Combination of hydrochemical graphical methods and multivariate statistical analysis to delineate groundwater bodies

The identification of hydrogeological boundaries and the assessment of the quantitative and qualitative state of groundwater are necessary for the delineation of groundwater bodies, according to the European Guidelines. In this context, this study attempts to verify the current delineation of groundwater bodies (GWBs) focusing on their hydrochemical features through the application of hydrogeochemical methods and multivariate statistical analyses. The areas of interest are three adjacent GWBs located in the north-western sector of Campania Region in Southern Italy: the Volturno-Regi Lagni Plain, a coastal plain constituted of fluvial, pyroclastic and marine sediments; the Plain of Naples, an innermost plain of fluvial and pyroclastic sediments and the Phlegrean Fields, an active volcanic area with a series of monogenic volcanic edifices. Physicochemical data measured in groundwater samples collected at more than 200 sampling points were considered. Results reveal five different hydrogeochemical processes variably influencing the chemical features of the three GWBs: dissolution of carbonate rocks, influence of volcanic deposits and/or gas/hydrothermal fluid upwelling, salinisation, reducing conditions, and anthropogenic contamination. Combining hydrochemical diagrams and multivariate statistical analysis (i.e., factor analysis) allows depicting areas characterised by one or more hydrogeochemical processes, mostly reflecting known processes, but also highlighting the influence of groundwater flowpaths on water chemistry. The current delineation of the three GWBs should be revised considering that some hydrochemical features and processes are peculiar to each GWB, but others are in common between two or more GWBs. Uncertainties in the delineation of GWBs based on their hydrochemical features could be reduced by improving the distribution of the monitoring well network to ensure a more homogeneous coverage.