Evaluation of Capping Materials to Reduce Zinc Flux from Sediments in a Former Mining Pit Lake

In the Zeida abandoned mine, pit lake waters exhibit alkaline pH and high conductivity. The concentrations of the total dissolved lead and zinc are very low due to their adsorption on clay minerals and iron oxyhydroxides. Conversely, arsenic concentrations in two lakes (ZL1 and ZA) exceeded WHO water quality guidelines. The As content is relatively high in ZL1 lake and exists mainly as As(V). In ZA lake, As(III) occurs in low concentration compared to the total dissolved arsenic, while dimethylarsenic acid [H2AsO2(CH3)2, DMA) prevails. This means that arsenic was methylated by organic matter produced by microorganisms such as chlorella. The sequential extraction of floor sediments in two lakes shows that the bioavailable arsenic contents change between the two lakes. In ZA lake, the sediments show high concentrations of lead and arsenic compared to ZL1 sediment since it is surrounded by mining waste tailings, which are rich in such chemical elements. An arsenic leaching test of ZA sediment shows that bioavailable arsenic is distributed in equal proportion between clay/carbonates, sulfide-organic matter, and iron oxides (HFO) phases, while in ZL1, most of the arsenic is linked to hydrous iron oxides (HFO).Thematic collection: This article is part of the Hydrochemistry related to exploration and environmental issues collection available at: https://www.lyellcollection.org/cc/hydrochemistry-related-to-exploration-and-environmental-issuesSupplementary material:https://doi.org/10.6084/m9.figshare.c.5545316

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Realizing Beneficial End Uses from Abandoned Pit Lakes

Minerals ◽

10.3390/min10020133 ◽

2020 ◽

Vol 10 (2) ◽

pp. 133 ◽

Cited By ~ 4

Author(s):

Cherie McCullough ◽

Martin Schultze ◽

Jerry Vandenberg

Keyword(s):

Water Quality ◽

Carbon Fixation ◽

Mine Closure ◽

Pit Lake ◽

Safety Issues ◽

Pit Lakes ◽

End Use ◽

Project Closure ◽

Active Recreation ◽

End Uses

Pit lakes can represent significant liabilities at mine closure. However, depending upon certain characteristics of which water quality is key, pit lakes often also present opportunities to provide significant regional benefit and address residual closure risks of both their own and overall project closure and even offset the environmental costs of mining by creating new end uses. These opportunities are widely dependent on water quality, slope stability, and safety issues. Unfortunately, many pit lakes have continued to be abandoned without repurposing for an end use. We reviewed published pit lake repurposing case studies of abandoned mine pit lakes. Beneficial end use type and outcome varied depending upon climate and commodity, but equally important were social and political dynamics that manifest as mining company commitments or regulatory requirements. Many end uses have been realized: passive and active recreation, nature conservation, fishery and aquaculture, drinking and industrial water storage, greenhouse carbon fixation, flood protection and waterway remediation, disposal of mine and other waste, mine water treatment and containment, and education and research. Common attributes and reasons that led to successful repurposing of abandoned pit lakes as beneficial end uses are discussed. Recommendations are given for all stages of mine closure planning to prevent pit lake abandonment and to achieve successful pit lake closure with beneficial end uses.

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

Minerals ◽

10.3390/min10080727 ◽

2020 ◽

Vol 10 (8) ◽

pp. 727 ◽

Cited By ~ 1

Author(s):

Ann Maest ◽

Robert Prucha ◽

Cameron Wobus

Keyword(s):

Water Quality ◽

Surface Water ◽

Failure Modes ◽

Porphyry Copper ◽

Water Quality Modeling ◽

Open Pit ◽

Mine Closure ◽

Pit Lake ◽

Water Modeling ◽

Surface Water Modeling

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.

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