Biogeochemical process studies on oil sand tailings used for land reclamation in Alberta, Canada

The Canadian oil sand region, located in the northern part of the province Alberta, holds the third largest oil reserves in the world with an estimated volume of 173 billion barrels of heavily biodegraded petroleum. Surface mining of oil sand deposits which are situated in low depth, has a huge impact on the surrounding environment due to the removement of vegetation, surface soils, and subsurface geological material (overburden). In 2011, an area of around 600 km2 was affected by oil sands mining. However, not only the mining process itself is an issue of growing public and environmental concern, also by-products resulting from the oil sand extraction with hot water cause additional environmental problems. The extraction water and residual oil are stored in tailings ponds, which surpass approximately 180 km2. The environmental impact of oil sand exploration and further processing is not only of growing concern regarding soil and vegetation removement but also pollution of the surrounding compartments soil, water and air, and the deterioration of the landscape and the ecosystem function are major problems. For landscape reconstructions the major solid waste or residual products of oil sand extraction, the coarse tailings sands, the mature fine tailings and the removed overburden are used. According to soil remediation standards, these reclamation sites should support a healthy plant community that will evolve towards an ecosystem comparable to that existing prior to disturbance. For a successful recultivation of the disturbed land into its natural environment soil and plant community development needs to be understood. Within the last decades especially soil structure, affected water, and plant growth and diversity have been studied for reclamation areas in Alberta. In contrast only little is known about the occurrence of residual oil-sand-derived organic matter, which is a contaminant in the soils from reclamation sites, and its possible fate with increasing age. The goal of this thesis is to understand how the initial organic matter composition of the oil sands evolves during oil sand processing and to gain information on the proportion, fate and type of oil-sand-derived organic matter in the developing soils on reclamation sites. Additionally the reactions of plants and microbial communities in terms of composition and adaptation to oil contamination of the growth substrate used for recultivation are studied. To assess the influence of oil sand processing and the progress of reclamation, field samples consisting of original oil sand material, mature fine tailings from a tailings pond and from drying cells of different ages as well as a variety of reclamation site samples were analyzed regarding biogeochemical parameter such as biomarkers, nitrogen, sulphur and oxygen (NSO) compounds, pore water ion concentrations, cell counts and microbial community structure. Oil sand related compounds are found in all samples from each step of the process chain. By comparing selected biomarkers and biomarker ratios from the different sample types it is shown that neither heat nor biological processes in tailings ponds, drying cells or reclamation sites have altered the composition of organic matter constituents to any significant extent. This demonstrates the resistance of the analyzed biomarkers against the extraction processes and biodegradation. Contrary to the oil-related biomarkers, changes in PAH abundances and patterns in the mature fine tailings and especially in the reclamation samples are observed, indicated by a loss or depletion of naphthalenes, phenanthrenes and chrysenes. These specific oil-derived organic constituents seem to be utilized as carbon/energy sources by microorganisms in the reclamation samples, leading to a reduction of hazardous substances in the developing soils. Designated cell abundances on reclamation sites are up to ten times higher compared to the original oil sand material. Here nitrate and roots in the newly developing soil seem to be the most important stimulants for microbial growth. In addition to the oil-sand-derived hydrocarbons, heteroatom-containing compounds were analyzed by ultra-high-resolution mass spectrometry regarding process-related changes of polar organic compounds. Based on bulk compound class distributions specific compositional features were identified that are related to the different steps of the process chain. As an example oxygen containing compounds like naphthenic acids are most pronounced in the oil sand samples and decrease along the process chain, whereas saturated fatty acids are most abundant in reclamation samples and represent their increasing input into the developing soils. These fatty acids are related to a microbial and cuticular wax origin. In contrast to the O2 class the N1 class decreases from the oil sand samples to the reclamation samples. This class is dominated by pyrrolic nitrogen compounds such as carbazoles, benzocarbazoles and dibenzocarbazoles, respectively. Detailed analysis of the N1 class shows that the reclamation samples do not contain any unique N1 compound and an almost invariant double bond equivalent distribution along the process chain. This implies that all nitrogen-containing constituents must derive from the oil sand, which makes them sensitive tracers of oil-sand-derived organic material even in soils from relatively old reclamation sites. A greenhouse experiment was set up to investigate effects of the different reclamations substrates on plants and soil microbial community by characterizing the phospholipid fatty acid composition of substrate and root material. Therefore three different reclamation substrates and two different plant species (Elymus trachycaulus and Lotus corniculatus) were analyzed. The analysis of the phospholipid fatty acid profiles in the substrates shows an enrichment of Gram-negative bacteria especially in the tailings sand/mature fine tailings substrates containing residual oil-sand-derived organic matter. These bacteria seem to be capable of surviving high amounts of hydrocarbon contamination, even though oil-sand-derived organic compounds are very resistant to biodegradation and do not serve as a good carbon and/or energy source. In contrast phospholipid fatty acid data reveal that the development of fungi in the substrate is inhibited by oil-sand-derived organic matter. The two plant types respond differently to the substrates. The phospholipid fatty acid inventory indicates a significantly higher fungal biomass and a generally higher microbial biomass for all growing substrates in the Lotus corniculatus roots. These results show that not only the substrate itself but especially the plant type influences the proportion of bacteria to mycorrhizal fungi and that the plant Lotus corniculatus might be a favourable pioneering plant for oil sand reclamation sites. It can be concluded, that the establishment of reclamation sites that evolve towards an ecosystem existing prior to mining operations is very challenging. Mature fine tailings and tailings sands show only slight to no changes of oil-sand-derived hydrocarbons and NSO compounds before they are used as reclamation substrates. As soon as the tailings sands are used in landscape reconstruction, specific oil-sand-derived pollutants were degraded by an active and specified microbial community. With increasing age of the sites the oil-sand-derived pollutants decrease and the input of exogenous organic material, mainly derived from plants and trees increases, which supports the aspired ecosystem restoration. However, recalcitrant pyrrolic nitrogen compounds were found on all reclamation sites and thus serve as an efficient and sensitive tool to detect even highly diluted petroleum residues in soils or other types of contaminated environmental samples.

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