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AbstractA mathematical model was used to predict the dispersion and degradation of methyl parathion (MEP) in freshwater ponds. Basic assumptions of the model were that loss processes could be adequately described in terms of simple partition phenomena and first-order rate kinetics. Predictions of the model were compared with experimentally-obtained data for concentrations of MEP in water and sediment. The rate of loss from water was much faster and accumulation in sediment was much less than was predicted. Sediment-catalysed hydrolysis or biodegradation of adsorbed MEP may account for differences between predictions and observations.The effects of MEP on Daphnia, aquatic insects and rainbow trout were similar to those observed in the laboratory. On the other hand, indirect biological effects occurred that could not be predicted on the basis of laboratory tests. An increase in populations of the crustacean Diaptomus sp. in treated ponds was attributed to mortality of competitors (Daphnia spp.) and predators (Cyclops and aquatic insects). An algal bloom in one of three treated ponds could have been induced by mortality of herbivorous mayfly larvae and Daphnia. Growth of rainbow trout was depressed probably because of mortality of aquatic invertebrates that are important items in their diet.In pond studies with the insecticide cypermethrin only about one-tenth of surface-applied material was subsequently found in subsurface water. Dispersion of this chemical into the aqueous phase was limited by sorption onto aquatic vegetation and suspended sediment. It was shown that only the dissolved material is toxic to aquatic organisms, and once again the toxicity of the dissolved fraction was in accord with laboratory tests.In general, we conclude that a combination of laboratory and field testing is required before a reasonable estimate can be made of the fate and effects of chemicals in natural waters.

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