Carbon isotopes to constrain the origin and circulation pattern of groundwater in the north-western part of the Bohemian Cretaceous Basin (Czech Republic)

Abstract The Bohemian Cretaceous Basin represents a complex hydrogeological system composed of several aquifers with very favourable hydrogeological properties. These aquifers have been exploited for many years. The sustainability of such resources might be guaranteed by well organised water management, which requires a detailed knowledge about the functioning of the hydrogeological system. Although many efforts have previously been made to evaluate groundwater residence time, the many intricate geochemical processes complicate groundwater dating. The current study clarifies the functioning of this complex hydrogeological basin using hydrogeochemical and isotopic investigations. Chemical data and a combination of 13 C and 14 C isotopes within the Cenomanian and the Turonian layers indicate groundwater interactions with deep-seated CO 2 , rock matrix, surface waters and fossil organic matter. Very depleted δ 13 C values (average δ 13 C ∼ −13.4‰) suggest interactions with fossil organic matter, whereas enriched values account for the interaction with deep CO 2 gas ascending from the upper mantle via the numerous faults and fractures, and also, to a lesser extent, from calcite dissolution. Geochemical processes that take place in the system cause a clear depletion in 14 C that greatly complicates groundwater residence time evaluation. Different dilution correction models have been applied considering the different C origins. The stable isotope content, mainly 18 O values, indicates both the contribution of modern precipitation and the partial infiltration of palaeowaters during colder climatic conditions from the end of the Pleistocene. The apparent 14 C groundwater ages range from modern to 11.1 ka BP, which suggests some post glacial infiltration from melting ice sheets. Finally, all the acquired information was used to propose a conceptual model of C origin within the basin.