Geochemical characterization of water quality in karst systems of Greece

Abstract

Karst systems represent the main drinking water source for 20-25% of world’s population, although covering only 7-10% of the global land surface (Ford & Williams, 2007). Karst aquifers are highly vulnerable to external pollution, so their protection and management is of critical importance to sustain water resources. In Greece, starting from the 1970’s, water demands for agricultural, domestic and industrial use increased significantly, mainly in coastal areas (Daskalaki & Voudouris, 2008). The main Hellenic aquifers are hosted in alluvial deposits, in Neogene deposits and in carbonate rocks. The latter cover about 35% of the country and are located in Western, Central and Southern Greece (Daskalaki & Voudouris, 2008). Karst aquifers are developed in limestones and dolomites (Triassic – Cretaceous), and in marbles (Paleozoic – Mesozoic). Their hydrogeological behaviour is controlled by tectonic deformation. About 45% of them is located inland, while the rest is in coastal areas (Voudouris & Kazakis, 2018). During several field sampling campaigns from 2016 to 2020, 126 karst water samples were collected in Greece. Physicochemical parameters (temperature, pH, electric conductivity and redox potential) were measured in situ. Analyses of major ions and trace elements were performed at the laboratories of INGVPalermo. Results were compared with the limits set by the Directive 98/83/EC that fixes quality standards for drinking water. Temperatures of sampled waters ranged from 8.7 to 31 °C, pH from 6.5 to 8.4, whilst Total Dissolved Solids from 206 to 25,617 mg L-1. Most of the samples showed a typical alkaline-earth bicarbonate composition, whilst those sited along the coastline presented elevated concentrations of Na+ (up to 7,680 mg L-1), Cl- (up to 14,200 mg L-1) and SO4 2- (up to 1,940 mg L-1), sometimes exceeding the EC limits, suggesting a seawater contamination. Furthermore, karst springs contaminated by seawater, displayed high concentrations of B (up to 3,870 μg L-1) and Sr (up to 7,080 μg L-1). Nitrate concentrations were always below the EC limit (50 mg L-1), indicating a low contamination from fertilizers. Few low chloride waters showed a metal enrichment, such as Tempi springs (Thessaly) that presented enrichments in Sr (up to 242 μg L-1), Mo (up to 2.27 μg L-1), Cs (up to 1.57 μg L-1) and As (up to 17 μg L-1). Such enrichments could be attributed to the local petrological environment. Generally, karst water samples can be considered suitable for human consumption. Water quality degradation of Hellenic karst springs is mainly due to seawater intrusion or to local geogenic contamination.