Uranium anomaly in groundwater of the hard rock aquifer system in southeast Brazil

Abstract

This study presents a hydrogeological characterization that fills knowledge gaps in an area where a complex geological scenario connects two fractured (granite/metamorphic rocks and diabase) and one porous (sedimentary rocks) aquifer systems. Previous groundwater studies from the four local deep supply wells suggested the sedimentary aquifer as the source of uranium (∼60 μg/L) in one of the wells. Despite the water extraction interruption from this well, this finding needed to be supported by evidence provided by the geological and structural characteristics of the aquifers. Data collected through video inspection, caliper, optical (OPTV), and acoustic borehole televiewers (ATV) revealed the predominance of the crystalline aquifer in the area. As recorded by video inspection, the well's borehole with high U intersects the crystalline rocks at several open fractures and presents the highest gamma radiation values. Therefore, we infer that the origin of dissolved U is the weathering of primary minerals locally occurring in those rocks. The U leaching and complexation as calcium-uranyl-carbonates favors its solubility in that specific aqueous medium, as shown by hydrogeochemical modeling using PHREEQC. Additionally, the major ions and trace elements concentrations allow the division of the wells into two groups: a) two with higher U concentrations, influenced by the granite/metamorphic aquifer and possibly longer residence times, and b) the other two wells with lower U concentrations, higher nitrate concentrations, influenced by the diabase aquifer, possibly have younger residence times.