A multi-scale assessment of the ecological risk of magnesium sulfate to aquatic biota of Magela Creek, Northern Territory, Australia

Abstract

Magnesium sulfate (MgSO₄) is common in mine waste waters, including Ranger Uranium Mine (RUM), Northern Territory, Australia. However, magnesium sulfate is generally assumed of low toxicity. This work derived an holistic understanding of the response of aquatic biota to elevated magnesium sulfate concentrations through re-examining data from previous studies and collecting new data at a variety of spatial and temporal scales. 

Single-species laboratory toxicity testing determined magnesium was most toxic to Amerianna cumingi at a LOEC of 1.6 mg/L. Toxicity was shown to be due to magnesium rather than sulfate. However, magnesium was not toxic to sensitive laboratory test species Hydra viridissima at concentrations of 10 mg/L, when the magnesium:calcium ratio was maintained at 9:1 or below. The toxicity of magnesium in Magela Creek waters is, therefore, likely due to the extremely soft nature of the receiving waters and is ameliorated with elevated calcium concentrations. 

Community-level effects of elevated magnesium sulfate concentrations were assessed across a range of biotic communities with natural creek pools and artificial enclosure experiments. The latter experiment demonstrated an apparent response of microinvertebrate community structure to elevated magnesium at 23 mg/L and above. 

Previous field-scale studies were re-examined along with field responses of taxa studied in laboratory tests. The strongest response to elevated magnesium was by the microinvertebrate zooplankton community. However, many previous surveys were potentially confounded by catchment differences between reference water bodies and water bodies receiving RUM mine waters. Nevertheless, collectively, field-scale studies indicated elevated magnesium concentrations would not alter aquatic biotic community structure at concentrations above 10 mg/L, at magnesium:calcium ratios of 9:1 or less. 

Good agreement was found between magnesium guideline values derived by single-species laboratory testing (0.6 mg/L) and community-level mesocosm experiments (0.8 mg/L). A final recommended extremely soft-water guideline value of 0.8 mg/L was given by the mesocosm experiments.

Date of Award	Sept 2006
Original language	English
Supervisor	Michael Douglas (Supervisor)