Modelling of the dissolution and reprecipitation of uranium under oxidising conditions in the zone of shallow groundwater circulation

Generic hydrochemical modelling of a grantoid-groundwater system, using the Russian software "HydroGeo", has been carried out with an emphasis on simulating the accumulation of uranium in the aqueous phase. The baseline model run simulates shallow granitoid aquifers (U content 5 ppm) under conditions broadly representative of southern Norway and southwestern Siberia: i.e. temperature 10 °C, equilibrated with a soil gas partial CO₂ pressure (P_CO2, open system) of 10^-2.5 atm. and a mildly oxidising redox environment (Eh = +50 mV). Modelling indicates that aqueous uranium accumulates in parallel with total dissolved solids (or groundwater mineralisation M - regarded as an indicator of degree of hydrochemical evolution), accumulating most rapidly when M = 550-1000 mg L^-1. Accumulation slows at the onset of saturation and precipitation of secondary uranium minerals at M = c. 1000 mg L^-1 (which, under baseline modelling conditions, also corresponds approximately to calcite saturation and transition to Na-HCO₃ hydrofacies). The secondary minerals are typically "black" uranium oxides of mixed oxidation state (e.g. U₃O₇ and U₄O₉). For rock U content of 5-50 ppm, it is possible to generate a wide variety of aqueous uranium concentrations, up to a maximum of just over 1 mg L^-1, but with typical concentrations of up to 10 μg L^-1 for modest degrees of hydrochemical maturity (as indicated by M). These observations correspond extremely well with real groundwater analyses from the Altai-Sayan region of Russia and Norwegian crystalline bedrock aquifers. The timing (with respect to M) and degree of aqueous uranium accumulation are also sensitive to Eh (greater mobilisation at higher Eh), uranium content of rocks (aqueous concentration increases as rock content increases) and P_CO2 (low P_CO2 favours higher pH, rapid accumulation of aqueous U and earlier saturation with respect to uranium minerals).