Application of tritium in precipitation and baseflow in Japan: a case study of groundwater transit times and storage in Hokkaido watersheds
Abstract. In this study, we demonstrate the application of tritium in precipitation and baseflow to estimate groundwater transit times and storage volumes in Hokkaido, Japan. To establish the long-term history of tritium concentration in Japanese precipitation, we used tritium data from the global network of isotopes in precipitation and from local studies in Japan. The record developed for Tokyo area precipitation was scaled for Hokkaido using tritium values for precipitation based on wine grown at Hokkaido. Then, tritium concentrations measured with high accuracy in river water from Hokkaido, Japan, were compared to this scaled precipitation record and used to estimate groundwater mean transit times (MTTs). A total of 16 river water samples in Hokkaido were collected in June, July, and October 2014 at 12 locations with altitudes between 22 and 831 m above sea level and catchment areas between 14 and 377 km2. Measured tritium concentrations were between 4.07 (± 0.07) TU and 5.29 (± 0.09) TU in June, 5.06 (± 0.09) TU in July, and between 3.75 (± 0.07) TU and 4.85 (± 0.07) TU in October. We utilised TracerLPM (Jurgens et al., 2012) for MTT estimation and introduced a Visual Basic module to automatically simulate tritium concentrations and relative errors for selected ranges of MTTs, exponential–piston ratios, and scaling factors of tritium input. Using the exponential (70 %) piston flow (30 %) model (E70 %PM), we simulated unique MTTs for seven river samples collected in six Hokkaido headwater catchments because their low tritium concentrations were no longer ambiguous. These river catchments are clustered in similar hydrogeological settings of Quaternary lava as well as Tertiary propylite formations near Sapporo city. However, nine river samples from six other catchments produced up to three possible MTT values with E70 % PM due to the interference by the tritium from the atmospheric hydrogen bomb testing 5–6 decades ago. For these catchments, we show that tritium in Japanese groundwater will reach natural levels in a decade, when one tritium measurement will be sufficient to estimate a unique MTT. Using a series of tritium measurements over the next few years with 3-year intervals will enable us to estimate the correct MTT without ambiguity in this period. These unique MTTs will allow estimation of groundwater storage volumes for water resources management during droughts and improvement of numerical model simulations. For example, the groundwater storage ranges between 0.013 and 5.07 km3 with saturated water thickness from 0.2 and 24 m. In summary, we emphasise three important points from our findings: (1) one tritium measurement is already sufficient to estimate MTTs for some Japanese catchments, (2) the hydrogeological settings control the tritium transit times of subsurface groundwater storage during baseflow, and (3) in the future, one tritium measurement will be sufficient to estimate MTTs in most Japanese watersheds.