THE ROLE OF CHEMICAL REDUCTION IN REDUCING U CONCENTRATIONS IN NATURAL GROUNDWATERS: A CASE STUDY FROM INDIA

For redox active elements, stable isotope ratios are a powerful tool for identifying if chemical reduction affects the amount of these elements in natural waters. Because elevated U concentrations are toxic, assessing long term immobility due to reduction relative to transient immobility is important. Previous work has shown that measurement of ²³⁸U/²³⁵U (δ²³⁸U) provides a method that discriminates reduction from sorption. Experiments and natural observations agree that as U concentration decrease by reduction in waters, the δ²³⁸U of the water decreases (Bopp et al., EST, 2010). Correspondingly, formation of reduced UO₂ produces a solid with isotopically heavier U.

Coyte et al. (2018) studied 324 natural waters from India examining U concentration, ²³⁴U/²³⁸U activity ratios, as well as a variety of geochemical parameters showing large variations in U concentrations, likely reflecting changes in U solubility derived from factors including uranium complexation with carbonate and varying redox conditions. Here, we present new δ²³⁸U data for 18 of these samples to examine if evidence of U reduction can be discerned. Samples were analyzed by a NuPlasma inductively coupled plasma mass spectrometer (MC-ICPMS) using the double spike technique. Spiking the samples with a ²³³U-²³⁶U tracer allows correction for the instrumental isotope mass bias. Results are reported relative to NIST 112A standard with standards IRMM18a and NIST 129A used for QA. As U concentration range between 22.7 to 319.7 ppb, δ²³⁸U varies from -0.71 to -0.09 per mil. Four replicates were processed with an average difference in δ²³⁸U of 0.03 per mil. Notably, a few of the lowest U concentration waters do have low δ²³⁸U, consistent with reduction, but most data are scattered around a value of -0.20, close to the value of the bulk earth. The relationship between uranium concentration and δ²³⁸U is a promising indicator that low U concentrations in the Indian groundwater are directly associated to U immobilized by chemical reduction.