The extraction and upgrading of bitumen have been identified as sources of enhanced atmospheric deposition of pollutant elements to ecosystems in the Athabasca Oil Sands Region (AOSR) in northern Alberta, Canada. Bitumen extraction became increasingly efficient, and oil prices surged in the 1990s, resulting in rapid expansion and increased production over the last two decades. Here, we examine temporal and spatial trends in wintertime atmospheric deposition of pollutant elements in 1978, 1981, 2008, and 2011–2016 at broad spatial scales using snowpack measurements. A hybrid source analysis was conducted, including (i) simple and multiple linear regression (MLR) of identified source locations and elemental deposition, (ii) spatially resolved aluminum enrichment factors (Al EFs), and (iii) positive matrix factorization (PMF) to determine source profiles. Temporal trends revealed a general decrease in atmospheric loadings; however, near-field V, Ti, and Al loadings in 2016 were an order of magnitude greater than at reference sites in the Peace Athabasca Delta. MLRs demonstrated that the two largest bitumen producers were major contributors of key pollutant elements (e.g., V, Al, Ti, W, Ga, Fe, Be, Cs, Co, Mo, Rb, Pb, As, U, and Ba) including rare earth elements (Y, Ce, and La). Using Al EFs, fugitive (geogenic) dust was identified as the main contributor for most elements, except for V and W, which were directly introduced by industrial sources. Results strongly suggest that fugitive dusts from petcoke piles and roads are important wintertime pollution vectors in the AOSR.

Environmental contextEnvironmental quality guidelines are often based on an ‘additive’ approach using single metal toxicity values. We evaluated the ‘additive’ approach by testing it on three priority pollutant metals (Ni, Cu, Cd), and found that the toxicity of the metal mixture was less than additive when dose was expressed as total metal concentration, but it was additive when dose was expressed as bioavailable metal. We suggest that for metal mixtures, a more realistic indicator of risk is provided by calculations based on the bioavailable form of metals. AbstractAquatic environments containing elevated metal concentrations, such as natural waters receiving mining effluents, are often a mixture of metals because mineral deposits are commonly an association of multiple metals. Water quality guidelines for protection of aquatic life are not designed for multiple toxicants but are overwhelmingly based on dose–response studies of a single toxicant and assuming additivity. Resolving the uncertainty in risk assessment for metal mixtures in waters surrounding Canada’s many current and legacy extractive mining sites is a high priority for both government and base metal mining companies. Our study evaluated the ‘concentration addition’ approach to predicting the chronic toxicity of a ternary metal mixture (Ni, Cu, Cd) to Lemna minor (a free-floating macrophyte used in biomonitoring of mining effluents) using either total metal concentration as dose or free-ion activity. The aim was to fill several data gaps in mixture toxicity studies, such as: inclusion of water chemistry to calculate metal speciation, test species other than the commonly studied rainbow trout and cladocerans, and test mixture effects on chronic toxicity. Results indicate that toxicity of Ni, Cu plus Cd to L. minor was less than additive (overestimated toxicity) when expressed as total metal concentration but was additive when expressed as free ion (the bioavailable form). We suggest that applying single-element quality guidelines ‘additively’ for plants is likely to overestimate risk to the ecosystem from metal mixtures, and that the use of a concentration addition approach based on the bioavailable form of metals provides a more realistic indicator of risk.

Lemna minor, a free-floating macrophyte, is used for biomonitoring of mine effluent quality under the Metal Mining Effluent Regulations (MMER) of the Environmental Effects Monitoring (EEM) program in Canada and is known to be sensitive to trace metals commonly discharged in mine effluents such as Ni. Environment Canada's standard toxicity testing protocol recommends frond count (FC) and dry weight (DW) as the 2 required toxicity endpoints-this is similar to other major protocols such as those by the US Environmental Protection Agency (USEPA) and the Organisation for Economic Co-operation and Development (OECD)-that both require frond growth or biomass endpoints. However, we suggest that similar to terrestrial plants, average root length (RL) of aquatic plants will be an optimal and relevant endpoint. As expected, results demonstrate that RL is the ideal endpoint based on the 3 criteria: accuracy (i.e., toxicological sensitivity to contaminant), precision (i.e., lowest variance), and ecological relevance (metal mining effluents). Roots are known to play a major role in nutrient uptake in conditions of low nutrient conditions-thus having ecological relevance to freshwater from mining regions. Root length was the most sensitive and precise endpoint in this study where water chemistry varied greatly (pH and varying concentrations of Ca, Mg, Na, K, dissolved organic carbon, and an anthropogenic organic contaminant, sodium isopropyl xanthates) to match mining effluent ranges. Although frond count was a close second, dry weight proved to be an unreliable endpoint. We conclude that toxicity testing for the floating macrophyte should require average RL measurement as a primary endpoint.

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