Paleolimnological evaluation of metal(loid) enrichment from oil sands and gold mining operations in northwestern Canada.

Abundant reserves of metals and oil have spurred large-scale mining developments across northwestern Canada during the past 80 years. Historically, the associated emissions footprint of hazardous metal(loid)s has been difficult to identify, in part, because monitoring records are too short and sparse to have characterized their natural concentrations before mining began. Stratigraphic analysis of lake sediment cores has been employed where concerns of pollution exist to determine pre-disturbance metal(loid) concentrations and quantify the degree of enrichment since mining began. Here, we synthesize the current state of knowledge via systematic re-analysis of temporal variation in sediment metal(loid) concentrations from 51 lakes across four key regions spanning 670 km from bitumen mining in the Alberta Oil Sands Region (AOSR) to gold mining (Giant and Con mines) at Yellowknife in central Northwest Territories. Our compilation includes upland and floodplain lakes at varying distances from the mines to evaluate dispersal of pollution-indicator metal(loid)s from bitumen (vanadium and nickel) and gold mining (arsenic and antimony) via atmospheric and fluvial pathways. Results demonstrate 'severe' enrichment of vanadium and nickel at near-field sites (≤20 km) within the AOSR and 'severe' (near-field; ≤ 40 km) to 'considerable' (far-field; 40-80 km) enrichment of arsenic and antimony due to gold mining at Yellowknife via atmospheric pathways, but no evidence of enrichment of vanadium or nickel via atmospheric or fluvial pathways at the Peace-Athabasca Delta and Slave River Delta. Findings can be used by decision makers to evaluate risks associated with contaminant dispersal by the large-scale mining activities. In addition, we reflect upon methodological approaches to be considered when evaluating paleolimnological data for evidence of anthropogenic contributions to metal(loid) deposition and advocate for proactive inclusion of paleolimnology in the early design stage of environmental contaminant monitoring programs.

Quantifying arsenic post-depositional mobility in lake sediments impacted by gold ore roasting in sub-arctic Canada using inverse diagenetic modelling.

Lake sediments are widely used as environmental archives to reconstruct past changes in contaminants deposition, provided that they remain immobile after deposition. Arsenic (As) is a redox-sensitive element that may be redistributed in the sediments during early diagenesis, for instance along with iron and manganese, and thus depth profiles of As might not provide a reliable, unaltered record of past deposition. Here, we use inverse diagenetic modelling to calculate fluxes of As across the sediment-water interface and interpret As sedimentary records in eight lakes along a 80 km transect from the Giant and Con mines, Northwest Territories, Canada. The sediment cores were dated using 210Pb methods and analyzed for solid-phase and porewater As, Fe, Mn and organic C concentrations. We reconstructed the history of As deposition by correcting for the varying mobility patterns and calculated contemporary As deposition fluxes. Correction for diagenesis was substantial for three of the eight lakes, suggesting that lakes with lower sedimentation rates, which allows longer residence of As within the reactive zones defined by the model, enhance the influence of diagenesis. Results show that solid phase As peaks coincides with the period of high emissions from past gold ore roasting activities. Results also show that sediments sustained present-day As fluxes to the water column of study lakes within 50 km of the mines, while sediment in study lakes further than 50 km acted as As sinks instead.