Recent scientific literature underlines the consensus between oil sands stakeholders regarding the need to separate mining-related and naturally occurring contaminants in the Lower Athabasca region where mining operations and geological units can both release contaminants to groundwater and to the river. This report discusses two new isotopic approaches developed to make such discrimination for organic contaminants and metals.
This test project investigates a groundwater flow system adjacent to a tailings pond operation located about 3 km from the Athabasca River. Its aims are to inform environmental management decisions by specifically: (i) developing new methods for distinguishing natural and mining- related inputs of organic acids and metals in groundwater; and (ii) estimating, at the local scale, the loads of mining-related and natural contaminants in groundwater within the selected glacio- fluvial aquifer system.
A two dimensional conceptual model was developed to simulate the groundwater flow and mass transport based upon two flow-parallel alignments of wells between the tailings pond and the Athabasca River. Two novel isotopic fingerprinting methods were developed for discriminating dissolved constituents and specifically identifying the sources of “naphthenic acids” (NAs) present in groundwater, and of lead (Pb) and zinc (Zn) in various phases of the local aquifer system. Acid extractable organics (AEO) containing classically defined NAs represent a diverse class of organic compounds that are very difficult to characterize chemically. These acids are naturally present in bitumen and become concentrated in oil sands tailings pond water. Here the new method of intramolecular carbon isotopic (δ13C) analysis of carboxyl (-COOH) groups is developed to distinguish mining-related contaminants from natural background organic acids. Importantly, prior to their isotopic analysis, the process water AEO were isolated from different groups of interfering compounds present in natural organic matter. Similarly, the development of the novel lead (Pb) and zinc (Zn) isotopic methods for partial leach protocols and their application to the oil sand extraction context were accomplished in order to discriminate natural from mining-related metal inputs in groundwater and surface water. In addition, reactive thermodynamic modeling was also done using major ion hydrogeochemistry, petrographic observations and X-ray spectral analysis of aquifer material.
Results and Interpretations
The hydrogeological modeling indicates that if mining-related dissolved constituents transiting through the most conductive zone of the surficial aquifer were currently transported from the tailings pond to a modeled discharge zone, the transit would require approximately 19 years. If transiting outside the preferential flow path, the time required would be between 156 and 230 years. These estimates suggest that mining-related AEO may be reaching the river, but only in very small amounts (non-detectable). Furthermore, the metal isotopes and major ion concentrations indicate that mining-related metals are attenuated along the groundwater flow path, with practically no load being delivered to the Athabasca River, at the scale of this test project.
Local Test Study Distinguishes Natural From Anthropogenic Groundwater Contaminants i
Mining-derived AEO concentrations between 7.3 and 14.1 mg/L in groundwater at approximately 1.6 km down-gradient from the edge of the tailings pond are relatively high compared to the ambient level of less than 1 mg/L for groundwater in the glacio-fluvial aquifer of the studied area. This observation suggests that, on a local scale, groundwater contamination with mining-related AEO may be an issue. The proportions of mining-related AEO in groundwater as determined by high-resolution mass spectrometry and intramolecular C analyses both show strong decreasing trends with distance from the tailings pond. These trends are coherent with the 14C content in the bulk AEO which suggests that the organic acids contain a significant proportion of modern carbon (i.e., natural, non-bitumen origin) in the furthest wells of the studied alignments. Note also that the river AEO load is dominated by the natural type. These trends in the water samples clearly reflect the mixing of fossil (radio-carbon dead), mining- related AEO with younger, natural AEO.
Major ion and trace-metal distribution, petrographic observations and thermodynamic modeling indicate that metal concentrations in groundwater show a strong attenuation after exiting the tailings pond. In particular, modeling suggests that the groundwater is supersaturated with respect to iron and manganese oxyhydroxides. The main consequences of this saturation are: 1) trace metals are adsorbed onto solids and thus are sequestered out of groundwater; and 2) a pool of metals has possibly accumulated in the aquifer between the tailings pond and the first monitoring wells. Most importantly, distinctive Pb and Zn isotopic ranges exist for McMurray Formation and Athabasca River samples, indicating that these isotopes distinguish mining-related from natural Pb and Zn. The Pb isotopic signatures used to estimate preliminary Pb proportions in the tailings pond from the two largest components mixed during industrial processing thus suggest that 60 to 70% comes from McMurray Formation, and 30 to 40%, from the Athabasca River water.