Due to their extreme anisotropy and heterogeneity, the hydrogeological characterization of karst terrains is notoriously difficult. Approximately 50% of Croatia consist of karst aquifers, in particular the touristically important region along the Adriatic coast. Being an important water reservoir for this area, we have chosen the Gacka area of Croatia as a typical example. We studied the mixing of groundwater and the responses to recharge in several karst springs by using environmental tracers, including stable isotopes in water (¹⁸O, ²H), tritium (³H), chlorofluorocarbons (CFCs) and noble gases (³He, ⁴He, Ne, Ar, Kr, Xe). The variation of the stable isotopic signal (δ¹⁸O and δ²H) in the spring discharges provides qualitative information on the mixing of waters with different transit time. Assuming piston flow in the system, the CFC and ³H data indicate that the waters contain mainly post-1980 recharge. The CFC concentrations in the spring waters were found to be significantly different between wet and dry weather periods. The higher CFC values during the wet weather period indicate that the waters contained some water components that have relatively short mean residence times. An important finding of the study was that the sole application of tritium-helium methods does not permit an accurate age determination. This is due to mass exchange between liquid and gaseous phases occurring in karst aquifer systems particularly in dry weather periods. During such a dry weather period, more space in the karst groundwater system is not filled with water compared with a wet weather period. It is shown in this study, however, that the combined use of stable isotopes (¹⁸O, ²H), tritium, CFCs and noble gases is able to account for groundwater mixing and responses to recharge in this specific scenario. Compared to single-tracer studies, multi-tracer techniques will allow a much better understanding and characterization of these systems. This information is vital for the assessment of resources and potential contamination of the groundwater in this special environment.

The tritium-helium (³H/³He) dating method has been applied to the Chalk (fractured microporous limestone) aquifer in the UK for the first time. An evaluation of the results from diffusion cell versus pumped tube sampling showed generally good agreement between the two techniques. Measurements of noble gas (Ne, Ar, Kr and Xe) concentrations revealed typically low amounts of excess air in the aquifer, with little variation around a mean of 1.3 ccSTP/kg suggesting the predominance of steady recharge via the microporosity rather than via the fracture network. Chalk boreholes are generally unlined, with discrete inflows from a few fracture-related flow horizons. Despite this, attempts to detect age layering in the water column by suspension of diffusion samplers or by slow-pumping were unsuccessful. However, when short-screen piezometers were used, better evidence for an age-depth relationship was obtained. Assuming a piston-flow model of water movement, a vertical flow rate of ~3.3 m/yr was indicated. However, a more complex picture of movement was obtained by comparing total ³H activity (including the ³He decay equivalent) against SF₆ concentration, which suggested the existence of various modes of mixing. This would be consistent with the high degree of fracturing that exists in the Chalk.

Groundwater recharge amounts and residence times were estimated in the quaternary aquifer of the Jizera Mountains, Czech Republic, using noble gases, ²H, ¹⁸O, ³H and CFCs. Tracer ages from years to decades were determined for water in a shallow sedimentary aquifer (three boreholes of 10, 20 and 30 m depth). Approximately 90–180 mm of precipitation (7–14% of annual precipitation) infiltrates to deep percolation. A mean percolation velocity of 0.6 m/y in the sedimentary Uhlířská aquifer was estimated based on an apparent groundwater age of 40 years at 30 m depth. A lumped parameter approach was used to calculate groundwater residence times based on synoptic evaluation of ³H/³He, ³H and CFCs. Groundwater apparent ages in the shallowest borehole (10 m depth) determined by ³H/³He are younger than those determined by CFCs. This discrepancy is caused by partial re-equilibration of water with modern air, and by admixing of water that has zero age. The pronounced variations of δ¹⁸O values in the borehole may be ascribed to a young component quickly recharged from the air-exchanged stream water or through preferential flowpaths.

Quantification of natural groundwater recharge in three study sites within the Great Hungarian Plain was performed using environmental tracer techniques, based on utilization of tritium and helium-3 isotopes in groundwater samples taken from multilevel well-nests. Transport models were calibrated by the measured ³H activities at different depths below surface. The 1963 tritium bomb-peak was used to determine the average natural groundwater recharge. Rates of 48 ± 6 mm/yr, 62 ± 8 mm/yr and 27 ± 3 mm/yr, respectively were obtained. The ³H/³He dating technique was also used to determine age profiles at the three sites, giving recharge rates of 48 ± 6 mm/yr, 63 ± 9 mm/yr and 22 ± 4 mm/yr respectively. Although the recharge rates calculated by the two methods agree well with each other, these two approaches to recovering recharge rates are based on different recharge properties. Modelling of the bomb peak distribution is mainly affected by the position of the bomb peak, hence the recharge rate obtained is not necessarily reliable for recent decades. In contrast, the ³H/³He age-depth profile averages the last 4–5 decades, and therefore may provide a better estimation of long term recharge. A third approach to calculating recharge rates using a simple soil moisture—stable isotope approach was found to only be reliable over the most recent few years.

The earthquake off the Pacific coast of Japan and the subsequent tsunami on March 11, 2011, triggered a series of accidents in the Fukushima Daiichi Nuclear Power Plant (FNPP1). The accidents caused the release of a mixture of radioactive substances into the environment. This study measured the concentration of tritium (³H) and iodine-129 (¹²⁹I) in rainwater samples collected at Tsukuba, 170 km southwest of the plant, during the year following the accident. High ³H concentrations were observed in the rainwater samples collected within one month after the FNPP1 accident. ³H concentrations decreased steadily over time and returned to the levels before the accident. Concentrations of ¹²⁹I also decreased over time. However, pulses of high ¹²⁹I concentrations were observed at several other times following the accident. The ¹²⁹I concentrations were found to be correlated with iron concentrations in rainwater. It is likely that iron oxide, which can absorb iodate ions (IO₃^–), was the carrier of radiogenic iodine. This study concludes that ¹²⁹I and also ¹³¹I, which is one of the most harmful radionuclides produced in nuclear reactors, can be redistributed to the atmosphere in the months following the deposition of radiogenic iodine on the ground.