Lawrence Berkeley National Laboratory

Biostimulation to induce reduction of soluble U(VI) to relatively immobile U(IV) is an effective strategy for decreasing aqueous U(VI) concentrations in contaminated groundwater systems. If oxidation of U(IV) occurs following the biostimulation phase, U(VI) concentrations increase, challenging the long-term effectiveness of this technique. However, detecting U(IV) oxidation through dissolved U concentrations alone can prove difficult in locations with few groundwater wells to track the addition of U to a mass of groundwater. We propose the ²³⁸U/²³⁵U ratio of aqueous U as an independent, reliable tracer of U(IV) remobilization via oxidation or mobilization of colloids. Reduction of U(VI) produces ²³⁸U-enriched U(IV), whereas remobilization of solid U(IV) should not induce isotopic fractionation. The incorporation of remobilized U(IV) with a high ²³⁸U/²³⁵U ratio into the aqueous U(VI) pool produces an increase in ²³⁸U/²³⁵U of aqueous U(VI). During several injections of nitrate to induce U(IV) oxidation, ²³⁸U/²³⁵U consistently increased, suggesting ²³⁸U/²³⁵U is broadly applicable for detecting mobilization of U(IV).