Mining operations at Giant and Con mines (Northwest Territories, Canada) resulted in the release of >20,000 tonnes of arsenic trioxide (As2O3) into the atmosphere, mainly during the 1950s, which were deposited on the surrounding landscape. Studies of arsenic concentrations in lake water and sediment have concluded that no potential ecosystem health effects exist beyond a 40-km radius of the mines. However, paleolimnological studies at distances well beyond 100-km have identified elevated arsenic concentrations aligning with the timing of peak emissions. To improve characterization of the legacy footprint of emissions, spatiotemporal patterns of metal deposition were reconstructed from the analysis of sediment cores at lakes located 10-40 km (near-field) and 50-80 km (far-field) along the prevailing northwesterly wind direction (NW) and 20-40 km to the northeast (NE). Results based on concentrations of mining-associated metal(loids) (arsenic, antimony, lead) in radiometrically-dated (210Pb, 137Cs) sediment cores, enrichment factors, and total excess inventories for arsenic and antimony assert that deposition of these pollutants was greatest closest to the mines and along the prevailing wind direction (NW). Enrichment is evident as far as 80-km to the NW (considerable for arsenic; severe for antimony) and 40-km to the NE (considerable for arsenic; severe for antimony) suggesting pollution from the mines likely travelled distances beyond those explored here. Additionally, the presence of elevated metal concentrations in uppermost sediment strata at near-field lakes suggest that deposition of anthropogenic-sourced metals from lake catchments remains ongoing. Differences in the degree of enrichment and stratigraphic profiles among lake groups are likely due to availability of catchment-sourced legacy metals and post-depositional mobilization from stores in lake sediment. Long-term sources of legacy metals in the near-field environment urge further research on metal mobilization linkages between terrestrial and aquatic ecosystems.

Potential for downstream delivery of contaminants via Athabasca River floodwaters to lakes of the Peace-Athabasca Delta (PAD), northeastern Alberta (58.6°N, 111.8°W), has raised local to international concern. Prior investigations have shown metals concentrations in sediment of lakes supplied by river floodwaters are not enriched above pre-industrial baselines. Additional real-time aquatic ecosystem monitoring approaches are needed to complement sediment-based techniques where time intervals captured are uncertain. Here, we quantify enrichment of eight metals (Be, Cd, Cr, Cu, Ni, Pb, V, Zn) at the base of aquatic food webs, relative to sediment-based pre-industrial baselines, via analysis of biofilm-sediment mixtures accrued on artificial substrate samplers deployed during summers of 2017 and 2018 in >40 lakes spanning hydrological gradients of the PAD. Widespread flooding in spring 2018 allows for assessment of metals enrichment by Athabasca River floodwaters. A main finding is that river floodwaters are not implicated as a pathway of metals enrichment to biofilm-sediment mixtures in PAD lakes from upstream sources. MANOVA tests revealed no significant difference in residual concentrations of all eight metals in lakes that did not flood versus lakes that flooded during one or both study years. Also, no enrichment was detected for concentrations of biologically inert metals (Be, Cr, Pb), and those related to oil-sands development (Ni, V). Enrichment of Cd, Cu, and Zn at non-flooded lakes, however, suggests uptake of biologically active metals complicates comparisons of organic-rich biofilm-sediment mixtures to sediment-derived baselines for these metals. Results lend confidence that this novel approach could be adopted for lake monitoring within the Wood Buffalo National Park Action Plan.