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Soil Survey Technical Note 11

Acid Sulfate Soils in the Coastal and Subaqueous Environment


This technical note provides information on the potential of coastal and subaqueous soils to form acid sulfate soils. It provides a definition of acid sulfate soils and describes the impacts, treatment and management, and testing of these soils. The procedures for testing and classifying these soils are provided in the Keys to Taxonomy (Soil Survey Staff, 2014a), Soil Survey Field and Laboratory Methods Manual (Soil Survey Staff, 2014b), and Field Book for Describing and Sampling Soils (Schoeneberger et al., 2012).


Acid sulfate soils are soils in which sulfuric acid either will be produced, is being produced, or has been produced in amounts that have a lasting effect on main soil characteristics (Pons, 1973). Acid sulfate soils form when sulfide minerals, such as pyrite, and/or elemental sulfur in reduced sulfidic sediments oxidize upon exposure to air. If sulfide-bearing subaqueous soils are dredged and placed in a subaerial environment, sulfides will oxidize, creating sulfuric acid. The sulfuric acid will drastically lower soil pH (to less than 4) and result in acid sulfate soil formation (Fanning and Fanning, 1989). Soils with the potential to become acid sulfate soils pose no threat unless exposed to atmospheric oxygen.

There are three types of acid sulfate soils: potential, actual, and postactive. Potential acid sulfate soils (PASS) refer to waterlogged, anaerobic soils that contain high levels of sulfidic materials. Their field pH is generally 4 or greater (SACPB, 2003). Actual acid sulfate soils have a sulfuric horizon and are highly acidic due to aeration of soil materials rich in sulfides. Acid sulfate soils are considered postactive when weathering and pedogenesis have reached the stage at which sulfide minerals are no longer present near the surface and pH has risen above 4 (Fanning, 2012).

The Keys to Soil Taxonomy classifies soils at the great group level as Sulfohemists, Sulfosaprists, Sulfihemists, Sulfisaprists, Sulfiwassists, Sulfaquents, Sulfiwassents, Sulfaquepts, Sulfudepts, or Sulfaquerts and at the subgroup level as sulfic, sulfuric, or sulfaqueptic.


Sulfidization, or the accumulation of sulfides, is an important soil-forming process by which sulfide minerals (mainly iron sulfides) form and accumulate in anaerobic soil materials (Fanning and Fanning, 1989). This process occurs most frequently in marine or estuarine soil environments where large quantities of sulfate sulfur are available from seawater (for chemical reduction to sulfide) and where iron is available in the form of iron oxides or oxyhydroxides in the soil.

Sulfides most commonly accumulate in marine, estuarine, or river settings and occur predominantly in low-lying areas near the coast, such as coastal flood plains, tidal marshes, rivers and creeks, deltas, coastal flats, backswamps, and mangrove areas. In these settings, sulfate, the second most common anion in seawater, is reduced to sulfide through the metabolism of sulfate-reducing bacteria in the subsurface anaerobic soil (Jorgensen, 1977; Day et al., 1989).

A group of bacteria (sulfate-reducing bacteria) use sulfate (SO4 2–) instead of oxygen in respiration to convert the sulfate to sulfide (S2–) (EPHCNRMMC, 2011). Sulfate-reducing bacteria are found in five phylogenetic lineages, with most isolated strains being organotrophic mesophilic Deltaproteobacteria (Enning and Garrelfs, 2014). In soils under oxygen-depleted conditions, iron combines with sulfur from sulfate to form iron sulfides, in particular pyrite. The sulfide subsequently becomes trapped in sediment by binding with metal ions such as Fe (Jorgensen, 1977). A range of metal sulfide minerals are produced, including mackinawite [(Fe,Ni)1 + xS], greigite (Fe3S4), and pyrite (FeS2) (Rickard and Luther, 2007).

Iron monosulfides (FeS) are often associated with organic-rich new estuarine sediments and oxidize rapidly when exposed to oxygen. Soil materials rich in organic materials and iron monosulfides are called monosulfidic black ooze (MBO) (Fyfe et al., 2006). MBOs are black, often oily in appearance, and greatly enriched in monosulfides high in organic matter. They can form thick accumulations within landscapes of acid sulfate soils.


Acid sulfate soils are most common in coastal regions but may also occur in inland waterways, wetlands, drainage channels, saline seepage areas, and sometimes in uplands derived from marine or estuarine sediments. Signs of acid sulfate soil include visual indicators such as orange-brown water and soil, oil-like slicks and subsurface MBOs, the presence of salinity or salt crusts, and vegetation dieback or shifts to acid-tolerant species (DSEWPC, 2012).

