Temporal variations in (134)Cs and (137)Cs concentrations in seawater along the Shimokita Peninsula and the northern Sanriku coast in northeastern Japan, one year after the Fukushima Dai-ichi Nuclear Power Plant accident

Abstract

Ninety-six seawater samples were collected between May 2011 and March 2012 at 6 sites along the Shimokita Peninsula and the northern Sanriku coast, 250-450 km north of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). Cesium-134 and (137)Cs concentrations were determined by low-background γ-spectrometry. During May-June 2011, (134)Cs and (137)Cs concentrations in surface waters decreased from 1.0-2.8 to 0.7-1.5 mBq/L and from 2.1-3.9 to 1.9-3.0 mBq/L, respectively. These decreases were due to diffusion and advection in the ocean after atmospheric input of the FDNPP-derived radionuclides. However, in July-August 2011, the concentrations of both radionuclides in the water samples collected on the Pacific side of the Shimokita Peninsula and the northern Sanriku coast exhibited 30-50-fold increases (∼40 mBq/L for (134)Cs and ∼50 mBq/L for (137)Cs) over concentrations observed at these sampling sites in June 2011 in contrast to the gradual decreases in the concentrations on the Tsugaru Strait side of the Shimokita Peninsula. These results suggest that radiocesium-contaminated waters offshore in the Pacific Ocean were transported to coastal regions along the Pacific side of the Shimokita Peninsula and the northern Sanriku coast by ocean currents.

Keywords: (134)Cs; (137)Cs; Coastal seawater; Fukushima Dai-ichi Nuclear Power; Northern Sanriku; Plant accident.