PNNL

Abstract

Biogeochemical redox processes that govern radionuclide mobility in sediments are highly sensitive to forcing by the water cycle. For example, episodic draining and intrusion of oxidants into reduced zones during dry seasons can create biogeochemical hot moments that change microbial activity, affect redox status of minerals, initiate changes in sediment gas and water transport, and stimulate release of organic carbon, iron, and sulfur. In the upper Colorado River Basin, water-saturation of organic-enriched sediments locally creates reducing conditions that accumulate U. Subsequently, fluctuating hydrological conditions introduce oxidants which may reach internal portions of these sediments and reverse their role to become secondary sources of U. Knowledge of the impact of hydrological variability on the alternating import and export of contaminants, including U, is required to predict contaminant mobility and short- and long-term impacts on water quality. In this study, we tracked U, Fe, and S oxidation state and speciation to characterize the variability in redox processes and related U solubility within a shallow fine-textured organic-enriched sediment at the legacy U ore processing site at Shiprock, NM. Previous studies have reported uranium speciation and behavior in permanently saturated fine-grained naturally reduced sediments. This is the first report of U behavior in fine-grained reduced sediments that are experience cyclic redox cycling due to seasonal fluctuations in moisture content. Our results support previous observations that reducing conditions are needed to accumulate U in sediments, but they dispute the expectation that U predominantly accumulates as U(IV); our data reveal that U may accumulate as U(VI) in equal proportion to U(IV). Surprisingly, high abundances of U(VI) confined in the reduced sediments suggests that redox cycling is needed to promote its accumulation. We propose a new process model, where redox oscillations driven by annual water table fluctuations, accompanied by strong evapotranspiration in low permeability sediments, promote conversion of U(IV) to relatively immobile U(VI), which suggests long-term U immobilization resistant to redox perturbations. This observation contradicts the common idea that U(IV) accumulated in reducing conditions is systematically solubilized and transported to groundwater during re-oxidation.