Ecotoxicological assessment of manganese

Executive summary

Manganese (Mn) is a ubiquitous element in the earth's mantle and a key contaminant of Ranger mine process water. Manganese toxicity is dependent on pH and water hardness, which is consistent with what is known for other metals. However, Mn aquatic chemistry is also a complex function of the pH and redox micro-environment with Mn primarily existing as soluble Mn(II) and insoluble Mn(IV) oxidation states. The risks of Mn toxicity to aquatic biota of Magela Creek have been considered low to date. However, groundwater modeling of Pit 1 and Pit 3 closures has found that elevated concentrations of Mn may reach Magela Creek and indicated that Mn will be a key contaminant of concern. Additionally, the likelihood of higher Mn concentrations being released to Magela Creek may increase following the commissioning of the brine concentrator plant. Insufficient Mn toxicity data existed for local species in local natural waters to be able to (i) conclude with high confidence that no adverse effects would be expected given the current water quality and (ii) predict at what Mn concentrations adverse effects would be expected to occur. A site-specific assessment of Mn is of particular pertinence given the low water hardness and relatively low pH of natural waters of the Alligator Rivers Region, which could potentially result in higher than expected (i.e. from existing literature) Mn toxicity. The aims of this study were to:

1. Assess the toxicity of manganese (Mn) in Magela Creek water (pH ~6–6.5) to six tropical freshwater species.
2. Derive a site-specific Trigger Value (TV) for Mn in Magela Creek.
3. Recommend Limit, Focus and Action Trigger Values, which can be incorporated into the Water Quality Objective (WQO) for Magela Creek.

The TVs derived in this project were incorporated into the water quality trigger framework for Magela Creek that has been described by Iles (2004). The framework consists of a hierarchy of TVs (Focus, Action and Limits) and exceedance of these TVs initiate increasingly strict reporting and investigation actions by the mine's operator.

The six local freshwater species tested in this study had a broad range of sensitivities to Mn in the soft surface waters of Ngarradj and Magela Creeks. For three of the species, Mn toxicity was higher than many of the species reported in the literature, which was probably due to the low concentration of Ca²⁺ in the natural waters. The low pH may have decreased to the toxicity of Mn to Chlorella sp., but increased the potential for Mn²⁺ to remain dissolved and, hence bioavailable. A loss of Mn was observed on the final day of a number of the H. viridissima toxicity tests but the Mn could not be recovered from the test system. This observation may be a result of the previously reported complex speciation of Mn. We accounted for such issues through extensive analysis of Mn (0.1 μm filtered and total) at the start and end of the tests. Toxicity estimates were adjusted using the measured Mn concentrations. The Species Sensitivity Distribution (SSD), which used the three international toxicity estimates derived under relevant physicochemical conditions, produced a 99% TV that can be implemented in Magela Creek.

It is recommended that a 99% protection TV of 75 μg L^-1 Mn be applied at MG009. The Focus and Action TVs should be 35 and 45 μg L^-1, respectively. These TVs are rounded out from the calculated 99% TV of 73 μg L^-1 and the 95th and 80th confidence intervals of 33 and 46 μg L^-1, respectively.