Acid Sulfate Soil (AAS) and its impacts in Sri

Lanka

S. Santharooban & s.d. Muralitharan

Introduction

• Acid Sulfate Soil (ASS) is the common name given to soils and sediments containing iron sulfide mineral such as Pyrite.

• When pyrite containing soil is waterlogged or covered, there is no problem to environment.

• But when exposed to oxygen, these soil produce sulfuric acid, release toxic quantities of Al and Fe.

The term ASS include both

Actual acid sulfate soils (AASS)

Potential acid sulfate soils (PASS)

Potential acid sulfate soils (PASS) are pyrite-bearing sediments that have the potential to

oxidise and generate sulfuric acid when exposed to oxygen.

PASS layers may be present 5 m above MSL.

Actual acid sulfate soils (AASS)

When iron sulfide are exposed air, it produce sulfuric acid. The soil containing sulfuric acid is known as actual acid sulfate soil. pH of AASS is usually less than 4.

Geological background of ASS

ASS is formed from the pyrite containing soil.

Pyrite consist of Iron and Sulfur

Formation of pyrite

Certain anaerobic bacteria produce pyrite when there are more favourable conditions for these bacterias

For pyrite to form, it requires

a supply of sulphur (usually from seawater)

anaerobic (oxygen free) conditions a supply of energy for bacteria (usually

rotting organic matter e.g. mangrove leaves)

a system to remove reaction products (e.g. tidal flushing of the system)

a source of iron (most often from terrestrial sediments)

temperatures greater than 100C

Formation of ASS

When pyrite containing soil is exposed to air,

FeS2 (Pyrite) + Oxygen + Water

Variety of iron compounds & H2SO4

accelerated by bacteria such as Acidithiobacillus ferrooxidans

How pyrite exposed to air?

• This is mostly due to human activities such as • Mangrove destruction• Excavation in coastal area for shrimp farm

Pyrite

Now pyrite soil is covered by water logging and mangrove swamps

Pyrite soil is now exposed to airO2 O2

FeS2 FeS2

Shrimp Farmas are established

What are the chemical reactions, involved?

Oxidation of pyrite by oxygen is slow

Generation of Fe3+ is mediated by iron-oxidising bacteria, particularly Thiobacillus ferooxidans .

Step 1

Step 2

Step 3

Here the Fe3+ is a more effective oxidant. oxidation of pyrite by Fe3+ much faster than the reaction of pyrite with oxygen.

So, Overall reaction is as follows

It depends on low pH (pH < 4) for Fe3+ to remain soluble otherwise it is precipitated as ochre.

so rapid oxidation of pyrite only takes place at very low pH.

Where acid sulfate soils are found?

• ASS generally found in less than 5 m above MSL, especially in low lying areas, estuaries, lagoon, salt marshes, wetlands, etc.

• mostly in coastal wetlands where development pressures are intense.

• Some inland marshes subject to saline seepage also develop acid sulfate soils.

• Any coastal wetlands which have peat.

• Sulfur is found in three forms in peat such as • part of the organic matter, • as mineral sulfide, and • mineral sulfate.

• In coals, sulfur occurs mainly as pyrite and organic sulfur.

Peat Coal +diagnesis

Organic Sulfur

Sulfate

More pyrite

• These inorganic sulfur, sulfur minerals associated with the formation of ASS in wetlands and other peat existing environment. • Peaty soil can easily be transformed into the acid sulfate soils, once is exposed to the aerobic environment.

World statistics on ASS

• Recent estimates suggests a total of 24 million ha land have where AASS and PASS are present world-wide.

• Mostly they are found in highly populated area.• About 12.5 million ha of total ASS cover found in

low lying coastal lands of – South East and East Asia, – West Africa and – Latin America.

