Trace elements in thermomineral waters in Greece

Abstract

Trace elements have a fundamental role in natural and anthropogenic systems. In waters, they present a great variability of concentrations that mostly depends on the degree of gas-water-rock interactions and geochemical conditions such as pH, temperature, redox and/or exchange reactions, etc. Even though, they are present in very low contents in host-rocks, elevated concentrations in ground or surface waters may have a hazardous impact on human health and thus, it is important to both quantify and understand their behavior in natural systems. Here we present the results of about 300 cold and thermal mineral waters collected along the entire Hellenic territory. Physicochemical parameters (temperature, pH, electrical conductivity and Eh) were measured in situ, whilst samples were analyzed by ICP-MS for their trace elements’ content. The great variability in hydrogeological settings justifies the wide range of temperatures (6.5 - 98°C), pH (1.96 - 11.98) and Total Dissolved Solids (TDS) (0.06 - 43 g/L). Based on the combination of pH, T and TDS, samples were divided into 5 classes: i) thermal waters; ii) thermal waters affected by seawater contamination; iii) cold CO2- rich waters; iv) hyperalkaline waters; and v) acidic waters. The great variability in chemical composition of the sampled waters is reflected in the large range of trace element contents (four to five orders of magnitude). Thermal waters affected by seawater contamination show the strongest enrichments in Li (up to 17600 μg/L), B (up to 38200 μg/L), Sr (up to 80000 μg/L) and Rb (up to 9230 μg/L), mostly deriving from water-rock interaction. Cold CO2-rich waters display elevated concentrations of Mn (up to 3970 μg/L), Ni (up to 111 μg/L) and Fe (up to 218000 μg/L), whilst at the water outflow an extensive precipitation of iron oxi-hydroxides is observed. Hyperalkaline waters are generally strongly depleted in trace elements due to the precipitation of secondary minerals, however they are enriched in Al (up to 421 μg/L). Aluminum becomes soluble at extreme pH conditions and therefore also acidic waters present enhanced concentrations (up to 100000 μg/L). Acidic waters show also enrichments in Fe (up to 58400 μg/L), Mn (up to 15600 μg/L) and Ni (up to 101 μg/L). In some cases, the maximum contaminant levels (MCLs) fixed by the Directive 98/83/EC for drinking water, are strongly exceeded in the under investigation waters. Such elevated concentrations of harmful elements may create hazards to human health either via direct consumption of cold mineral waters or through mixing of highly mineralized waters – even in small proportions - with shallow groundwater. For instance, As (MCL 10 μg/L) in the sampled waters reaches concentrations up to 1820 μg/L that derive from high temperature water-rock interaction within the hydrothermal circuit.