Application of the chlorine-36 method for the characterization of the groundwater circulation in tectonically active areas: examples from northwestern Anatolia/Turkey

For groundwaters of tectonic active regions the problem of determining the residence time above the range of the tritium method arises. The usually applied isotope techniques are often not appropriate because of (i) the high amount of water-rock interaction, (ii) the underground production of nuclides in crystalline fissured rocks under geothermal conditions, (iii) the occurrence of highly saline waters at boiling temperature affected by waterrock interaction processes, as also the high fluxes of CO₂. However, the ³⁶Cl evolution will be affected by these processes, too. Nevertheless it gives an independent tool to investigate the groundwater evolution. Therefore the application of the ³⁶Cl method looks very promising. Within this publication the interpretation of ³⁶Cl measurements of three study areas along the North Anatolian Fault zone (from East to West) of Kuzuluk/Adapazari, Bursa and of Canakkale (Tuzla/Kestanbol) are discussed. By considering for each area the sources of ³⁶Cl and the water-rock interaction with respect to the chloride cycle, information on the ongoing processes of groundwater circulation and evolution have been derived. In addition, ranges of residence times of these waters were gained which are compatible with the results of geological, hydrogeological and hydrochemical investigations (Balderer et al ., 1991; Greber, 1992, 1994; lmbach, 1992, 1994; Imbach and Balderer, 1990; Mutzenberg, 1989) as well as with already developed conceptual groundwater flow models.