minerals Review Geochemical Stability of Oil Sands Tailings in Mine Closure Landforms Heidi L. Cossey 1, Anya E. Batycky 1 , Heather Kaminsky 2 and Ania C. Ulrich 1,* 1 Department of Civil & Environmental Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada; [email protected] (H.L.C.); [email protected] (A.E.B.) 2 Centre for Oil Sands Sustainability, Northern Alberta Institute of Technology (NAIT), Edmonton, AB T5G 0Y2, Canada; [email protected] * Correspondence: [email protected]; Tel.: +1-780-492-8293 Abstract: Oil sands surface mining in Alberta has generated over a billion cubic metres of waste, known as tailings, consisting of sands, silts, clays, and process-affected water that contains toxic organic compounds and chemical constituents. All of these tailings will eventually be reclaimed and integrated into one of two types of mine closure landforms: end pit lakes (EPLs) or terrestrial landforms with a wetland feature. In EPLs, tailings deposits are capped with several metres of water while in terrestrial landforms, tailings are capped with solid materials, such as sand or overburden. Because tailings landforms are relatively new, past research has heavily focused on the geotechnical and biogeochemical characteristics of tailings in temporary storage ponds, referred to as tailings ponds. As such, the geochemical stability of tailings landforms remains largely unknown. This review discusses five mechanisms of geochemical change expected in tailings landforms: consolidation, chemical mass loading via pore water fluxes, biogeochemical cycling, polymer degradation, and surface water and groundwater interactions. Key considerations and knowledge gaps with regard Citation: Cossey, H.L.; Batycky, A.E.; Kaminsky, H.; Ulrich, A.C. to the long-term geochemical stability of tailings landforms are identified, including salt fluxes Geochemical Stability of Oil Sands and subsequent water quality, bioremediation and biogenic greenhouse gas emissions, and the Tailings in Mine Closure Landforms. biogeochemical implications of various tailings treatment methods meant to improve geotechnical Minerals 2021, 11, 830. properties of tailings, such as flocculant (polyacrylamide) and coagulant (gypsum) addition. https://doi.org/10.3390/ min11080830 Keywords: reclamation; end pit lake; consolidation; biogeochemistry; methanogenesis; sulfur cycling; polyacrylamide; salt flux Academic Editors: Benoît Plante, Thomas Pabst and David Wilson Received: 18 June 2021 1. Introduction to Oil Sands Tailings Accepted: 28 July 2021 Published: 30 July 2021 Alberta, Canada is home to the third largest oil reserve in the world [1]. Most of Alberta’s oil is unconventional oil because it is trapped within oil sands and cannot be Publisher’s Note: MDPI stays neutral extracted using the natural pressure differential created by drilling an oil well. Oil sands with regard to jurisdictional claims in consist of sand, silt, clay, water, and a heavy oil referred to as bitumen. Surface mining published maps and institutional affil- (ex situ recovery) is used to extract oil sands reserves with less than 75 m of overburden, iations. while deeper reserves must be extracted using in situ recovery methods (not discussed here) [1,2]. Oil sands exist beneath approximately 142,200 km2 of land in Alberta and are situated in three distinct regions: Peace River, Athabasca, and Cold Lake [1]. The Athabasca region, situated in northeastern Alberta, is the only region in Alberta in which oil sands ore can be extracted through surface mining. Surface mining is advantageous where possible Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. because it results in significantly greater bitumen recovery than in situ recovery methods [3]. This article is an open access article However, surface mining also requires more land disturbance and results in large volumes distributed under the terms and of waste, known as tailings, which consist of sand, silt, clay, oil sands process-affected conditions of the Creative Commons water (OSPW), and unrecovered bitumen. Surface mining in the Athabasca oil sands has 2 3 Attribution (CC BY) license (https:// disturbed approximately 895 km of land and created over 1.3 billion m of tailings [1,4]. creativecommons.org/licenses/by/ Tailings are temporarily stored above ground in dams referred to as tailings ponds but 4.0/). eventually must be reclaimed and integrated into mine closure landforms. Minerals 2021, 11, 830. https://doi.org/10.3390/min11080830 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 830 2 of 39 Alberta’s oil sands operate under a zero-effluent discharge policy [5]. Approximately 80 to 85% of OSPW generated during surface mining can be recovered and recycled back into the extraction process [6]. The remainder