Toxicity of magnesium sulphate to local tropical aquatic species: Derivation of a site-specific trigger value

2003

Internal Report 421
Camilleri C, Hogan A, McCullough C & van Dam R
Supervising Scientist Division
Department of the Environment, Water, Heritage and the Arts

About the report

Mining in the Magela Creek catchment, Northern Territory, has occurred for over 20 years with the operation of ERA Ranger uranium mine. This is an area identified as having World Heritage status as well as a high conservation value, and therefore requires reliable measures of protection from mining activities. In addition to uranium, magnesium sulphate (MgSO4) is a common mine contaminant arising from runoff over waste rock. However very little data existed on the toxicity of MgSO4 to local aquatic species. Concern over potential increase in salinity downstream from the mine from MgSO4 has resulted in an investigation of MgSO4 toxicity to local aquatic organisms, with the aim to derive a site-specific high reliability trigger value (TV) for MgSO4 in Magela Creek.

MgSO4 toxicity was assessed using five freshwater species: a green alga (Chlorella sp.; 72-h population growth), cladoceran (Moinodaphnia macleayi; 3-brood reproduction), cnidarian (Hydra viridissima; 96-h population growth), snail (Amerianna cumingii; 8 day reproduction) and fish (Mogurnda mogurnda: 96-h survival). No-observed-effect concentrations of MgSO4 varied from 11 mg/L for green hydra to 1086 mg/L for Chlorella sp However, green hydra were more than 15 times more sensitive than the other test organisms. An interim trigger value for MgSO4 in Magela Creek is 0.81 mg/L. The implications of this will be discussed. Further research effort is required to better understand the relative contributions of the anion and cation to MgSO4 toxicity, as well as the effect of the Mg:Ca ratio on MgSO4 toxicity.