Evaluation of models used to assess the fate of chemicals in aquatic systems

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Abstract
Five reference chemicals, 2,5,4′‐trichlorobiphenyl (3‐CB), chloroform, parathionmethyl (MEP), pentachlorophenol (PCP) and 3,4‐dichloroaniline (DCA), were applied to experimental outdoor ponds by sub‐surface injection. Samples of water, sediment and vegetation were removed and analysed for residues at various intervals after treatment. The fate of these chemicals in the ponds was predicted through the use of process analyses and mathematical models. The predicted rates of loss were compared with experimental observations. Data obtained for 3‐CB were fitted to a three‐compartment model and this was used to calculate the rates of transport between water and sediment, and between water and vegetation. These processes are primarily dependent on turbulent mixing. In experiments with MEP it was shown that biodegradation was primarily associated with bacterial populations in the sediment rather than with those suspended in the water. The rate of biodegradation in sediment was relatively fast and there was no detectable lag phase. The sediment could therefore be considered a sink for MEP and the overall rate of loss of the latter was dependent on its rate of transport to the sediment. In an experiment with chloroform there was reasonably good agreement between predicted and observed rates of evaporation. However, observed rates were always greater than predicted rates, suggesting a systematic bias that may warrant further investigation. In experiments with PCP and DCA there was good agreement between predicted and observed rates of phototransformation.