Pesticides in groundwater: modelling and data analysis of the past, present and future

Pesticides are the most frequently detected groundwater contaminants in Denmark. However, there is still a great deal of debate about the fate of pesticides and their future occurrence in our environment. We do not really understand the link between past usage and current observations, and are not well equipped to predict future trends in pesticide concentrations in groundwater. For example, we do not understand the difference between the loads from point and diffuse sources, or the impact of impurities and degradation products. We must also face the challenge that arises from the need to jointly manage our groundwater and surface water resources. Here, observed pesticide data is analyzed and combined with models to address these questions and needs. Groundwater and surface water pesticide observations reflect the fact that these two hydrological components have a strong interaction. For example, many older and banned pesticides are detected in streams and reflect the groundwater baseflow contribution to stream flow. Models of groundwater age and pesticide transport demonstrate the importance of geology and pumping regime in determining observed groundwater concentrations. Finally, management issues are addressed, eg. it is shown that it is important for pesticide management to consider both pumping strategies and manage surface application.