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 acidi c material is leached from the soil to water in the canal system. Eval uation of water chemistry data indicate that jurbanite is the acidic m ineral which is accumulated at the soil surface and subsequently leach ed to the canals during the early rains. The results obtained correspo nd 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 t hat Fe(II) is oxidized to Fe(III) according to a pseudo first-order re action with a half-time of about 4 h and precipitated as hydroxide. In stead aluminium, produced by weathering, has proved to be the most aci dic ion in the water.