Chemical conditions in acidic water in the Plain of Reeds, Viet Nam

In the Plain of Reeds in the Mekong delta (Viet Nam) about 600,000 ha is covered by pyrite soils. At the beginning of the rainy season acidic material is leached from the soil to water in the canal system. Evaluation of water chemistry data indicate that jurbanite is the acidic mineral which is accumulated at the soil surface and subsequently leached to the canals during the early rains. The results obtained correspond well with published results for acidic soils in other areas. It has earlier been assumed that Fe(II) is the major acidic ion in the water and oxidized slowly. The measurements made in acidic canal indicate that Fe(II) is oxidized to Fe(III) according to a pseudo first-order reaction with a half-time of about 4 h and precipitated as hydroxide. Instead aluminium, produced by weathering, has proved to be the most acidic ion in the water.