Highly treated mine waters may require major ion addition before environmental release

► A treated waste water product was assessed using ecotoxicology and TIE methods. ► Water quality of the distillate was markedly improved but residual toxicity remained. ► Major ion deficiency was identified as the primary cause of effects to Hydra sp. ► Residual metal was measured at concentrations that may have contributed to toxicity. ► Treated waters may have inadvertent environmental effects that need to be managed.