Behavior of pesticides in water-sediment systems.

Many experimental reports on the fate of pesticides in either laboratory or outdoor water-sediment systems have been obtained from both research and regulatory aspects that show some trends in distribution and degradation for each chemical class of pesticides. Adsorption, diffusion, hydrolysis, and biodegradation processes are important in controlling the behavior of pesticides in these water-sediment systems. Through these investigations, the contribution of suspended particles and dissolved organic matter has become more accepted in relation to these processes. Not only the physicochemical properties and degradability of a pesticide but also the characteristics of the many phases composing a water-sediment system determine the actual pesticide behavior, and therefore we should appropriately design an experimental system by considering the real situation of the natural aqueous environment to be examined. Many factors controlling experimental results in a laboratory system such as water-sediment ratio, depth of water and sediment phases, and mixing of water column have been clarified; however, there are still many issues to be examined. For example, a pesticide is always used as a formulation, but its effects on pesticide behavior in a water-sediment system have not been extensively examined. When its behavior in a natural aquatic system is considered, the effect and importance of photolysis are necessary to examine as an individual degradation process, but photolysis has been only briefly discussed in outdoor microcosm and mesocosm studies. Many studies discuss the distribution and degradation pathways of a pesticide, but its transport between water and sediment phases has scarcely been investigated because of its complexity, especially for a pesticide that is moderately or easily degraded in a water-sediment system. This form of investigation would be very useful when metabolites or degradates having more toxicological impact on aquatic species and sediment dwellers are found. From this point of view, the behavior of a pesticide and its metabolite(s) in an interstitial sediment porewater should become another critical point to be examined in the future. Other issues to be investigated further are the relevant processes in the neighborhood of interfaces. In an air-water interface, the effect of a surface microlayer has been examined mainly through microcosm and mesocosm studies, but the contribution of interfaces to either volatilization or photodegradation should be examined in more detail to precisely estimate dissipation profiles of a pesticide in the real aquatic environment. Furthermore, the enrichment of a pesticide in this interface should be investigated in relation to an emergence of chironomids. Recently, many kinetic approaches have been attempted to more effectively use experimental data in prediction of the fate of a pesticide by the aid of a simulation model. Most existing rate data usually represent apparent dissipation rates but not degradation rates, and therefore separation of the degradation rate from dissipation by considering adsorption-desorption and transport processes would be of immense value.