Geochemical Controls on Arsenic, Uranium, and Molybdenum Mobility in a Low-Level Radioactive Waste Management Area in Ontario, Canada

Abstract

Contaminant leaching from industrial wastes can have widespread negative impacts on water quality, ecosystem processes, and human health. Low-level radioactive wastes derived from uranium (U) ore processing contain several potentially-hazardous metal(loid)s including U, arsenic (As) and molybdenum (Mo). Predicting the release, transport and attenuation of these and other contaminants is challenging due to complex geochemical behaviour impacting their mobility in soils, sediments and groundwater. The release, transport and attenuation of U, As and Mo in these systems is strongly controlled by pH and redox conditions, which influence both aqueous speciation and mineral solubility. Research was conducted to examine geochemical controls on As, U and Mo mobility within soil contaminated with low-level radioactive waste stored in Waste Management Area F (WMAF) at Chalk River Laboratories (Chalk River, Ontario, Canada). Core samples were collected as a function of depth at several locations in WMAF. Geochemical and mineralogical characteristics of these samples was examined and a subset of samples containing elevated concentrations As, U and Mo was selected for additional study. Sequential extractions and synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy identified major forms As, U and Mo present in this subset of samples. Column experiments were conducted to examine release of these elements under anoxic and oxic conditions. Batch experiments were performed to quantify potential for As, U and Mo attenuation by sediments underlying WMAF. Results demonstrated that As and U release are strongly controlled by redox conditions and Mo is strongly controlled by pH. In general, As release was greatest under anoxic conditions, while U release was greatest under oxic conditions. In contrast, elevated Mo concentration were observed in leachates generated under both anoxic and oxic conditions. The subsequent mobility of As, U and Mo was reduced via sorption to underlying sediments; however, potential exists for their transport in the groundwater flow system.