Jason M. E. Ahad

Century-Long Source Apportionment of PAHs in Athabasca Oil Sands Region Lakes Using Diagnostic Ratios and Compound-Specific Carbon Isotope Signatures

Evaluating the impact that airborne contamination associated with Athabasca oil sands (AOS) mining operations has on the surrounding boreal forest ecosystem requires a rigorous approach to source discrimination. This study presents a century-long historical record of source apportionment of polycyclic aromatic hydrocarbons (PAHs) in dated sediments from two headwater lakes located approximately 40 and 55 km east from the main area of open pit mining activities. Concentrations of the 16 Environmental Protection Agency (EPA) priority PAHs in addition to retene, dibenzothiophene (DBT), and six alkylated groups were measured, and both PAH molecular diagnostic ratios and carbon isotopic signatures (δ(13)C) of individual PAHs were used to differentiate natural from anthropogenic inputs. Although concentrations of PAHs in these lakes were low and below the Canadian Council of Ministers of the Environment (CCME) guidelines, diagnostic ratios pointed to an increasingly larger input of petroleum-derived (i.e., petrogenic) PAHs over the past 30 years concomitant with δ(13)C values progressively shifting to the value of unprocessed AOS bitumen. This petrogenic source is attributed to the deposition of bitumen in dust particles associated with wind erosion from open pit mines.

Assessing microbial uptake of petroleum hydrocarbons in groundwater systems using natural abundance radiocarbon.

Carbon sources utilized by the active microbial communities in shallow groundwater systems underlying three petroleum service stations were characterized using natural abundance radiocarbon ((14)C). Total organic carbon (TOC) Delta(14)C values ranged from -314 to -972 per thousand and petroleum-extracted residues (EXT-RES) ranged from -293 to -971 per thousand. Phospholipid fatty acids (PLFAs)-biomarkers for active microbial populations-ranged from -405 to -885 per thousand and a comparison of these values with potential carbon sources pointed to significant microbial assimilation of (14)C-free fossil carbon. The most (14)C-depleted PLFAs were found in the samples with the highest concentrations of total petroleum hydrocarbons (TPHs). A radiocarbon mass balance indicated up to 43% of the carbon in microbial PLFAs was derived from TPHs, providing direct evidence for biodegradation at two of three sites. At lower levels of TPHs Delta(14)C values of PLFAs were generally similar to or more enriched than all other carbon in the system indicating microbial utilization of a more (14)C-enriched carbon source and no resolvable evidence for microbial incorporation of petroleum-derived carbon. Results from this study suggest that it is possible to delineate petroleum biodegradation in groundwater systems using these techniques even in complex situations where there exists a wide range in the ages of natural organic matter (i.e., EXT-RES).

Source Apportionment of Background PAHs in the Peace-Athabasca Delta (Alberta, Canada) Using Molecular Level Radiocarbon Analysis

The downstream accumulation of polycyclic aromatic hydrocarbons (PAHs) in the Peace-Athabasca Delta (PAD), an ecologically important landscape, is a key issue of concern given the rapid development of the oil sands industry in Northern Alberta, Canada. In addition to PAHs derived from industrial activity (i.e., oil sands mining) within the Athabasca watershed, however, forest fires and erosion of fossil fuel deposits within both the Athabasca and Peace watersheds are two potentially important natural sources of PAHs delivered to the PAD. Consequently, evaluating the environmental impact of mining activities requires a quantitative understanding of natural, background PAHs. Here, we utilize molecular-level natural-abundance radiocarbon measurements on an amalgamated sediment record from a Peace River flood-susceptible oxbow lake in the northern Peace sector of the PAD to quantitatively discriminate sources of naturally occurring alkylated PAHs (fossil and modern biomass). A radiocarbon mass balance quantified a predominantly natural petrogenic source (93% petrogenic, 7% forest fire) for alkylated PAHs during the past ∼50 years. Additionally, a significant petrogenic component determined for retene, a compound usually considered a biomarker for softwood combustion, suggests that its use as a unique forest fire indicator may not be suitable in PAD sediments receiving Peace watershed-derived fluvial inputs.

Isotopic Evidence for Oil Sands Petroleum Coke in the Peace-Athabasca Delta.

