Natural uranium-limiting processes in the groundwater of the former uranium mine Königstein (Germany)

Uranium is a redox-sensitive element in the environment that interacts with numerous compounds in groundwater. Understanding uranium behavior at the groundwater–solid interface is crucial for evaluating remediation efforts, both in restoring pre-mining conditions and assessing potential anthropogenic impacts. This study aims to identify key reactions that regulate uranium concentrations in groundwater, providing plausible limits for natural uranium levels. Investigation focus on the 4th aquifer (depth of app. 200 m), mainly sandstone, located at a former uranium mining site in Königstein (Germany).

Samples from drill surrounding the deposit were analyzed though shaking tests (batch tests), aqua regia digestions, and organic carbon content determination. Furthermore, minerals were determined using thin section microscopy, and element distributions were visualised using micro-X-Ray fluorescence analysis (µXRF).

A total of 25 g of air-dry, crushed sandstone was weighed into 50 ml centrifuge tubes which were then filled with sampled groundwater from the 4th aquifer and shaken for 27 days. Element concentration, phosphate content, and carbonate hardness were subsequently analyzed using ICP-OES, ICP-MS, ion chromatography, photometry and titration methods. Batch tests and acid digestion were performed in duplicates.

The uranium concentration in the aerobic oxygenated zone in the 4th aquifer was found to be between 1.1 and 31.9 µg/l. The solid concentration of uranium in the sandstones was between 0.1 and 41 ppm. Based on the experiments most important factors associated with high uranium concentrations in the aquifer were the amount of uranium in the bedrock, the redox potential, the pH, the carbonate hardness and the dissolved reactive phosphate content. Oversaturation of some ternary uranyl phosphate minerals determined using PHREEQC and the PSI thermodynamic database (2020), could explain a limitation of the uranium concentration in solution, due to mineral precipitation. However, these minerals have not yet been identified using previous analytical methods. Further, surface complexation was not yet implemented in the modeling approaches.

With regard to the pre-mining state of uranium levels in groundwater and the influence of anthropogenic changes, solution and precipitation reactions of ternary uranyl phosphates and complexation reactions with ternary uranyl carbonates should be considered.