The formation of acid sulfate soil may occur naturally or be influenced by human activities that alter hydrologic regimes and mobilize salt. Anthropogenic structures, such as dams, irrigation, and earthwork construction, or anthropogenic activities, such as water control, dredging, mining, and land-clearing, can lower the water table. Acid sulfate soils form in these areas due to the development of waterlogged and oxygen-free conditions, combined with new sources of sulfates (from either saline ground water or contributions from marine sedimentary rock deposits) and the presence of organic matter and metals such as iron.


Sulfidification can fundamentally change the way aquatic ecosystems function. If deposited near water, acid sulfate soils can create runoff into aquatic systems that is high in aluminum, iron, manganese, copper, and lead, all of which become more soluble at a low pH (Demas et al., 2004). Pulses of acidic, metal-laden water entering estuarine and coastal environments can cause massive kills of fish, crustaceans, shellfish, and other organisms. Moreover, exposure to acidic water can damage fish skin and lead to infection by fungus. Research suggests a strong association between acidity, aluminum, and gill damage in fish (NWPASS, 2000). Harmful algal blooms can also be triggered by acidic water containing dissolved iron and silica (SACPB, 2003).

Acidic scalds or drain spoils are common in areas affected by acid sulfate soils and cause major habitat degradation and loss of biodiversity (SACPB, 2003). In such cases, plant communities decrease in diversity and become dominated by acid-tolerant plants, or soils become unvegetated.

Prolonged exposure of coastal acid sulfate soils to air also causes “soil ripening,” an irreversible loss of water-holding capacity that results in physical, chemical, and biological changes to the soil (SACPB, 2003). Soils have been shown to shrink as much as 50 percent or more, by volume, particularly if peat topsoil is oxidized or areas are drained (SACPB, 2003). This causes subsidence in drained areas.

Saltwater intrusion into previously freshwater soils introduces sulfates. Extended saltwater inundation into freshwater areas enhances sulfate reduction, the primary cause of subsidence and soil mineralization (Hackney and Williams, 2012). Mineralization of peat releases carbon dioxide and methane as well as other elements and results in subsidence. Methyl mercury, which is an environmental concern, can be released during the mineralization process. Sulfate-reducing bacteria methylate mercury when sulfate is present, even at very low levels. Methyl mercury is soluble and bioaccumulates, possibly resulting in high levels of mercury in food (Atkeson and Axelrad, 2004). Hackney and Williams (2012) found that free phosphate was released when sulfate was added to organic soils under anaerobic conditions and not under aerobic conditions. They suggest that the release was the result of sulfate-driven mineralization.

For every ton of sulfidic matter that is oxidized, 1.6 tons of sulfuric acid are produced (NWPASS, 2000). The pH, which normally is near neutral before drainage or exposure, will drop below 3 (Soil Survey Staff, 2014a). In some coastal environments, inputs of calcareous sediment may neutralize the acidity generated during oxidation of sulfides, but this is more often the exception than the rule (Payne and Stolt, 2017).

The impacts of acid sulfate soils are numerous (Sammut and Lines-Kelly, 1996) and include—

  • Mobilization of Fe, Al, Mn, and Cd by sulfuric acid and lower soil pH, making some soils toxic to plant growth and causing scalding (similar to salinity).
  • Damage to infrastructure (such as buildings, foundations, drainage systems, and roads) due to corrosion of concrete, iron, steel, and some aluminum alloys.
  • Blocked drainage systems due to the formation of iron oxides.
  • Release of sufficient sulfuric acid and aluminum to cause fish disease and mortality.
  • Mobilization of aluminum and heavy metals such as cadmium, which can be adsorbed by fish and aquatic life.
  • Rust-colored stains and slimes.
  • Irreversible soil shrinkage.
  • Low-bearing capacity of soils.
  • Human health problems, including heavy metals (such as aluminum, iron, and arsenic) in drinking water, dermatitis, and eye inflammations.
  • Offensive sulfidic odor.
  • Decreased availability of some nutrients for plants.
  • Decrease in animal productivity due to reduced pasture quality and an increased uptake of aluminum and iron by grazing animals.
  • Smothering of aquatic plants by iron precipitates.
  • Deoxygenation.

Treatment and Management

Ideally, areas of high risk for coastal acid sulfate soil should not be disturbed by development activities, utilization of dredge materials, and beach and dune nourishment projects. The cost to the surrounding environment and inevitably to the development itself, through the release of acid and metal ions into the soil and ground water, outweighs any short-term gain. Construction on acid sulfate soils is not recommended due to the potential for infrastructure damage. Where acid sulfate soils have been disturbed in the past, structures have subsided, building materials have been corroded, and agricultural or aquacultural productivity has been markedly reduced (NWPASS, 2000).

