Evaporative emissions from tailings ponds are not likely an important source of airborne PAHs in the Athabasca oil sands region

In their paper, Parajulee and Wania (1) use a multimedia fate model to argue that emissions of polycyclic aromatic hydrocarbons (PAHs) in environmental impact assessments conducted to approve developments in the Athabasca oil sands region (AOSR) are likely underestimated. The discrepancy between their model and reported emissions was mainly attributed to indirect evaporative releases of PAHs from tailings ponds (TPs).

With the exception of naphthenic acids, dissolved concentrations of most organic contaminants in TPs and in adjacent shallow groundwater are very low (2⇓–4). Recently, Wang et al. (3) reported concentrations <0.8 µg/L for total US Environmental Protection Agency PAHs in two unspecified TPs. In another study, dissolved concentrations of phenanthrene (PHE), pyrene (PYR), and benzo(a)pyrene (BaP)—the three proxy PAHs used in the model—in two different TPs measured in summer 2011 were each <2.0 µg/L (4). These values ranged from around 10 to several hundred times lower than the simulated summer 2009 concentrations reported by Parajulee and Wania (1). A reason for the very large discrepancy between simulated and measured dissolved PAH concentrations in TPs was not provided. Clearly the model is unable to accurately simulate what has been measured in the field and underestimates the role of sorption in tailings sediments, the most likely process for removal of PAHs, even those with relatively higher K_AW values (e.g., PHE).

Using lichens as a tool for receptor modeling of air pollution in the AOSR, Studabaker et al. (5) found significant correlations between concentrations of total PAHs and metals deriving from lithogenic sources in samples located up to around 200 km away from the main area of mining activities. In conjunction with the observation that PAH concentrations drop off sharply as the distance from the mines increases, these data pointed to mine dust as an important source for airborne PAHs. Another study used diagnostic ratios and compound-specific δ¹³C signatures to delineate historical sources of PAHs in sediment cores from two lakes located 40 and 55 km east of the main area of mining operations (6). An increasingly larger input of petroleum-derived PAHs over the last 30 y was attributed to the deposition of bitumen in dust particles associated with wind erosion from open pit mines. Although Parajulee and Wania hint that dust could be an important source of atmospheric PAHs with relatively low K_AW values (e.g., BaP), it was the δ¹³C signatures of PAHs with relatively higher K_AW values (C1-fluorene and dibenzothiophene) that provided evidence for a fugitive dust origin in recent sediments (6). Emissions from upgrading facilities have also been suggested as a principal source of PAHs to the surrounding AOSR environment (7, 8). Evaluating the relative importance of these two sources should be a goal of future research.

The capacity for multimedia fate models to closely simulate the transport and fate of PAHs relies on an accurate identification of the major source inputs. With their model, Parajulee and Wania miss the significance of a previously reported important source of mining-related PAHs in the AOSR—fugitive dust—and likely overstate the importance of evaporative releases from TPs. Such an oversight does not support informed oil sands management strategies.