Scott McMaster

Environmental context: As a part of the rehabilitation of Ranger Uranium Mine, Northern Australia, closure criteria for water concentrations of uranium and other contaminants in local waterbodies have been developed. Increased concentrations of uranium in the water column can result in an accumulation of uranium in the sediment and be hazardous to benthic organisms. A uranium partitioning relationship was derived to predict sediment uranium concentrations from water column concentrations. After almost four decades of mineral processing, the Ranger Uranium Mine (RUM) is set to cease operations in 2021. Beyond this period, rehabilitation of the Ranger Project Area (RPA) will continue into 2026. As part of the rehabilitation of RUM, water quality guideline values (WQGVs) for U and other metals in local waterbodies have been developed to protect aquatic organisms from the effects of metal toxicity. However, benthic organisms will also be exposed to U as it partitions from the water column to the sediments. This important component of the ecosystem may then be exposed to increased U concentrations through ingestion and adsorption of U from sediments. As increased concentrations of U in the water column can result in an accumulation of U in sediment, a U partitioning relationship was derived to enable prediction of sediment U concentrations when water column concentrations were at the WQGV. In this work, the effect of U counter ions and dissolved organic carbon on uranium speciation was modelled and demonstrated a high affinity for the formation of uranyl-organic complexes. Uranium partitioning was studied using experimental U-spiked sediment data as well as water/sediment data collected from multiple sites on and adjacent to the RPA. Using a Freundlich isotherm, and a water-column U concentration of 2.8 µg L−1 (representing the WQGV), the acid-extractable U in sediment was calculated to be 48 mg kg−1, i.e. lower than the interim sediment quality guideline value (SQGV) of 94 mg kg−1. This offers assurance that a WQGV implementation of 2.8 µg L−1 U will not lead to a level of U accumulation in sediment that will impact benthic communities.