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Geochemistry of thermal waters in the northeastern part of the Eger Rift, Czech Republic: implications for 14C dating

Ondrej Sracek1, Tereza Dupalová2, Zbyněk Vencelides3, Karel Žák4
1 Faculty of Science, Department of Geology, Palacky University
2 Charles University
3 OPV s.r.o.
4 Academy of Science

O 12.10 in Grundwasserqualität

29.05.2014, 16:20-16:40, H19, NW II

 

The investigation of the thermal waters in Ústí nad Labem area in the northeastern part of the Eger Rift aimed to determine their origin and age. Waters from geothermal reservoirs are exploited from wells from up to 616 m deep. For a comparison, thermal waters of the adjacent Teplice Spa area were also incorporated into the study. Results based on water and isotopic chemistry indicated mixing of groundwaters from aquifers of the Bohemian Cretaceous Basin (BCB) with ground water derived from crystalline rocks of the Erzgebirge Mts., represented by thermal waters in Teplice Spa, in approximately equal proportions. Compared to thermal waters in Děčín, which are of Ca-HCO3 type, two types of thermal waters in Ústí nad Labem have been identified: Na-HCO3-Cl-SO4 type with high TDS values and Na-Ca-SO4-HCO3 type with low TDS values. Quartz geothermometer indicated a similar depth of circulation, from 1.2 to 1.4 km at both Ústí nad Labem and Děčín. Carbon isotope data and speciation and inverse geochemical modeling suggest a large input of endogenous CO2 at Ústí nad Labem in the case of high TDS groundwater. Besides CO2 input, both silicate dissolution and cation exchange coupled with dissolution of carbonates may explain the high mineralization of thermal waters equally well. This is a consequence of similar δ13C and 14C values in endogenous CO2 and carbonates (both C sources have 14C activity 0 pmc, endogenous CO2 has δ13C around -3 ‰, carbonates in the range from -4 to + 3 ‰ V-PDB). The source of Cl- seems to be relict brine formed in Tertiary lakes, which infiltrated into the deep rift zone and is continuously being flushed out. The difference in water chemistry between high and low mineralization ground waters in Ústí nad Labem is caused by position of the high TDS ground water wells in endogenous CO2 emanation centers linked to channel-like conduits. This results in high dissolution rates of minerals and in different δ13C(DIC) and 14C(DIC)  fingerprints. A combined δ34S(SO4) and δ18O(SO4) study of dissolved sulfate indicate multiple sulfate sources, including sulfate from relict brines and oxidation of H2S. The study clearly demonstrates potential problems of 14C dating encountered at sites where multiple sources of carbon are present. In such cases several evolutionary ground water scenarios and ages are possible.

 



 

Dupalová T., Sracek O., Vencelides Z., Žák K., 2012. The origin of thermal waters in the northeastern part of the Eger Rift, Czech Republic, Applied Geochemistry, 27, 3, 689-702.

 



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Letzte Änderung 26.02.2014