Mining Impacts · impact
Pit lakes and perpetual treatment
Water · After closure
- Category
- Water
- Timescale
- After closure
Mitigation
Financial assurance sized to perpetual care, passive treatment wetlands where the load allows, and backfilling or flooding designed to limit oxidation from the start.
Case
The Berkeley Pit at Butte, Montana has been filling since pumping stopped in 1982; its water is acidic and metal-laden, and treatment is expected to continue indefinitely.
Mechanism
When pumping stops, groundwater returns and fills the void. If the walls are sulfide-bearing, the lake that forms is acidic and metal-rich, and it will keep forming for as long as water flows — which means the treatment plant becomes a permanent installation and the liability outlives the company that created it.
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