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Lakes of the Bécancour River Basin (BRB), located in the former asbestos mining region of Thetford Mines (southern Quebec, Canada), are heavily exposed to pollution. A previous paleolimnological investigation revealed that Trout Lake underwent severe eutrophication following the drainage of former Black Lake for mining purposes in the 1950s. Questions remained regarding the causes and level of degradation of other BRB lakes located upstream (Lake Bécancour) and further downstream (lakes William and Joseph), which also suffer from eutrophication. This paper presents diatom analyses performed on sediment cores from these lakes, along with complementary carbon (C) and nitrogen (N) data, that fill this knowledge gap. It also provides new insights into the natural limnological history of the Thetford Mines region. Low planktonic to benthic diatom ratios in oldest Lake Bécancour sediments revealed lower water levels during the Holocene Thermal Maximum (ca. 9000–5500 cal years BP). They later increased while diatom-inferred water pH decreased in response to cooler and wetter climatic conditions. Lake William responded more strongly than other BRB lakes to anthropogenic settlement activities in its watershed, with the emergence of Aulacoseira subarctica marking its transition to a mesotrophic state at about 1885 CE. Similar to Trout Lake, the drainage of Black Lake further enhanced the eutrophication of lakes William and Joseph, as illustrated by increases in sediment δ13C, decreases in C/N ratios, and proliferation of nutrient-dependent species such as Cyclostephanos invisitatus/makarovae. Our study highlights long-lasting and dramatic effects of such hydrological interventions on multi-lake systems. [ABSTRACT FROM AUTHOR]

The Bécancour River in southern Quebec (Canada) has evolved under the influence of the long asbestos mining history of the Thetford Mines region (1877–2011 Common Era; CE). This river originates at the outflow of Lake Bécancour, flows through the old mining sites, and then widens to successively form Stater Pond, Trout Lake, Lake William, and Lake Joseph (25 km from the closest mine). Our objective was to assess asbestos fiber contamination in dated sediment cores retrieved at these sites by means of transmission electron microscopy. Chrysotile was the most abundant form of asbestos identified in the samples. Asbestiform actinolite and tremolite were also found. Our results revealed that precolonial sediments deposited at 4 out of the 5 study sites naturally contained asbestos fibers (up to 3.5 wt%), with mass accumulation rates historically reaching up to 1.0 mg cm−2 yr−1. However, fiber concentrations were greatly enhanced in the anthropogenic horizon of all sediment cores. The highest concentrations (up to 6.9 wt%) were found in layers from Stater Pond and Trout Lake deposited during the maximum of regional mining activities (1945‒1980 CE). Likewise, reconstructed mass accumulation rates indicated that asbestos fiber influx peaked (up to 38.6 mg cm−2 yr−1) during this epoch in all water bodies located downstream from Thetford Mines at magnitudes consistent with their proximity to the mining sites. They decreased after 1980 EC (≤ 24.4 mg cm−2 yr−1) following the decline of regional mining activities and improvement of air quality. Modern sediments of all study sites (including upstream Lake Bécancour) were nevertheless still highly enriched in asbestos fibers (up to 4.4 wt%), which can be attributed to intensive mining waste erosion in the lake catchments. Our study highlights that asbestos fiber contamination is important and widespread in the Bécancour River basin, which raises human and ecosystem health concerns.