dating back to the 1800s, PAG tailings and waste rock, a wet environment subject to extreme storms and high peak flows, in addition to topographic constraints, scarcity of suitable construction materials, and geohazard risks associated with steep, rocky terrain. Sources of ML/ARD for the site were characterized through site-wide hydrologic and water quality modelling assessments, which were used to identify and quantify sources of loading from historic and contemporary mining activities, and assess the impact of proposed water management plans and external influences such as upstream hydroelectric developments, and climate change. Measures implemented to mitigate ML/ARD risks include design and implementation of water and granular covers to prevent oxidation and mobilization of tailings within the TSF, reconstruction of water treatment sludge clarification ponds and spillways to reduce the risk of failure, and development of a long-term water treatment sludge management plan that accounts for the possibility of perpetual water treatment from underground workings. Flood management measures implemented include design and construction of a TSF closure spillway in bedrock, to safely route peak PMF flows in excess of 500 m3/s without mobilizing impounded tailings, as well as upgrades to the tailings dams to reduce the risk of external and internal erosion, such as a granular tailings beach cover, additional riprap protection and a downstream inverted filter berm. Much of the site is characterized by high geohazard risks due to steep, rocky terrain and frequent rockfall. The water management plan was designed to eliminate reliance on diversion channels and access roads that are subject to risk of failure due to rockfall, and to facilitate safe monitoring and sampling in key discharge areas. This includes construction of an upgraded TSF seepage monitoring system allowing safe measurement and sampling of seepage, and scaling and ongoing monitoring of high-risk rock surfaces. Geohazards, steep topography and scarcity of suitable granular materials introduced additional challenges to the design and construction of water management infrastructure. However, the development and implementation of water management solutions of Premier Gold Mine achieved the desired outcomes, by meeting challenges with suitable solutions.