THE USE OF GEOCHEMICAL SPECIATION MODELING TO PREDICT THE IMPACT OF URANIUM TO FRESH-WATER BIOTA

Uranium is the prime potential contaminant in mine waste waters that m ay be released from the Ranger Uranium Mine (RUM) into the receiving w aters of the Magela Creek, Alligator Rivers Region, Northern Australia . The potential ecological impact of the migration of uranium, that wo uld result from an elevation in its concentration above background, in the Magela Creek downstream of the RUM, has been experimentally inves tigated by integrating biomonitoring with geochemical speciation model ling. The freshwater bivalve Velesunio angasi, abundant throughout the Magela Creek catchment, was exposed to a variety of uranium concentra tions in a synthetic Magela Creek water, at four pH levels (5.0, 5.3, 5.5 and 6.0), in the presence (3.05 and 7.50 mg l(-1)) and absence of a model fulvic acid (FA), and its behavioural response was measured. S peciation modelling, using the HARPHRQ code, provided evidence that UO 22+ and UO2OH+ are the uranium species most responsible (ca. 96%) for eliciting an adverse behavioural response when UO22+ is assigned twice the toxic effect of UO2OH+. This finding rejects the notion that biot a respond specifically to the sum total of inorganic uranyl species.