Summary:
The hydrogeochemistry of the wider area of Psachna - Politika in central Euboea Island has been extensively studied in the past due to the intense anthropogenic activities such as mining, agriculture, seawater intrusion and geogenic influences (i.e. ultramafic soils).
This study includes hydrogeochemical data from 176 groundwater samples from eight different previous surveys conducted in the same area over a 15-year period (2005-2019).
In the present study, recorded values of the physical parameters (i.e., D.O., TDS, E.C., pH and Eh), the major ions (i.e., Ca2+, Mg2+, Na+, K+, NO3-, NO2-, NH4+, Cl-, SO42-, HCO3- and PO43-) and potentially toxic elements and other trace elements (i.e., Ag, As, B, Ba, Br, Cd, Co, Cr, Cr6+, Cu, Fe, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Si, Sr, U, V, and Zn). A co-evaluation of geological-hydrogeological-hydrochemical data, land uses, statistical processing, temporal and spatial distribution maps, and Pearson correlation coefficients was performed, with a focus on a comprehensive examination of physicochemical and chemical parameters.
Significant concentrations of Cl- (up to 1460 mg/L), NO3- (up to 540 mg/L), Cr (up to 359 g/L), and Cr6+ (up to 359 g/L) have been routinely measured over the last 15 years. As can be seen from the data processing, the reasons are water-rock-soil interaction, intensive agricultural activities, seawater intrusion in the area's coastal zone, and over-pumping of the aquifer and the following process of reverse cation exchange. The Cr6+/Cr ratio is in all samples very high (mean value=89%) and there is a statistically significant very strong correlation between the two parameters (r=0.99).
The synergistic role of anthropogenic activities in groundwater quality in an ultramafic geological environment is primarily reflected in the high concentrations of Cr and Cr6+, showing a spatial and statistical correlation with NO3- ; the main source of NO3- in the area is the intense use of fertilizers. Also, the temporal distribution of the percentage changes of the average annual concentrations of Cr follows the same pattern as that of NO3- and Cr6+, which suggests that the change of one parameter is inextricably linked to the other.
Regarding the slightly increased groundwater concentrations of As and Se, these were investigated and found to be associated with seawater intrusion in the study area, but due to the strongly oxidizing environment and high concentrations of NO3-, their concentrations in groundwater fluctuate at low levels.
Furthermore, low U concentrations in groundwater are being systematically found in the region; there are no U-bearing minerals in the area. Fertilizers are the primary source of U in groundwater resources, according to spatial distribution maps and statistically significant strong correlations of U with NO3- and SO42-.
Finally, the non-carcinogenic risk to public health from the chronic use of water with high concentrations of NO3- and Cr6+ was assessed. Specifically, health risk indicators were calculated from their intake due to ingestion and skin contact, in which case they enter the body through the skin during the bath. The index values revealed that the majority of the samples pose serious risks to public health in the study area.
