Remediation of broadacre coastal acid sulfate soils

Remediation strategies for acid sulfate soils have evolved greatly in recent years, largely due to rapid developments in the understanding of scientific and technical issues.

Application of lime

An early strategy to address the effects of acid sulfate soils was the application of agricultural lime to surface soils, particularly liming of the spoil mounds deposited during drain construction. However, liming of surface soils was found to be largely ineffective due to the limited penetration of lime. It was also later found that surface soils were not the main source of acidity: discharges of acid arise predominantly from subsoils and groundwater.

Revegetation of acid sulfate scalds was another major early focus and over the years great effort went into getting something to grow in scalded sites, preferably something palatable to livestock.

Active floodgate management

Poor quality acidic water generally occurs behind closed floodgates. Opening the gates aimed to dilute and neutralise this acidic drain water. Active floodgate management subsequently became a widely practised strategy and drain water quality clearly improved, at least in terms of such parameters as acidity and dissolved oxygen.

However, as far as its impact on ameliorating acid sulfate discharges was concerned, the policy was supported by very little research.

Containing the products of oxidation

It was subsequently found that acid export from the soil to drains and then creeks occurs via interconnected macropores and the planar cracks that form when soils shrinks as they dry out and oxidise. These macropores and cracks are major pathways for the discharge of acid. Critically, it was discovered that this could occur simply due to tidal fluctuations in the drain through opened floodgates.

As a result, the focus in recent years has been on containing acid sulfate oxidation products. This is often achieved by infilling drains and installing in-drain weirs, which retain water and significantly reduce acid flux by lowering the frequency and volume of flows. Water retention may involve changes in land management, such as a return to wet pasture grazing, or land-use change, involving wetland rehabilitation.

Site before remediation Site after remediation

Click photos to enlarge

A site on the Macleay floodplain before and after remediation by restoring wet pasture. Photos: M. Tulau, S. Henderson.

In many cases, however, it is not possible to return to a previous condition. As noted previously, the oxidation process involves the release of aluminium which can lead to irreversible shrinkage and lowering of ground surfaces at many sites.

There is also concern about chemical transformations: while reduction processes have been observed in rewetted anoxic materials, a surface layer of pyrite or monosulfides can reform in the uppermost layers of the soil. During dry weather this reformed pyrite can oxidise, causing an accumulation of acidic solutes in surface soils.

Overall, containment strategies can lead to many improvements from an acidity perspective, though the reality of remediation is far more nuanced in practice.

More recently, the need for remediation of former wetland areas has been furthered by the opportunities for carbon sequestration in the context of global warming. Sea level rise may also affect remediation strategies at some sites in the medium to long term.

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Page last updated: 11 October 2013