Following four decades of mineral exploration, discovery, project development and minerals processing, the Ranger Uranium Mine (RUM) at Jabiru, Northern Territory, will cease operations in 2021. Beyond this period, rehabilitation of the Ranger Project Area (RPA) will continue into 2026. As a part of the rehabilitation of RUM, water quality guideline values (WQGV) for uranium and other metals in local waterbodies have been developed to protect aquatic organisms from the effects of metal toxicity. Benthic organisms will also be exposed to uranium as it partitions from the water column to the sediments. This important component of the ecosystem may then be exposed to increased uranium concentrations through ingestion and adsorption of uranium from sediments or the pore-waters.
Increased concentrations of uranium in the water column may result in an accumulation of uranium in the sediment component. A study requested by the Environmental Research Institute of the Supervising Scientist (ERISS) developed a uranium partitioning relationship to enable prediction of sediment concentrations when water column concentrations were at the WQGV. The effect of uranium counter ions and dissolved organic carbon on uranium speciation was modelled; and demonstrated a high affinity for the formation of uranyl-organic complexes. Using a Freundlich isotherm, if the water-column uranium concentration was at the WQGV of 2.8 µg/L, then the acid extractable uranium in sediment was calculated to be lower than a proposed sediment quality guideline value (SQGV). This offers assurance that implementation of a 2.8 µg U/L WQGV will not lead to an accumulation of uranium to a level that will affect benthic communities.