Excess air of groundwater in Chubu region estimated from dissolved nitrogen and argon concentrations

－Impact of excess air on SF₆ age －

Abstract

To evaluate the impact of excess air on the SF₆-based groundwater age estimation, the dissolved nitrogen and argon concentrations in groundwater at 70 points in the Chubu region were measured, and the excess air was estimated. The groundwater from all points contained supersaturated nitrogen and argon, indicating that excess air was generated during groundwater recharge. The average excess air calculated based on dissolved nitrogen content was 2.2 cc STP / kg for the springs in a small mountainous catchment area, 2.8 cc STP / kg for the springs at the foot of a Quaternary volcano, and 3.7 cc STP / kg for the groundwater in an alluvial fan. The groundwater residence time was estimated to be 4 to 18 years longer based on the excess air correction of SF₆ . The corrected residence time was consistent with the residence time estimated from other age-dating tracers or hydrogeological settings. These results indicate that excess air correction is an important process in SF₆ dating applications in Japan, especially in highly permeable areas such as Quaternary volcanoes and alluvial fans.