Reported world statistics on ASS (estimates in thousands of hectares)

Australia 3000 Galloway;

Bangladesh 226 Rahman 1990

Brazil 1111 FAO 1974

China 100 Gong Zi Tong 1990

Central America 650 FAO 1974

India 293 Dent 1990

Indonesia 4109 Soekardi 1990

Kampuchia 211 Dent 1990

Kenya <100 Sombroek et al. 1980

Sweden 140 Oborn 1994

Sri Lanka 20 Dent 1990

Madagascar 528 FAO 1974

Malaysia 657 Dent 1990

Myanmar 1200 estimate

Nigeria 1000 estimate

North America 100 estimate

Philippines <500 Brinkman + Singh 1982

Senegal 600 Khouma and Toue 1982

Thailand 1500 Krishnamra 1990

Uruguay 37 FAO 1974

Venezuela 2000 van Breman 1980

Vietnam 2140 Bui Quang Tran 1990

Occurrence of ASS in Sri Lanka

• Most of the inter-tidal areas in Sri Lanka have a pyritic zone which is acidic or potentially acidic.

• The West and South-West coastal belt of Sri Lanka enclose an area of 30000 ha of low lying lands potential for presence of Acid Sulfate Soils.

• The South-Western coastal belt consist – Negombo lagoon, – Lunawa lagoon, – Muthurajawela salt marshes – Wetlands, – mangrove vegetations etc.

• Muthurajawela swamp extent to an area of 20 km2 and is estimated to contain 50 billion MT of peat that contains about 25-40% carbon.

• Peat layers are extensively seen in Muthurajawela swamp .

• Age of peaty layers are about 30000 years. • The extent of peat in other areas are less extensive in

a irregular fashion.

ASS mainly found in South Western Coastal region of the Sri Lanka.

Mainly due to peat deposit

• The south-western coastal belt of Sri Lanka is subjected to frequent flooding and salt water intrusions.

• Hence the potential on formation of ASS is high. • Some studies showed that PASS and ASSS are found in

Malimboda and Kapduwa in Matara ditrict. • Also most developing cities of the country are located along

the south-western coastal belt.• Because of the lack of proper understanding about the PASS,

those lands used to many development activities. • Hence, possibility of the formation of AASS from the PASS in

the southern coastal belt is rather high.• That will perhaps affect many of the agricultural lands in the

future.

Impacts of ASS

• Soil pH become less than 4 and may be as low as 2. i.e. Production of H2SO4

• brings toxic concentrations of Al 3+, As ion and heavy metals into solution.

Primary impacts are:

This will develop secondary effects such as • Environmental effects• Ecological effects• Health effects• Economic and engineering effects

Environmental effects • When acid reach the aquatic environments this can:

– kill fish, crustaceans, annelid worms, shellfish and oysters

– change aquatic plant communities – cause fish diseases (breaks down defenses against

diseases)

Lesion on fish body

In fish, it can induce the Epizootic Ulcerative Syndrome (EUS) diseased condition

Ecological effects 1. Habitat degradation

In waterway habitats, drainage from oxidised acid sulfate soils:– destroys food resources – displaces biota – precipitates iron on vegetation and microhabitat – alters the chemical and physical properties of the

water – degrades spawning and nursery grounds

2. Poor plant productivity

Poor plant productivity and stunted growth at low soil pH can be caused by:– toxic effects of aluminium, iron and manganese – deficiency in plant base minerals such as calcium,

magnesium and potassium – low availability of nutrients – increased attacks by plant pathogens – decrease in soil microbes, particularly those responsible

for nitrogen fixation – stunting of roots producing water stress – heavy deposits of ochre can choke vegetation and block

drains.

Health effects

The possible health impacts could include:– stunted growth, poor health and mental

impairment caused by drinking or bathing in aluminium-rich waters

– dermatitis as a result of skin contact with acid soil materials

Economic and engineering effects

include – corrosion of steel and concrete, – uneven subsidence, – very high permeability of undisturbed mud but

low permeability and slow consolidation of reworked material, and

– the blockage of drains by ochre.– Abandoning of aquaculture (e.g. shrimp farm)

industry

Current Management Methods

1. Chemical neutralisation – By the application of lime, dolomite, calcite or magnesite

2. Site selection criteria – Identification of PASS enables decisions making

3. Water management – Seawater is often used to neutralise, dilute and remove acid

and iron flocks. 4. Capping, compaction and lining

– Compressed laterite is sometimes used to create a barrier between ASS and the pond water and also to reduce contact of run-off with ASS.

– Plastic liners have also been used