Monday 5 March 2018

Geochemical signatures of the groundwaters from Ourense thermal springs, Galicia, Spain

Sustainable Water Resources Management

Different hot springs and boreholes in the city of Ourense, Galicia, Spain, have been studied to determine the mineral equilibrium conditions of the discharged groundwaters and the reservoir temperatures predicted by the equilibrium conditions. Ourense is located in the Miño River’s valley. The area is characterized by two fault systems, which determine groundwater circulation. A NW trending fault system is the permeable system that transfers groundwater and heat to springs in the Miño River valley as it is evident from the location of the springs in the region. Groundwaters traveling and discharging from granitic and schistose rocks are mainly bicarbonate waters. In comparison, groundwaters traveling and discharging from granodiorite rocks can be bicarbonate, sulfate or chloride waters. Different equilibrium activity diagrams for the dominant cations in groundwater (Na+, K+, Ca2+, Mg2+) have been constructed to correlate water equilibrium conditions with the mineral assemblage, K-feldspar, clinochlore, muscovite, quartz, and calcite. Granites and schists are the rocks within which groundwater circulation approaches mineral equilibrium with equilibrium temperatures around 140–160 °C. Groundwaters circulating throughout granodiorite seem a little bit high in Mg2+ to reach equilibrium conditions. Miño River’s tectonic valley presents strong morphological contrasts in terms of faulting and rock types to the north of the river that allow increased longitude and depth of groundwater circulation. These conditions allow close achievement of water–mineral equilibrium conditions. More research is needed to know the extension of this energy resource and optimize its use.