Long-term time series of environmental tracers reveal recharge and discharge conditions in shallow karst aquifers in Hungary and Slovakia

Abstract

Study region: Three karst regions in Hungary and Slovakia. Study focus: Time series of environmental tracers both in the groundwater recharge and discharge provide important insights into how a karst water system works. The aim of the present work was to study the response of discharging karst waters to recharge using time series of environmental tracers, such as tritium, stable water isotopes, noble gases and SF6. New hydrological insights for the region: Our results show that sampling frequency plays a significant role in detecting short residence times (months): the seasonality of 18O isotope composition of a selected karst spring indicates a 10 % contribution of recent water with a residence time of half a year. The contribution of an older component can be proven by the decrease of the tritium content of the waters, which compares to the decreasing trend of the tritium time series of the precipitation. However, the tritium concentrations are just slightly lower than those of the precipitation and the recharge water, hence the residence times of these shallow springs are supposed to be short. 3H/3He and SF6 apparent ages confirm this to be between 0 and 10 years, with a median of 1.4 years. Our study demonstrates that long-term time series are preferable to provide better estimation to the age distribution than individual, short-term investigations.

Keywords

• Karst hydrology
• Environmental tracers
• Tritium
• Oxygen isotopes
• Residence time