Acid Sulfate Soil Acid sulfate soils are soils that contain metal sulfides (mainly pyrite and black iron monosulfides)that can oxidize to generate acidity upon exposure to atmospheric oxygen and moisture. There are two end member types of acid sulfate soil; actual acid sulfate soil (AASS) in which most of the sulfides have oxidized and the soil is highly acidic, and potential acid sulfate soil (PASS) in which the sulfides have not yet oxidized and the soil may be neutral or mildly alkaline. There are also many soils that are only partly oxidised in between these two extremes and they can have different acidities depending on the sulfide mineral content, the abundance of acid neutralizing minerals in the soil (mainly calcium carbonate), how much oxidation has taken place and how much acid has been flushed into nearby ecosystems. All types of acid sulfate soils are widely distributed around the world and can cause severe agricultural, engineering and environmental problems if they are not properly managed. Potential acid sulfate soils are naturally occurring soils, sediments or organic substrates (e.g. peat) that formed under waterlogged conditions where there was sufficient sulfate present to sustain populations of sulfate reducing bacteria. Fresh PASS contains fine-grained iron sulfide minerals that are commonly highly reactive, but so long as they are undisturbed and remain below the water table they cause no environmental harm or damage to human infrastructure. However if these soils are drained, excavated or exposed to air by any lowering of the water table, the sulfides will readily react with oxygen to form sulfuric acid and dissolved iron that can react with water and oxygen to produce still more acid. Release of this sulfuric acid when PASS reacts to form AASS can in turn mobilise iron, aluminum, and other heavy metals (e.g. copper, lead, zinc & cadmium) within the soil. Furthermore, if arsenopyrite is one of the sulfide minerals present, as it is in several parts of the world, the breakdown of the sulfides can release hazardous quantities of arsenic. Once mobilized in this way, the acid and metals can have many adverse impacts: killing vegetation by acid scalding or metal toxicity, seeping into and acidifying groundwater and water bodies, killing fish and other aquatic organisms, increasing the incidence of some fish diseases such as red spot disease, poisoning humans and other animals consuming affected groundwater, and degrading concrete and steel structures to the point of failure. The soils and sediments that are most prone to becoming acid sulfate soils are those that formed within the last 10,000 years, after the last major sea level rise when sea level rose to slightly above present levels. When the sea level rose and inundated the landand sufficient organic matter was available, anaerobic sulfate reducing bacteria (e.g. Desulphovibrio desulphuricans) were able to reduce sulfate in the seawater to sulfide that could then react with iron oxides, and any other available metals, to form metal sulfides. Warm temperatures and calm water are more favorable conditions for these bacteria, creating a greater potential for formation of iron sulfides in tropical waterlogged environments, such as mangrove swamps, salt marshes, seagrass beds, harbours and estuaries; in wave dominated environments near-surface sediments don’t get to accumulate large amounts of sulfides but they can still accumulate in subsurface sediment. However, acid sulfate soils are not restricted to the coastal zone and they can also form in inland waterlogged soils if there is enough sulfate available for the bacteria, in salt lake sediments and saline river sediments, in over used and fertilized rice paddy fields, and in drains or ponds affected by sulfate-rich acid mine drainage water.
