The synergistic role of agricultural activities in groundwater quality in ultramafic environments: the case of the Psachna basin, central Euboea, Greece

In the present study, we approach the geochemical processes affecting the hydrochemistry and resulting in elevated concentrations of hexavalent chromium (Cr6+) in groundwater of the Psachna basin, central Euboea, Greece. Sixty-five groundwater samples and 16 topsoil (5-20cm) samples were studied in order to examine groundwater and soil quality in relation to geogenic processes and anthropogenic activities. Specifically, the origin of Cr and Cr6+ in groundwater was investigated by co-evaluating (a) hydrochemical cross plots of major ions; (b) spatial distribution maps of Cl-, Mg2+, NO3-, and Cr6+; (c) multivariate statistical analyses such as factor analysis (FA) and hierarchical cluster analysis (HCA) of groundwater geochemistry; (d) chemical analyses of soil samples; and (e) chemical analyses of fertilizers. The major factors that control the hydrochemistry of the study area are reverse ion exchange, dissolution of silicate minerals, and intense agricultural activities. According to FA, three factors explain 73.2% of the total variance of data, whereas according to HCA, the groundwater samples were classified into three groups indicating both geogenic (water-rock interaction) and anthropogenic (agricultural activities) impact. The high concentration of NO3-, up to 540mgL(-1); the strong positive correlation between NO3- and Cr as well as between NO3- and other parameters such as SO42- and Mg2+ in groundwater samples; and the very high content of P, up to 2444mgkg(-1), in soil samples of the Psachna basin, imply the synergistic, although commonly neglected, role of the use of fertilizers in groundwater quality.