Helium isotopic fingerprints of the heavy land subsidence left in groundwater of the Saga Plain, near Beppu–Shimabara graben, Kyushu, Japan

The Saga Plain is near Beppu–Shimabara graben, a region of potential active volcanism. In the graben, mantle He, which has a high 3He/4He ratio of 1.1 × 10−5, escapes easily from the underlying subduction zone. In groundwater of the Saga Plain, except in the Shiroishi district, this ratio gradually increased as the dissolved He content increased, to a maximum of 5 × 10−6. In central Shiroishi, however, the ratio reached a minimum of 8.7 × 10−7 with increasing dissolved He content, suggesting that groundwater in central Shiroishi has selectively accumulated radiogenic He, which has a very low ratio of 1 × 10−8, rather than reflecting the regional He, which is rich in mantle He. This can be explained if groundwater in Shiroishi has become mixed with fossil pore water drawn from impermeable marine clay aquitard layers. The withdrawal of pore water has also caused severe land subsidence in central Shiroishi.