Pesticide biotransformation in surface waters: Multivariate analyses of environmental factors at field sites

Transformation rates of four widely used pesticides were determined in surface waters that were characterised on the basis of hydrological status and physico-chemical, biochemical and chemical composition. Large variations in transformation rates were observed, ranging from 0.004 to 0.01 day−1 (half-life = 70–173 days) for aldicarb, 0.005 to 0.57 day−1 (half-life = 1–139 days) for simazine, 0.002 to 0.43 day−1 for MCPA (half-life = 2–347 days) and 0.0005 to 0.24 day−1 (half-life = 3–1400 days) for mecoprop. Principal component analyses and step-wise multiple regression analyses were carried out, combining field data and laboratory observations, to reveal the discriminating environmental variables that determine the transformation rates of aldicarb, simazine, MCPA and mecoprop in various aqueous systems. A large set of environmental variables (286 observations) was reduced to three underlying components, explaining 84% of the total variance in the data set. The first component contains variables that promote biorespiratory processes, in which a relationship appears between sorption potential, N sources and microbial activity. The second component is the macro/miconutrient group. The third component is the phosphorus group. Rapid transformation of these pesticides generally occurs in small hydrological systems such as field ditches and channels. Large water bodies such as main discharge channels or lakes seem to enhance the persistence of all four pesticides. Besides the hydrological status of the water course, historical application of the pesticide and subsequent adaptation of biorespiratory processes appear to be the most discriminating environmental factors that determine transformation rates of the pesticides studied.