Behaviour of pesticides in a water/sediment system under laboratory and field conditions

Experiments were conducted in order to evaluate the behaviour of some pesticides commonly used in rice production. At a laboratory scale, but exposed to the environmental conditions, a known concentration of pesticides in water Rowed through a cement channel over a sediment layer, at a constant flow rate. The collected water was homogenised and analysed by gas chromatography/HPLC to assess the concentration of each pesticide. Under these conditions, the reduction of each pesticide concentration on the nature of it and on the water flow rate. The lower the flow rate, the grater the. rapesticides in the water that ran over the sediment layer. Experiments were also carried out under field conditions, using a 1200 in length, 2 in width and 0.3 in depth rice field channel. A pesticide mixture of known concentration was poured an the flowing water and mixed thoroughly. After the arrival of this contaminated water at preestablished sampling points down the strewn, the concentration of each pesticide in water was determined. The longer the distance of the sampling points downstream from the starting point, the lower the pesticide concentration found. Only a few pesticides were detected at the 1200 m sampling point, but at very low concentration. The others were not detected. It was pointed out that the channel length was the most important factor in the reduction of the pesticide concentration in the flowing water. The influence of the flow rate on this reduction could not be dearly related as in the laboratory experiments.