Coastal floodplains in eastern Australia have many thousands of kilometres of constructed drains and large areas of acid sulfate soils (ASS). Drains in areas with shallow ASS may intercept acid groundwater and can export this acid, along with iron and aluminium, to the estuary (Figure 1).
Apart from readily visible impacts like fish kills, acid sulfate drainage water can cause many hidden impacts. These include dissolving the shells of oysters, restricting fish and prawn migration, and damage to the skin and gills of fish which increase susceptibility to infections and diseases such as ‘red spot’ (Figure 2).
Drain management can help reduce the export of acid to the estuary. The first thing to do is find out if your drain exports acid (see 'Further information' on the next page). If it does there are three main management options available including
Controlled opening of floodgates helps by neutralising and diluting acidity with river water (see Managing floodgates to improve drain water quality). However, it is far better to contain the acid within the soil profile and prevent it from getting into the drain in the first place. This can be achieved using a number of methods, including retaining drain water and re-shaping or infilling drains.
One of the main ways that acid gets from the soil into the drain is by seepage of groundwater. This seepage is driven by ‘groundwater gradients’ – the difference between the groundwater level and the drain water level. Low tide draw down of drain water levels can increase these gradients (Figure 3).
Using a weir or a similar structure to retain water in the drain and prevent tidal draw down of drain water levels can be a simple, cheap and very effective means of reducing acid groundwater seepage (Figure 4). It is a particularly useful approach if the acid sulfate soils are highly permeable.
Infilling or shallowing and widening of drains so they no longer intercept acid sulfate soils, but still remove surface water, can also be an effective way of reducing the amount of acid exported to the river.
There is another method which has been used with success by the sugar cane industry that incorporates some drain infilling and shallowing. This is based on efficient drainage of surface water and keeping the groundwater level lower. This allows the soil to store rainfall, reduces waterlogging and reduces the time that groundwater seeps into the drain. This method requires laser levelling of the site, shallow drains, fewer drains, regular drain bank liming and liming of pump out water. It is a higher cost option that applies to land used for high water use, deeprooted crops. It works best in soils with low permeability.
Acid sulfate soils are quite complex and have greatly varying properties. For example, the permeability of ASS can vary by over 1000 times between sites.
Before making changes to your drain management to reduce acid export, it is a very wise idea to get some expert assistance.
Various state and local government organisations have expertise in assessing acid sulfate soils. They can help you identify some of the key properties that are important to know before making management changes.
For further information, advice and assistance on floodgate and drain management to reduce acid export contact NSW DPI.
Citation: Johnston S (2004) Floodgate and drain management on coastal floodplains leaflet 2: Managing drains to reduce acid export. (NSW Agriculture: Wollongbar)