This Internet case study is copyrighted (©) 1999 by Kevin A. Morin and Nora M. Hutt.
Click here for a PDF version of this MDAG.com Internet Case Study 14
1. INTRODUCTION
The Island Copper Minesite is located on the northern end of Vancouver Island, in the Canadian Province of British Columbia. The layout and drainage chemistry of this site are described in the March 1999 Internet Case Study entitled Onset of Acidic Drainage from a Mine-Rock Pile, and are depicted in Figures 1 and 2. The minesite lies along the north side of Rupert Inlet, which is connected to the Pacific Ocean through narrows and straits.
Click on Figures 1 and 2 to Enlarge Them.
(Enlargements are stored at another location, so please wait after clicking)
During operation, the on-land dumps to the north, west, and east of the pit drained through ditches to a treatment pond to the south of the pit (Figure 1). Pit water was also pumped to this pond. After batch treatment, the water was released to Rupert Inlet.
The Beach Dump extends into Rupert Inlet and was separated from the adjacent pit by a cutoff wall that minimized groundwater flow and marine seepage into the pit. This dump was found not to be a significant source of acidity and aqueous metals. As a result, the two major closure activities for it were to move low-grade ore from it into the mill at the end of operation, and to excavate small bays into its shoreline to enhance colonization of marine life (Figure 3). The colonization occurred rapidly within two years after mine closure.
Click on Figure 3 to Enlarge It
The closure of Island Copper was directed towards low-maintenance solutions for acidic drainage, with low risks of failure and low environmental consequences upon failure. Because predictions indicated acidic drainage from the on-land dumps and pit walls would persist for many decades to millennia, long-term water treatment was not the preferred option.
The selected closure plan involved the flooding of the pit for technical and aesthetic reasons. This would also greatly reduce the area of pit wall exposed to air and actively generating acidity and releasing metals. As well, the Northwest Dump was pushed into the pit, although some of the waste rock is still exposed on the upper pit wall (Figure 4).
Click on Figure 4 to Enlarge It
Pit flooding involved excavating a trench through the Beach Dump and then breaching the cutoff wall (Figures 5 and 6), which had to be timed to avoid drawing many fish into the pit. Once the pit was filled, the trench was partially refilled, isolating the pit water from Rupert Inlet.
Click on Figures 5 and 6 to Enlarge Them
As time has passed, a thin fresh-water lens has developed above the salt water in the pit. This water is derived from rainfall and seeps into the Beach Dump.
The deeper, salt water is relatively stagnant and the intent is to develop extremely reducing conditions with active sulphide production near the bottom to cause metals to precipitate as metal-sulphide minerals.
To eliminate the need for long-term water treatment, the acidic drainage from the on-land dumps is directed into a collection pond (Figure 7) and is then fed into a pipeline system (Figure 8) that carries the acidic water deep into the water column. This water is then diluted into the salt water. This dilution will manage water chemistry for many decades to millennia and, if strong sulphide production begins, this is expected to lower aqueous metal concentrations further.
Click on Figures 7 and 8 to Enlarge Them
Monitoring of the water column continues and shows that the acidic drainage is diluted in the saltwater column and that some sulphide production is beginning. As a result, Island Copper has implemented a low-maintenance closure plan for control of acidic drainage and protection of the adjacent environment (Figure 9).
Click on Figure 9 to Enlarge It
© 1999 Kevin A. Morin and Nora M. Hutt
For more case studies, see Environmental Geochemistry of Minesite
Drainage: Practical Theory and Case Studies.
Created by K.Morin