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Tritium (3H) as a constituent of the water molecule is an important natural tracer in hydrological sciences. The anthropogenic tritium introduced into the atmosphere unintentionally became an excellent tracer of processes on a time scale of up to 100 years. A prerequisite for tritium applications is to know the distribution of tritium activity in precipitation. Here we present a database of isoscapes derived from 41 stations for amount-weighted annual mean tritium activity in precipitation for the period 1976 to 2017 on spatially continuous interpolated 1 km×1 km grids for the Adriatic–Pannonian region (called the AP3H_v1 database), with a special focus on post-2010 years, which are not represented by existing global models. Five stations were used for out-of-sample evaluation of the model performance, independently confirming its capability of reproducing the spatiotemporal tritium variability in the region. The AP3H database is capable of providing reliable spatiotemporal input for hydrogeological application at any place within Slovenia, Hungary, and their surroundings. Results also show a decrease in the average spatial representativity of the stations regarding tritium activity in precipitation from ∼440 km in 1970s, when bomb tritium still prevailed in precipitation, to ∼235 km in the 2010s. The post-2010 isoscapes can serve as benchmarks for background tritium activity for the region, helping to determine potential future local increases in technogenic tritium from these backgrounds. The gridded tritium isoscape is available in NetCDF-4 at https://doi.org/10.1594/PANGAEA.896938 (Kern et al., 2019).

Tritium is an important natural tracer in hydrological sciences. The dataset used in the present study is compiled of ~7, 700 monthly precipitation tritium activity values from 71 stations, covering the period from Jan 1961 to Dec 2017. As a pre-processing step, a sequential univariate outlier detection procedure was applied in which the time series of the stations were pairwise compared for each year. Variogram analysis was then applied to the precipitation amount weighted annual averages of tritium activity. Variograms applicable for interpolation were obtained from years 1977, 1982, 2007, 2010, 2011 and 2012. These where further on used as the reference years, and their variograms were used as the weights for kriging in the intermediate years. It suggests a decrease in spatial representativity of the precipitation monitoring stations from 590 km in 1977 to ~270 km in 2012, which might reflect the diminishing influence of broadly homogenous anthropogenic disturbance (i.e., global effect of bomb tests) on natural tritium levels of precipitation. Since 3H activity in the region approximated the natural level in precipitation by the early-1990s, it can be expected that the ~270 km range obtained for the 2012 reflects the natural 3H variability. The final product of the research was a database of the interpolated 1x1 km grids of each year’s tritium activity.

Systematic studies of isotope composition of monthly precipitation in Croatia and Slovenia have been performed since 1976 and 1981, respectively. The monitoring included measurements of radioactive isotope of hydrogen – tritium (3H) as well as stable isotope ratios of hydrogen (2H/1H) and oxygen (18O/16O). Isotope data are partly available in the Global Network of Isotopes in Precipitation (GNIP) at https://nucleus.iaea.org/wiser. This work presents details on long-term monitoring of tritium activity concentration in precipitation at Zagreb, Croatia, and Ljubljana and Portorož, Slovenia, performed in collaboration between Ruđer Bošković Institute (RBI) and Jožef Stefan Institute (JSI). Sampling locations have been changed during the long-term monitoring. Local contamination with technogenic tritium was noticed at the RBI site and therefore precipitation has been collected at the Zagreb-Grič site at the Meteorological and Hydrological Service of Croatia, in the center of Zagreb since 1996. Monitoring in Ljubljana was performed between 1981 and 1993 at the synoptic station Ljubljana–Bežigrad located at the Hydrometeorological Survey of Slovenia (today Slovenian Environment Agency – ARSO), between 1993 and 2000 at the JSI (station Ljubljana–IJS), and finally, since 2000 at the Reactor Centre of the JSI (station Ljubljana–Reaktor) in the vicinity of Ljubljana. In addition, monitoring has been performed at the ARSO synoptic station Portorož airport since 2000. Zagreb and Ljubljana are both continental stations while Portorož represents a coastal station. The long-term data records showed tritium pattern typical of continental stations of the Northern Hemisphere. Seasonal variations were superposed on the basic decreasing trend of mean annual values until app. 1997. The maximal 3H activity concentration at station Zagreb was observed between May- July, mostly in June. A secondary maximum was also observed three times in January and February. The lowest 3H activity concentrations were almost uniformly distributed from October to February, with a slightly more frequent occurrence in December. The seasonal distribution at the Ljubljana station was characterized by the most frequent appearance of the maximum in July, no secondary winter maximum, and the minimum was most frequently measured in November, although it was distributed from October to March. The ratio of the maximum to minimum value for Zagreb and Ljubljana precipitation ranged from 2.2 to 5.7 and from 2.3 to 5.5, respectively, without a significant trend. Data recorded during last 2 decades, however, show almost constant mean annual 3H activity concentration ranging between 5 and 10 TU with the average of about 9 TU for the continental stations Ljubljana and Zagreb, while for the coastal station Portorož lower value of about 7 TU was observed. Seasonal variation remained with winter activities close to the natural pre-bomb 3H activity concentrations (≤5 TU), and summer values rarely above 20 TU.