SULFIDIC MATERIALS IN THE WESTERN MEKONG DELTA, VIETNAM

Sulfidic materials in the western Mekong delta, Vietnam, are of two main kinds. The older ones, deposited more than about 5500 years ago, are several meters deep, have high contents of finely distributed organic matter and few macropores. The younger ones are the most extensive. They are generally less than 1.5 to 3 m deep over non-sulfidic material, have low organic matter contents and common tubular macropores which contain partly decomposed root remnants in the deeper horizons. The distribution of pyrite in the older materials is relatively uniform. In the younger materials, the pyrite is mostly found in the root remnants and other concentrations of organic matter. The boundary between the two materials is locally marked by a major shell and sand ridge. Contents of actual + potential acid are about 0.2 to 0.8 kmol/m3 soil in the upper 0.8 m of both kinds of acid sulfate materials. Acid contents increase with depth to about 0.4 to 3 kmol/m3 soil at 1.5 to 2.5 m in the older materials. In the younger sediments, acid contents decrease to generally less than 0.1 kmol/m3 soil within 2 m depth.