Mapping Soil Physical Structure of Loamy Agricultural Fields for Assessing Localised Potential Leaching Risks.

During the last decades detection of pesticides and their metabolites in groundwater has increased, forcing several drinking water wells to shut down. The Danish Pesticide Leaching Assessment Programme (PLAP), initiated in 1998, evaluates the leaching risk of pesticides and their metabolites. In PLAP, the leaching of 41 pesticides and 40 metabolites has been investigated on 5 conventionally cultivated agricultural fields in Denmark. One of these, Silstrup, is located in the north-western part of Jutland. Until recently TPMP (5-(trifluoromethyl)-2(1H)-pyridinone), a metabolite of the pesticide fluazifop-P-butyl sold as e.g. Fusilade X-tra or Fusilade Max, was not considered a potential problematic metabolite. However, at the PLAP field in Silstrup the fluazifop-P-butyl application was directly linked with TFMP detections in a downstream monitoring well just 39 days after application. A field in Silstrup was evaluated based on soil texture, structural parameters, tritium breakthrough curves, and colloid- and phosphorus leaching to investigate the link between the leaching of pesticides such as TFMP and soil structure. Bulk soil was sampled from the A-horizon in a 15 x 15 m grid across the field, and according to soil texture analyses the clay content was ranging from 14.2 to 18.9%, whereas the organic carbon (OC) content was ranging between 1.7 and 2.2%. Clay content increased to the North and OC content to the South. It was found that there is a risk for pronounced leaching to take place from the northern part of the field where the highest clay contents and bulk densities were found, and also the largest amounts of particles and phosphorus were leached from this area. A positive correlation was obtained between particle leaching and particular phosphorus leaching, indicating that colloid-facilitated transport plays an important role in the leaching of phosphorus and other strongly sorbing compounds. Interesting negative correlations was obtained between bulk density and the 5% tritium tracer arrival time, suggesting soil compaction level to be one of the controls for formation of functional macropore networks.