An experimental plot has been established since January 2001 on a calcareous soil in southern England to investigate the fate and transport of diuron (N`-[3,4-dichlorophenyl]-N,Ndimethylurea), a commonly used phenylurea herbicide. In late January 2002 an agricultural grade of diuron was applied to the existing plot and to an adjacent plot on 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 125 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,N-methylurea (DCPMU), N`-3,4-dichlorophenylurea (DCPU) and 3,4-dichloroaniline (DCA). The centrifuged soil was retained and then extracted the solid phase material with methanol prior to HPLC analysis for the same suite of analytes. Variability within the same plot between different sample periods and between plots makes understanding and quantifying the system difficult. The generally more moist Plot B contains higher concentrations of diuron and metabolites in the porewaters than Plot A whereas Plot A contains generally higher concentrations in the solid phase than Plot B. In year 2, the rate of degradation over a 50 day period is faster in both plots than it was in Plot A in year 1. After 120 days metabolites account for more than 50% of total pesticide found in solid phase in both plots. In the liquid phase metabolites account for 33% in Plot A and 51% in Plot B. The rate of degradation is faster in Plot B than Plot A for both solid phase and porewaters. In all cases, the dominant metabolite is DCPMU. The highest percentage of DCA occurs in solid phase and porewaters of Plot A which might suggest some enhanced degradation of DCPU or could be a residual from the 2001 application. During the study period roughly 100% of residues were accounted for. Metabolites are more strongly retained in the solid phase than the parent compounds as demonstrated by the generally higher Kd values. From highest to lowest Kd DCA>DCPMU>DCPU>Diuron. Although greater rates of degradation are associated with the solid phase, this also suggests that metabolites can remain as a potential source of groundwater contamination long after the parent compound has disappeared from both porewaters and solid phase.
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