GROUNDWATER QUALITY ISSUES IN THE FLORINA AREA (N. GREECE)

Abstract

The Florina basin developed in the Pelagonian Zone, the westernmost zone of the Internal Hellenides, in response to NE–SW extension in the Late Miocene and to a subsequent Pleistocene episode of NW–SE extension. Continuous sedimentation resulted in the accumulation of a 560 m thick succession of Late Miocene to Early Pleistocene lake sediments with intercalated lignites and alluvial deposits. The presence of intercalated volcanic ash beds of Pliocene age evidences volcanic activity related to the exstensional tectonics. Groundwaters in the central part of the Florina plain display high levels of dissolved gases, which often separate in a free gas phase. Their composition is dominated by carbon dioxide, which accounts for 85-99% of these gases. Apart from small amounts of atmospheric gases, minor components are CH4 (0.05-0.4%) and He (3-30 ppm). Carbon isotopic composition ranging from –1.6 to 0.3‰ (vs. VPDB) testifies for a deep (magmatic-hydrothermal) origin of CO2 and also He isotopic composition (0.24-0.55 R/Ra) reveals a small (3.5-8.4%) but significant mantle contribution. Furthermore the water composition of a deep well (Mesochori) shows important contribution from a hydrothermal component, displaying very high Li and B contents and a clear isotopic shift on a δD-δ18O diagram. Geothermometric estimates of the deep reservoir are in the range 150-180 °C. The uprise of mantle gases is related to the main tectonic structures, which probably allowed also magma intrusion episodes whose heat flow sustain the deep hydrothermal system. Because of the huge input of CO2, the shallow groundwaters of the studied area become acidic and consequently strongly aggressive with respect to the host rocks. At the sampling point many waters display pH values down to 5.5, being generally under the lower limit for drinking waters. Intense rock leaching results in metal release to the solution and enhanced metal fluxes in the aqueous system. As such, magmatic-hydrothermal CO2 input produces a “natural pollution” of the aquifer, where maximum admissible concentrations (MAC) fixed by European Union for drinking waters are exceeded at least for Ni, Mn and Fe in most of the analysed samples. Measured values reach respectively up to 30, 1700 and 55000 µg/l (MACs 20, 50 and 200 µg/l). This natural contamination combines with the pollution due to agricultural practices in the Florina plain, which is responsible for elevated nitrate contents (up to 90 mg/l) often exceeding maximum admissible concentration (50 mg/l). The interaction of natural and anthropogenic contamination of the shallow groundwater resources in the Florina area leads to serious water quality issues.