CONTAMINATION OF SURFACE-WATER AS AFFECTED BY LAND-USE IN ACID SULFATE SOILS IN THE MEKONG RIVER DELTA, VIETNAM

Leaching toxic substances out of the rootzone is an effective measure for improving soil quality and crop yield in acid sulfate soils (ASS). However, leaching of ASS may pollute the environment. We quantified t he concentration and amount of pollution from leaching of ASS for rice , pineapple and yam cultivation in a Typic Sulfaquept in the Mekong Ri ver Delta, Vietnam, The fields studied were originally uncultivated an d had been reclaimed 2 months, and 1 and 2 years prior to the initiati on of the study. Pineapple and yam were cultivated on raised beds cons tructed using soil excavated from adjacent lateral ditches. The pH of the drainage water ranged from 2.9 to 3.9 and aluminum concentration f rom 3 to 13 mmol(+) 1(-1), The mean monthly aluminum concentration in the leachate from pineapple and yam raised beds was about three times greater than that from rice fields. The monthly total amount of alumin um released by the upland raised beds could be as high as 16690 mol ha (-1), and was three to five times greater than that from rice fields. Consolidation and crust forming in pineapple raised beds reduced the c oncentration and amount of aluminum released with respect to the age o f the raised beds. Pollution from ASS leaching was probably greatest i n June due to a combination of the highest total amount of aluminum re leased to the canal network and low river discharge. Environmental haz ards make it imperative to plan carefully the reclamation of ASS, such that the toxicity carrying capacity of the surface water is not excee ded.