Controls on the Immobilization of Radium and Uranium in Natural Aquifers.

Significant expense and effort has been spent to quantify the transport of radionuclides in groundwater and remediate hazardous concentrations to safe levels. Typically, these elements pose a unique hazard due to their radiotoxicity, while the differential decay rate between different isotopes serve as useful tracers for hydrologic processes. Here, we focus on two elements in the uranium series: radium, a contaminant associated with hydraulic fracturing and tracer for submarine groundwater discharge, and uranium, a persistent contaminant from nuclear energy and weapons development despite remediation efforts. Both elements sorb readily to aquifer surfaces, and can be (co)precipitated to form environmentally stable solid phases. For radium, sorption competes with co-precipitation with barium or strontium sulfate minerals, while for uranium, solids form during complexation with phosphate or reduction by microbes. Our work seeks to further geochemical understanding of these radionuclides to improve transport predictions and develop prospective methods for engineering long term in-situ immobilization of these elements. We use batch radium experiments to quantify the intricate competition between barite co-precipitation and differential sorption to common aquifer minerals, and batch uranium studies to show that combining well known sequestration strategies (i.e. trapping in silica, phosphorous precpitation) can improve uranium sequestration as compared to a single chemistry. We use this batch work to inform studies in etched silicon micromodels examining these geochemistries under flow conditions and mapping geochemical heterogeneity in-situ. The presence of heterogeneity in the pore sizes and structure particularly help interrogate the effect of tight pore spaces on uranium and radium trapping in the long term. The results of our work suggest promising methods for improved uranium remediation, and highlight the variety of processes to consider when using radium as a tracer.