Pesticide and element release from a paddy soil in central Vietnam: role of DOC and oxidation state during flooding

Insecticide and element release from submerged soils in paddy rice farming systems causes potential hazard to local drinking water wells during the annual monsoon flood season. Intact soil columns collected from paddy fields in the Hue province in Vietnam was submerged with 100 mm artificial floodwater. Experiments were performed under aerobic and anaerobic conditions with and without addition of dissolved organic carbon (DOC). The intact soil columns were spiked with the insecticides fenobucarb, endosulfan and dichlorodiphenyltrichloroethane (DDT), and later flooded. After 24, 48 and 72 h, the floodwater samples were collected to analyze release of elements and insecticides. Insecticide release into floodwater depends on their initial concentration in the soil and was enhanced by the presence of DOC under aerobic conditions more than under anaerobic conditions. Release of Al, Cu, Ni and Pb increased in the presence of DOC in floodwater under aerobic conditions. Meanwhile As, Co, Fe, Mn, Sb and Zn release only increased under anaerobic conditions with highest release without DOC added. Several elements and all three insecticides studied show increased release to floodwater in response to DOC and reducible elements respond to the anaerobic conditions in the soil source.