Floodgate and drain management on coastal floodplains: Basic principles. Leaflet 2. Managing drains to reduce acid export
Date: 27 May 2004 Author: Scott Johnston
Figure 1. A toxic plume of acidic water entering an estuary -
a result of drained acid sulfate soil (Photo: Scott Johnston)
Figure 2. Bream and mullet with lesions caused by ‘redspot’
disease. This disease has been linked to acid drainage water,
which causes skin damage and allows a fungus to invade the
skin, causing lesions (Photo: Richard Callinan)
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).
What you can do
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
- retaining drain water
- re-shaping and infilling of drains.
Figure 3 (above). A diagram representing the seepage of acid
groundwater to a drain. Low tides cause the drain water level
to drop lower than the groundwater. The larger the difference,
or gradient, the more groundwater seepage.
Figure 4 (above). Keeping drain water levels high can reduce
groundwater gradients and greatly decrease the amount
of acid groundwater seepage.
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.
Things to consider
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.
Further information
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)