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Geofluids
Volume 2018 (2018), Article ID 7823195, 21 pages
https://doi.org/10.1155/2018/7823195
Research Article

An Evaluation of the Long-Term Stagnancy of Porewater in the Neogene Sedimentary Rocks in Northern Japan

1Nuclear Fuel Cycle Backend Research Center, Abiko Research Laboratory, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-shi, Chiba-ken 270-1194, Japan
2Underground Research Center, Japan Atomic Energy Agency, 432-2 Hokushin, Horonobe-chou, Teshio-gun, Hokkaido 098-3224, Japan

Correspondence should be addressed to Kotaro Nakata

Received 18 June 2017; Accepted 19 November 2017; Published 18 January 2018

Academic Editor: Christophe Renac

Copyright © 2018 Kotaro Nakata et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A groundwater dating for very old porewater using 36Cl and 4He was applied to the Koetoi and Wakkanai formations distributed in the northernmost part in Japan. Measured 36Cl/Cl in the Koetoi Formation was 2.6 ± 2.0 × 10−15 and that in the Wakkanai Formation was 8.1 ± 2.5 × 10−15. These values are similar to 36Cl/Cl in situ secular equilibrium calculated from chemical compositions of core suggesting that Cl ions and porewater have remained in the formations for much longer than half-life of 36Cl . He concentration in porewater ranged from 1.1 × 10−6 to 2.6 × 10−5 () and it is much higher than water saturated with air indicating that both formations contain very old porewater. However, the possibility of mixing of young water was indicated because He concentration was lower than that calculated by multiplication of in situ He production and time after the uplift. This possibility was also supported by Cl, δD, and δ18O data. After combining information on 36Cl/Cl, 4He, and δD and δ18O, it was inferred that the porewater in the deep part of the Wakkanai Formation might have been stagnant since the uplift. The porewater in the Koetoi Formation and the shallow part of the Wakkanai Formation were found to be affected by young surface water.