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2F - Site Specific Testing (MOP/102) Toxicity of the mine effluent salt, magnesium sulphate, to aquatic biota in Magela Creek, Northern Territory, Australia. McCullough, Clint1, 2, Humphrey, Chris2, Douglas, Michael1, Hogan, Alicia2, Shiel, Russell3, Gell, Peter3, van Dam, Rick4, 1 Northern Territory University, Darwin, Northern Territory, Australia2 Environmental Research Institute of the Supervising Scientist (eriss), Darwin, Northern Territory, Australia3 University of Adelaide, Adelaide, South Australia, Australia4 Sinclair Knight Mertz, Sydney, New South Wales, Australia ABSTRACT- Magnesium sulphate (MgSO4) is a common mine contaminant. However, the salinity arising from this salt has received very little ecotoxicological assessment. Because MgSO4 is a major constituent of Ranger Uranium Mine (RUM) wastewaters, a study has been initiated to examine its effects on the ecology of Magela Creek downstream of RUM through laboratory ecotoxicity tests on specific species and field mesocosm experiments on various aquatic communities. The Magela Creek is located within the World Heritage listed Kakadu National Park in the Wet-Dry tropics of Australia's Northern Territory. Experiments have indicated that the toxicity is associated with the cation magnesium, rather than the anion sulphate, at a definitive LOEC range of 4.6 mg/L for the local endemic species Hydra viridissima, to 143.2 mg/L for the single-celled alga, Chlorella sp. Further experiments have found that this toxicity is due to the extremely soft nature of the receiving waters and is ameliorated with increasing ambient calcium concentration. Field-based mesocosm tests have used 3,000 L fibreglass tubs placed in the creek bed that were seeded with resident creek communities during recessional flow (early Dry season). The tubs were sampled for macro- and microinvertebrates, diatoms on artificial substrates, and chlorophyll pigments (as a surrogate for phytoplanktonic abundance and type). Sampling of biota was conducted prior to dosing of the tubs, then twice after dosing for each replicate of the five treatments of geometrically increasing concentrations of MgSO4. These chronic data will be combined to derive a local "ecosystem-level" NOEC for magnesium sulphate, taking into account agreement/conflict between single-species laboratory derived data (with their higher precision but lower ecological relevance) and extensive, larger scale, field, community-level data from mesocosm experiments (statistically "noisier" but more ecologically valid). Key words: mesocosm, Magnesium sulphate, community, salinity |
