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Open AccessArticle

Hydrologic and Water Quality Modeling of the Pebble Mine Project Pit Lake and Downstream Environment after Mine Closure

1
Buka Environmental, Boulder, CO 80302, USA
2
Integrated Hydro, Fort Collins, CO 80525, USA
3
Lynker Technologies, Boulder, CO 80301, USA
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(8), 727; https://doi.org/10.3390/min10080727
Received: 30 June 2020 / Revised: 6 August 2020 / Accepted: 11 August 2020 / Published: 18 August 2020
(This article belongs to the Special Issue Environmental Geochemistry of Mineral Deposits)
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. View Full-Text
Keywords: pit lake; geochemical modeling; hydrologic modeling; acid mine drainage; ecological services; post-closure; failure modes; mine water management; perpetual treatment pit lake; geochemical modeling; hydrologic modeling; acid mine drainage; ecological services; post-closure; failure modes; mine water management; perpetual treatment
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MDPI and ACS Style

Maest, A.; Prucha, R.; Wobus, C. Hydrologic and Water Quality Modeling of the Pebble Mine Project Pit Lake and Downstream Environment after Mine Closure. Minerals 2020, 10, 727. https://doi.org/10.3390/min10080727

AMA Style

Maest A, Prucha R, Wobus C. Hydrologic and Water Quality Modeling of the Pebble Mine Project Pit Lake and Downstream Environment after Mine Closure. Minerals. 2020; 10(8):727. https://doi.org/10.3390/min10080727

Chicago/Turabian Style

Maest, Ann; Prucha, Robert; Wobus, Cameron. 2020. "Hydrologic and Water Quality Modeling of the Pebble Mine Project Pit Lake and Downstream Environment after Mine Closure" Minerals 10, no. 8: 727. https://doi.org/10.3390/min10080727

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