Evaluating upward trends in groundwater nitrate concentrations: an example in an alluvial plain of the Campania region (Southern Italy)

To achieve the objectives of the Water Framework Directive's, the identification and reversal of significant upward trends in pollutant concentrations in groundwater is crucial. A statistically significant increase in a pollutant, groups of pollutants or pollution indicators in groundwater bodies, means an upward trend in concentration values, calculated using a recognized statistical method, at least 90% significant. Following this aim, we focused on a groundwater body of southern Italy, with high concentration of nitrate and, therefore, defined "at risk", on the basis of its bad quality status. In this area, we calculated the trends of the time series of nitrate concentrations in groundwater. We described the adopted procedure for trend analysis, following the guidelines proposed by the Italian Institute for Environmental Protection and Research (ISPRA). We used the Mann-Kendall method for calculating the statistical significance of the upward trend of time series and, successively, the Sen method for estimating the value of the trend (angular coefficient). We applied these methods also to forecast nitrate concentrations in groundwater in 2021 and in 2027. We found differences in the sampling stations in terms of groundwater quality and trends, as a result of different environmental factors. Peculiarly, the location of the wells presenting upward trends seems to be in areas with high population density and intensive agriculture.