The dynamic water–sediment system: results from an intensive pesticide monitoring campaign

To gain insight into the dynamics of pesticides in the different compartments of a river system, an intensive monitoring campaign was set up. An extensive dataset is useful when planning to model the fate of pesticides in river systems. In this study we focus on the Nil, a small, hilly basin situated in the central part of Belgium. Two automatic samplers were placed in the river basin, i.e., one at the mouth and one upstream, taking composite samples of water and suspended solids. Undisturbed sediment samples were taken by means of a macro-core and immediately frozen with CO₂-ice. The samples were sliced and analysed for pesticides in pore water and on sediment. The results of the study revealed the dynamics of the different compartments of a river system. The water compartment showed hourly variations in pesticide peak concentrations, regularly exceeding the standards. The amount of pesticides transported by suspended solids increased after a rainfall event. The concentration of pesticides in pore water, measured on a monthly basis, followed the trends of the water compartment remarkably well, but in a significantly lower concentration. These observations may be explained by combined diffusion, biodegradation and chemical reactions occurring in the water column and the sediment. Further insight into the importance of each of these processes can be gained by dynamic modelling, an approach that is currently being followed.