Application of tritium in precipitation and river water in Japan: A case study of groundwater transit times and storage in Hokkaido watersheds
Authors: Gusyev Maksym  A. , Morgenstern Uwe , Stewart Michael  K. , Yamazaki Yusuke , Kashiwaya Kazuhisa , Nishihara Terumasa , Kuribayashi Daisuke , Sawano Hisaya , Iwami Yoichi 
Published: April 2016

Abstract. In this study, we demonstrate the application of tritium in precipitation and river water 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 (GNIP) 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 determine groundwater mean transit times (MTTs). Seventeen river water samples in Hokkaido were collected in June, July and October 2014 at twelve locations with altitudes between 22 and 831 m above mean sea level and catchment areas between 45 and 377 km2. Measured tritium concentrations ranged between 4.07 (&amp;pm;0.07) TU and 5.29 (&amp;pm;0.09) TU in June, 5.09 (&amp;pm;0.09) TU in July, and between 3.75 (&amp;pm;0.07) TU and 5.01 (&amp;pm;0.08) TU in October. We utilized TracerLPM (Jurgens et al., 2012) for MTT estimation and introduced a Visual Basic module to automatically simulate tritium concentrations and relative errors for selected range of MTTs, exponential-piston ratio, and scaling factors of tritium input. Using the exponential(70 %)-piston flow(30 %) model (E70 %PM), we found unique MTTs for seven river samples collected in six Hokkaido headwater catchments because their low tritium concentrations are not ambiguous anymore. These river catchments are clustered in similar hydrogeological settings of Quaternary lava as well as Tertiary propylite formations nearby Sapporo city. However ten river samples of other six 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 robust MTT. Using a series of tritium measurements over the next few years with 3 year intervals will enable us to determine the correct MTT without ambiguity in this period. These unique MTTs allow estimation of groundwater storage volumes for water resources management during droughts and improvement of numerical model simulations. In summary, we emphasise three important points from our findings: (1) one tritium measurement is already sufficient to estimate MTT for some Japanese catchments, (2) the hydrogeological settings control transit times with tritium of groundwater watersheds at baseflow, and (3) in future one tritium measurement will be sufficient to estimate MTT in most Japanese watersheds.