Uranium remobilisation in anoxic deep rock-groundwater system in response to late Quaternary climate changes – Results from Forsmark, Sweden

• Published in: Chemical geology Vol. 584; pp. 120551 - 1-120551-16
• Main Authors: Suksi, Juhani, Tullborg, Eva-Lena, Pidchenko, Ivan, Krall, Lindsay, Sandström, Björn, Kaksonen, Kai, Vitova, Tonya, Kvashnina, Kristina O., Göttlicher, Jörg
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05.12.2021; Elsevier
• Subjects: Climate changes; Fracture coatings; Glaciation cycles; Groundwater; Natural Science; Naturvetenskap; Redox conditions; Sciences of the Universe; U mobility; U redistribution; U series modelling; Redox conditions; Glaciation cycles; Groundwater; Climate changes; U series modelling; U redistribution; Fracture coatings; U mobility; MIGRATION; COMPLEX; THORIUM; REDOX CONDITIONS; SPECTROSCOPY; DISEQUILIBRIUM; NATURAL URANIUM; RECOIL; DECAY SERIES
• ISSN: 0009-2541; 1872-6836; 1872-6836
• DOI: 10.1016/j.chemgeo.2021.120551

Unusually high uranium (U) concentrations (up to 175 μg/L) have been measured in groundwater at depths between 400 and 650 m at the Forsmark site, eastern Sweden. Since it is unlikely that such high concentrations formed under the stagnant and low redox groundwater conditions that currently prevail, this study employs U-series isotopes to understand how the recent evolution (<1 Ma) of the flow system has influenced the observed U distribution. Material from fractures as deep as 700 m along the assumed flow route was subject to U-series disequilibrium (USD) measurements, as well as sequential extractions (SE) and U redox-state analyses that revealed the U-series activity ratios in the bulk and soluble fraction of the fracture precipitates. Uranium isotope data collected over several years of annual groundwater monitoring were scrutinized to evaluate the U sources and U exchange in fractures located in high-U groundwater sections. Numerical simulations with the experimental data were used to study evolution of U-series isotope composition in a fracture in the highest U section at ~500 m depth under various U mobility scenarios. The results show that U redistribution in fractures with certain dissolution/deposition flux ratios during periodic water intrusions, driven by glaciation and deglaciation events during the last 120 ka, can explain the U anomaly in the groundwater. [Display omitted] •U mobilisation as response to late Quaternary climate changes.•Novel U-series modelling quantifies U mobility under glaciation related processes.•U(VI) redistribution explains elevated U in the deep reducing groundwater.