Acid sulfate soil

However, if the soils are drained, excavated or otherwise exposed to air, the sulfides react with oxygen to form sulfuric acid.

[1] Under these anaerobic conditions, lithotrophic bacteria such as Desulfovibrio desulfuricans obtain oxygen for respiration through the reduction of sulfate ions in sea or groundwater, producing hydrogen sulfide.

[1] Up to a point, warmer temperatures are more favourable conditions for these bacteria, creating a greater potential for formation of iron sulfides.

Tropical waterlogged environments, such as mangrove swamps or estuaries, may contain higher levels of pyrite than those formed in more temperate climates.

The impacts of acid sulfate soil leachate may persist over a long time, and/or peak seasonally (after dry periods with the first rains).

The process of acidification is accompanied by the formation of high amounts of aluminium (Al3+, released from clay minerals under influence of the acidity), which are harmful to vegetation.

Acid sulfate soils exhibit an array of colors ranging from black, brown, blue-gray, red, orange and yellow.

Acid sulfate soils are widespread around coastal regions, and are also locally associated with freshwater wetlands and saline sulfate-rich groundwater in some agricultural areas.

[9][10] Acid sulfate soil disturbance is often associated with dredging, excavation dewatering activities during canal, housing and marina developments.

[11] Disturbing potential acid sulfate soils can have a destructive effect on plant and fish life, and on aquatic ecosystems.

When cultivated, acid sulfate soils cannot be kept wet continuously because of climatic dry spells and shortages of irrigation water, surface drainage may help to remove the acidic and toxic chemicals (formed in the dry spells) during rainy periods.

[18] A study in South Kalimantan, Indonesia, in a perhumid climate, has shown that the acid sulfate soils with a widely spaced subsurface drainage system have yielded promising results for the cultivation of upland rice, peanut and soybean.

[19] The local population, of old, had already settled in this area and 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.

In the second half of the 20th century, in many parts of the world, waterlogged and potentially acid sulfate soils have been drained aggressively to make them productive for agriculture.

[8] The soils are unproductive, the lands look barren and the water is very clear (again, due to the flocculating effect of Al+3), devoid of silt and life.

An array of technical manuals, maps, databases, and other forms of guidance for identifying, sampling, and/or managing acid sulfate soils have been published by Federal and State/Territory governments over the years.

The national sampling and identification manual provides: background information on acid sulfate soil formation and disturbance processes and environmental impacts; the minimum requirements of a desktop assessment and site inspection; and a guide to sampling and field testing.

Management strategies discussed in the manual include: avoidance, minimisation of disturbance, neutralisation, hydraulic separation, strategic reburial / interment, and stockpiling.

The Queensland government Acid Sulfate Soils: Laboratory Methods Guidelines[37] is recommended for a discussion on the analytical techniques.

Although the National guidance constitutes the primary authority on the subject, and covers a broader range of issues, this manual provides more commentary on the analytical techniques such as SPOCAS.

Inland acid sulfate soil systems across Australia: CRC LEME Open File Report 249[24][39] (Fitzpatrick and Shand, 2008) provides a comprehensive introduction to the subject as well as more detailed discussion on select aspects such as the mineralogy of and toxic gas emissions from acid sulfate soils.

The standard Australian ABA system[31] describes three operationally defined pools of acidity (mol H+ per tonne dry soil).

Potential Sulfidic Acidity (PSA) is a function of the concentration of Reduced Inorganic Sulfur (RIS) in a soil sample.

[37] The chromium reduction method is summarized in Soil Chemical Methods[40]:The Cr reduction method… is based on the conversion of reduced inorganic S to H2S by a hot acidic CrCl2 solution; the evolved gas is trapped quantitatively in a Zn acetate solution as solid ZnS.

[42] Therefore, soils with high amounts of Retained Acidity may require special management considerations to mitigate gradual acidification over tens to hundreds of years.

[43] Monosulfidic material will change from pitch black to light brown within minutes of being exposed to air (see images below).