Recovering and managing acid sulfate soils
Summary
Acid sulfate soils occur naturally on Australiaʼs coastal plains and when they are exposed to oxygen large quantities of sulfuric acid are produced, causing contamination of soil and water systems with acid, metals or arsenic. Despite the consequences of draining acid sulfate soils, soil scientists still do not wholly understand the complex processes underpinning acidification. In particular, it is not always clear whether soil acidification is caused by oxidation of pyrite or other fors of sulphur in soil. The authors of this study concluded that pyrite oxidation was the primary cause of acidification for the samples under consideration.
Research findings
Acid sulfate soils occur naturally on Australiaʼs coastal plains, usually as waterlogged layers of soil containing the iron sulfide mineral pyrite. Although acid sulfate soils are generally harmless, when they are exposed to oxygen through excavation or drainage, large quantities of sulfuric acid are produced, causing contamination of soil and water systems with acid, metals or arsenic. The consequences of disturbing acid sulfate soils can be serious – causing fish kills, corroding infrastructure, and compromising wetland ecology, groundwater resources, and agricultural productivity.
Potentially harmful acid sulfate soils occur primarily in Australiaʼs coastal flood plains, where escalating rates of drainage for agricultural and urban development have created increasing acidification problems. Important agricultural industries, such as dairy farming and sugar cane production, are especially vulnerable, as they are frequently situated on floodplains underlain by acid sulfate soils. Fuelling the acidification of coastal regions is the current drive to drain wetlands and convert them into productive agricultural land. Unfortunately, drainage of acid sulfate soils can instigate the acidification process, with dire consequences for farmland productivity and waterway health.
While acid sulfate soils have primarily created issues in coastal regions, it is not only coastal agriculture and settlements that have been affected. Inland agricultural areas can also be impacted, particularly those areas affected by rising water tables. In these areas, drainage efforts aimed at mitigating salinity can actually initiate problems with acid sulfate soils. Despite the consequences of draining acid sulfate soils, soil scientists still do not wholly understand the complex processes underpinning acidification. While many studies have concluded that oxidation of pyrite (FeS2) by molecular oxygen (O2) and ferric iron (Fe3+) are the primary contributors to soil acidification, other studies have observed acidification without detectible pyrite oxidation. There has, therefore, been some speculation that oxidation of other sulfur species may also contribute to soil acidification.
A study by Ward et al. (2004) published in the Australian Journal of Soil Research seeks to shed light on this issue by analyzing the complex chemical processes underpinning acidification. The authors took samples from two locations on the McLeods Creek coastal floodplain in northeastern New South Wales. These samples were incubated in the laboratory to simulate the natural oxidation process. Analysis of the samples was undertaken for 36 days post-incubation to determine the fraction of ʻacid volatile sulfurʼ (SAV) that could potentially contribute to acidification through its oxidation.
Both samples experienced significant acidification during the course of the study, with the pH of one sample decreasing from 6 to 4.3, and the pH of another sample decreasing from 8.3 to 7.5. Although the authors found that the fraction of SAV in both samples increased significantly during the first eight days of the study, and that its oxidation did coincide with some acidification, Ward et al. (2004) concluded that this process was not the primary cause of the acidification as the quantity of SAV oxidised was far less than the quantity of pyrite oxidised. The authors therefore concluded that pyrite oxidation was, indeed, the primary cause of acidification for the samples under consideration.
It can be extremely costly and difficult to rehabilitate soil and water systems once acidified; avoiding acidification via thorough soil mapping processes is, therefore, by far the most effective management strategy. Mapping of acid sulfate soils, as well as the rehabilitation of acid affected land, hinge on an understanding of the complex chemical processes leading to acidification. It is therefore critical that planners and developers, farmers and land managers, in both urban and agricultural communities, have access to the implications of research into these processes – research that often remains only in academic circles.
While studies such as that undertaken by Ward et al. (2004) attempt to clarify some of the unknowns behind soil acidification, it is clear that further research is required to grapple with some of the less understood factors contributing to acidification. It is also imperative that research findings are better targeted towards land managers to help avoid and ameliorate the potential impacts of acid sulfate soils.
Reference
Ward N, Sullivan LA & Bush RT (2004) The process of sulfide oxidation in some acid sulfate soil materials. Australian Journal of Soil Research 42: 449-458. Read Abstract.
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