The Occurrences of Ca2UO2(CO3)3 Complex in Fe(II) Containing Deep Groundwater at Forsmark, Eastern Sweden☆

Abstract Elevated U concentrations, most evident in a section ∼500 mbsl, have been measured in deep Fe(II)-containing groundwater at Forsmark, eastern Sweden and have prompted detailed geochemical and isotopic investigations. The highest U contents (up to 175 μg/L) are associated with HCO 3 − of 120-135 mg/L and Ca 2+ of 900-1050 mg/L. Geochemical modelling shows that elevated dissolved U can be stabilized by Ca-uranyl-carbonate complexes. Indeed, time resolved luminescence spectrometry confirmed the Ca 2 UO 2 (CO 3 ) 3 0 complex, which is identified in deep reducing groundwater for the first time. U isotopes have been monitored in several sections with high U, and show stable but fracture specific activity ratios (ARs) around 1.5 to 2, although the U concentration varies. This is explained by mobilization of a solid phase with the same AR present in the fracture system close to the sampled sections. The AR >1 in this solid phase indicates a Quaternary age.