BIOGEOCHEMISTRY OF MEROMICTIC PIT LAKES AND PERMEABLE REACTIVE BARRIERS AT THE CLUFF LAKE URANIUM MINE by Konstantin Von Gunten A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Earth and Atmospheric Sciences University of Alberta © Konstantin Von Gunten, 2019 ABSTRACT Mining generates not only vast amounts of waste rock and tailings but is also responsible for far- reaching contamination of soil, groundwater, and surface water, which often requires remediation. This thesis focused on the biogeochemistry of two types of remediation technologies applied at the decommissioned Cluff Lake uranium (U) mine in Northern Saskatchewan. The first remediation technique is pit lakes from open-pit mining operations. Pits created by mining are left to flood with surface and groundwater to prevent excessive oxidation of exposed rocks and release of contaminants. At Cluff Lake, two such pits exist, the older D-pit and the younger DJX-pit, which are geochemically different. The pits are contaminated with U, arsenic (As), and nickel (Ni) and were previously described as meromictic. It was found that in the D-pit, meromixis stability, pH conditions, and contaminant distribution were controlled by Fe cycling. In the DJX-pit, two chemoclines were characterized, both being linked to sharp U and Ni concentration gradients. Meromixis was stabilized by calcium (Ca) carbonate dissolution and precipitation. It was found that aluminum oxyhydroxide colloids might play an important role in contaminant removal. The role of colloids in contaminant sequestration and their accumulation in sediments was further investigated. The most common colloidal particles found in the pits consisted of Ca-O, Fe-O, and Ca-S-O. A high abundance of metals was found in colloidal fractions, especially in aged samples, suggesting that colloids can act as metal accumulators. With the help of sediment traps, the precipitation of Fe-O, Fe-S, Al-Si, and Ce-P phases, with traces of U and Ni, was demonstrated. The stability of metals in bottom sediment followed the order Ni<U<As. U-bearing phases confirmed by spectroscopy and diffraction, such as vandendriesscheite and monazite, were found to increase the overall U stability. Sediment chemistry was the primary driver for microbial community composition in the sediments, with low species richness and diversity in deeper and more contaminated sediments. The meromictic pit lakes were found to be an efficient remediation method, but future development of the pits need to be monitored to assure ongoing remediation success and to prevent the release of sequestered contaminants. Abstract II The second remediation technology was a permeable reactive barrier (PRB). Two such barriers, made of peat, gravel, lime, and limestone, were installed in a mining-affected wetland. Previously installed groundwater wells allowed an in-depth analysis of groundwater passing through the first PRB. Both PRBs were effective in removing U, Cu, and Zn from groundwater but were less efficient for Ni and Co. In the alkaline environment of the PRB, significant portions of Ni, Co, Cu, U, and Fe were associated with colloids, while in the more acidic environments of the surrounding wetland, ionic species and complexes dominated. The presence of colloidal fractions favored the removal of Cu and U, which were found to more strongly bind to the solid phase, suggesting ongoing metal sequestration processes. Uranium removal was further enhanced by chemical and biological U(VI) reduction in the oxygen-depleted conditions of the PRBs. The less efficient removal of Ni and Co, being major target metals, was explained by their high solubility, their limited association with colloids, and unfavorable redox and pH conditions created by the alkaline PRBs, considerations that are critical in the design of future PRBs for the remediation of similar systems. The biogeochemical approach used in this thesis was found suitable to investigate the role of elemental cycling, contaminant mobilization, and the role of microorganisms to assess the efficiency of two common remediation techniques for mining sites in northern Canada. While meromixis was found to be effective in stabilizing contaminants in the pit lakes, any perturbation of this delicate system, such as acid generation, could compromise its performance. This study demonstrated how important it is to properly design a permeable reactive barrier. Chemical incompatibilities (e.g., inappropriate pH conditions for sulfide precipitation), unreactive carbon sources, and issues arising during the scale-up of barrier systems could lead to underperformance and ineffectiveness of the technology in removing target compounds from groundwater. The results from this work could provide an important