The symbiotic relationship of sediment and biofilm dynamics at the sediment water interface of oil sands industrial tailings ponds
Graphical abstract
Introduction
Extraction of bitumen from surface mined raw ore involves a caustic hot water extraction process, separating the bitumen from the clays and sands, however, small amounts of bitumen are left over in the waste stream. These fresh water extraction wastes are termed fluid fine tailings (FFT), and are stored in large settling basins called tailings ponds. The tailings ponds allow for FFT settling and consolidation and for the recycling of oil sands process-affected water (OSPW) for reuse in the extraction process. Further, there is a provincially mandated zero-discharge policy requiring containment of oil sands OSPM and FFT onsite. The consolidation and containment of various contaminants within these tailings environments is vital to the efficacy of long-term reclamation strategies. End-pit lake (EPL) and dry-land reclamation of these tailings requires the contaminants such as naphthenic acids and residual bitumen to be contained/sequestered in the FFT, and avoid remobilization due to external forces such as the addition or removal of surface waters. However to date, no research has been performed to directly examine the stability of these tailings when subjected to water turbulence/shearing forces. The vast majority of FFT experiments involve stationary, static-microcosm designs, despite the obvious dynamic nature of tailings pond environments. These tailings basins are frequently disturbed by the inflow of fresh tailings, dredging and are even subjected to wave-induced shearing. The stabilization effect of biofilms have been previously examined in other altered/disturbed freshwater sites, and has shown to be a significant and influential mechanism in reducing the ability for sediments and contaminants to become re-mobilized into the water column (e.g. Garcia-Aragon et al., 2011; Stone et al., 2011).
In the past few decades, recognizing the importance of bacterial assemblages and their interactions and effects on their surrounding environment has become significant in many scientific fields of study (e.g. Stasik et al., 2014, Chi Fru et al., 2013). One such field involves the stabilizing effect that biofilm has on aquatic sediments, causing a reduction to sediment re-suspension and the subsequent re-mobilization of contaminants (e.g., Grabowski et al., 2011, Gerbersdorf and Wierpecht, 2015). This stabilization is attributed primarily to the attachment of the microbial cells to particulate surfaces through a matrix comprised of extracellular polymeric substances (EPS) (Donlan, 2002), although consolidation (Droppo and Amos, 2001), dewatering (Tolhurst et al., 2000) and electromagnetic interactions (Mehta, 1989) between particles also have an effect on the stabilization of bed sediments. EPS structures are commonly made up of lipids, polysaccharides, and nucleic acids (Flemming and Wingender, 2010). Variability between these EPS matrices are not fully understood, but are attributed to the substrate itself, the species of microbial organism and medium in which it grows (e.g. Donlan, 2002, Donlan et al., 1994, Fletcher, 1988). Variations in microbial community structure proves interesting as specific microbial species can be responsible for measured increases or decreases in critical sediment shear strength (i.e. ability to withstand the forces causing re-suspension), due to variation of the biofilm structure (e.g. Droppo and Amos, 2001, Droppo et al., 1997; Paterson, 1997). Erosion of bed sediment only occurs once the critical shear strength of the sediment is surpassed by an external hydrodynamic or biological force (e.g. currents, tides, movement of organisms), greater than this critical shear value. Exceeding this critical shear strength causes re-suspension of particles and transportation, until these suspended solids reach a hydraulic environment where they may settle often aided by flocculation (clumping of particles in the water column).
Several studies have observed the effects referred to as “biostabilization” (Paterson, 1989) within both freshwater and marine sediments subjected to various shearing forces, noting the cohesive effects of microbial biofilms (e.g. De Boer, 1981, Grant, 1988, Noffke, 1998, Noffke and Krumbein, 1999, Friend et al., 2008, Gerbersdorf and Wierpecht, 2015). Such studies have varied from tidal flats, to inland streams and rivers, to large lakes and marine environments (e.g. Cuadrado et al., 2011, Stone et al., 2011). This increased cohesion of sediments is a significant consideration in contaminated aquatic ecosystems, in which it is advantageous to avoid the re-suspension of heavy metals and toxic substances back into the water column (e.g. Azetsu-Scott et al., 2007).
In this study, we measured the biostabilization controls of several fluid fine tailings materials. An erosion microcosm UMCES-Gust Erosion Microcosm System (U-GEMS) was used to apply a known and incrementally increasing shear stress to cores containing FFT of varying ages to assess the effect that biostabilization has on stability and in limiting re-suspension at the tailings-water interface. Further a UHS meter was used to determine density gradients below the sediment-water interface, identifying physical variations caused through biofilm establishment. Microbial community analysis within each tailings material was studied to define roles of specific classes of microorganisms in the stability and structure of these respective FFT products. Finally, scanning electron microscopy was used to identify the presence of biofilm structures formed within the investigated tailings treatments. This is the first study to investigate and measure causal differences for tailings under dynamic environments, and illustrates how hydrodynamic forces may affect the erosion and consolidation of the FFT as they are prepared for future wetland or EPL reclamation.
Section snippets
Experimental design – U-GEMS microcosm
The critical sediment shear strength of two tailings pond materials was determined using the UMCES-Gust Erosion Microcosm System (U-GEMS). The U-GEMS system measures the erodibility of two sediment cores simultaneously (see cores in section 2.2), using a range of shear forces from 0.005 Pa to 0.7 Pa. A uniform shear force was applied to the entire sediment interface through means of a rotating erosion head at the water surface and central suction (Fig. 1). The rotation of the water column
GUST microcosm shear strength
The turbidity recorded during the initial erosion experiment (21 days of settling) was clearly different between biotic and abiotic FFT for both pond materials (Note that high humic acids within abiotic P1A columns resulted in elevated NTU measurements due to their darker coloration (Fig. 3b). Such elevated initial NTU does not influence observed erosion results, comparisons and interpretations of critical shear stress and re-suspension observations). Fig. 3, Fig. 5 show the turbidity (total
Conclusions
A novel erosion system (the U-GEMS microcosm), density gradient analysis and microbial assessments/observations allowed for the investigation of the mechanisms responsible for the stabilization of FFT. They provide a means by which to compare and contrast re-suspension characteristics of various materials, including FFT from oil sands tailings ponds. These dynamic studies give greater insight into the physical, biological and chemical processes affecting the water-sediment interface within
Acknowledgements
The authors wish to thank Suncor Energy, Inc. for their support of this project. Their cooperation and active participation in the collection of oil sands process material made much of this work possible. A big thanks goes to Robert Pasuta (McMaster University Nuclear Reactor Supervisor) for providing off-site research facilities and logistical support for the GI treatments. We also wish to thank collaborative reviewers for their constructive comments that helped improve this manuscript
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