Spatio-temporal variability of groundwater hydrochemical features: examples in different hydrogeological settings (Piedmont and Campania plains, Italy)

Groundwater chemical characteristics are influenced by natural processes and anthropogenic activities, which can change over time. The aims of this study are to identify the main ongoing processes and to compare the spatio-temporal hydrogeochemical variations in two different study areas in Italy: an alluvial plain at the foothills of the Alps relief in Piedmont Region (1840 km²) and a coastal alluvial-pyroclastic plain in Campania Region (1630 km²). The main physico-chemical parameters from the Regional Groundwater Monitoring network databases, referred to biannual sampling, were analyzed. The average values referring to the 2015-2020 period were used for groundwater chemical characterization. Statistical methodologies (Mann-Kendall trend test) were applied to assess the evolution of parameters in the period 2000-2020. In the Piedmont plain, groundwater is characterized by prevalent oxidizing conditions. Ca-HCO3–facies is prevalent, and a good correlation between EC-HCO3– and NO3–-Cl– is observed. All ions show low concentrations close to the Alps, which increase along the groundwater flow due to mineral/rock dissolution processes and anthropogenic impacts, with a diffuse nitrate contamination. Moreover, groundwater chemistry is more affected by natural processes than anthropogenic influences. Long-term trends reveal a significant decrease in SO42– concentration and an increase of Na+ and Cl–. In the Campanian plain, groundwater chemistry is influenced by volcanic edifices and related pyroclastic products and the closeness to calcareous rock formations. Ca-HCO3 facies is prevalent, with a gradual transition to alkaline type from the mountains towards the coast. Moreover, ions concentration distributions highlighted the occurrence of seawater intrusion (Na+, Cl–) and anthropogenic impact (K+, NO3–, SO42–) on groundwater quality. Long-term trends show significant increases in ion concentrations in the innermost part of the plain and decreases in the coastal areas for most of the ions (exceptions are Na+, NO3– and SO42–). The spatial and temporal elaborations show a significant hydrochemical variability in both study areas, mainly reflecting the geological characteristics of the aquifers and surrounding rock formations, but also influenced by land-use/land-cover and anthropogenic pressures (e.g., agricultural, urban and peri-urban areas) that differently changed over the 2000-2020 period in the study areas.