Assessment of sulfidic sediments in Coonancoocabill Lagoon

"October 2008".

Project Number: Survey and Risk Assessment of Sulfidic Sediments in Coonancoocabill Lagoon - M/BUS/282.

MDFRC item.

Sediment and water quality was determined at 12 sites in Coonancoocabill Lagoon to determine if sulfidic sediments were present in the wetland. Overall, the sediment and water quality parameters at most sites indicate that sulfidic sediments are not prevalent in the wetland, but very high levels of reduced sulfur were found at two sites in the wetland - both deep pools associated with ancestral river bends. Notwithstanding the presence of reduced sulfur at both these sites, there would appear to be a low risk of acidification of the wetland as a whole if the water levels in the wetland are manipulated using a regulating structure (although localised acidification may occur if the remanent pools are taken to near dryness). It is noteworthy that the wetland was close to dry at time of sampling with no detectable ecological consequences. It is important to note that the sediments have a low acid neutralising capacity (as evidenced by the low pHKCl values) indicating that the lagoon would be susceptible to acidification if reduced sulfur was to become more prevalent in the sediments. Therefore, a principle objective for any management of the wetland should include preventing formation of new sulfidic material. Before regulating water levels in the wetland it is recommended that: 1. The Kirkup Road Drain be totally cut-off from the lagoon, especially ensuring leakage from the regulator is fixed. 2. A hydrogeological study of the region be undertaken to determine possible groundwater inputs to the wetland. 3. Any drawdown/refilling of the wetland be accompanied by a monitoring program to look at changes in sediment and water chemistry. In particular, weekly monitoring of pH, dissolved oxygen and salinity is recommended at at least 4 sites in the lagoon including the deep holes at sites e and j. 4. A contingency plan is drawn up to detail appropriate management responses to any deterioration in water quality associated with draw down or refilling the wetland.