Biogeochemistry of meromictic pit lakes and permeable reactive barriers at the Cluff Lake uranium mine

  • Author / Creator
    Von Gunten, Konstantin
  • 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. 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.

  • Subjects / Keywords
  • Graduation date
    Fall 2019
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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