Impacts of asbestos mining activities on lake ecosystems: insights from a multi-proxy paleolimnological investigation

The impacts of asbestos mining activities and wastes on aquatic ecosystems are generally assumed to be minimal, yet have been poorly studied. To evaluate their importance, we analyzed several sediment cores collected in the Bécancour River Basin, notably in 4 fluvial lakes located downstream from Thetford Mines (Quebec, Canada): Stater Pond, Trout Lake, Lake William, and Lake Joseph. This region has been the center of more than a century of asbestos mining activities (1877–2011 CE), which resulted in the accumulation of huge piles of wastes (tailings and waste rock) on riverbanks. Age-depth models, primarily derived from radiometric dating (¹³⁷Cs, ²¹⁰Pb, ¹⁴C), revealed extreme increases in sediment accumulation rates in Stater Pond and lakes Trout and William, corresponding with the 1955–1959 CE drainage and excavation of an upstream lake for mining purposes. This event also corresponded with their strong eutrophication, as revealed by sudden changes in diatom assemblage composition (e.g., proliferation of Cyclostephanos invisitatus/makarovae, Cyclotella meneghiniana). ICP-MS/ICP-AES analyses revealed that post-1960 sediments at Stater Pond and lakes Trout and William, which maintained very high accumulation rates, were distinctively enriched in magnesium, chromium, and nickel. This provided evidence that they are contaminated by asbestos mining wastes, hence that the piles on the riverbanks are exposed to heavy erosion. Analyses by transmission electron microscopy demonstrated that post-1960 sediments contain important asbestos fiber concentrations (up to 6.9 wt%). Evidence of asbestos mining contamination has also been found in Lake Joseph, which showed that it spreads over ≥ 25 km in the Bécancour River system. We conclude that asbestos mining activities and wastes are susceptible to cause high sediment loads, as well as metal and fiber contamination in aquatic ecosystems (among other undesirable effects). Therefore, more efforts should be invested in the restoration of such mining sites and in controlling pollution they cause.