Processes Controlling Groundwater Quality in Uranium In-Situ Recovery (ISR) Mining

Science Center Objects

The project's primary objective is to evaluate the reducing capacity of an aquifer down-gradient of a roll-front ore zone to assess the mobility of uranium and other associated elements (e.g. arsenic, selenium, molybdenum, and sulfur). Assessing the reducing capacity of the aquifer requires characterization of the mineralogy, geochemistry, and microbiology and their variation across the aquifer.

Science Issue and Relevance

In-situ recovery (ISR) mining methods offer advantages over conventional mining in minimal surface disturbance, and lack of tailings piles and milling facilities. ISR methods generally have a lower surface impact than conventional methods; however, uncertainties exist regarding the aquifer and geochemical conditions that complicate and/or assist mining and groundwater remediation. To protect potential use of down-gradient water resources, more information is needed regarding the long-term and down-gradient effects of ISR mining and restoration on groundwater quality and the biogeochemical processes that control these effects. Down gradient processes in the reduced portion of the aquifer have been identified as an important understudied aspect of ISR mining and are one of the major concerns in mine permitting and aquifer quality. Understanding the mineralogy and geochemistry down gradient is important from the perspective of the mining company as far as the extent of post-mining remediation necessary and is also important from a groundwater quality perspective if the reducing capacity is overestimated.

U ISR process diagram

In-Situ Uranium Recovery Process. Infographic from 2018-2019 Information Digest, NUREG 1350, Volume 30. Published in August 2018. (Public domain.)

Methodology to Address Issue

Characterization of Core Material from the Down Gradient, Reduced Zone: We will obtain a fresh core sample down gradient from the ore zone and conduct analyses to characterize the sample by conducting bulk major and trace element chemistry, mineralogical analyses, DNA sequencing to determine miocrobial populations, and analysis to understand potential abiotic (not derived from living organisms) redox (chemical reduction-oxidation reaction) controlling phases.

Chemical Extractions and Batch Experiments of Core Material: We will perform chemical extractions to determine the relative abundance of uranium and other co-occurring elements of potential concern (e.g. selenium and vanadium) in specific mineral phases (carbonate/bicarbonate, extractable iron (II) and (III) in (oxyhydr)oxide, acid volatile sulfides, and organic carbon). To further evaluate the mechanisms affecting the aquifer's reducing capacity, we will conduct batch and/or column experiments using variations of reduced and oxidized waters both with and without uranium (VI) additions. The resulting solutions will be analyzed for major and trace elements, carbon content, redox speciation (e.g. iron and sulfur), and isotopic analysis (sulfur, oxygen, carbon, ±uranium). A key component to understanding the reducing capacity will be identifying the mineral phases that immobilize uranium introduced in batch experiments and understanding the stability of those phases, so we will analyze solid material remaining after batch experiments.

Geochemical Modeling: Using the results from the core characterization, chemical extractions, and batch experiments, we will construct geochemical models to assess the reducing capacity of the aquifer down-gradient from the mining zone. A better understanding of the association of uranium (and other elements) with specific mineral phases in the reduced zone will help refine geochemical models and provide insight into the effects of aquifer heterogeneities on the reducing capacity down gradient from ISR mining sites.

Pilot Project Year: A one year scoping pilot was conducted during 2015. Our objectives during this pilot phase were to identify process-oriented research needed to increase the understanding of geochemical and microbiological processes controlling the long-term effects of in-situ recovery (ISR) / in-situ leach (ISL) uranium mining and restoration techniques on groundwater quality. We conducted a literature review of sandstone-hosted uranium deposits and in-situ recovery (ISR) mining to compile information where long-term effects to groundwater quality have been documented, and we analyzed the information to identify knowledge and data gaps in geochemical and microbiological process that may be responsible for observed effects. We also conducted a review of existing ISR mine sites to select potential study sites where acquisition of fresh aquifer sediment is possible and adequate supporting data exist or are available through cooperation with a mining company. These efforts resulted in our current project activities.

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