To avoid disturbing coastal acid sulfate soils and creating the need for subsequent remedial works or rehabilitation, alternative approaches need to be considered before any earthworks are undertaken. These include—

  • Relocating the development to a low-risk area.
  • Reserving areas of high risk for environmental protection.
  • Redesigning site layouts to avoid potential acid sulfate soils.
  • Using only clean fill, not material from potential acid sulfate soil areas (SACPB, 2003).

Where developments already exist in coastal acid sulfate soils or where they may occur within the coastal zone at risk of environmental or structural damage, remedial actions will be necessary to reduce any adverse impacts and rehabilitate the site and surrounding affected areas (SACPB, 2003). The main strategies for the treatment and management of coastal acid sulfate soils include—

  • Avoidance, leaving coastal acid sulfate soils in an undisturbed state.
  • Minimization of disturbance, not undertaking any activity that results in the release, or accumulation and potential future release, of acid from the oxidation of undisturbed PASS and preventing any lowering of the permanent water table.
  • Strategic reburial or reinternment below the water table, preventing oxidation of soils through long-term or permanent storage in an anoxic environment.
  • Hydraulic separation techniques, removing fine particles of pyrite and monosulfides (PASS fines). (The process generally involves suspending PASS fines in a slurry and separating them from larger particles by either sluicing or cycloning (Queensland Government, 2002).)


Sulfidic soil materials as characterized in the Keys to Soil Taxonomy (Soil Survey Staff, 2014a) commonly occur in intratidal zones adjacent to oceans and are saturated most or all of the time. Current taxonomic criteria (Soil Survey Staff, 2014a) define sulfidic material as waterlogged mineral, organic, or mixed soil material that has a pH of 3.5 or higher, contains oxidizable sulfur compounds and, if incubated as a 1-cm thick layer under moist, aerobic conditions (field capacity) at room temperature, shows a drop in pH of 0.5 or more units to a pH value of 4.0 or less (1:1 by weight in water or in a minimum of water to permit measurement) within 16 weeks or, if the pH is still dropping after 16 weeks, until the pH reaches a nearly constant value (van Breemen, 1982; Soil Survey Staff, 2014a).

The intent of the method is to determine if known or suspected sulfidic materials will oxidize to form a sulfuric horizon. The transition from sulfidic materials to a sulfuric horizon normally requires very few years and may occur within a few weeks. Although not currently recognized as sulfidic materials in Soil Taxonomy, many coastal subaqueous soils experience a significant drop in pH, though not to below pH 4.0 (Payne and Stolt, 2017).

The sulfuric horizon is 15 cm or more thick and is composed of either mineral or organic soil material that has a pH value of 3.5 or less (1:1 by weight in water or in a minimum of water to permit measurement) and shows evidence that the low pH value is caused by sulfuric acid. The evidence is one or more of the following:

  1. Jarosite concentrations
  2. Directly underlying sulfidic materials
  3. 0.05 percent or more water-soluble sulfate

Two methods that relate to criteria in Soil Taxonomy (Soil Survey Staff, 1999) are as follows:

  1. Oxidized pH (incubation) (method 4C1a1a3)

Oxidized pH is used to test for the presence of sulfidic material and to predict the occurrence of sulfuric horizons. It is an indicator of potential acid sulfate soils. Soils are considered potential acid sulfate soils if the sulfide material is waterlogged mineral, organic, or mixed soil material with a pH of 3.5 or higher and, if incubated as a 1-cm thick layer under moist, aerobic conditions (field capacity) at room temperature, shows a drop in pH of 0.5 or more units to a pH value of 4.0 or less within 16 weeks or longer, if the pH is still dropping after 16 weeks, until the pH reaches a nearly constant value (Soil Survey Staff, 2014a).

  1. Hydrogen Peroxide Test, Delta pH; Presence of Reduced Monosulfides

A 3-percent H2O2 solution is applied to soil immediately after exposure to the air (e.g., freshly broken ped or core interior). A positive reaction, resulting in a color change, indicates the presence of reduced FeS, which quickly oxidize and change color upon application of hydrogen peroxide. “Peroxide color change” is an immediate (within 10 seconds), discernible color change upon addition of H2O2. This method is only for detection of monosulfides and is not applicable to other sulfides (e.g., pyrite, marcasite, and FeS2) (Soil Survey Staff, 2014b). Soils are considered potential acid sulfate soils if they contain reduced monosulfides.


