British Columbia Mine Reclamation Symposium

Investigations of metal mobility and bioavailability in spill-affected lakes Stecko, Pierre; Batchelar, Katharina; Hughes, Colleen; Anglin, Lyn


A foundational failure of the perimeter embankment of the Mount Polley Tailings Storage Facility (TSF) on August 4th 2014 resulted in a breach that released approximately 25 million cubic meters of debris (water and solids that consisted of tailings, construction materials, and scoured sediment and soil). The debris flowed into Polley Lake, along the length of Hazeltine Creek, and into Quesnel Lake. Sediment quality investigations indicated that sediments within the debris path contained concentrations of copper that were consistently greater than baseline and/or reference concentrations and British Columbia’s upper working sediment quality guideline, concentrations of arsenic and iron that were greater than baseline and/or reference concentrations and British Columbia’s lower working sediment quality guidelines, and concentrations of manganese that were greater than reference concentrations (in affected areas of Quesnel Lake but not Polley Lake) and British Columbia’s lower working sediment quality guideline. All of the elevated metals have been previously observed at concentrations greater than working sediment quality guidelines at un-impacted areas of the Polley Lake and Quesnel Lake watersheds. A number of lines of investigation were carried out to characterize the mobility and potential bioavailability of metals within debris-impacted lake sediments, including selective chemical extractions, characterization of metal binding constituents (e.g., organic matter and volatile sulphides), application of diffusive gradients in thin films (DGT) devices, sediment toxicity testing, and benthic invertebrate community monitoring. Selective chemical extractions indicated that concentrations of the metals most elevated in the debris path were predominantly present in non-mobile forms and that concentrations in forms that are considered to be potentially mobile and/or bioavailable (exchangeable, carbonate, and easily reducible forms) were generally below lower working sediment quality guidelines. For copper, this finding was aligned with its known presence in residual alumino-silicate minerals as well as with years of geochemical test work conducted on Mount Polley tailings. DGT devices, which measure free and labile metals, indicated low free and labile concentrations that have been stable and, for copper, were much lower than those associated with effects observed elsewhere. Standard sediment toxicity tests of Chironomus dilutus and Hyalella azteca documented no unacceptable responses to survival or growth of either organism in Polley Lake. Sediment toxicity test results for Quesnel Lake were complicated by the very low organic carbon content of debris-impacted sediments and thus observed survival and growth responses appeared to be at least partly associated with adverse physical conditions. Benthic invertebrate community monitoring has documented recovery in Polley Lake and in the breach-influenced area of Quesnel Lake, as apparent in endpoints of density (organisms per square meter), taxon richness (the number of different organisms present), and taxon composition. In Polley Lake, recovery of these endpoints was to conditions similar to pre-breach and reference. Recoveries were supported by sediment transplant experiments in deep depositional areas of Quesnel Lake (cross-over design) which indicated that debris-impacted sediments could be readily colonized when moved to reference areas. Although the lines of evidence were not completely aligned, overall integration of the weight of evidence collected from 2014 to 2017 suggests limited mobility and bioavailability of metals in debris-impacted sediments of Polley and Quesnel lakes.

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