Abstract
The karst aquifer of the Ljubljanica River catchment, which has numerous springs and sinks, presents an interesting environment for studying hydrogeological processes. This study aims to explore the behavior of U isotopes and to evaluate their use as tracers of hydrogeochemical processes as an alternative to classical geochemical tracers (i.e., physicochemical parameters, elemental ratios, and alkalinity) involved in water–rock interactions and water flow in this karst water system. Basic hydrochemical parameters, as well as the spatiotemporal variations of total U concentrations, 234U/238U activity ratios, and δ238U values, were monitored in water samples from springs and sinks under different hydrological conditions. The bedrock as the source of dissolved and detrital U was also analyzed. Multi-collector inductively couple plasma-mass spectrometry results reveal variations of the 234U/238U activity ratios, which are consistently negatively correlated with the discharge at most analyzed sites. Large 238U/235U isotope fractionation occurred during bedrock weathering, and the large variability of the measured δ238U values is seemingly unrelated to the lithological characteristics of the bedrock or discharge. Our results confirm that 234U/238U activity ratios in water can be used as a tracer for studying changes in groundwater flows and the mixing of waters of different origins under different hydrological conditions.
Highlights
Around 13%–14% of Earth’s land surface is characterized by karst features, which most commonly develop in carbonate rocks [1], and ~10% of the world’s population relies on karst aquifers for its drinking water supply [2]
The present study aimed to provide a baseline for U isotopes under the diverse hydrological conditions in the karst aquifer of the Ljubljanica river catchment (Slovenia) and to explore the potential of using U concentrations and isotope compositions in springs and streams as alternative tracers to classical ones to analyze the sources of water and solutes and water mixing under different flow regimes in this complex groundwater system
This study showed that classical geochemical tracers of the origin of water in the Ljubljanica River catchment reflect the chemical composition of bedrock and provides some information about the groundwater flow and mixing of water in different parts of the aquifer
Summary
Around 13%–14% of Earth’s land surface is characterized by karst features, which most commonly develop in carbonate rocks [1], and ~10% of the world’s population relies on karst aquifers for its drinking water supply [2]. Karst aquifers show spatially variable rapid infiltration, and their water flow is controlled by heterogeneous permeability. The characterization of water circulation in karst systems is challenging, it is essential for sustainable water management in these areas. Environmental tracers are commonly used in hydrogeology to investigate the recharge, groundwater flow, and water–rock interactions in karst aquifers [6]. Because flow paths are complex, classical hydrogeological approaches are sometimes not enough to properly investigate the groundwater hydrodynamics [7]. The use of geochemical and physicochemical parameters in combination with uranium (U) isotopic composition could significantly improve the understanding of complex karst hydrodynamics
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