The National Leader for Technical Soil Services, NRCS, Washington, DC.


Atkeson, T., and D. Axelrad. 2004. Everglades consolidated report. Chapter 2B: Mercury monitoring, research and environmental assessment.

Day, J.W., Jr., C.A.S. Hall, W.M. Kemp, and A. Yáñez-Arancibia. 1989. Estuarine ecology. John Wiley and Sons, New York, NY.

Demas, S.Y., A.M. Hall, D.S. Fanning, M.C. Rabenhorst, and E.K. Dzantor. 2004. Acid sulfate soils in dredged materials from tidal Pocomoke Sound in Somerset County, MD, USA. Australian Journal of Soil Research 42:537–545.

Department of Sustainability, Environment, Water, Population and Communities (DSEWPC). 2012. Inland acid sulfate soil and water quality. Australian Government.

Ennings, D., and J. Garrelfs. 2014. Corrosion of iron by sulfate-reducing bacteria: New views of an old problem. Applied Environmental Microbiology 80(4):1226–1236.

Environment Protection and Heritage Council and the Natural Resource Management Ministerial Council (EPHCNRMMC). 2011. National guidance for the management of acid sulfate soils in inland aquatic ecosystems, Canberra, ACT.

Fanning, D.S. 2012. Acid sulfate soils. In: S.E. Jorgensen (ed.) Encyclopedia of environmental management, Taylor and Francis, New York, NY.

Fanning, D.S., and M.C.B. Fanning. 1989. Soil morphology, genesis, and classification. John Wiley and Sons, New York, NY.

Fyfe, D.M., L.A. Sullivan, R.T. Bush, and N.J. Ward. 2006. Oxidation pathways of monosulfidic black ooze. Proceedings of 18th World Congress of Soil Science, Philadelphia, PA, 9–15 July. International Union of Soil Sciences.

Hackney, C.T., and A. Williams. 2012. Impact of sea level rise and salt intrusion on everglades peat: Review and recommendations. Final report dated June 22, 2012 submitted to the U.S. Army Corps of Engineers, Jacksonville District.

Jorgensen, B.B. 1977. The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark). Limnology and Oceanography 22:814–831.

National Working Party on Acid Sulfate Soils (NWPASS). 2000. National strategy for the management of coastal acid sulfate soils. NSW Agriculture Wollongbar Agricultural Institute, p. 39.

Payne, M.K., and M.H. Stolt. 2017. Understanding sulfide distribution in subaqueous soil systems in southern New England, USA. Geoderma 308:207–214.

Pons, L.J. 1973. Outline of the genesis, characteristics, classifications and improvement of acid sulphate soils. In: H. Dost (ed.) International Symposium on Acid Sulphate Soils, introductory papers and bibliography. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. pp. 3–27.

Queensland Government. 2002. State planning policy 2/02. Planning and managing development involving acid sulfate soils. Department of Local Government and Planning and Department of Natural Resources and Mines, Queensland, Australia.

Rickard, D., and G. Luther. 2007. Chemistry of iron sulfides. Chemical Reviews 107:514–562.

Sammut, J., and R. Lines-Kelly. 1996. An introduction to acid sulfate soils. Seafood Council, ASSMAC, Department of Education, Science and Training.

Schoeneberger, P.J., D.A. Wysocki, E.C. Benham, and Soil Survey Staff. 2012. Field book for describing and sampling soils. Version 3.0. Natural Resources Conservation Service, National Soil Survey Center, Lincoln, NE.

Soil Survey Staff. 1999. Soil taxonomy: A basic system of soil classification for making and interpreting soil surveys. 2nd edition. Natural Resources Conservation Service. U.S. Department of Agriculture Handbook 436.

Soil Survey Staff. 2014a. Keys to Soil Taxonomy, 12th ed. U.S. Department of Agriculture, Natural Resources Conservation Service, Washington, DC.

Soil Survey Staff. 2014b. Soil survey field and laboratory methods manual. Soil Survey Investigations Report No. 51, Version 2.0. R. Burt and Soil Survey Staff (eds.). U.S. Department of Agriculture, Natural Resources Conservation Service.

South Australian Coast Protection Board (SACPB). 2003. Coastline–A strategy for implementing CPB policies on coastal acid sulfate soils in South Australia. No 33. pp. 1–12.

van Breemen, N. 1982. Genesis, morphology, and classification of acid sulfate soils in coastal plains. In: J.A. Kittrick, D.S. Fanning, and L.R. Hosner (eds.) Acid sulfate weathering. SSSA Special Publication 10. pp. 95–108.