Radium isotopes (226Ra and 228Ra) in Na–Cl type groundwaters from Tohoku District (Aomori, Akita and Yamagata Prefectures) in Japan

Abstract

A total of 28 Na–Cl type groundwater samples were collected from Aomori, Akita and Yamagata Prefectures, in the Tohoku District of Japan, and their radium isotope (226Ra and 228Ra) concentrations were measured along with their chemical components and stable isotope ratios (δ2H and δ18Os). The 226Ra concentrations in groundwater samples varied widely, ranging from 8.8 to 1587 mBq kg−1. These concentrations showed an increasing tendency with the increase of the total dissolved solid (TDS) contents. The 228Ra/226Ra activity ratios were in the range from 0.3 to 4.2, with most data being around 0.5–2. These ratios were within those of 232Th/238U found in granitic and related rocks and so on in Japan, indicating that Ra isotopes mainly ejected into the groundwater by the alpha-recoil process. The relationship between 226Ra and other parameters suggested that Ra isotopes in groundwater samples in this study were mainly constrained by adsorption–desorption reactions depending on salinity with wide variation. Clear correlation between 226Ra-Ca, 226Ra-Sr, 226Ra–Ba and 226Ra–TDS observed in sulfate-free groundwater samples indicated that Ra isotopes of them were constrained by adsorption–desorption reactions depending on salinity under reducing condition. In contrast, relationship of 226Ra-Ca, 226Ra-Sr, 226Ra–Ba and 226Ra–TDS in sulfate-containing groundwater samples varied widely, and then, removal or enhanced mobility of Ra isotopes of them were observed.