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

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 3H, which becomes tritiogenic 3He (tri3He) through β-decay, and 36Cl. In the northern hemisphere, the maximum fallout deposition of 36Cl from nuclear testing was observed during 1954–1957, and that of 3H in 1963. Most 3H (half-life 12.3 y) in the rock porewater had decayed to tri3He during the 38 years between 1963 and 2001. In the borehole, bomb pulses were observed in profiles of tri3He and 36Cl/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 36Cl/Cl bomb pulse and 2.7 m/y from the depth of the tri3He 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.