Application of tritium in precipitation and baseflow in Japan: a case study of groundwater transit times and storage in Hokkaido watersheds
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...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-07-01
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| Series: | Hydrology and Earth System Sciences |
| Online Access: | http://www.hydrol-earth-syst-sci.net/20/3043/2016/hess-20-3043-2016.pdf |
| Summary: | 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 km<sup>2</sup>. 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 km<sup>3</sup> 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. |
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| ISSN: | 1027-5606 1607-7938 |