Iron is a major problem in Southeast Asia for aquaculture ponds in coastal areas. In Asia some 7 million hectares (Mha) are acid sulphate soils, heavy in iron.
The high iron concentrations come from either pyrites or accumulated iron in mangrove swamps. When the ground is disturbed with pond excavation, oxygen is introduced. This produces iron complexes and sulphuric acid (H2SO4).
Pyrites compete with aquatic life for more oxygen to oxidise and become ferric ion (Fe3+).
Water and pond bottom becomes acidic, and iron is observed in the pond.