Monash University, Victoria, Australia
When settlers first arrived in eastern Australia, the large expansive coastal floodplains appeared to have the fertile soils and reliable water sources for successful agriculture. However, the floodplains were often waterlogged and contained excess surface water which required drainage to allow for the expansion of rural settlements and year-round agriculture. Thousands of kilometres of drains and hundreds of floodgates were built, changing the coastal floodplains forever.
Unknown at the time, the thin veneer of fertile alluvial soil was underlain by sulfidic sediments. When these sediments are exposed to air and oxidised, such as through drainage, acid sulfate soils are formed, which produce acidity and mobilise trace metals such as iron, aluminium, zinc, nickel and manganese.
This process results in the acidification of soil water, shallow groundwater, and eventually, surface waters. Now, more than a hundred years later, these coastal floodplains are host to a range of soil and water quality issues which present a serious challenge for floodplain management.
Fish kill events now periodically affect the estuaries which drain coastal floodplains following recurring summer floods to create estuarine “dead zones.” These fish kills are caused by hypoxia and linked to the interaction between the historic drainage systems with acid sulfate soils and common vegetation species. Floodplain drainage has changed the vegetation assemblages, shifting communities from wetland dominant species to dryland dominant species which are intolerant of waterlogging. Inundation of the floodplain during summer floods kills the dryland vegetation, which provides a labile carbon source as it rapidly decomposes. Decomposition processes consume the dissolved oxygen in the overlying floodwaters, causing the surface water to turn hypoxic. These events are known locally as “blackwater” events due to the black-coloured water, high in trace metals and low in oxygen, which forms after about a week.
Acid sulfate soils commonly have surface accumulations of iron, manganese and sulfate minerals. These redox sensitive species further fuel hypoxia as conditions on the floodplain shift to favour anaerobic decomposition. The extensive drainage systems then transport the deoxygenated floodwaters rapidly to the main estuarine channel. Where these floodwaters once remained on the floodplain for approximately 100 days or more in natural swamps and wetlands behind natural levees, enhanced drainage can remove floodwaters in approximately 3-4 days. Hypoxic floodwaters further consume dissolved oxygen in the estuary, causing fish and other aquatic life to die en masse.
It is likely that these events will increase in the future. Under climate change, more extreme events, such as floods, will occur and together with higher temperatures, could result in regular events which decimate the fish stocks in these important waterways. Despite the devastating effects, little is known about the processes which drive the formation of blackwater. Vanessa Wong (Monash University) is currently working with a team from Southern Cross University (http://www.scu.edu.au/geoscience/) to understand these processes so that the frequency and magnitude of these events can be reduced.