The impact of a catastrophic mine tailings impoundment spill into one of North America's largest fjord lakes: Quesnel Lake, British Columbia, Canada

Abstract

On 4 August 2014, a catastrophic breach of the Mount Polley mine tailings impoundment released ~25 M m3 of tailings and water and scoured an unknown quantity of overburden into the West Basin of Quesnel Lake. We document Quesnel Lake and Quesnel River observations for 2 months postspill. Breach inflows raised Quesnel Lake by 7.7 cm, equivalent to ~21 M m3. The West Basin hypolimnion was modified immediately, exhibiting increased temperature (~5°C to 6–7.5°C), conductivity (110 to 160 μS/cm), and turbidity (<1 to 200–1000 nephelometric turbidity units (NTU)). Cooscillating seiches moved West Basin hypolimnetic water both westward and eastward contaminating the Main Basin. Postspill, high‐turbidity water propagated eastward (~1 cm/s), introducing a persistent ~20 m thick layer below the thermocline and an ~30 m thick layer at the bottom. The contaminant introduction, mobilization, and bioaccumulation may pose risks to resident and anadromous fish stocks, which support recreational, commercial, and First Nations fisheries.

Sediment-associated contaminant transport and storage dynamics in the Quesnel River from the Mount Polley Mine disaster

“…While Q2 and Q3 have lower concentrations than Q1, the Cariboo River may still have an impact on the trace element concentrations at these downstream sites, though it is not relevant to this study. suggest that the lake mixing was delivering the fine sediment, a combination of supplied tailings and scoured sediment, that was trapped below the thermocline (Petticrew et al 2015) into the Quesnel River. Fall overturn maxima were also observed for As, Fe, P, V and Zn with increases in trace element concentrations in November-January 2015.…”

“…There were four observed temporal patterns for the nine trace elements as indicated in section 3.1.3, and of those As, Cu, Fe, P, V and Zn at the headwaters of the Quesnel River concentration is presumed to be due to the delivery of the large mass of contaminated sediments stored in the hypolimnion of Quesnel Lake following the breach. As a consequence of the breach, fine sediment was delivered into Quesnel Lake and these fines remained suspended in the deeper (>30 m), colder and denser waters of the hypolimnion (Petticrew et al 2015). Summer stratification prevented mixing of these materials into the upper warmer waters (epilimnion), but as the water column of the lake cooled with decreasing autumn air temperatures, the lake's mixed layer became deeper, and entrained fine sediment from below 30 m and discharged it into the Quesnel River.…”

“…decreasing particle size) during the late fall and winter seasons was again most likely due to the physical limnology of Quesnel Lake acting to deliver fine particles of breach material both suspended in the water column and re-suspended from the breach lake deposit during lake mixing. Petticrew et al (2015) indicate that the material suspended in the lake water column post-breach had a D50 = ca. 1 m. The delivery of these fine particles to the Quesnel River as the mixing depth of the lake increased was also supported by data produced by Imperial Metals, showing the rapid increase of river turbidity close to Q1…”

“…The readily bioavailable fractions are considered to be the exchangeable and carbonate fractions with the reducible fraction being potentially bioavailable with changing redox conditions. Due to the high costs associated with the (Petticrew et al 2015), it is estimated that 18.5 billion kg of tailings was released or 24 million kg of Cu, with 4.08 million kg of Cu that is considered bioavailable in the tailings material alone. For the samples analyzed by sequential extraction, total Cu was much higher in the mine tailings (>1300 ppm) and…”

“…The West Basin of Quesnel Lake experiences two isothermal periods which enables overturn events, one during winter (beginning in early January) and the other in the spring (beginning in mid to late April; Petticrew et al 2015).…”