of the OSPW cannot be recovered because it is trapped between mineral grains in the tailings and in its place, fresh water from the Athabasca River is used in the extraction process. The water holding capacity of tailings contributes to a number of tailings management and reclamation challenges, such as physical and geochemical stability, tailings impoundment volumes, and land use requirements. Some of the other key environmental issues associated with oil sands tailings include OSPW toxicity [7–9], biogenic greenhouse gas emissions, namely methane (CH4) and carbon dioxide (CO2)[10–15], and potential seepage of OSPW from tailings into underlying clay till and sand channel sediments [16]. Tailings accumulation and land use has prompted concerns from stakeholders over the permanency of tailings ponds, while toxicity and potential contamination of the surrounding environment from OSPW has raised questions over whether tailings reclamation can be achieved [17]. To date, there have been limited reclamation certificates issued for oil sands sites and none in relation to tailings. A lack of certified, reclaimed land is a representation of the time it takes for tailings to be reclaimed, and the limited knowledge that currently exists for their reclamation. The geochemical stability of tailings is intricately connected with tailings manage- ment and reclamation and is key to the long-term success of tailings landforms. Tailings geochemistry varies with each oil sands ore deposit and operator. Currently, there are four oil sands companies operating surface mines in Alberta: Suncor Energy Inc. (Sun- cor), Syncrude Canada Ltd. (Syncrude), Canadian Natural Upgrading Ltd. (CNUL) and Canadian Natural Resources Ltd. (CNRL) (CNUL and CNRL are collectively referred to as Canadian Natural), and Imperial Oil Resources Ltd. (Imperial), listed in order from oldest to newest surface mining operations. Each operator uses different tailings treatment methods and generates tailings with unique geochemical characteristics. This variability presents additional challenges to operators and regulators in predicting and assessing the long-term geochemical stability of tailings in closure landforms. Because tailings land- forms are a relatively recent development, research to date has heavily focused on the biogeochemistry and geotechnical stability of tailings in tailings ponds. As such there are significant knowledge gaps surrounding the long-term geochemical stability of tailings in closure landforms. This review focuses on five mechanisms of geochemical change that are expected to have the greatest impact on the geochemical stability of tailings in closure landforms: consolidation, chemical mass loading via pore water fluxes, biogeochemical cycling, polymer degradation, and surface water and groundwater interactions. This review identifies key considerations and knowledge gaps surrounding the geochemical stability of tailings in closure landforms to direct future research and development, reduce the long-term environmental impacts of mine closure practices, and moderate the economic and environmental liabilities associated with oil sands tailings reclamation. 1.1. Tailings Generation Bitumen is extracted from surface mined oil sands ore using the Clark Hot Water Process (HWP) patented by Dr. Karl Clark in 1929 [2]. The process has since been im- proved to achieve a recovery of greater than 90% from high grade ore containing > 10 wt% bitumen [18]. In HWP, caustic soda (sodium hydroxide, NaOH) and hot water are mixed with crushed oil sands ore, allowing bitumen to froth and float to the surface of primary and secondary separation vessels while solids settle to the bottom [19]. NaOH addition promotes bitumen recovery by increasing the pH of the crushed ore mixture and causing or- ganic acids that are naturally present in bitumen to become water-soluble surfactants [2,20]. This effectively reduces surface and interfacial tensions in the mixture and causes the ore structure to disintegrate. However, while NaOH addition is beneficial for bitumen recovery, it also produces tailings that are difficult to process. Because of the reduced interfacial tensions, NaOH addition produces water-in-bitumen emulsions and dispersion of clays [2]. The water-in-bitumen emulsions must undergo further treatment (froth treatment) to sep- Minerals 2021, 11, 830 3 of 39 arate the bitumen. The organic acids released from bitumen during the HWP are called naphthenic acid fraction compounds (NAFCs), which are a broad family of polar organic carboxylic acids, some of which are acutely and chronically toxic to a number of aquatic organisms [9,21–25]. While the bitumen froth undergoes froth treatment with a diluent to reduce bitumen viscosity and enhance separation,