Pesticides and their metabolites in groundwater : diuron in the Isle of Thanet aquifer of southeast England

Pollution from diuron and its metabolites is, like nitrate pollution, a ubiquitous problem within the Isle of Thanet aquifer. The widespread diuron pollution is symptomatic of a much larger problem of historic pollution within the aquifer system. Diuron was observed in 90% of samples (0.4-214 ng/L). In 60% of these metabolites were more prevalent than diuron. Mean total diuron concentrations were above the Prescribed Concentration Vale (PCV) at nearly 30% of sites and showed a large concentration range (2.2-1161 ng/L). Maximum concentrations for diuron and its metabolites were in the following order DCA>DCPU>Diuron>DCPMU, all four compounds exceeded the PCV on at least one occasion. There is some evidence to suggest that diuron occurrence could be spatially related to areas of urban and industrial development. Results from groundwater tracers (SF6) support the idea that high diuron concentrations were associated with the unusually high recharge observed in the winter/autumn of 2000-2001. The Isle of Thanet Chalk aquifer has characteristically short residence times, as evidenced by the response to historic reductions in applications of other pesticides such as atrazine and simazine and recent groundwater tracer studies. This is both a benefit as well as a drawback in terms of overall groundwater quality. While the problem of diuron may be only transient, given improved application practices, the aquifer is still vulnerable in the short-term depending on the quantity of diuron stored in the unsaturated zone. Much more needs to be understood about the transport and occurrence of diuron in the unsaturated zone to determine more fully the future risk to groundwater from pesticides such as diuron. Despite some evidence to suggest that the metabolites of diuron may be more toxic than the parent compound, there is currently no requirement to monitor metabolite concentrations in groundwater. Should this change there could be potential implications in terms of future management of the groundwater resource in the Isle Thanet aquifer, and across the UK as a whole.