Understanding surface water – ground water interactions to help manage wetlands

Australia’s wetlands are of great national significance with many appearing on the Ramsar List of Wetlands of International Importance. They replenish and purify water resources and provide social and financial benefits. However, many of these wetlands are under threat. Water for a Healthy Country Flagship scientists are working to provide the knowledge and tools to support the management, protection and restoration of these important water ecosystems.

One such area where Flagship scientists are at work is the Lower River Murray, where there are more than 1100 wetlands. Before river regulation, these wetlands went through seasonal wetting and drying cycles, providing an environment that supported a diverse range of plant and animal life. However, many of these wetlands are now permanently inundated and have an increased salt concentration which is detrimentally affecting species diversity.

Wetland managers are attempting to rehabilitate these wetlands, re-introducing cycles of wet and dry spells, and returning the wetlands to a more natural state. The two main factors which contribute to this rehabilitation are altered hydrology and salinity. As a result many managers are changing the hydrology of the wetlands despite a lack of scientific knowledge about the consequences of these changes.

By investigating the interactions between surface water and ground water of three wetlands of the Lower River Murray, Water for a Healthy Country scientists, along with collaborators from the South Australian Murray Darling Basin Natural Resources Management Board and Flinders University, are increasing understanding and helping wetland managers to make informed decisions on the management regimes of wetlands in this region.

The study focussed on three wetlands with very different hydrological regimes:

1. Lake Littra has no connection to the river and relies upon flooding to provide a supply of surface water,
2. Hart Lagoon has a permanent connection to the river through a single inlet/outlet, while a second inlet allows surface water flows through the wetland during floods, and
3. Banrock Station wetland has two permanent connections to the river on either side of a weir, this results in a 3 metre difference in river levels between the inlet and outlet and ensures that there is a significant flow of surface water through the wetland.

At all three wetlands electromagnetic surveys were performed to determine areas that were more conductive and therefore likely to be storing salt in the sediments. Surface water levels were also measured and piezometers were used to record groundwater levels. Ground and surface water samples were collected and analysed for anions, cations and stable isotopes of water. Soil samples were analysed for gravimetric water content, chloride concentration and stable isotopes of water. This data was collected from June 2006 through to September 2007.

Analysis of the data showed that during wet conditions when the wetlands were full, Lake Littra and Banrock Station wetland function as recharge wetlands (i.e. surface water was recharging the groundwater) while Hart Lagoon acts as a flow-through wetland (i.e. the groundwater discharges to the wetland, and the surface water in the wetland also recharges the groundwater depending upon the location in the wetland).

The study found that when the surface water was removed from each of the wetlands they all behave similarly. In the recharge wetlands groundwater flow was reversed so that it was no longer flowing away from the wetlands but towards them. The flow-through wetland had a similar result in that the gradient of the water flow flattened so that water again was not flowing away from the wetland. In all three cases without surface water the wetlands became groundwater discharge features on the floodplain. Being the lowest point of the floodplain, groundwater flowed toward the wetland where it was evaporated and salt could concentrate. This shows that removing surface water from the wetlands will lead to salinisation.

If the wetlands on the floodplains have their surface water connection to the River Murray severed and forced into a dry spell, the groundwater flow direction will reverse causing the wetlands to act as groundwater discharge features which would lead to salinisation. The time taken for this to occur will depend on various characteristics so will be different for each wetland.

Using this knowledge, wetland managers will be better equipped to develop rehabilitation plans that incorporate the risk of salinisation in controlled wetting and drying cycles for the rehabilitation of wetlands in the Lower River Murray, Additionally in times of drought this research will help in selecting wetlands to be dried to save water for human consumption.

This work was funded by the Centre for Natural Resource Management and the National Action Plan for Salinity.

Contact:
Russell Crosbie, CSIRO Land and Water
Email: Russell.Crosbie@csiro.au

Reference:
Crosbie, R., McEwan, K., Jolly, I., Holland, K., and Lamontage, S. (2007) Surface water – groundwater interactions in three River Murray floodplain wetlands: Results from field studies. CSIRO: Water for a Healthy Country National Research Flagship Report, 53 pp.
http://www.clw.csiro.au/forms/publications/details.aspx?ID=176130