New assessment techniques for groundwater movement are being recommended for projects that involve making changes to drainage of backswamps in acid sulfate soil affected areas.

A study by Industry & Investment NSW (I&I NSW) on coastal floodplains in NSW has demonstrated that groundwater movement can be extremely variable within individual floodplains but is capable of extraordinarily high flows, which are relatively common.

“The results indicate the need for site specific assessments of groundwater flow via pit techniques, when making changes in backswamp water flows during acid sulfate soil remediation projects,” said researcher, Phil Hirst.

“This is particularly important if the proposed changes include attempts to retain acid within the landscape or involve floodgate opening and exchange of saline estuarine waters within drains.”

Mr Hirst said the project investigated the behaviour of groundwater in NSW North Coast acid sulfate soils, providing valuable information for the management of these soils.

“Tests were carried out in the Tweed, Richmond, Clarence, Hastings, Macleay and Taree catchments as well as at five high-risk backswamps,” he said.

“Networks of small tunnels in acid sulfate soils, created by old roots, have been found to play a significant role in the export of acid from these soils on the North Coast.

“We observed that groundwater escapes into the drainage system up to 1000 times faster when these tunnels, called macropores, are present.

“This massive variability in the rate at which water moves through these soils is a major factor in how they should be managed.”

I & I NSW soils and organics researcher leader, Dr Peter Slavich, said coastal acid sulfate soils typically contain large stores of soluble acidity and trace metals, which can be mobilised and transported to adjacent waterways, causing considerable environmental degradation.

“Groundwater flow is the main transport pathway of acidic products to drainage systems for soils with macropores.

“The groundwater flow in the sulfuric horizon is an important soil property to evaluate when developing remediation strategies to reduce acid export.
“A typical acid sulfate soil profile consists of an oxidised, acidic, sulfuric horizon overlying unoxidised, sulfidic material.

“This difference in physical structure can result in groundwater movement decreasing markedly with depth.”

Dr Slavich said acid sulfate soils is the common name given to soils containing iron sulfides, found mostly in coastal low-lying areas, such as mangrove swamps.

When the iron sulfides are exposed to air, through, for example, floodplain drainage, they produce sulfuric acid.

The acid moves through the soils, acidifying soil water, groundwater and eventually surface water.

“To reduce the impacts of acid sulfate soils, we need to slow the export of groundwater from the soils into the drains because the bulk of acidity entering the drainage system is directly from groundwater,” said Dr Slavich.

“Unfortunately dense networks of these macropores can speed up the groundwater flow dramatically.”

“While ‘plugging’ up the pores is not a viable option, reducing the gradient that drives the groundwater through the pores is more achievable with low cost in-drain water retention structures.

“These can be designed to keep the water in the drains at a similar level to the groundwater and reduce the gradient that moves groundwater sideways into the drains.”

Contact Phillip Hirst, Grafton, (02) 6640 1600.