Groundwater flow traced by bomb pulses of 36Cl and tritiogenic 3He in a borehole

Abstract In 2001, a borehole was drilled to 400 m depth below the ground surface of a tableland in northern Japan, where horizontal porous sedimentary rock strata are deposited. Fresh drill-core and groundwater samples were collected from the borehole for estimating global fallout bomb pulses of 3 H, which becomes tritiogenic 3 He (tri 3 He) through β-decay, and 36 Cl. In the northern hemisphere, the maximum fallout deposition of 36 Cl from nuclear testing was observed during 1954–1957, and that of 3 H in 1963. Most 3 H (half-life 12.3 y) in the rock porewater had decayed to tri 3 He during the 38 years between 1963 and 2001. In the borehole, bomb pulses were observed in profiles of tri 3 He and 36 Cl/Cl at 101 m and around 138 m depth below the ground surface, respectively. The downward groundwater velocity was estimated to be 2.9–3.1 m/y from the depth of the 36 Cl/Cl bomb pulse and 2.7 m/y from the depth of the tri 3 He bomb pulse. Although the two velocity estimates differ by 10–15%, it is possible to model groundwater flow in the tableland as downward piston flow without diffusion.