Acid sulphate soils

The expression “Acid Sulphate Soils” pertains to soils, in which the soil formation process has produced, is producing or will produce sulphuric acids in amounts that have a long-term effect on main soil characteristics. The acid sulphate soils include formations with sulphuric horizon pH <3.5. Acid sulphate soils are often limited to include soils with cat clay phenomenon which is a combination of evident pyrite (iron sulphide i.e. FeS2) often visible as straw-yellow jarositic mottles and a very low pH. In an undisturbed and waterlogged state these soils are relatively harmless.

Yellow pyrite in clay on recently excavated canal bank forming part of drained peat water management system (Photo: J. Jauhiainen).

Yellow pyrite in clay on recently excavated canal bank forming part of drained peat water management system (Photo: J. Jauhiainen).

There are approximately 12.6 Mha of recent, coastal sulphide sediments throughout the world with major deposits in the Asia-Pacific region. The majority of acid sulphate soils occurs in coastal areas and develop in recent or sub-recent marine sediments. Pyrite layers are formed under tidal conditions, thus found in low-lying areas near the coast. They are still being formed in mangrove forests and salt marshes, estuarine and tidal lakes. This links this soil type to tropical wetland and peatland development. Onset of peat accumulation near sea coastlines results usually from a complex interaction between time of exposed ground after water withdrawal, moderately sloping topography, and wet climate. Mangrove, including Melaleuca spp., and some reed species have a fair tolerance to such a condition. Peat formation in the tropics starts on the inland side of mangrove when the fine sediments get trapped into tangled mangrove roots. As the coastline withdraws towards sea, new organic deposits get less frequently inundated by salty tidal water. Over time, these mangrove areas build up and eventually form domes that are rarely flooded. Anaerobic conditions and high sodium sulphide concentration restricts development of decomposing bacteria and slows organic matter decomposition, and peat starts to accumulate. Vegetation will gradually differentiate from mangrove towards peat swamp vegetation and characteristic high water table condition in peat protects the pyrite containing layers or other existent mineral soil type.

However, when disturbed and exposed to oxygen through drainage or excavation, these soils produce sulphuric acid and thiosulphate. The soils in which these processes are active are defined also as “actual acid sulphate soils” (ASS). The formed acids can be partially neutralized by neutralizing bacteria and appropriate chemical environment in soil component while the rest can leach and cause severe acidification in drainage water. The acid attacks clay particles and causes the dissolution of structural aluminium, iron, manganese, and other heavy metals which become available to biota in potentially toxic quantities in soil solution. Combined acidification and toxic elements cause ecologically and economically huge losses such as a loss of vegetation. Furthermore extensive mortality and collapses in the reproduction success of aquatic life may result from acidification and toxins released.

Major land development projects have been commenced either without independent environmental impact assessment or proper understanding of the tropical peatland ecosystem, or without consideration of acid sulphate soil related hazards under the peat. One of the largest single development projects in SE-Asia has been the Million-Hectare Mega Rice Project (MRP) – an ambitious attempt aiming to convert tropical peat area into rice fields in Central Kalimantan, Indonesia.

Former tropical peat area where the peat has already disappeared and agriculture is attempted to continue on exposed acid sulphate soils. Mekong delta, Vietnam. Photo: J. Jauhiainen

Former tropical peat area where the peat has already disappeared and agriculture is attempted to continue on exposed acid sulphate soils. Mekong delta, Vietnam. Photo: J. Jauhiainen

Key words in finnish: happamat sulfaattimaat, happamat maat, happamuus, suo, tropiikki, pyriitti, rautasulfidi, hapettomuus, myrkyllisyys, turve, sedimentti, mangrove