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Publications
Supervising Scientist, Darwin, 2004
ISBN 0 642 24391 3
ISSN 0 158-4030
This programme assesses the aquatic ecotoxicity of regionally relevant toxicants with the aim of developing or enhancing site-specific water quality guidelines. It is directly linked to the landscape analysis programme, particularly the development of a conceptual transport pathways model for Ranger uranium mine, where information is required on the toxicity, bioaccumulation, trophic transfer and risks of key metals within the aquatic pathways.
Key activities in 2003-04 included:
A conceptual model of contaminant pathways from the operational phase of Ranger uranium mine is being developed to better place the off-site contaminant issues at Ranger in a risk management context. A key underlying point in the development of the model is that the Supervising Scientist's research planning and prioritisation activities have always been underpinned by a good conceptual understanding of the various contaminant pathways. Thus, the approach being adopted here is not new, and simply represents a process of updating existing, and incorporating new, knowledge and understanding.
To date, the process of conceptual model development has involved an internal expert panel approach identifying the relevant details within the following model elements:
and then for each stressor, its:
Key stressors that were identified include (but are not limited to) uranium, radionuclides, magnesium sulfate, manganese, ammonia, suspended sediment, radon-222 (and its progeny), gamma dose and weed propagules. At least nine key transport pathways were identified, the key ones being: controlled and semi-controlled release of contaminated water to Magela Creek surface water from at least three locations; water seepage from on-site waterbodies into groundwater and potentially surface water; spray and flood irrigation to land of contaminated water; trophic transfer to mobile species visiting on-site waterbodies; and atmospheric dispersion.
The development of the conceptual model is continuing and requires further discussion, clarification and verification from senior Supervising Scientist Division staff and external stakeholders including ARRTC and ERA before being finalised. The relative status of research and associated information addressing the stressors and their behaviour and effects within the various transport pathways will be determined. Many of the above stressors and pathways either have been, or are currently being, addressed by the SSD research and monitoring programme. Thus, where possible, the risks associated with each pathway/stressor sub-model will be quantified and compared.
As an example, the risks of uranium in surface water have been well-researched and can be quantified by comparing the exposure, represented by the measured concentrations of uranium over time in Magela Creek downstream of Ranger with the effects, represented by the sensitivity of local aquatic biota (plant and animal) to uranium as determined using laboratory toxicity tests. Plotting the uranium concentration and toxicity datasets as cumulative probability distributions, as illustrated in Figure 3.6, clearly shows that there is no overlap between the concentrations of uranium downstream of Ranger (that is, the exposure) with those known to cause adverse toxic effects to a selection of local aquatic biota (that is, the effects). As the risk is a function of the degree of overlap between the exposure and the effects distributions, the lack of overlap, which corresponds to a 'margin-of-safety' of approximately an order of magnitude, indicates negligible risk to the aquatic biota of Magela Creek from uranium in surface water as a result of current mining operations. However, the surface water pathway contains additional mining-related stressors to uranium (for example, magnesium sulfate, manganese, radionuclides) for which it is anticipated that similar quantitative risk estimation approaches will be applied.
When complete, the conceptual model will represent an iteration of previous models, having incorporated new knowledge and understanding of the relevant processes and issues. It will provide the framework for a quantitative comparison of risks and uncertainties for all stressors/pathways, such as that presented for uranium in surface water. It will also enable historical and current activities and priorities to be evaluated, and future priorities to be determined. Further, it will act as a knowledge and communication management tool, within a risk management framework that links clearly to the ongoing management of mining operations.
Figure 3.6 Risk estimation of uranium in Magela Creek by comparison of concentrations of uranium in Magela Creek upstream (+) and downstream (♦) of Ranger (2001-2004) with the toxicity of uranium to five local aquatic species (•). The solid line through the toxicity data represents the line-of-fit (broken lines around the fitted line represent the 95% confidence limits) based on a log-logistic distribution, and enables the calculation of concentrations of uranium predicted to protect (or affect) a given percentage of species. The dotted arrow points to the uranium 'Limit' for Magela Creek of 6 µg/L, which represents the concentration predicted by the log-logistic model to protect at least 99% of species, and which is not to be exceeded (Bayliss & van Dam, in prep).