Fractures in the Danish clay soils cause detections of pesticide residues in groundwater.

Published 22-12-2016

Results from the Danish Pesticide Leaching Assessment Programme (PLAP) show that clay till soils are more vulnerable to leaching of pesticide residues compared to sandy soils.

PLAP was initiated in 1998 as an intensive monitoring programme aimed at evaluating the leaching risk of pesticides and/or their degradation products under field conditions. The project is funded by the Danish EPA, and the work is carried out jointly by GEUS and Department of Agroecology and Department of Bioscience at Aarhus University. The PLAP consists of five test fields, which are agricultured conventionally using approved pesticides, and it is investigated whether the pesticides or their degradation products leach to the groundwater in concentrations above the allowed limit.

Two of test fields in PLAP are situated on sandy soil and three on clay till soils, and together they represent common Danish climate and soil conditions. Although clay till soil appears impermeable and dense, it can contain fractures that facilitate fast water transport to great depths, and because of this fast transport the pesticides do not have time to adsorb or degrade fully before reaching the groundwater. Results from PLAP show this effect since more pesticides are detected in both drainage and groundwater beneath clay till soils than under sandy soils. Approximately 40% of the Danish soils consist of fractured clay till, and it has been decided to include a sixth field in the PLAP. The new field is situated on fractured clay till above limestone, and it will hopefully provide more insight into the mechanisms that are at play.

During the latest monitoring period PLAP evaluated the leaching risk of 15 pesticides and 22 degradation products. Out of the 37 compounds, six pesticides and seven degradation products were detected in groundwater samples, and of these, two pesticides and four degradation products were found in concentrations exceeding the limit.

Watch a movie about the new PLAP field: (remember to choose English subtitles!)

Download the latest monitoring results from PLAP: