Abstract
The Pebble Project in Alaska is one of the world’s largest undeveloped copper deposits. The Environmental Impact Statement (EIS) proposes a 20-year open-pit extraction, sulfide flotation, and deposition of separated pyritic tailings and potentially acid-generating waste rock in the pit at closure. The pit will require perpetual pump and treat management. We conducted geochemical and integrated groundwater–surface water modeling and streamflow mixing calculations to examine alternative conceptual models and future mine abandonment leading to failure of the water management scheme 100 years after mine closure. Using EIS source water chemistry and volumes and assuming a well-mixed pit lake, PHREEQC modeling predicts an acidic (pH 3.5) pit lake with elevated copper concentrations (130 mg/L) under post-closure conditions. The results are similar to water quality in the Berkeley Pit in Montana, USA, another porphyry copper deposit pit lake in rocks with low neutralization potential. Integrated groundwater–surface water modeling using MIKE SHE examined the effects of the failure mode for the proposed 20-year and reasonably foreseeable 78-year expansion. Simulations predict that if pumping fails, the 20-year pit lake will irreversibly overtop within 3 to 4 years and mix with the South Fork Koktuli River, which contains salmon spawning and rearing habitat. The 78-year pit lake overtops more rapidly, within 1 year, and discharges into Upper Talarik Creek. Mixing calculations for the 20-year pit show that this spillover would lead to exceedances of Alaska’s copper surface water criteria in the river by a factor of 500–1000 times at 35 miles downstream. The combined modeling efforts show the importance of examining long-term failure modes, especially in areas with high potential impacts to stream ecological services.
Highlights
Characterization and modeling of water-related environmental changes at mine sites involve several earth sciences disciplines, including petrology, mineralogy, groundwater and surface water hydrology/hydrogeology, meteorology, and geochemistry (e.g., [1])
We examined the effects of an alternative pit lake conceptual model, which allows mixing, and a reasonably foreseeable water management failure scenario for the Pebble Mine Project in Alaska, USA
We modeled distributed weather inputs, the post-closure hourly dynamics of the pit lake flows and levels, and the surrounding coupled hydrologic system using the fully integrated, physically based code MIKE SHE
Summary
Characterization and modeling of water-related environmental changes at mine sites involve several earth sciences disciplines, including petrology, mineralogy, groundwater and surface water hydrology/hydrogeology, meteorology, and geochemistry (e.g., [1]). Conceptual hydrogeologic models for mines must include mine-related sources, pathways for movement of mine-influenced water, and potentially affected receptors. The elements of a conceptual site model can change throughout a mine’s life. Mine-related sources could expand and even disappear as a mine progresses from construction through operations to closure and post-closure. Physical and biogeochemical processes might be excluded due to a lack of information or investigation.
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