Effects of water hardness and alkalinity on the toxicity of uranium to a tropical freshwater hydra (Hydra viridissima)

In tropical Australian freshwaters, uranium (U) is of potential ecotoxicolo gical concern, largely as a consequence of mining activities. Although the toxicity of uranium to Australian freshwater biota is comprehensive, by wor ld standards, few data are available on the effects of physicochemical vari ables, such as hardness, alkalinity, pH and organic matter, on uranium spec iation and bioavailability. This study determined the individual effects of water hardness (6.6, 165 and 330 mg l(-1) as CaCO3) and alkalinity (4.0 an d 102 mg l(-1) as CaCO3), at a constant pH (6.0), on the toxicity (96 h pop ulation growth) of uranium to Hydra viridissima (green hydra). A 50-fold in crease in hardness (Ca and Mg concentration) resulted in a 92% (two-fold) d ecrease in the toxicity of uranium to H. viridissima [i.e. an increase in t he EC50 value and 95% confidence interval from 114 (107-121) to 219 (192-24 6) mug l(-1)]. Conversely, at a constant hardness (165 mg l(-1) as CaCO3), the toxicity of uranium to H. viridissima was not significantly (P > 0.05) affected by a 25-fold increase in alkalinity (carbonate concentration) [i.e . EC50 values of 177 (166-188) and 171 (150-192) mug l(-1) at 4.0 and 102 m g l(-1) as CaCO3, respectively]. A knowledge of the relationship between wa ter chemistry variables, including hardness and alkalinity, and uranium tox icity is useful for predicting the potential ecological detriment in aquati c systems, and can be used to relax national water quality guidelines on a site-specific basis.