Mining operations often use passive and/or active water treatments to improve water quality prior to environmental release. Key considerations in choosing a treatment process include the extent to which the water quality is actually improved, and the potential residual environmental risks of the release of such water. However, there are few published studies concerning the environmental impacts of treated waste waters. This study used toxicity identification evaluation (TIE) methods to quantify and identify the "toxic" constituents of a highly-treated water (distillate) produced by brine concentration of a mining process water. Exposure of five freshwater species (Chlorella sp., Lemna aequinoctialis, Hydra viridissima, Moinodaphnia macleayi and Mogurnda mogurnda) to a concentration range of the distillate (0, 25, 50 and 100%) found that it was toxic to H. viridissima (50-100% effect when exposed to 100% distillate). TIE tests demonstrated that the effect wasn't due to residual ammonia (~1 mg L(-1)N) or trace organics, and unlikely to be due to manganese (Mn; 130-230 μg L(-1)). Conversely, addition of 0.2 and 0.5 mg L(-1) calcium improved the growth rate of H. viridissima by 61 and 66%, respectively, while addition of calcium, sodium and potassium (0.5, 1.0 and 0.4 mg L(-1), respectively) to levels comparable to that in the local aquatic environment resulted in 100% recovery. Further assessment on the likelihood of residual metal toxicity indicated that Mn concentrations in the distillate were at levels that could inhibit the growth of H. viridissima. Ultimately, the results demonstrated that ion deficiency should be considered as a potential stressor in risk/impact assessments of the discharge of treated wastewaters, and these may need to be supplemented with the deficient ions to reduce environmental impacts. The findings have highlighted the need for water managers to consider the possibility of unintended environmental risks from the discharge of highly-treated wastewaters.