The continued growth of mining and upgrading activities in Canadas Athabasca oil sands (AOS) region has led to concerns about emissions of contaminants such as polycyclic aromatic hydrocarbons (PAHs). Whereas a recent increase in PAH emissions has been demonstrated within around 50 km of the main center of surface mining and upgrading operations, the exact nature of the predominant source(s) and the geographical extent of the deposition are still under debate. Here, we report a century-long source apportionment of PAHs using dual (δ(2)H, δ(13)C) compound-specific isotope analysis on phenanthrene deposited in a lake from the Athabasca sector of the Peace-Athabasca Delta situated ∼150 km downstream (north) of the main center of mining operations. The isotopic signatures in the core were compared to those of the main potential sources in this region (i.e., unprocessed AOS bitumen, upgrader residual coke, forest fires, coal, gasoline and diesel soot). A significant concurrent increase (∼55.0‰) in δ(2)H and decrease (∼1.5‰) in δ(13)C of phenanthrene over the last three decades pointed to an increasingly greater component of petcoke-derived PAHs. This study is the first to quantify long-range (i.e., >100 km) transport of a previously under-considered anthropogenic PAH source in the AOS region.

Carbon isotope effects associated with Fenton-like degradation of toluene: Potential for differentiation of abiotic and biotic degradation

Hydrogen peroxide (H(2)O(2))-mediated oxygenation to enhance subsurface aerobic biodegradation is a frequently employed remediation technique. However, it may be unclear whether observed organic contaminant mass loss is caused by biodegradation or chemical oxidation via hydroxyl radicals generated during catalyzed Fenton-like reactions. Compound-specific carbon isotope analysis has the potential to discriminate between these processes. Here we report laboratory experiments demonstrating no significant carbon isotope fractionation during Fenton-like hydroxyl radical oxidation of toluene. This implies that observation of significant isotopic fractionation of toluene at a site undergoing H(2)O(2)-mediated remediation would provide direct evidence of biodegradation. We applied this approach at a field site that had undergone 27 months of H(2)O(2)-mediated subsurface oxygenation. Despite substantial decreases (>68%) in groundwater toluene concentrations carbon isotope signatures of toluene (delta(13)C(tol)) showed no significant variation (mean=-27.5+/-0.3 per thousand, n=13) over a range of concentrations from 11.1 to 669.0 mg L(-1). Given that aerobic degradation by ring attack has also been shown to result in no significant isotopic fractionation during degradation, at this site we were unable to discern the mechanism of degradation. However, such differentiation is possible at sites where aerobic degradation by methyl group attack results in significant isotopic fractionation.

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).

Evaluating in situ biodegradation of 13C-labelled naphthenic acids in groundwater near oil sands tailings ponds

Potential seepage of naphthenic acids (NAs) from tailings ponds into surface water and groundwater is one of the main environmental concerns associated with the Canadian Athabasca oil sands mining operations. Here we report the application of 13C-labelled NA surrogate compounds to evaluate intrinsic biodegradation along groundwater flow-paths originating from oil sands tailings ponds at two different sites: a glacio-fluvial aquifer (Site 1) and a low-lying wetland (Site 2). Microcosms containing the carboxyl group labelled (99%) NA surrogates (cyclohexanecarboxylic acid, CHCA; 1,2-cyclohexanedicarboxylic acid, CHDCA; 1-adamantanecarboxylic acid, ACA) were lowered into monitoring wells for several months to allow sufficient time for substrate degradation and formation of a biofilm in conditions characteristic of the local aquifer. Phospholipid fatty acids (PLFAs), biomarkers for the active microbial population, were extracted from the biofilms for stable carbon isotope (δ13C) analysis. At Site 1, highly 13C-enriched δ13C values (up to ~+7100‰) confirmed the in situ microbial breakdown of CHCA and CHDCA. At Site 2, δ13C-PLFA values from -60.6 to -24.5‰ indicated uptake of a 13C-depleted substrate such as biogenic methane and not 13C-labelled ACA. Determination of the microbial community using 16s RNA sequencing confirmed the presence of methane-oxidizing bacteria in the subsurface at Site 2. The in situ biodegradation of NAs at Site 1 demonstrates that the indigenous microbial population in the shallow subsurface near tailings ponds can readily break down some of these compounds prior to surface water discharge. The lack of evidence for microbial uptake of 13C-labelled ACA at Site 2 demonstrates that other NAs, in particular tricyclic diamondoid acids, may persist in the environment following seepage from tailings ponds or natural sources.