Elevated groundwater concentrations of arsenic and chromium in ultramafic environments controlled by seawater intrusion, the nitrogen cycle, and anthropogenic activities: The case of the Gerania Mountains, NE Peloponnese, Greece

This study aimed to investigate the role of seawater intrusion, nitrogen (N)-cycle, and anthropogenic activities on the elevated groundwater concentrations of oxyanion-forming trace elements in an ultramafic environment and to study the factors controlling the hydrogeochemical characteristics of groundwater in a broader, ultramafic-dominated area of the Gerania Mountains in NE Peloponnese, Greece. Sixty-eight groundwater samples from the a) Schinos area, b) Loutraki area, and c) springs in the Gerania Mountains were analysed for an extensive set of physical and chemical parameters. Elevated concentrations of As, up to 15 μg/L, along the coastline in the Schinos area, and abnormally high concentrations of Cr6+, up to 460 μg/L, in the central part of the Schinos area, were recorded. The strong spatial correlation of Cr6+, NO3-, and P in the central part of the Schinos area, indicated that the release and subsequent oxidation of geogenic Cr in groundwater can be triggered by N and P anthropogenic inputs, such as a) septic tank effluents or N-bearing fertilisers that lead to nitrification and subsequent soil acidification and drive Cr3+ oxidation by the NO2−/NO3− redox couple and b) P-bearing fertilisers that cause the desorption of anionic Cr6+ with simultaneous PO43- adsorption and contribute additional Cr and other trace elements to the soils. Unlike Cr, geogenic As follows a different pattern of groundwater release mechanism, which is driven by seawater intrusion and initial stages of denitrification along a flowpath near the Schinos coastline, as pointed out by the strong positive correlation coefficient between As and Cl− (0.78). The increased salinity controls the mobilisation of As and other oxyanion-forming trace elements, such as B, Se, and V, resulting in their elevated groundwater concentrations through a) their desorption from the surface of Fe/Mn-oxyhydroxides under alkaline pH conditions, b) the reductive dissolution of Fe/Mn-oxyhydroxides under local hypoxic conditions, and c) an increase in the ionic strength of groundwater favouring the dissolution of Fe/Mn-oxyhydroxides. •Release and oxidation of geogenic Cr in groundwater can be triggered by N and P anthropogenic inputs.•Abnormally high Cr6+ in groundwater in ultramafic environments is related to septic tank effluents and fertilisers.•Seawater intrusion and denitrification enhance geogenic As mobility in coastal groundwater.•Factor analysis and hierarchical cluster analysis show strong corr