Restoration Insights Gained from a Field Deployment of Dithionite and Acetate at a Uranium In Situ Recovery Mine

Abstract

Mining uranium by in situ recovery (ISR) typically involves injecting an oxidant and a complexing agent to mobilize and extract uranium in a saturated ore zone. This strategy involves less infrastructure and invasive techniques than traditional mining, but ISR often results in persistently elevated concentrations of U and other contaminants of concern in groundwater after mining. These concentrations may remain elevated for an extended period without remediation. Here, we describe a field experiment at an ISR facility in which both a chemical reductant (sodium dithionite) and a biostimulant (sodium acetate) were sequentially introduced into a previously mined ore zone in an attempt to establish reducing geochemical conditions that, in principle, should decrease and stabilize aqueous U concentrations. While several lines of evidence indicated that reducing conditions were established, U concentrations did not decrease, and in fact increased after the amendment deployments. We discuss likely reasons for this behavior, and we also discuss how the results provide insights into improvements that could be made to the restoration process to benefit from the seemingly detrimental behavior.

Multi‐Isotope Based Identification and Quantification of Oxygen Consuming Processes in Uranium Hosting Aquifers With CO2 + O2 In Situ Leaching

“…Therefore, δ 34 S SO4 and δ 18 O SO4 in GC groundwater were both inherited from rainwater and modified by bacterial sulfate reduction. However, SO 4 2− reduction would be inhibited under strongly oxic conditions in production wells (Reimus et al., 2022), which limited the contribution of sulfate reduction to positive δ 34 S and δ 18 O in both M2 and M1. Lower δ 34 S SO4 and δ 18 O SO4 in production wells would be caused by pyrite oxidation in the aquifer.…”

Geoenvironmental Model for Roll-Type Uranium Deposits in the Texas Gulf Coast

“…Experimental work has also shown that reductants downgradient from mining areas may help attenuate elevated U concentrations [142]. Ultimately, the success of restoration is subject to the geochemical constraints at each site combined with the choice of restoration techniques [143], which vary from site to site, and even possibly vary within different production areas at the same mines.…”

Natural Attenuation of Groundwater Uranium in Post-Neutral-Mining Sites Evidenced from Multiple Isotopes and Dissolved Organic Matter

“…Although neutral in situ leaching (ISL) is an economically advantageous mining method for extracting low-grade sandstone uranium (U), , restoration of affected groundwaters to premining (baseline) water quality is subject to considerable uncertainty. U­(VI) can be removed from groundwater by sorption, precipitation, and reduction, which are expected to play an important role in U natural attenuation. − Reduction of U­(VI) is preferred due to its long-term stability, which converts U­(VI) to U­(IV) and precipitates from solution. , Sandstone-hosted U deposits are commonly associated with reductants (e.g., pyrite and OM). − Multiple electron donors (e.g., OM and pyrite) and biogeochemical processes (e.g., sulfate bioreduction) can promote U­(VI) reduction. , …”