Field Application of 238 U/ 235 U 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 ²³⁸ U/ ²³⁵ U ratio of aqueous U as an independent, reliable tracer of U(IV) remobilization via oxidation or mobilization of colloids. Reduction of U(VI) produces ²³⁸ U-enriched U(IV), whereas remobilization of solid U(IV) should not induce isotopic fractionation. The incorporation of remobilized U(IV) with a high ²³⁸ U/ ²³⁵ U ratio into the aqueous U(VI) pool produces an increase in ²³⁸ U/ ²³⁵ U of aqueous U(VI). During several injections of nitrate to induce U(IV) oxidation, ²³⁸ U/ ²³⁵ U consistently increased, suggesting ²³⁸ U/ ²³⁵ U is broadly applicable for detecting mobilization of U(IV).