A field study to determine the fate of diuron and its metabolites in calcareous sediments

An experimental plot has been established on a calcareous soil in southern England to investigate the fate and transport of diuron (N`-[3,4-dichlorophenyl]-N,N-dimethylurea), a commonly used phenylurea herbicide. In early January 2001 an agricultural grade of diuron was applied to the soil surface at a rate of 6.7 kg/ha along with a potassium bromide conservative tracer applied at 200 kg/ha. Hand augured samples were taken at regular intervals over the next 50 days, and then again after 338 days, with samples collected down to 54 cm. Porewaters were extracted from the soil cores by using high speed centrifugation and the supernatant fluids were retained for analysis by HPLC for diuron and three of its metabolites, N`-[3,4-dichlorophenyl]-N,Nmethylurea (DCPMU), N`-3,4-dichlorophenylurea (DCPU) and 3,4-dichloroaniline (DCA). The centrifuged soil was retained and then extracted with methanol prior to HPLC analysis for the same suite of phenylureas. A mass balance approach showed large variations in diuron distribution but on average accounted for 104% of the diuron applied. Concentrations of diuron and its metabolites were roughly five times higher in the soil than in the soil porewaters. After 50 days metabolites comprised 10% of the total diuron present in the porewater and 20% of the total diuron sorbed to the soil matrix. After 36 days a large pulse of diuron and DCPMU appeared in the porewaters and soil matrix at a depth of 54 cm, travelling an average of 0.15 cm/d faster than Br. A preferential route for diuron transport is suggested. There is evidence to suggest that degradation occurs at depth as well as at the soil surface. Metabolites generally appear to move more slowly than the parent compound. All metabolites were encountered but interpreting transport and degradation processes simultaneously proved beyond the capability of the study. Diuron was detected once in a shallow (5 m) observation well situated on the experimental plot. High concentrations of diuron and metabolites were still present in the soil and soil solutions after 50 days. After 338 days diuron was absent from the porewaters but still resided in the solid phase. The metabolite DCPU became the dominant compound in the soil solution whereas DCPMU was dominant in the sorbed phase. Both compounds remained as a source of potential groundwater contamination.