Controls on water acidification and de-oxygenation in an estuarine waterway, eastern Australia

The quality of soil and water was investigated in an estuarine floodplain system, eastern Australia. The backswamp portion of the floodplain is underlain by sulfidic sediments at depths about 0.5e0.9 m below the ground surface. Actual acid sulfate soils have developed due to sulfide oxidation as a consequence of land drainage since the early 1900s. These acid sulfate soils have a high measured total actual acidity (TAA, up to 500 mmolHC/kg). However, only a very small proportion (!2%) of this TAA occurs in a water-soluble form. Water quality monitoring in the creek (Rocky Mouth Creek) draining the estuarine embayment during the period from May 1998 to July 2000 shows that acidic flows (pH!4:5) of several months occurred intermittently in the upper reach of the creek. This may be attributed to the hydrolysis of Fe3C after the oxidation of Fe2C that is exported into the creek from acid sulfate soils through an artificial drain network. It is hypothesized that Fe2C is being generated by biological iron reduction, which consumes HC and thereby drives the conversion of retained acids to soluble acids. This allows the release of retained acids and subsequently the translocation of acids from soils to the adjacent waterway. Monitoring results also show clear responses of pH and dissolved oxygen (DO) to heavy rainfall events during the period of alternating high (O6) pH-dominated flows. Frequently, pH and DO levels in the creek water drop during flooding. Results from field investigation and experimental simulation suggest that DO depletion associated with organic matter decomposition takes place rapidly in the floodwater inundating the soils and this DO-depleted water has a significant capacity to further de-oxygenate any receiving water. However, the consumption of DO in floodwater is not clearly related to oxidation of Fe2C. It is not certain what other DO consumers are involved in the process and further research is needed to fill this knowledge gap.