Groundwater pollution by nitrates from agricultural sources is a common environmental
issue. In order to support risk analysis, hazard maps are used to classify land uses according to their
potential of pollution. The aim of this study is to propose a new hazard index based on nitrogen
input and its connection with nitrate concentration in groundwater. The effectiveness of the Nitrogen
Input Hazard Index was tested in the Gallocanta Groundwater Basin (Spain), a highly polluted area,
declared as a Nitrate Vulnerable Zone. Agricultural data at a plot scale were used to estimate the
nitrogen fertilizer requirement of each crop, and the correlation between nitrogen input and nitrate
concentration in groundwater was explored. The resulting hazard map allows us to delimit the most
hazardous areas, which can be used to implement more accurate nitrate pollution control programs.
The index was proven to successfully estimate nitrogen input influence over groundwater nitrate
concentration, and to be able to create hazard maps. The criterion used to create categories was
empirically based on nitrate concentration thresholds established by the EU Nitrate Directive. The
Nitrogen Input Hazard Index may be a useful tool to support risk analyses of agricultural activities
in vulnerable areas, where nitrate pollution could endanger human water supply.
