Year of Publication: 2014
Abstract:
Emissions of organic substances with potential toxicity to humans and the environment are a major concern surrounding the rapid industrial development in the Athabasca oil sands region (AOSR). Although concentrations of polycyclic aromatic hydrocarbons (PAHs) in some environmental samples have been reported, a comprehensive picture of organic contaminant sources, pathways, and sinks within the AOSR has yet to be elucidated. We sought to use a dynamic multimedia environmental fate model to reconcile the emissions and residue levels reported for three representative PAHs in the AOSR. Data describing emissions to air compiled from two official sources result in simulated concentrations in air, soil, water, and foliage that tend to fall close to or below the minimum measured concentrations of phenanthrene, pyrene, and benzo(a)pyrene in the environment. Accounting for evaporative emissions (e.g., from tailings pond disposal) provides a more realistic representation of PAH distribution in the AOSR. Such indirect emissions to air were found to be a greater contributor of PAHs to the AOSR atmosphere relative to reported direct emissions to air. The indirect pathway transporting uncontrolled releases of PAHs to aquatic systems via the atmosphere may be as significant a contributor of PAHs to aquatic systems as other supply pathways. Emission density estimates for the three PAHs that account for tailings pond disposal are much closer to estimated global averages than estimates based on the available emissions datasets, which fall close to the global minima. Our results highlight the need for improved accounting of PAH emissions from oil sands operations, especially in light of continued expansion of these operations.
The bitumen deposits underlying Alberta, Canada, represent the third largest proven reserve of crude oil in the world (1). It is predicted that investment in oil sands developments and its operations will contribute 2.28 trillion CAD to Canada’s gross domestic product from 2010 to 2035 (2). Presently, most of the bitumen is extracted through surface-mining processes that necessitate clearing of overlying vegetation, resulting in loss of habitat, migration corridors, and breeding grounds (3). The surface mineable deposits cover 4,800 km2 of the largest oil sands deposit, the Athabasca oil sands, and surround the region of Fort McMurray in northeastern Alberta.
During the surface mining process, bitumen is extracted from up to 100 m below the surface and is separated from other oil sands constituents using hot water (40–60 °C) and frothing processes. The residual fluids from the extraction process are transported to on-site settling basins, commonly known as “tailings ponds,” and consist of a small percentage of residual bitumen in addition to sand, clay, dissolved metals, and organic compounds, including polycyclic aromatic hydrocarbons (PAHs), 16 of which are listed as priority pollutants by the US Environmental Protection Agency (EPA). Following extraction and separation of bitumen from the oil sands, the bitumen is subjected to upgrading, which often involves high temperatures (∼460–500 °C). The leftover material from this process—known as petroleum “coke” and enriched in heavier compounds, such as heavier PAHs—may be shipped abroad or left on-site (4) (e.g., as capping over tailings areas in a recently developed reclamation strategy), and is also subject to wind erosion and transport.
The Alberta government enforces a zero-discharge policy, which resulted in the on-site storage of ∼720 million m3 of oil sands process water in 2009 (5). Despite the presence of seepage-capture facilities and interceptor ditches to limit export of process waters from tailings ponds (6), it has been estimated that seepage from various tailings areas travels to the Athabasca River at a rate of 0.0864–5.6 million L/d (7). In addition, the quantities of PAHs reported by oil sands developers in the Athabasca Oil Sands region (AOSR) to the Canadian government’s National Pollutant Release Inventory (NPRI) (8) as disposal to tailings ponds are up to five orders-of-magnitude larger than quantities reported as direct atmospheric emissions, highlighting the possibility of volatilization of PAHs from these ponds and their subsequent deposition to soils and waters. Other sources of PAHs resulting from oil sands operations include emissions from industry-associated vehicle traffic and stacks, in addition to wind erosion and transport of exposed bitumen from mine faces.
Past studies have found various aquatic species native to the AOSR suffer adverse health effects when exposed to oil sands process water and sediments produced in the region (e.g., refs. 9⇓⇓–12). The toxic nature of oil sands process waters discerned through controlled field and laboratory studies and the carcinogenic nature of some PAHs warrant concern when considered alongside the concentrated presence of PAHs in tailings ponds, and the pathways connecting these ponds to freshwater bodies in the AOSR. Recent findings by Kelly et al. (13) suggest that development in the AOSR during the 2 y preceding their study was related to increased concentrations of dissolved PAHs observed in the Athabasca River and its tributaries, and that these concentrations fell within a range “likely toxic” to fish embryos.
The impact of oil sands development on PAH cycling through the AOSR remains unclear, in part because of monitoring programs that have been deemed inadequate by various review panels (e.g., refs. 14⇓–16), and the difficulty in ascribing observed environmental residue levels to natural sources versus anthropogenic activity. However, a recent assessment of PAHs in lake-sediment cores provides compelling evidence that oil sands development has led to a significant increase in PAH levels in the AOSR environment (17). Although there have been studies providing insight into concentrations of PAHs in air (18), water (13), snow (13), sediments (10, 17, 19⇓–21), tailings pond pore water (22), and tailings pond sediments (10, 21) in the AOSR, thorough characterization of PAH sources, pathways, and sinks within the AOSR is still lacking.
Given environmental monitoring data and emissions estimates, a multimedia environmental fate model can serve as a cost-effective method to describe PAH transport and fate in a particular region, not only helping to elucidate important sources, pathways, and sinks, but also highlighting those environmental parameters and processes that require better characterization through field studies (23⇓⇓⇓⇓⇓–29). Environmental monitoring data relevant to the AOSR can be found in environmental impact assessment (EIA) reports compiled by consulting companies (30), in addition to studies reporting environmental residue levels, such as those mentioned above. Emissions estimates are available in EIA reports as well as in an online repository maintained by the NPRI. In light of the uncertainty surrounding water quality and contaminant cycling in the AOSR, the objectives of the present study are to: (i) assess whether reported emissions of PAHs in the AOSR can be reconciled with measured concentrations in air, water, soil, and foliage found in the AOSR and similar boreal environments; (ii) make an estimate of PAH emissions if reported emissions of PAHs are not found to be reasonable, in addition to first estimates of some alkylated PAHs and dibenzothiophene (DBT), considered oil sands marker compounds; and (iii) elucidate major transport pathways of PAHs in the AOSR, specifically with respect to supplying PAHs to aquatic systems.
