Internal Report 535
Supervising Scientist Division
Department of the Environment, Water, Heritage and the Arts, 2008
About the report
Chronic toxicity no-observed-effect concentration (NOEC) data for five local freshwater pecies have been used to derive a high reliability water quality ‘Limit’ of 6 µg/L for uranium in Magela Creek downstream of Ranger Uranium Mine. Two of the NOEC values, 400 and 810 µg/L, represent estimates for two fish species, the purple-spotted gudgeon, Mogurnda mogurnda and the chequered rainbowfish, Melanotaenia splendida inornata, respectively, based on mortality after only 7 days exposure (+ 7 days post-exposure for M. mogurnda). Although such endpoints satisfy a current national water quality guideline criterion for a ‘chronic’ endpoint, their appropriateness as indicators of longer-term, sub-lethal effects has been questioned.
The aims of this study were to (i) develop a 28 day chronic toxicity test for two species, M. splendida inornata and M. mogurnda, and (ii) use the resultant toxicity test protocol to determine the chronic toxicity of uranium to these species.
Toxicity test development experiments for M. splendida inornata focusing on diet and feeding regimes proved mostly unsuccessful, with a range of complex diets unable to result in greater than 40% larval survival over 28 d. Based on a minimum acceptable criterion for control larval survival of =80%, M. splendida inornata was considered inappropriate for a 28d chronic toxicity test.
Test development experiments for M. mogurnda were successful in characterising the partitioning of uranium in the test environment and identifying a diet and feeding regime that resulted in >70% larval survival and strong larval growth over 28 d. Subsequent modifications to the diet/feeding regime resulted in larval survival rates of >90% in the toxicity tests, well above the control survival criterion. The resultant 28 d larval growth and survival test protocol for M. mogurnda was used to assess the chronic toxicity of uranium. Newly hatched larvae exposed to 3180 µg/L uranium resulted in 100% larval mortality within the first 24 hours of exposure. Larvae exposed to 1,400 µg/L uranium exhibited significant 13% and 30% reductions in length (P < 0.001) and dry weight (P < 0.001), respectively. Based on larval length and dry weight, the lowest-observed-effect concentration (LOEC) and the No-Observed-Effect-Concentration (NOEC) to larval M. mogurnda was 1400 and 770 µg/l U, respectively. These results are similar to the existing toxicity data based on shorter exposure durations, and suggest that the first two weeks post-hatch are critical in terms of the effects of uranium.