Distribution of artificial radionuclides (^110mAg, ^129mTe, ¹³⁴Cs, ¹³⁷Cs) in surface soils from Miyagi Prefecture, northeast Japan, following the 2011 Fukushima Dai-ichi nuclear power plant accident

The Tohoku earthquake and tsunami occurred off the Pacific coast of Japan in March 2011 and caused the Fukushima Dai-ichi nuclear power plant accident. To assess the distribution of artificial radionuclides in Miyagi Prefecture (northeastern Japan), we measured the activity concentrations of ^110mAg, ^129mTe, ¹³⁴Cs, and ¹³⁷Cs in surface soil samples that were collected from 60-190 km north of the power plant approximately 1 month (April 16-29, 2011) after the earthquake. Since the studied samples were obtained from depths of 0-1 cm, the activity concentrations we determined were higher than those reported by the Japanese Government, whose measurements were based on soil samples obtained from depths of 0-5 cm. We found relatively high concentrations of radionuclides in surface soils in the southern and northern parts of Miyagi Prefecture. In the south, close to the border with Fukushima Prefecture, the total activity concentrations in the period April 16-29, 2011, reached approximately 27,600 Bq/kg dry soil. Relatively high total concentrations (2600-6600 Bq/kg dry soil) were also found in the northern part of Miyagi Prefecture (north Osaki, Kurihara, and Kesen-numa), but relatively low concentrations (400-1900 Bq/kg dry soil) were found in Sendai city and other areas in central Miyagi Prefecture (Higashimatsushima, south Osaki, Ishinomaki, and Onagawa). The concentrations found are consistent with the spatial variation in radiation dose rates in the air in Miyagi Prefecture.