Fate of two currently-used pesticides in water-sediment systems

Experiments were undertaken to understand the fate of selected pesticides in water-sediment systems and to determine whether laboratory experimental data coupled with mathematical modelling is able to represent behaviour in outdoor condition. Two pesticides were selected for study; thiamethoxam has a low sorption coefficient (Koc) and is susceptible to hydrolysis and photolysis, whilst metalaxyl-M has moderate sorption but is stable to hydrolysis and photolysis. A sequence of experiments was carried out starting with investigation of simple, single-phase systems and building additional complexity first in the laboratory and subsequently in the field. Hydrolysis of thiamethoxam was pH-dependent; the rate was very slow under acidic and neutral conditions and significantly higher at alkaline pH (p<0.001). Photolysis of thiamethoxam was also strongly influenced by pH (p<0.001) with the rate of photolysis 4.30 and 3.85 times faster at pH 10 than at pH 9 for pure and natural water, respectively. The presence of nitrate anions significantly (p<0.05) decreased rate of photolysis of thiamethoxam and this was attributed to a direct competition for absorption of light. Sorption of the two pesticides to a natural sediment (Koc 32.6 and 36.6 L kg-1 for thiamethoxam and metalaxyl-M, respectively) suggested that these pesticide are weakly sorbed and likely to be present predominantly in the water phase. Pesticides in water-sediment systems with plants (Myriophyllum spicatum) degraded much faster than in systems without plants under both laboratory and outdoor conditions. Plants had direct effects through sorbing and taking up pesticide, but the dominant influence was indirect due to changing pH to alkaline conditions and thus increasing the rate of photolysis and hydrolysis. Degradation in water broadly translated from the laboratory to outdoor experiment, but sorption behaviour in simplified systems greatly overestimated pesticide sorption in complex systems in outdoor experiments. There was less intense contact between pesticide and sediment in vessels under outdoor conditions and pesticide degraded from water much more quickly, limiting the time available for sorption. A previously unreported tendency for photolysis of metalaxyl-M in outdoor experiments showed the importance of selecting laboratory conditions able to mimic behaviour of pesticides under outdoor conditions. TOXSWA predictions for fate of the two pesticides in outdoor experiments showed a reasonable match to measured data for the water phase, although residues were overestimated at initial and after a few days of experiment. Model efficiency was -1.81 and 0.944 for thiamethoxam and metalaxyl-M, respectively. The simulations showed a reasonable match with measured data in plants for thiamethoxam, but underestimated concentrations of metalaxyl-M in plants. Concentrations of both pesticides in sediment were overestimated by the model at model efficiencies of -84.0 and -2.14 for thiamethoxam and metalaxyl-M, respectively. Further work could apply alternative models such as EXAMS or develop a new model able to account for all relevant processes acting on pesticides in surface waters. This work considered two pesticides in detail and further research with a wider range of pesticides is needed to develop generalised conclusions.