Indicator Kriging approach for the assessment of groundwater nitrate pollution in the Salento aquifer (Southern Italy)

Groundwater, being the largest distributed freshwater supply, plays a central role in sustaining ecosystems and enabling human adaptation to climate change. The strategic importance of freshwater for global water and food security will intensify as the frequency of extreme weather events such as floods and droughts events. Many aquifers are severely stressed due to unsustainable management of the water resource that compromises its quantity and quality.

Today, nitrate pollution is the most common form of groundwater contamination worldwide. Many Mediterranean regions present worrying concentrations of nitrate in groundwater and are consequently one of the most polluted territories across the world. Nitrate monitoring is fundamental since excessive concentrations in water resources may affect the quality of crops and causes several human health disorders. For this purpose, this study aimed to deal with the evolution in space and time of nitrate concentrations in the coastal karst aquifer of Salento (Puglia, Southern Italy), whose water demand for drinking and irrigation purposes relies on groundwater. The intrinsic vulnerability of this territory is critical due to its complex geomorphological and structural characteristics, the presence of saltwater beneath freshwater, intensive exploitation, and climate change.

Several different chemical surveys from 1995 to 2021 were organized into two-time datasets to focus on the spatio-temporal evolution of nitrate concentrations. The geostatistical Indicator Kriging (IK) method was used for this purpose. This is a spatial interpolation technique aimed at estimating the conditional cumulative distribution function of a variable at an unsampled location. Indicator Kriging analyses were performed for a direct estimation of the local conditional probabilities of nitrate concentrations for the two reference periods using sampled available wells. Probability maps representing the spatial distribution of water quality have been obtained as result. Results highlight a critical situation in terms of nitrate pollution, as most of the territory has experienced an increase over the past 25 years, progressively affecting large areas.