toolset to assess the suitability of remediation techniques for similar mining sites and to evaluate potential risks with regard to changing environmental conditions, which could affect their performance. Abstract III PREFACE Since its discovery, uranium became an important resource. It developed a somewhat bad reputation in the general public due to its radioactivity, its history of being a critical component for the manufacturing of nuclear weapons and its more recent use in depleted uranium projectiles used by various militaries. It is, however, an important metal for Canada’s and world’s economy due to its indispensable use as a supplier of energy. This function generates other problems, much less known to the public, namely the contamination of the environment caused by uranium mining. As with all mining activities, when U is mined, geological material is exposed to the surface conditions, e.g., humidity, temperature changes, and physical stress, which lead to its oxidation and dissolution of metals (e.g., U, Pb, Ni, Co) and metalloids (e.g., As, Se, Sb) from the rock, leading to a phenomenon known as acid mine drainage. In such way, contaminants can reach surface- and groundwater, soil and air, with imminent contact to organisms causing toxicity effects and disrupting surface biogeochemical cycles. Where a legal framework exists and where financial means and skilled workers are available, such contamination can be mitigated with remediation (clean-up operations) and reclamation (return of the landscape to its original state) techniques, which can be very successful or sometimes fail due to lack of diligence or unexpected outcomes. This thesis deals with two types of remediation technologies and investigates associated biogeochemical cycles in a decommissioned uranium mine at Cluff Lake. The first remediation technique is pit lakes, which are created after open pit mines are flooded. The idea behind such lakes is the formation of a meromictic water body, in which the deeper water does not seasonally mix with the surface water, a feature that allows isolating deeper, contaminated water from shallow water that might come in contact with flora and fauna. The second remediation technique is permeable reactive barriers, which are generally used to treat contaminated groundwater. Reactive medium (e.g., zerovalent iron, lime, organic material) is buried in the flow path of an affected groundwater plume, so that contaminants in the water can react with it. This required the medium to be highly permeable, to not impede the groundwater flow and additionally the medium has to be properly chosen based on the contaminant. This thesis consists of four chapters. The first chapter is an introduction to uranium biogeochemistry and mining and to the field site investigated in this thesis, the Cluff Lake uranium mine. Each of the following chapters focused on a certain biogeochemical aspect of the Cluff Lake decommissioning and remediation activities. All projects were supervised by Kurt Konhauser and Daniel Alessi. The second chapter focuses on the aqueous biogeochemistry of two pit lakes at Cluff Lake. This chapter was published in the Canadian Journal of Earth Sciences, 55(5):463-474 (National Research Council Research Press) with the co-authors Tyler Warchola, Mark Donner, Manuel Cossio, Weiduo Hao, Preface IV Christopher Boothman, Jonathan Lloyd, Tariq Siddique, Camille Partin, Shannon Flynn, Arden Rosaasen, Kurt Konhauser and Daniel Alessi. Tyler Warchola performed Fe(II)/Fe(III) analyzes, Mark Donner analyzed As and Se speciation under the supervision of Tariq Siddique, Manuel Cossio assisted during field work, Weiduo Hao provided support during sample processing and assisted in gathering necessary historical information, Christopher Boothman and Jonathan Lloyd assisted with the DNA extractions from water samples, sequencing 16S rRNA genes and the bioinformatics. Camille Partin assisted in gathering necessary geological background information, Shannon Flynn assisted during ICP-MS analyzes of water samples and Arden Rosaasen provided historical monitoring records and pit lake models. The third chapter focuses on the sediments and colloids biogeochemistry in the two pit lakes at Cluff Lake. This chapter was submitted to the Geochmica et Cosmochimica Acta (Elsevier) with the co-authors Brendan Bishop, Isabel Plata-Enriquez, Samrat Alam, Peter Blanchard, Leslie Robbins, Renfei Feng, Kurt Konhauser, and Daniel Alessi. Brendan Bishop assisted during field work and sample processing, Isabel Plata-Enriquez assisted during sequential extractions of sediment samples, Samrat Alam, Peter Blanchard, Leslie Robbins, and Renfei Fend assisted during synchrotron-based analyzes. The fourth chapter focuses