Field Application of ²³⁸U/²³⁵U Measurements To Detect Reoxidation and Mobilization of U(IV)

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)