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Case Study


Remediating acid sulfate soils

Date: 25 August 2013
Category: Case Studies

East Trinity inlet, a quiet tidal creek bordering the township of Cairns, Queensland, has witnessed a transformation in the last decade that has made it one of the world’s most successful demonstrations of how to restore an area severely affected by acid-sulfate soils.

The acid sleeper

The acid crisis at East Trinity began in the 1970s when developers drained and cleared 740 hectares of tidal wetland to grow sugarcane. This dried underlying acid-sulfate soils, causing them to release slugs of acid whenever they were soaked by rain. Such acid release can in turn mobilise toxic levels of iron, aluminium and heavy metals, and can also degrade steel pipes and concrete structures to the point of failure.

Over the subsequent 25 years, it is estimated that 72,000 tonnes of sulphuric acid were released into Trinity Inlet. Creeks became sterile, stained with the iron-red deposits from the acidification process. Adjacent mangrove ecosystems and associated wetlands were badly degraded, experiencing repeated fish kills in areas that were prime nurseries for coastal and reef waters. Birds and other plants and animals also disappeared.

The restoration

In 2001 the Queensland Government purchased the land to remediate the acidic soils and protect the natural green backdrop to Cairns. A team led by CRC CARE Program Leader Dr Richard Bush of Southern Cross University took advantage of the enclosing sea wall built by the original developers to exclude the tide and drain the site.  They re-introduced a partial tidal exchange through adjustable floodgates, allowing the most acidic sediments to be strategically re-flooded, which prevented them producing more acid. Where the runoff was still too acidic they added hydrated lime using specially designed equipment.

Gradually the mangroves began to colonise areas newly flooded with seawater. A wider range of mangrove species than the scientists had dared hope for have re-established, and fish and other marine life are now common in creeks that once ran so acid that nothing could survive in them. Birdlife is also returning, with over 40 species now present in previously abandoned areas.

Better LATE than never

Dr Bush and his team essentially reversed the chemistry of what took place when the acidic soils were drained. Not only is this approach – known as lime-assisted tidal exchange, or LATE – capable of remediating acid sulfate soils of below pH 3 to near-neutral in around one year, but it is also extremely cost effective.  Dr Bush estimates that treating all the acid-sulfate soil at East Trinity according to recommended traditional practices would cost over $300 million and require complete vegetation clearing.  CRC CARE’s process is returning nature for a mere fraction of that.

To keep the acid at bay, however, means that across a third of the site, soils will have to remain permanently a tidal wetland, unsuited to major developments such as marinas or urban housing. However Cairns could inherit a huge nature park with potential for an eco-tourist attraction right on its doorstep.

Educational resource

The success of this innovative project has led to a decision by the Queensland Government to adopt the strategy across the entire East Trinity site. The work has also been linked to a national program of training, education and public awareness about acid-sulfate soils, benefiting not only scientists, experts and natural resource managers, but also chemistry teachers and students, wildlife enthusiasts and the general public.

International application

With an estimated 40 million hectares of similar acid coastal wetlands round the world and at least 4 million in Australia, including parts of the Murray-Darling system, there is clear scope to apply much of the science and experience developed at Cairns. Students from Stanford and Hong Kong universities have already visited the site, and results of the work have been published and presented at numerous international conferences.

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