Occurrence of hyperalkaline spring water in the Brezanska river, Central Serbia

Abstract

Hyperalkaline (pH>10.5) springs are extremely rare. There are only about ten in the entire world. This type of water has been identified in an ultramafic massif in central Serbia, on Mt. Studena, during the course of investigations conducted in 2017. The investigations included prospecting, terrain mapping and sampling, followed by assessment of data collected in situ and from laboratory analyses. The hyperalkaline spring is of the ascending type, its yield is very low (~0.05 L/s), and the water emerges in a diffuse fashion along fractures. The spring water is of the Ca-OH type, the pH level is 11.6 and the water temperature 11.2°C. The drainage area of the spring, like the entire Mt. Studena, is built up of Paleozoic ultramafic rocks, which geotectonically belong to the External Vardar Subzone. The spring is situated in the floodplain of the Brezanska River, whose channel is the local base level of erosion. The primary point of groundwater emergence is in solid ultramafic rocks, within a zone of fractures up to 0.5 mm wide, over a land area of ~1.5 m2. The paper describes a conceptual model of the formation of hyperalkaline water for the ascending spring in the ultramafic valley of the Brezanska River, on the northern slopes of Mt. Studena. The pH level of the groundwater is indicative of contemporary serpentinization, deep groundwater circulation and a prolonged contact between groundwater and ultramafic rocks along secondary fracture systems.