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Acid sulfate soil

What are the effects of acid sulfate soils?

With increased acidity (lower pH) in acid sulfate soils, iron and aluminium may become soluble in toxic quantities, with their precipitates affecting water quality and coating streambanks and benthic (sediment-dwelling) organisms.

Deoxygenated water may also be related to acid sulfate soil oxidation, together with the decomposition of pasture plants killed by waterlogging. Secondary oxidation of the Fe2+ produced by acid sulfate soil oxidation consumes oxygen and lowers the level of dissolved oxygen in streams. Silica and other compounds dissolved from acid sulfate soils by acid may also cause algal blooms.

Fresh water is particularly affected by acid due to its lack of buffering ions. Substances produced by acid sulfate soil also tend to be poorly diluted because of the generally low flows of Australian rivers.

Low pH and aluminium toxicity can cause massive kills of aquatic life, including large numbers of fish, despite the ability of mobile organisms to often avoid poor quality water. Red spot disease of fish (Epizootic Ulcerative Syndrome) has also been linked to acid water.

Mullet affected by red spot disease.

In addition to the acute effects of acid sulfate soils, there are also chronic effects on aquatic systems. These effects are common and widespread. They include diseases and reduced hatching, survival and growth rates for a wide range of aquatic species. Impacts often observed are:

habitat degradation
fish kills
outbreaks of fish disease
reduced resources for aquatic food
reduced ability of fish to migrate
reduced recruitment of fish
changes to communities of water plants
weed invasion by acid-tolerant plants
indirect degradation of water quality

Other potential impacts include the release of heavy metals from contaminated sediments, human and animal illhealth caused by polluted water, decreased productivity of agricultural land, and damage to infrastructure such as bridges.

Oxidation and subsequent reactions also change the soil fabric. Oxidation products such as aluminium ions flocculate the clays, which do not redisperse during re-wetting, leading to irreversible shrinkage and lowering of ground surfaces.

Acid sulfate soils have economic impacts on most industries on the NSW coast including recreational fishing, commercial fishing, oyster growing and other aquaculture industries, cropping, grazing and dairying.

In 1995, over $1 million worth of sea mullet was discarded by NSW commercial fishers because of red spot disease (Epizootic Ulcerative Syndrome). Losses in production of Sydney rock oysters from acid sulfate soil impacts were estimated at $7 million over a period of six years. Overall, the cost to NSW fishery resources due to acid sulfate soils has ranged from $2.2 to $23 million per annum. Recreational fishing and tourism has been affected in some areas.

Acid sulfate soils underlie significant areas of coastal Australia. The cost of testing, treating and monitoring of acid sulfate soils (depending on the pyrite content) can add substantially to the cost of urban development in coastal areas.

Concrete corrosion of a floodgate.

The costs associated with damage to public and private infrastructure such as corrosion of drinking water pipes, bridge footings and floodgates is significant. Local government has spent millions of dollars in NSW replacing iron water pipes corroded by acid groundwater in acid sulfate soil areas. Concrete corrosion of road and rail bridges and other structures has been observed from the Shoalhaven in southern NSW to Cairns in northern Queensland.

The impacts of acid sulfate soil oxidation constitute the most acute water-based environmental problem in coastal areas of NSW, and are comparable in environmental and economic terms to the effects of salinity on inland waters.

It is now recognised that certain environmental effects of the oxidation of acid sulfate soils can last for hundreds or thousands of years.