Mapping groundwater vulnerability to nitrate pollution in the Segura Hydrographic District (Spain) by the LU-IV Procedure

Abstract

The lack of significant improvements in the year-on-year evolution of nitrate levels in the groundwater bodies of the Segura Hydrographic District underscores the need to review the Nitrate Vulnerable Zones (NVZs) designations and adopt additional measures. In this study, the digital map of groundwater vulnerability to nitrate pollution of the Segura Hydrographic District (LU-IV map), generated using the LU-IV Procedure, was employed to assess the appropriateness of the NVZs designated to date within the District. The procedure generates the map of specific vulnerability to nitrate pollution by combining a map of groundwater intrinsic vulnerability (based on four environmental parameters that represent the vulnerability associated with the lithology of the vadose zone, the depth of the water table, the topographic slope and the water inputs) and a map that assesses the vulnerability associated with land use (based on the annual N-surpluses for different types of crops) by using the Over tool of Spatial Analyst in ArcGIS. The high significance of the point-biserial correlation analyses (the first validation method for the vulnerability maps) confirmed the coherence of the spatial distributions of the designated NVZs and the areas of high to extreme vulnerability extracted from the LU-IV map, respect the nitrate distribution in groundwater bodies. However, the correlation matrix between the coverage percentages of NVZs, high to extreme vulnerability zones extracted from the LU-IV map and at-risk and polluted areas analysed by catchment areas (the second validation method for the vulnerability maps) revealed weak correlations of pollution indicators with NVZs, but strong correlations with vul­nerable zones extracted from the LU-IV map. The sensitivity of the LU-IV Procedure to the source-pathway-recep­tor approach explains these results (due to its high mobility, nitrate can move through the hydrological catchments from higher to lower elevation areas). The NVZs designated within the Segura Hydrographic District cover 34% of its surface area, whereas the high to extreme vulnerability areas resulting from the LU-IV Procedure cover 26%. Discrepancies between the two vulnerability maps arise from the differing criteria and methodologies. The high-to-extreme vulnerability map extracted from the LU-IV Procedure offers a much more precise representation, enabling the identification of new vulnerable zones (abundant and scattered) that have yet to be designated and highlighting the over-dimensioning of some of the existing NVZs. Given these results, the inadequacy of the designated NVZs cannot be considered a significant factor justifying the lack of improvements in contaminated aquifers. Nevertheless, NVZs could improve by introducing adjustments based on the LU-IV map. The authors recommend optimising the designations and focusing on the ineffectiveness or lack of implementation of action programmes within the NVZs.