Development of a vulnerability assessment framework to evaluate potential effects of mine water discharges from Ranger Uranium Mine, Northern Territory

Abstract

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.

Application of a vulnerability assessment framework to assess environmental risks of solutes at Ranger Mine, Northern Territory, Australia

“…The present targeted vulnerability assessment builds on the framework and findings of Richardson et al (2019) and BMT (2021). This involved the following tasks:…”

“…• Task 3: Identify ecological components to be assessed. Richardson et al (2019) and BMT (2021) defined the environmental and community values (ECVs) supported by waterbodies in the RPA and surrounds and underpinning ecological components. This considered planning documents defining ECVs of the RPA and surrounds, especially the Kakadu National Park Ramsar Site Ecological Character Description (BMT WBM 2010) and park management plans.…”

“…• Task 5: Assess vulnerability scores of ecological components. Ecological trait scores for each ecological component presented in Richardson et al (2019) were assessed to determine:…”

“…A refined list of ecological components for creek and billabong habitats in the RPA was determined based on a review of habitat preferences (BMT 2021) and workshop discussions. One ecological component identified by Richardson et al (2019) and BMT (2021) does not occur in the RPA (Yellow chat) and was removed from the list. Refer to Table 3 for a summary of ecological components assessed.…”

Multiple frameworks informing closure criteria at Ranger Mine

“…The VAF assesses: (i) exposure of ecological components based on water quality modelling and distribution of identified indicator species/communities, (ii) their direct and indirect sensitivity to contaminant exposure based on laboratory and field studies, and (iii) their capacity for recovery based on a review of the traits of ecological components. A separate paper in this series of proceedings (Richardson et al 2019) provides details on developing and applying the VAF to Ranger waterbodies. The findings provide information on the vulnerability of the important ecosystem components for water quality predicted to occur under modelled closure scenarios.…”