The natural origins of sulphate are rainwater and solutions of sedimentary evaporitic rocks, particularly gypsum ( CaSO4 ), but also pyrites ( FeS2 ) and, more rarely, igneous rocks ( galenite, blende, pyrite ).

 

Human origins are mainly the use of chemical fertilisers and detergents.

 

In general, the presence of sulphate in non-polluted natural water indicates the presence of gypsum or pyrites.

 

In anaerobic conditions, sulphate is transformed by bacteria into hydrogen sulphide that is toxic and corrosive.

Presence of sulphur in waters may lead to acceptance problems,

 

Hydrogen sulphide ( H2S ) is a gas recognisable by its rotten egg smell at low concentrations.

 

In higher doses, it becomes odourless and very dangerous if inhaled, causing frequent fatal accidents among sewer workers and possible problems in wells ( Laos, ACF 1996 ) when gypsum is present. 

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• AEP standard upper limit is 250 mg/l SO4.

• WHO recommends < 250 mg/l SO4.