Geology and Geomorphology Drive Polycyclic Aromatic Compound Concentrations and Composition in Rivers Draining the Alberta Oil Sands

Large-scale open-pit bitumen mining operations in Alberta, Canada, have raised concerns about contaminant releases to downstream ecosystems and communities. Among the contaminants of concern are polycyclic aromatic compounds (PACs), a toxic group of organic pollutants prevalent at high concentrations in bitumen. Here, we quantify PAC concentrations, loads, and yields in four rivers draining watersheds impacted by mining. These rivers also actively erode and incise the bitumen-bearing McMurray Formation, which has complicated previous attempts to distinguish natural from anthropogenic inputs. We collected 998 water quality samples from locations both upstream and downstream of mining, analyzed them for a broad suite of 48 unsubstituted and alkylated PAC homologues, and compared their compositional fingerprints to potential natural and anthropogenic sources. Erosion of bituminous outcrops, rather than industrial pollution, is the main driver behind PAC input and riverine transport, as supported by (i) discrepancies in loads and yields among watersheds with varying levels of industrial development, (ii) responses to hydrologic changes with respect to the distribution of mapped outcrops, and (iii) shifts in PAC relative abundances. This study presents evidence that PAC concentrations are primarily controlled by the presence of eroding bitumen-rich outcrops in conjunction with hydrographic profiles, rather than industrial impacts.