Translating the agricultural N surplus hazard into groundwater pollution risk: Implications for effectiveness of mitigation measures in nitrate vulnerable zones

In the Nitrate Vulnerable Zones farmers are required to implement measures to reduce the nitrogen (N) surplus. Nevertheless, in some cases the status of the water bodies show that the effect of these measures remains insufficient despite the global decrease in N surpluses. The present work aims to contribute with a method that produces an appropriate indicator for the N mitigation measures effectiveness for reducing groundwater nitrate pollution. The Global Risk Index (GRI) results from overlaying the agricultural N surplus hazard and aquifer vulnerability. It includes both irrigation activity and precipitation contribution to water recharge calculated at the municipality level. It integrates a range of regional datasets combined with monitored nitrate (NO3-) concentrations in groundwater under a GIS framework. Results show that the pollution status of the Tagus Vulnerable Zone (TVZ) aquifers has been aggravating in spite of the overall reduction in the N surpluses that resulted from the implementation of the Nitrates Directive measures. Twelve years after the TVZ designation, the GRI indicates high and moderate NO3- pollution risk, respectively in 33 % and 66 % of the territory. Scenario analysis indicates the potential of targeted measures for ending high risk areas and reducing moderate risk areas to 13 %. This supports that N mitigation measures must be reformulated and spatially targeted according to site specific hazards and vulnerabilities.