Seasonal sand-bed streams are common throughout the wet-dry tropics of northern Australia. While a resident fauna has been found to exist in the shallow groundwater of these beds during the dry season, little is known of the ecology of these communities, nor of the potential impacts upon exposure to contaminants. Lowland portions of Magela Creek comprise a seasonal sand-bed stream, located mostly within the boundaries of Kakadu National Park, but with portions of the creek flowing through the Ranger uranium mine lease which is excluded from the Park. The sands in the creek channel have a relatively high hydraulic conductivity and will be a pathway for the transport of contaminants, including MgSO4, from the mine site. As part of a broader ecological risk assessment project, we sought to determine whether shallow groundwater communities are at risk from elevated levels of MgSO4. To do this we collected water samples from 18 piezometers installed within the stream channel of a reach of Magela Creek, extending from approximately 3 km upstream of the Ranger uranium mine to 4 km downstream. In addition to describing the water chemistry, biotic communities were characterised using molecular techniques and traditional morphological methods. Our initial findings confirmed the presence of a groundwater contamination gradient. While the molecular data are still being analysed, initial analysis of the invertebrate communities showed lower dispersion in ordination space between exposed sites compared to reference sites, indicating less variability within the exposed communities.
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