Lower Lakes and tributaries
The regulation of the water level in the River Murray since the locks and barrages were completed in 1940 has led to the loss of the natural wetting–drying cycles across the Murray–Darling Basin. This has allowed more stable water levels protecting freshwater supplies for irrigation and maintaining navigation, and one of the consequences is the accumulation of acid sulfate soils (ASS) in the Lower Lakes.
From 2007–09 the Lower Lakes received record low inflows, due to a combination of drought and over-allocation of water in the Murray–Darling Basin. The low inflows meant that water levels fell nearly 2 m and exposed large areas of the Lower Lakes shoreline for the first time in over 100 years. The receding shoreline meant that the potential acid sulfate soils which had accumulated were exposed to oxygen and sulfuric acid was generated.
As the water levels returned, there were several areas around the Lakes where the surface water was affected by acidity leaching from the exposed acid sulfate soils. This caused the pH to drop and also allowed soluble metals to be transferred to the surface water. This caused detrimental effects to the environment and the ecosystem. The acidic areas were mainly on the shallow lake margins and embayments which had limited connection to the main lake water body. The total area that acidified was estimated to be 2,173 ha in 2009.
There were different severities and durations (ranging from weeks to months) when the acidity affected the surface water in the Lower Lakes.
In some areas the acid was neutralised naturally by dilution and alkalinity input following a rapid rise in lake levels following Murray–Darling Basin floodwater inflows during 2010. In addition, the Department of Environment, Water and Natural Resources undertook treatment of acidity via aerial limestone addition. This was conducted in Currency Creek and Boggy Lake, two of the most affected areas, and was highly successful in achieving neutralisation over large areas.
The main way to prevent acidification from occuring again is to ensure acid sulfate soils are kept wet and not exposed to air. Managing water levels in the Lower Lakes is one way that acid sulfate soils are being actively contained.
Bacteria in the soil can reverse the process of acid sulfate soils forming sulfuric acid and help return the contaminated environment to a healthy state. This is called bio-remediation. The bacteria use iron and organic matter, as well as sulfate in the acid to do this.
The EPA has worked with the DEWNR to monitor the Lower Lakes region through the drought and identify areas of high risk. Since the return of the water level, the EPA regularly monitors the water quality in Lake Alexandrina, Lake Albert and the tributaries to assess the recovery of the region's ecosystem. While the surface water is pH neutral at present following the return of flows, acidity is still present in the sediment and groundwater under the areas which were exposed during the 2007–09 drought.
Historical and recent monitoring of the Lower Lakes by the EPA