Novel U and Th isotopic tracers for characterization of karstic freshwater and recent tufa from the Krka River (Croatia)

The Krka River in Croatia is a specific groundwater-fed karstic river, characterized by complex hydrology and seasonally variable diffuse subsurface recharge. It represents a unique model system, where tufa is precipitating in a turbulent stream at morphologic discontinuities and in lentic environments. Tufa is especially attracting attention as a potential environmental archive that can provide insight into water-rock interactions, hydraulic connections, recharge, and terrestrial CO₂ cycling in terms of storage, evasion, and transfer to the ocean. In a dynamic karst river system with alternating lentic and turbulent lotic sections, the carbonate precipitation rarely occurs in isotopic equilibrium for either C or O isotopes. Therefore, the use of traditional isotopes in river water (d¹⁸O, d²H, d¹³C_DIC), tufa, surrounding bedrock, soil (d¹³C_CaCO3, d¹⁸O_CaCO3, d¹³C_POC) and geochemical parameters (Ca, Mg, Na, K, HCO₃^-) in river/carbonate system in combination with uranium (U) and thorium (Th) isotopic composition could increase the understanding of this complex karst hydrodynamic system and help with the identification and quantification of authigenic carbonate precipitated in the river.

River water samples, tufa, and surrounding bedrock and soil samples were collected at 11 locations, which were selected based on the spatial distribution of bedrock types and occurrence of tufa. Measurements of U and Th isotope ratios were carried out with multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) and for assessing U and Th concentrations, triple Quadrupole ICP-MS was used.

The study brought a new perspective to already known data on this highly sensitive karst eco-system. U concentration and the activity ratios of ²³⁴U/²³⁸U in the river show a decrease with the distance from the spring. U isotopic differences reflect the changing bedrock lithology and the mixing of waters from different sources. Therefore, U values show promise as a tracer for studying changes in host rock composition and hydraulic connections in the karst aquifer.

Tufa samples of the studied system demonstrate a much higher activity ratio of ²³⁴U/²³⁸U compared to the bedrock and soil. The ²³⁴U/²³⁸U ratio of carbonate in tufa is almost identical to that of the dissolved U in the river water, indicating that a majority of U present in tufa samples is co-precipitated with the carbonate from the river water. This assumption was confirmed with a much lower ²³⁴U/²³⁸U ratio of the non-carbonate fraction of tufa, which is comparable to that of the soil and bedrock, and the d¹⁸O and d¹³C values of carbonate in tufa, which confirmed its authigenic origin. 

The Th and U concentrations and their isotope ratios in carbonate materials from our study were shown to be reliable indicators of the storage of CO₂ as authigenic carbonate in tufa. Moreover, they were also useful for the determination of tufa with U bond to detrital material and consequently relevant for both the construction of the CO₂ mass balance in a karst aquifer, as well as for dating.