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Technical Memoranda 39
Rippon GD, leGras CAA, Hyne RV and Cusbert PJ
Supervising Scientist, 1992
ISSN 0810-9532
ISBN 0 644 25172 7
Due to the proposal to mine and process gold-bearing ore at Coronation Hill at the headwaters of the South Alligator River (SAR), the effect of cyanide in SAR water on local aquatic animals was studied using biological toxicity tests. The species tested were a water flea (Moinodaphnia macleayi), green hydra (Hydra viridissima), and the northern purple spotted gudgeon (Mogurnda mogurnda). The tests involved both lethal and sublethal endpoints, as well as chronic and acute exposures. Sodium cyanide dissolved in SAR water was used at concentrations of 2, 6.7, 20, 67 and 200 m g/L.
The number and size of water flea broods were significantly affected at 67 and 200 ug/L while no affect was seen for either hydra or the gudgeon embryos. Chemical analysis of the test waters showed, because of the high volatilisation of cyanide, that most of the cyanide was lost from test containers within 24 hours. The rate of loss was dependent on the surface area to volume ratio of the test container and solution; this complicates the interpretation of the results but does show the difficulty of trying to predict the fate, and therefore the toxicity, of any cyanide entry into the environment from such sources as a residue dam.