Nitrate concentrations in drainage water in marine clay areas : exploratory research of the causes of increased nitrate concentrations

The nitrate concentrations measured in drainage water and groundwater at LMM farms (farms participating in the National Manure Policy Effects Measurement Network (LLM)) in marine clay areas have decreased with 50% since the mid-nineties. The nitrate concentrations in marine clay areas are on average below 50 mg/L EU target value. A geographical analysis of the monitoring results shows, however, that the nitrate concentrations in 22% of the measurements, mainly taken in the south-western and central marine clay areas, exceed the EU target. The Ministry of Economic Affairs (EZ) commissioned a study to investigate the possible causes of the differences in nitrate concentrations between clay regions. This study shows that the differences in nitrate concentrations are largely due to the specific form of land use combined with the volume of seepage water occurring at LMM farms. This accounts for 43% to 62% of the variation in nitrate concentrations in samples taken from drain pipes at LMM farms from 2006 to 2008. Statistical analysis shows that the average nitrogen surplus of the soil balance of the LMM farms in marine clay areas does not explain the differences in measured (average) nitrate concentrations between these farms. However, the number of farms on clay soils of the LMM-dataset is limited (83 farms in total).