Assessing the hydrogeochemistry of groundwaters in ophiolite areas of Euboea Island, Greece, using multivariate statistical methods

• We assess the groundwater chemistry in ophiolite areas by 102 samples.
• Hierarchical cluster and principal component analysis were applied.
• Both natural and anthropogenic processes affect the chemical composition.
• Notable Cr enrichment is observed in the alluvial costal aquifer.

Major ion and selected trace element compositions were analyzed in 102 groundwater samples from central and northern areas of Euboea Island, Greece, where both serpentinite dissolution and anthropogenic activities (agricultural) are in action. By integrating hydrogeochemical and two multivariate statistical methods, hierarchical cluster analysis (HCA) and principal component analysis (PCA), the chemistry of these groundwaters was assessed with the aim to understand the evolution of groundwater and assign the major processes that exert control on its composition. HCA classified the groundwater samples into three chemically distinct groups (C1–C3) according to their dominant chemical composition. These three clusters were further categorized by their electrical conductivity values: C1 (median EC: 480 μS/cm), C2 (median EC: 608 μS/cm), C3 (median EC: 1020 μS/cm). PCA was performed to identify the underlying natural and anthropogenic processes affecting the chemistry of these groundwaters. The PCA results can be represented by two principal factors: (1) salinization by seawater intrusion and NO3− contamination; and (2) enrichment of groundwater with Mg2 +, HCO3− and Cr following groundwater–serpentinite interaction. A third trivial component is associated with more local effects of the geological substrate. The three components of the PCA account for 70% of the total variance in the data. Dissolved Cr concentrations (up to 71 μg/L) that rise environmental concerns are more pronounced in the groundwaters from the alluvial coastal area of Politika (C3 water samples). Results of this study demonstrate that appropriate measures should be taken to protect the vital groundwater resources in the alluvial coastal area including the regulation of the amount of chemical fertilizers applied to agricultural soils and the monitoring of groundwater pumping rates.