Radium geochemistry in Na–Cl type groundwater in Niigata Prefecture, Japan

Radium isotopes in 23 Na–Cl type groundwater sampled mainly from deep wells in Niigata Prefecture, which is the site of the largest oil- and gas-fields in Japan, were measured along with U isotopes, chemical components and hydrogen and oxygen isotope ratios to elucidate the distribution and behavior of Ra in a brackish environment underground. Also analyzed were U and Th isotopes in 38 rock samples collected from outcrops at 17 locations. Ra-226 concentrations (8.86–1637 mBq kg−1) of groundwater samples roughly correlated with total dissolved solid (TDS) concentrations and other alkaline earth contents. Their 228Ra/226Ra activity ratios (0.32–5.2) were similar to or higher than the 232Th/238U activity ratios (0.6–1.7) in the rocks. The most likely transport mechanism of Ra isotopes into groundwater was due to their α-recoil from the solid phase, probably from the water–rock interface where Th isotopes had accumulated, and adsorption/desorption reaction based on the increase in 226Ra contents with TDS.