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The metal sulfides formed in these sediments will remain stable while conditions remain anaerobic, which effectively means, so long as the sediment remains under water or below the water table. However, when these sulfides are exposed to air they start to oxidise and produce sulfuric acid as the PASS are converted to AASS. Much of the AASS in coastal lowlands started to form when sea level fell slightly (about 1 – 1.5 m) about 6,000 years ago but the conversion of PASS to AASS has been greatly accelerated in many areas by drainage work and other land use practices. Usually, the generation of PASS to AASS is slow but certain aerobic bacteria (Thiobacillus thiooxidans and Thiobacillus ferrooxidans) can increase the rate of sulfide oxidation but about one million times; once the pH of the soil gets below about 3.0 sulfide oxidation by ferric iron can also increase the rate of sulfide oxidation but about one million times. Hence, disturbing a soil that has been fairly innocuous for a long time can cause severe acid sulfate soil problems to develop very quickly and once the problems have developed they are difficult to reverse without extensive physical and chemical intervention. The impacts of acid sulfate soil leachate may persist over a long time and in some areas, PASS that were exposed to air hundreds of years ago are still releasing acid. In many areas acid that is slowly generated in the soil profile can be stored within the soil during a long dry season only to be flushed out into creeks and rivers when the next flood comes, often resulting in major fish kills and other environmental problems. Geographical distribution Acid sulfate soils have been documented over a large land area in Asia (about 18-20 million hectares); mainly in Cambodia, China, Indonesia, Malaysia, Thailand and Vietnam. They are widespread in coastal regions and are often associated with freshwater wetlands and saline sulfaterich groundwater in some agricultural areas; paddy fields are particularly vulnerable. In Australia, coastal acid sulfate soils occupy over 4 million hectares, underlying coastal estuaries and floodplains near where most Australians live. Acid sulfate soil disturbance is often associated with farm drainage, dredging, excavation, land reclamation operations, and dewatering activities during canal, housing and marina developments. Acid sulfate soils in the Mekong delta cover 1.6 million hectares, of which about 400,000 ha are in the Plain of Reeds and all of which have a very high acid generation potential when fully oxidised. Reclamation of the 150,000 ha of still uncultivated AASS became a national priority in 1990 and now attracts local farmers and migrants. However, these soils present major agronomic and environmental problems and farmers urgently need advice to reclaim them and considerable work will be required to make these soils productive because of their very high variability. Because the acid, together with associated toxic elements (heavy metals and other contaminants), can kill plants and animals, contaminate drinking water and food such as oysters, and corrode concrete and steel, land managers need to be able to identify those areas where development is either best avoided, or is going to need some special treatment. Broadening the Definition Traditionally, the use of the term “acid sulfate soil� was confined to coastal soils evolved from sulfidic estuarine and marine sediments. However, in recent years, inland soils and sediments that contain metal sulfides are increasingly referred to as acid sulfate soils and these are common in many rivers, salt lakes and salinas in arid and semi-arid areas and they are especially widespread in mine sites where mining activity exposes metal sulfide ores or sulfidic strata associated with coal and other resources. Problems resulting from the conversion of PASS to AASS have also been created in inland areas by highway and other construction activities that expose sulfide containing
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strata, and in coastal areas by dredging harbours and shipping channels and by land reclamation activities where excavated material includes sulfidic sediment. Acid rock drainage (ARD) or acid mine drainage (AMD) has attracted substantial management and research efforts from mining industry and scientific communities. Both, AMD and ARD share similar mineralogical and geochemical properties and they can be treated using similar control measures and remediation technologies. It is expected that a gathering of researchers and industry practitioners in both of these areas will be to the benefit of each other by allowing information exchanges between ARD specialists and ASS specialists. That such a combined meeting is being held at the South China Agricultural University in Guangzhou, China, in September 2008 reflects both the close links between ASS and ARD and the importance of solving problems associated with ASS and ARD in the southeast Asian region.
