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• SSR197 - Toxicity of Ranger mine RP2 and Pit 3 waters to native freshwater species: 2007 wet season (PDF - 859 KB)

Executive summary

Following extreme rainfall at ERA Ranger Mine in late February/early March 2007, a number of water management options were explored to reduce the volume of pond water being stored in Retention Pond 2 (RP2) and Pit 3. One possible option involved the short-term direct release of untreated Pond Water from RP2 and/or Pit 3 to Magela Creek. To provide information on the potential biological effects of such a strategy, the toxicities of RP2 water and Pit 3 water were assessed separately, using five and three native freshwater species, respectively. A key aim was to determine the maximum dilution of Pond Water that could be released whilst providing sufficient protection of the downstream aquatic ecosystems. It was also intended that the data would inform decisions about possible water management options for future wet seasons.

The toxicity of RP2 water (containing 1870 µg/L U - dissolved) varied markedly between the species tested, with IC₁₀ and IC₅₀ estimates in the range 0.6 - >100% and 1.8 - >100% RP2 water, respectively. Based on IC₅₀/LC₅₀ values, the order of sensitivity of the five test species was:

Moinodaphnia macleayi >> Hydra viridissima > Chlorella sp. > Lemna aequinoctialis >> Mogurnda mogurnda

Based on a species sensitivity distribution using species' IC₁₀ (or assumed equivalent) data, the dilution of RP2 water predicted to protect 99% of species was estimated to be 0.33% RP2 water (or 1 part RP2 water in ~300 parts Magela Creek water). At this dilution, the U concentration would have been approximately 6 µg/L U, the same as the current U Limit for Magela Creek, while the Mg concentration would have been approximately 0.5 mg/L, well below the recently proposed Limit for Magela Creek of 4.6 mg/L, and approximately equal to natural background concentrations.

The toxicity of Pit 3 water (~1620 µg/L U - dissolved) was assessed using M. macleayi, H. viridissima and L. aequinoctialis. The resultant IC₁₀ and IC₅₀ estimates were in the range 0.41 - 34% and 1.35 - >100% Pit 3 water, respectively. Based on IC₅₀ values, the order of sensitivity of the three test species was the same as that for RP2 water.

There was no significant difference between the toxicities of Pit 3 water and RP2 water to M. macleayi and H. viridissima. The general similarity in toxicity of the two water types to M. macleayi and H. viridissima was not unexpected given their similar uranium concentrations and the fact that RP2 receives water pumped from Pit 3. In contrast, the toxicity of Pit 3 water to L. aequinoctialis was significantly less than that of RP2 water. The difference in toxicity of the two water types to L. aequinoctialis may have been due to differences in (i) test organism responses between the tests, (ii) important physico-chemical variables that influence metal toxicity (eg. pH), and/or (iii) concentrations of, and interactions between, potentially toxic metals other than uranium (eg. Co, Cu, Mn, Ni, Pb and Zn).

At the time of completing the RP2 water and Pit 3 water toxicity assessments, flow in Magela Creek was insufficient for the direct release of Pond Water at acceptable/protective dilutions (ie. at least 1 in 300) to have significantly reduced the on-site water inventory. Consequently, this option was not progressed further. However, notwithstanding temporal variations in Pond Water composition/quality and how they may affect toxicity, the knowledge gained from the study will form a key part of the knowledge base required for the future evaluation of water management options at the Ranger Mine.

The results from the current study should not be used as the primary basis for determining protective dilutions of Pond Water that should be applied during future wet seasons. Any direct releases of Pond Water in the future will require new pre-release toxicity testing studies as soon as practicable prior to the intended time of discharge, recognising, however, the likely impracticality of running a full test suite at the time of release, given the likely narrow time window within which a significant volume of water could be released. Consequently, strategies for obtaining more timely toxicity information for Pond Waters may need to be considered, but would need to be rigorously tested and discussed and agreed by all stakeholders before being implemented.