Geochemical behavior of Ra in a uranium mill tailings pond of the Ningyo-toge mine area

Tanaka, Kazuya  ; Kurihara, Yuichi*  ; Tomita, Jumpei  ; Maamoun, I.; Yamasaki, Shinya*; Tokunaga, Kohei   ; Fukuyama, Kenjin*; Kozai, Naofumi   

We collected surface sediment and water samples at a U mill tailings pond in the Ningyo-toge center. Radioactivity concentrations of Ra in sediments were 12,000 - 29,000 Bq/kg while those in water ranged from 60 to 580 mBq/L. As a result, apparent distribution coefficients of Ra between ferric sediment and water were estimated to be 3.1 10 - 3.8 10 mL/g, which were higher than reported ones for ferrihydrite and goethite. This suggests that another host on which Ra is strongly fixed would be present in sediment if we assume equilibrium between sediment and water. It is possible that Mn(IV) oxide as a minor component contributed to an increase in the apparent distribution coefficients. Another possibility is upward flow of pore water containing Ra or diffusion of Ra released from lower sediment layers. Either scenario provides additional input of Ra to surface sediment. In this case, surface sediment and water did not reach equilibrium with each other, but Ra concentrations in surface sediments were governed by dynamics through input of Ra from porewater as well as groundwater in sediment-water interaction.