Water isotopes from precipitation to groundwater: insights from drip water monitoring at a transect of a karst aquifer (Jama v Dovčku, SE Slovenia)

The δ¹⁸O (isotopic composition of oxygen) paleothermometry relies on the temperature-dependent ¹⁸O/¹⁶O isotope fractionation between H₂O and CaCO₃ during calcite precipitation from carbonate-rich water. An understanding of the relationship between the δ¹⁸O values of drip water and local precipitation is thus essential for the uncertainty management of paleoclimate reconstructions from cave flowstone formations. Therefore, a two-year monitoring of the stable isotope composition of drip water at 15 drip sites in the cave (Jama v Dovčku, SE Slovenia), and a three-year monitoring of monthly precipitation at the nearest national precipitation station (Planina v Podbočju) were conducted as a part of a paleoclimate study using stalagmites as climate archives. The δ¹⁸O-δ²H correlations and d-excess were analysed in precipitation and 15 drips, where the mean groundwater travel times (MTT) were also calculated from the degree of damping of amplitudes of sine wave functions fitted to the monthly δ¹⁸O and δ²H data of precipitation and drip water.

The MTT was 1.43-4.24 years and 1.58-3.71 years calculated from δ¹⁸O and δ²H values, respectively. The estimates were not correlated with the roof thickness above the drips, nor with the mean δ¹⁸O values of drip water at individual sites, however, a moderate negative correlation (R = -0.53) was observed between the roof thickness and the average δ¹⁸O values of drip water. The average δ¹⁸O values of individual drips varied and the variability exceeded the interannual differences in annual average δ¹⁸O values of precipitation. The d-excess of precipitation and individual drips was the same within the measurement uncertainty of δ¹⁸O and δ²H values; however, the slopes of δ¹⁸O vs. δ²H correlation lines were at most drips lower than in precipitation. At four out of 15 drip sites with MTT larger than 2.5 years, the δ¹⁸O and δ²H values were not correlated at a level of p = 0.05. Several possible explanations were explored, e.g., evaporation in the epikarst, mixing of slow- and fast-moving water, and different fractions of young water at individual drips caused by complex geometry and intricate underground connections in karst aquifers.

Acknowledgment: This study was supported by the Slovenian Research and Innovation Agency, research project J1-2478, programme P1-0143, and the Slovenian National Commission for UNESCO (UNESCO Intergovernmental Hydrological Programme).