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 flowed 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 pestcide concentration in water depends on the nature of it and on the water flow rate. The lower the flow rate, the grater the reduction in the concentration of pesticides in the water that ran over the sediment layer. Experiments were also carried out under field conditions, using a 1200 m length, 2 m width and 0.3 m depth rice field channel. A pesticide mixture of known concentration was poured on the flowing water and mixed thoroughly. After the arrival of this contaminated water at preestablished sampling points down the stream, 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 clearly related as in the laboratory experiments.