Geochemical composition of sandstone samples from drill cores of the former uranium ore mine Königstein (Saxony) and the determination of hydrochemical equilibrium concentrations of uranium by means of batch tests

Geochemical background values are commonly used for the authorization of remediation measures. However, in the case of the former Königstein uranium ore mine, located in Saxony (Germany), natural uranium concentrations in groundwater cannot be directly assessed, due to active mining activities. The Königstein uranium ore depositis located within the 4th aquifer and consists of the Oberhäslich Formation and 'Wurm'-Sandstone.

This study aimed at deriving natural uranium content using batch shake test, assuming chemical equilibrium with regard to the speciation of uranium in solution and the binding to the rock matrix. For that purpose, representative subsamples from core material were taken for batch experiments and geochemical analyses. As uranium solubility strongly dependents on the redox state, the pH value, and the hydrochemistry of the target fluid, experiments were performed under various conditions.

The rock samples were analyzed with respect to geochemical and mineralogical compositions, while uranium concentration in fluid samples was measured using inductively coupled plasma - mass spectrometry. Concentrations of major cations were analyzed using cation chromatography techniques. The PHREEQC software was used to analyses the species distribution of uranium under the hydrochemical conditions of the the unaffected inflowing water from the inflow area.

Results show, that the sandstones in the middle of the 4th aquifer consist mainly of SiO₂ with more than 98 wt.-%. Two-layer clay minerals and iron oxides were identified in another sample with fractions more than 50 wt.-% kaolinite. Uranium was found in the anaerobic zone in one rock sample at 1820 ppm and once at only 25.6 ppm.

Equilibrium modeling revealed, that at a pH of 5.5 to 6 and under oxidizing conditions, uranium mainly occurs as UO₂²⁺, UO₂OH⁺, and UO₂CO₃. It can therefore be expected that the uranium species present as cations will likely be adsorbed by the solid matrix. Initial shaking tests showed that combining reduced rock materials with oxidizing water led to excessive uranium fractionation into the fluid phase. Batch tests using deionised water showed uranium concentrations between 2 and 2.5 mg/l, contrasting expectation of natural uranium concentrations in the lower µg/l range.