Impacts des activités minières d’amiante sur l’évolution du lac à la Truite d’Irlande, région de Thetford Mines (Québec, Canada) Impacts of asbestos mining activities on the evolution of Lac à la Truite d’Irlande, Thetford Mines region (Quebec, Canada)

Abstract

Lac à la Truite (Trout Lake) from the municipality of Irlande (southern Quebec, Canada) suffers from eutrophication and siltation. More than 100 years of mining activities in the Thetford Mines region (1877–2011 CE) and the accumulation of gigantic mining waste piles have often been blamed for these problems, but their real impacts have never been demonstrated. Paleolimnological analyses were undertaken on a sediment core collected in Lac à la Truite to provide insights into its long-term evolution since pre-mining times. Results indicate that its degradation was greatly enhanced by the drainage of upstream Lac Noir (Black Lake) from 1955 to 1959 CE for mining purposes. Radiometric dating (137Cs, 210Pb, 14C) showed that this event caused a strong increase in sediment inputs, which afterwards remained high (0.6–1.3 cm yr−1; 0.4–1.0 g cm−2 yr−1). Loss-on-ignition and spectrometry (ICP-AES/ICP-MS) analyses revealed that sediments deposited since 1960 CE mostly originate from the mining waste piles as they are organic-poor and highly enriched in metals associated with asbestos tailings (magnesium, chromium, nickel). During the drainage of Lac Noir, the emergence of diatom taxa typical of nutrient-rich waters in the sedimentary assemblages (e.g., Cyclotella meneghiniana, Cyclostephanos invisitatus) and the concomitant shift of biochemical proxies (C/N, δ13C) indicate that the disappearance of this water body has also caused the rapid eutrophication of Lac à la Truite by facilitating municipal wastewater inputs. Moreover, the pH and conductivity of the lake water are now considerably higher than during pre-mining times. Pollution generated by the mining waste piles and wastewater must be rapidly controlled to ensure the sustainability of Lac à la Truite.