The silica supersaturated waters of northern Evia and eastern central Greece

The area of north Evia and eastern central Greece is characterized by stron g geomorphological contrast and is built up mainly of consolidated rocks. U nconsolidated young sediments of Pleistocene to Holocene age cover the vall ey and basin flats, forming the most productive aquifers in this area. Howe ver, two more types of aquifers can be distinguished within the consolidate d rock,area. The first one is associated with karstified limestones and the second with strongly tectonized ultramafic-rocks. The schist-chert formati on, with intercalations of shales and cherts, seals the ultramafic masses u nderneath. Surface and spring waters associated with ultramafic rocks in north Evia an d eastern central Greece were studied. Two types of water can be distinguis hed: (1) high Mg2+ and SiO2, bicarbonate as the dominant anion, pH 7.4-9.2, temperature 9.5-16.3 degrees C, low TDS (total dissolved solutes) (459-103 7 mg/l), found both in peridotite and serpentinite areas, classified as Mg- HCO3 type; (2) high Ca2+, low Mg2+ and SiO2, hydroxyl ion as the major anio n, pH 11.2, temperature 28 degrees C, very low TDS (122 mg/l), found in per idotite areas, classified as Ca-OH type. The studied waters are highly supe rsaturated with respect to quartz, amorphous silica, brucite and most low t emperature magnesium silicates (antigorite, sepiolite, talc, etc.). These w aters show relatively narrow SiO2 concentration ranges and a trend parallel to the amorphous silica saturation surface. The silica supersaturated wate rs have the potential to precipitate silica and consequently could affect t he people of the local communities that use it as drinking water, causing h ealth problems (kidney stones). Copyright (C) 1999 John Wiley & Sons, Ltd.