Textural characteristics and heavy metal concentrations in billabong sediments from the Magela Creek system, northern Australia
Technical Memorandum 9
Thomas PA and Hart BT
ISBN 0 644 01282 X
About the report
The objective of this study was to assess the usefulness of Magela Creek billabong sediments to monitor the potential buildup in heavy metals as a result of the Ranger uranium mining operation and the regional township at Jabiru.
Major differences were found in the physical characteristics of the sediments both between billabongs and, in some cases, within the same billabong. The billabong sediments were found to reflect closely the hydrological characteristics during the Wet Season. The sediments in the backflow billabongs were fine-grained, being mostly silt and clay, and rich in organic matter (~20%). The floodplain billabongs also contained predominantly fine-grained sediments but the organic content was considerably lower (6 to 7%). The channel billabongs contained essentially sandy sediments although deeper sections and slower flowing areas contained some silt and clay. The textural characteristics of the Mayamarleprard Waterhole system (Y-shape and Island billabongs) were found to vary widely over the two billabongs.
The sediments in Hidden and Leichhardt billabongs were considerably different from the other billabongs. They consisted of a sticky, grey clay material. This same material appears to underlie much of Island Billabong and the Mudginberri Corridor area.
A large number of sediments from Magela Creek billabongs were analysed for iron, manganese, cadmium, copper, chromium, lead, zinc and mercury. From these data three main points emerge:
- The concentrations of all metals in the less than 63 µm fraction are very low when compared with published values for unpolluted sediments.
- The metals were evenly distributed over fractions less than 63 µm in size.
- A digestion procedure involving extraction of the sediments with H202 and HNO3 has proved to be most effective. Comparison with the results obtained using a complete digestion procedure (H202 /HNO3/HF) showed that most of the chromium (63%), much of zinc (46%), but only 25 to 36% of the iron, manganese, copper and lead were associated with the mineral phase of the sediments and would be unavailable to the biota.