Impacts of Acid Sulphate Soil Disturbing PASS can have a destructive effects on plant and fish life, and on coastal ecosystems. Flushing of acidic leachate to groundwater and surface waters can cause many adverse impacts, including: Ecological damage to aquatic and riparian ecosystems through fish kills, increased fish disease outbreaks, dominance of acid-tolerant species, precipitation of iron oxyhydroxides, etc. Effects on estuarine fisheries and aquaculture projects (increased disease, loss of spawning area, reduction in fish stocks, etc). Contamination of groundwater with arsenic, aluminum and other heavy metals. Reduction in agricultural productivity through acid and metal contamination of soils (predominantly by aluminum). Damage to sewerage and water supply infrastructure through the corrosion of concrete and steel pipes, damage to bridges, building foundations, roads, etc. Agricultural impacts In many areas, potential acid sulfate soils (also called cat-clays) are not cultivated because of the risk to the environment and because they are not very fertile. However, some are planted with rice, so that the soil can be kept wet to prevent oxidation. Subsurface drainage of these soils is not advisable. When cultivated, acid sulfate soils can seldom be kept wet continuously because of climatic dry spells and shortages of irrigation water and hence, some form of surface drainage may be necessary to help to remove the acid and toxic chemicals that formed in the dry season, during subsequent rainy periods. Eventually, surface drainage combined with sub-surface drainage (mole drains) and treatment of discharge water can help to reclaim acid sulphate soils, but the process may require many years or decades. The indigenous population of Guinea Bissau has thus managed to develop the soils, but it has taken them many years of careful management and toil.
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In an article on cautious land drainage the author describes the successful application of subsurface drainage in acid sulfate soils in coastal polders of Kerala State, India and also in the Sunderbans, West Bengal, India, where acid sulfate soils have been developed for agricultural use. A study in South Kalimantan, Indonesia, in a per-humid climate, has shown that the acid sulfate soils with a widely spaced subsurface drainage system have yielded promising results for the cultivation of upland (sic!) rice, peanuts and soy beans. People who had originally settled in this area were able to produce a variety of crops (including tree fruits), using hand-dug drains running from the river into the land until reaching the back swamps. The crop yields were modest, but provided enough income to make a decent living. Reclaimed cat-clays usually have a well-developed soil structure, and good permeability, but they are infertile due to the leaching that has occurred. In the second half of the 20th century, in many parts of the world, waterlogged soils and PASS have been drained aggressively to make them productive for agriculture. however, the results were disastrous. The soils are unproductive, although they can be colorful, the lands look barren and the water is very clear and devoid of silt and life. Discussion There is quite a bit of material published about acid sulfate soils in Vietnam and, there has even been a conference on the problem held in Ho Chi Minh City, Vietnam in 1992. There is no doubt that the problem is huge in Vietnam and that there are enormous potential opportunities in agriculture in Vietnam and there will be even more opportunities when canals and other water bodies are dredged and sediment is placed where it is exposed to oxygen. Sediment dredged from canals and the like are usually some of the worst possible material because it contains iron monosulfides, which are the most reactive acid generating forms of sulfide; these sediments already create huge problems elsewhere. Although the chemistry of acid sulfate soils was recognized only a few of decades ago, they cause huge problems worldwide and there have already been many international conferences on the topic (the next one is later this year in China). There are thousands of square kms of coastal land affected by acid sulfate soils around the world and each square km can generate tens of tons of sulfuric acid per year. Quite apart from damaging agriculture and aquatic ecosystems, the acid sulfate soils can cause serious damage to buildings and infrastructure such as bridges, concrete pipes and drains. The cost of the problems caused by acid sulfate soils is estimated to be in the hundreds of millions of dollars per year in Australia alone. Re-flooding does not necessarily solve the problem although regular flushing with seawater will fix eventually restore some stability because seawater has some available alkalinity (about 120 mg/L calcium carbonate equivalent alkalinity). When it comes to dredging canals and harbors, the problem can be even worse because here you have monosulfide black oozes and not just pyrite. The monosulfide black oozes are a more reactive form of iron sulfide and they can react to produce vast quantities of acid in as little as a few hours. These monosulfide black oozes are the main problem for shrimp farms and they get worse as dead algae and shrimp excrement accumulate in the ponds and cause geochemical conditions to become more reducing. Such reducing conditions favour the biogenic formation of monosulfide black oozes and then any oxidisation can rapidly produce vast quantities of acid. The acid is bad enough for the
