Mapping groundwater vulnerability to nitrate pollution from diffuse sources in the Ebro river basin (NE Spain)

Nitrate pollution from diffuse sources is one of the main causes of groundwater quality deterioration on a global scale. Recent research suggests the need to minimize uncertainty in assessing groundwater vulnerability through the development of a robust methodology for the evaluation of vulnerability, using a source-pathway-receptor approach at basin scale. The LU-IV procedure (Arauzo, 2017) was devised for mapping intrinsic groundwater vulnerability (risks associated with the physical environment) and specific vulnerability to nitrate pollution (risks associated with land use in areas that are intrinsically vulnerable), with the final goal of improving the nitrate vulnerable zones (NVZ) delineation. This methodology uses simple and readily available parameters combined in a GIS environment and allows us to accurately assess groundwater vulnerability in the entire catchment area that drains water (potentially polluted by nitrate) into a receptor aquifer. The application of the LU-IV procedure to the river Ebro basin (N.E. Spain) revealed a surface area of 18,757 km2 under high to extreme vulnerability to nitrate pollution, so that the affected territories could potentially be designated as NVZ. These results differ from the 9.796 km(2) designated as NVZ by the Spanish regional administrations with competences in NVZ designations in the Ebro basin. Although the officially designated NVZ are consistent with the results of this research, the application of the new procedure allowed us to identify 50% more surface area that could be classified as NVZ. Alluvial areas showed the highest degree of vulnerability (both intrinsic and specific) affecting most of their territory, which suggests the need to propose an integral protection of all alluvial surfaces, not only of the Ebro basin, but also at national and European level.