ABSTRACT

Radioactivity concentrations of ¹³¹I, ¹³⁴Cs and ¹³⁷Cs were determined for the river water sampled from the Edogawa River, the Arakawa River and the Tamagawa River, three major rivers spreading in or surrounding Tokyo Metropolis. Data from March 28 to April 7 are presented. Among the three radioactive nuclides, ¹³¹I recorded the highest radioactivity concentrations of 31 Bq/L and 11 Bq/L in water samples collected on March 28 from the Edogawa and the Arakawa Rivers, respectively. Two trends of the ¹³¹I concentration change occur from March 28 to April 7 for the Edogawa River water; the ¹³¹I concentration decreased abruptly from March 28 to March 30 and then decreased gradually until April 7. These trends suggest that ¹³¹I in the Edogawa River in late March primarily originated from fallout in the Tokyo-Chiba area, which dropped to the ground together with rain from March 21 to 24. The ¹³⁷Cs concentration in river water from the Edogawa River changed differently from the ¹³¹I concentration. The highest ¹³⁷Cs concentration was measured for the sample collected on March 31. Apparently, fractionation between ¹³¹I and ¹³⁷Cs occurred during the transportation from FDNPP to the metropolitan area and also during the transportation from the ground where radionuclides deposited to the sampling site of river water.