Assessing Pesticide Trends in Groundwater: A Comparative Study of the Netherlands and Denmark

The occurrence of pesticides and metabolites in groundwater are of concern, due to their risk for human health and the environment. These contaminants can reach groundwater through various pathways, including runoff from agricultural fields. Once in the groundwater, these chemicals can persist for long periods, depending on their chemical properties and the hydrogeological conditions of the aquifer. Understanding the contamination patterns in groundwater has been difficult due to the heterogeneous groundwater age and redox status over the depth range that is typically sampled.
Groundwater is a vital resource for drinking water, providing approximately two-thirds of the drinking water in the Netherlands and the entire supply in Denmark. Both countries monitor groundwater quality through dedicated networks, which report to the EU Water Framework Directive and the Groundwater Directive. In these monitoring networks, pesticides are routinely measured. Also, major parts of the groundwater monitoring network of Denmark and parts of the monitoring networks in the Netherlands have been age-dated, by tritium/helium for the Netherlands and by a combination of CFCs and tritium/helium for Denmark. By linking pesticide concentrations to recharge years derived from groundwater
ages, trends in pesticide leaching over time can be detected, correlating the presence of these substances to specific periods of farmland application. The availability of pesticides sales in both countries, make it possible to use groundwater ages to correlate pesticide detections in Danish and Dutch groundwater with pesticide sales.
The similarities between the monitoring networks allows for a comprehensive comparison between the pesticide data that was gathered in both countries. For the Dutch data a framework was recently published which allows the interpretation of large groundwater datasets on pesticides, relating them across various redox conditions, groundwater ages and land use types. A similar analysis was performed for the Danish monitoring network, where the effects of pesticide regulation on groundwater quality has been assessed. The results show, for example, that BAM is frequently detected in older groundwater in both
Denmark and the Netherlands. The comparison between the Dutch and Danish data improves the understanding on the trends and trend reversals of pesticides in relation to their emission histories.