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Energy Resources of Australia Ltd (ERA) is planning the closure of its Ranger Uranium Mine (RUM), with decommissioning planned to conclude in 2026. One challenge for closure is understanding the potential impacts of solutes, such as magnesium, that will continue to discharge from the site via groundwater and surface water. ERA has commissioned a water quality modelling study to predict solute concentrations in the receiving environments post-closure. The authors developed a vulnerability assessment framework (VAF) to aid the interpretation of modelling results, with an initial focus on the potential effects of magnesium on environmental and cultural values (ECVs) of the mine area. This paper describes the approach used to develop the VAF and the preliminary findings of the assessment. The VAF involved the following steps: (i) identification of ECVs, including 'key species' that are important from biodiversity and cultural perspectives, as well as important habitats and other groups; (ii) development of conceptual models of key processes and linkages with ECVs; (iii) assessment of the direct (i.e. toxicity) and indirect (i.e. food resources and habitats) sensitivity of ECVs to magnesium; (iv) assessment of the adaptive capacity of ECVs. The VAF was based on multiple information sources including ecotoxicology assessments and field studies. A multidisciplinary scientific advisory committee provided expert guidance and input into the vulnerability assessments. The preliminary results of this assessment indicated that all of the key species are largely insensitive to elevated magnesium concentrations (>10 mg/L). There are several functional groups that contain species that are sensitive to magnesium (<3 mg/L), especially algae and invertebrates. The resilience of these groups is a key focus of the vulnerability assessment noting many of the species have high adaptive capacity. Key knowledge gaps were identified, and work is planned for 2020-2021 to address these gaps and refine the VAF.

The Ranger Project Area (RPA), site of Energy Resources of Australia Ltd.'s Ranger Mine, is surrounded by (but separate from) Kakadu National Park (KNP) World Heritage Place and Ramsar wetland. Closure requirements differ for on and off the RPA.The Mirarr Indigenous landowners source food and drinking water up and downstream of the mine and wish to resume these activities on the site after closure. The regulatory Environmental Requirements (ERs) specify that waters and tailings from the mine must not impact the KNP values which includes the local Indigenous culture, health of the local people and the biodiversity and ecological processes of the region. The ERs also state that impacts on the RPA must be as low as reasonably achievable (ALARA). Closure criteria for water and sediment on and off the RPA need to support these diverse values and goals.The ANZG ( 2018) water quality monitoring framework (WQMF) was used to identify indicators to represent KNP values, human health and biodiversity and derive water and sediment quality criteria to support management of these values. Risk and vulnerability assessments were used, as relevant components in the WQMF, to assess the results of sediment and water quality monitoring and predicted post-closure water quality.ALARA is widely understood and applied to radiation hazards but not chemical hazards. A fourth framework is required to provide information that will be used to assess if impacts on the RPA are ALARA. This paper demonstrates the role of these frameworks in water and sediment closure criteria development at Ranger Mine.

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Energy Resources of Australia Ltd (ERA) is planning the closure of Ranger Mine. An ecological vulnerability assessment framework (VAF) was applied to water quality modelling results to understand the potential effects of magnesium (Mg) in mine affected waters on aquatic ecosystem values within the mine site. The VAF assessed three elements: (i) understanding contaminant exposure, which was based on water modelling and species/community (ecological component) distributions; (ii) sensitivity of the ecological component to the contaminant, as determined from laboratory ecotoxicity experiments and field-based studies (monitoring, mesocosm experiments), and (iii) the recovery capacity of the ecological component, based on a review of the traits of ecological components. The VAF was applied to four waterbodies -two billabongs and two seasonal creek sites. The VAF identified that at three of the waterbodies, only the most sensitive algae and invertebrates were predicted to be intermittently affected by Mg, but would recover during periods of low Mg. All other ecological components (including other invertebrates, plants, fish and other vertebrates) at these three waterbodies had low vulnerability. Results for the fourth water body indicated that most ecological components were potentially vulnerable to predicted Mg concentrations. The assessment findings for the fourth water body had low confidence due to knowledge gaps regarding the sensitivity of some ecological components, especially aquatic plants on which most species depend. The VAF findings were applied in conjunction with an environmental risk assessment to identify where closure strategies for contaminant management needed review and inform decisions on whether impacts from those strategies would be as low as reasonably achievable (ALARA).