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shrimp but it can also release a lot of metals (particularly aluminum) from the sediment and this can compound the problems for marine organisms. Normal fertilizers (both compost and inorganic compounds such as superphosphate) tend to increase boom-crash cycles for algal growth in water bodies and this leads to cycles in monosulfide black ooze formation and then acid generation. This process then leads to loss of productivity in shrimp farms and damage to agricultural land (particularly rice fields) and aquatic ecosystems near affected areas. No one knows the full extent of the problem in Vietnam or elsewhere in Asia. Even in Australia, where large financial and technical resources have been applied to solving problems associated with ASS, the distribution and seriousness of acid sulfate soils is still being mapped; Australia was the first place to start doing this work and remains the world leader on acid sulfate soil assessment. It is quite easy to do analyses to quantify the distribution of acid sulfate soils but the work takes a lot of time. It is also necessary to distinguish between AASS (i.e. those where the sulfides have already been oxidized and the acid already exists in the soil - TAA) and PASS (i.e. those soils containing sulfides that will oxidize given any exposure to oxygen - TPA) and to determine the proportions of actual and potential acidity in soils characterized by partial or incomplete oxidization. The Solution Normal fertilizers (both compost and inorganic compounds such as superphosphate) tend to increase boom-crash cycles for algal growth in water bodies and this leads to cycles in monosulfide black ooze formation and then acid generation. This process then leads to increased damage to shrimp farms, agricultural land (particularly rice fields) and aquatic ecosystems near affected areas. Our solution is to use our bio-converted material and amend it with Bauxsol™. This blended product will provide all the characteristics of a rich bio-fertilizer product while addressing the ASS remediation problem. Our process will blend the amendment to exacting conditions after testing the local environment (both water and sediment). Bauxsol™ addition either directly or as an additive with compost can solve this problem very effectively by neutralizing any acid that is produced thereby preventing the pH falling to the extent that ferric iron can become an oxidant and accelerate the decomposition of sulfides (by up to a million times). The Bauxsol™ also locks up potentially damaging elements (e.g. aluminum, arsenic, copper, etc.) and it is more effective than lime because it cannot be leached out by water. Furthermore, if not quite enough lime is added, the resulting bicarbonate ions can act as a catalyst and accelerate the decomposition of sulfides whereas Bauxsol™ does not add significant bicarbonate because it uses hydroxyl ions to neutralize acid rather than carbonate or bicarbonate. Optimization calculations will be based on soil analysis for each application of our blended product. Some further research must be done on the subject on the actual soils found in the area and a census undertaken to determine the extent of the problem. Our blended product will establish a new paradigm for agricultural amendments for the Vietnamese markets.
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Technical Adviser to ENVIRtrade: Dr. David M McConchie PhD Southern Cross University, Professor (personal chair), researcher, and consultant in Geochemistry, Geology, and Sedimentology at the Centre for Coastal Management, Southern Cross University. Research Interests: Research and consultancy work over the last 25 years has focused on the geochemistry of trace metals in sediment, water and biota; acid sulfate soils and acid mine drainage; early diagenetic mineral transformations; trace element speciation in sediments; the influence of biota on trace element distributions in sediment and water; the use of bauxite refinery residues in environmental remediation; and the use of geochemical engineering procedures in securing a sustainable future for mining and minerals processing industries. Much of this work has been in the new discipline of Engineering Geochemistry, which involves examining natural geochemical processes and using these processes in the management of environmental problems, particularly problems affecting active and derelict mine sites. Geochemical and sedimentological consultancy work has been carried out for a wide range of industry and government organizations, in Australia and overseas and emphasis in these studies has been placed on the development of practical solutions to environmental problems. Recent work on the use of Bauxsol™ (modified bauxite refinery residues) and Bauxsol™ blends to treat acid rock (mine) drainage water, tailings dam water, industrial waste waters, sewage effluent, nuclear wastes, acid sulfate soils and sulfidic mine tailings and waste rock, and to remove arsenic, radium and fluoride from human drinking waters has proved to be exceptionally effective and the technology is now being marketed worldwide.