Chronic toxicity of uranium to the tropical green alga Chlorella sp. for the derivation of a site specific Trigger Value for Magela Creek

About the report

Uranium mining in the Magela Creek catchment of Kakadu National Park has occurred for over twenty years. Due to a very high wet season rainfall in the region, controlled releases of water are an essential part of the water management program of the ERA Ranger Mine. The revised Australian and New Zealand Guidelines for Fresh and Marine Water Quality recommend a receiving water Trigger Value (TV) for uranium (U) of 0.5 µg/L for the protection of downstream aquatic life. The TV was considered to be of low reliability, due to an inadequate toxicity database and the subsequent derivation of the recommended value using the less preferred ‘safety factor’ approach. Given that the Magela Creek catchment is considered to be of high conservational and ecological value, a low reliability TV was considered inadequate and a site-specific assessment was considered essential. In order to derive a high reliability, site specific TV for U, chronic toxicity data for at least five local species from at least four taxonomic groups was required. However, appropriate data were limited to four local species from three taxonomic groups, namely the cladoceran, Moinodaphnia macleayi; the green hydra, Hydra viridissima; the purple spotted gudgeon, Mogurnda mogurnda and the chequered rainbowfish, Melanotaenia splendida inornata. The recent development of a unicellular algal toxicity test using the locally isolated Chlorella sp., enabled the inclusion of toxicity data for a primary producer, thus fulfilling the requirements for a high reliability sitespecific TV. This report describes a series of experiments undertaken to refine the test protocol, determine the toxicity of U to this species of alga and subsequently derive a new TV for U in Magela Creek.