Elsevier

Water Research

Volume 100, 1 September 2016, Pages 337-347
Water Research

The symbiotic relationship of sediment and biofilm dynamics at the sediment water interface of oil sands industrial tailings ponds

https://doi.org/10.1016/j.watres.2016.05.025Get rights and content

Highlights

  • Novel erosion systems give great insight into sediment-water interface dynamics.

  • Biofilm significantly influences tailings stability of oil sands tailings material.

  • Aged tailings measured higher shear strength thus resistant to particle erosion.

  • Implications for consolidation of fresh tailings and contaminant transport.

Abstract

Within the oil sands industry, tailings ponds are used as a means of retaining tailings until a reclamation technology such as end pit lakes (EPLs) can be developed and optimized to remediate such tailings with a water cap (although dry-land strategies for tailing reclamation are also being developed). EPLs have proven successful for other mining ventures (e.g. metal rock mines) in eventually mitigating contaminant loads to receiving waters once biochemical remediation has taken place (although the duration for this to occur may be decades). While the biological interactions at the sediment water interface of tailings ponds or EPLs have been shown to control biogeochemical processes (i.e. chemical fluxes and redox profiles), these have often been limited to static microcosm conditions. Results from such experiments may not tell the whole story given that the sediment water interface often represents a dynamic environment where erosion and deposition may be occurring in association with microbial growth and decay. Mobilization of sediments and associated contaminants may therefore have a profound effect on remediation rates and, as such, may decrease the effectiveness of EPLs as viable reclamation strategies for mining industries.

Using a novel core erosion system (U-GEMS), this paper examines how the microbial community can influence sediment water interface stability and how the biofilm community may change with tailings age and after disturbance (biofilm reestablishment). Shear strength, eroded mass measurements, density gradients, high-resolution microscopy, and microbial community analyses were made on 2 different aged tailings (fresh and ∼38 years) under biotic and abiotic conditions. The same experiments were repeated as duplicates with both sets of experiments having consolidation/biostabilization periods of 21 days. Results suggest that the stability of the tailings varies between types and conditions with the fresh biotic tailings experiencing up to 75% more biostabilization than the same abiotic tailings. Further, greater microbial diversity in the aged pond could be a contributing factor to the overall increase in stability of this material over the fresh tailings source.

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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