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Estimation of groundwater residence time in a geologically active region by coupling 4 He concentration with helium isotopic ratios
Author(s) -
Morikawa N.,
Kazahaya K.,
Yasuhara M.,
Inamura A.,
Nagao K.,
Sumino H.,
Ohwada M.
Publication year - 2005
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2004gl021501
Subject(s) - groundwater , aquifer , geology , residence time (fluid dynamics) , flux (metallurgy) , hydrology (agriculture) , geochemistry , materials science , geotechnical engineering , metallurgy
Because mixing of groundwater commonly occurs in geologically active regions such as subduction zones, sole use of a conventional method for groundwater dating is not applicable. Here we present a new model using dissolved 4 He concentration and He isotopic ratio as well as crustal He flux to estimate the very long residence time of groundwater where mixing of waters of different origin occurs. We applied this model to the Kobe area, southwest Japan, where Arima‐type thermal brine with a high 3 He/ 4 He (1 × 10 −5 ) component is incorporated into meteoric water. A groundwater residence time of 25–230 kyr is estimated for the deep groundwater (up to 1500 m depth) aquifers. The flux of Arima‐type brine is also estimated to be 2 × 10 5 m 3 yr −1 in the studied area (20 km × 5 km). These attempts contribute to evaluating the long‐term stability of geological formations over the time range of 10 4 –10 5 years and to considering the deep‐seated fluid flux to shallower aquifers.