Assessment of groundwater nitrate pollution using the Indicator Kriging approach

Abstract

This paper presents the results of investigations on groundwater nitrate pollution to assess its spatial and temporal evolution through the indicator kriging method. With this approach, the direct estimation of the local conditional probabilities of nitrate concentrations permits the identification of areas more vulnerable to pollution without establishing a precise estimate of the physical quantities under investigation. The aspect of interest is the possibility of defining areas of increased vulnerability to nitrate pollution, even in the case of sample points non-homogeneously distributed. In this case, using classical geostatistical tools such as ordinary kriging would lead to incorrect assessments. On the other hand, using the Indicator Kriging indicates areas most prone to deterioration over time and, therefore, need attention. The selected case study can be considered a benchmark due to its high geomorphologic and structural complexity and the scarce availability of homogeneous monitoring data. Results indicated an increasing trend in nitrate pollution over time, often exceeding the recommended threshold for drinking water use stated by the EU Nitrate Directive. Moreover, there is no correspondence to either land use variation or the use of fertilisers over the same period. This behaviour could depend on a long delay between the leaching of nitrates from the soil to the groundwater. This finding suggests the importance of systematic monitoring of groundwater quality, especially for those vulnerable systems characterized by slow response time to external pressures and of great significance in water resource supplies.