Records of the riverine discharge of 129I in riverbank sediment after the Fukushima accident

Although 129 I discharge from watersheds is fundamental for assessing long-term radiation effects on aquatic ecosystems, 129 I originating from the Fukushima nuclear accident is yet be evaluated. This study investigated the transport behavior of 129 I by riverbank surveys conducted from 2013 to 2015 in a watershed where the 129 I/ 137 Cs activity ratio is low in the mountainous area and high in the plain as of 2011. Until 2015, the 129 I/ 137 Cs activity ratio of the levee crown in the studied watershed was similar to that of the surrounding area in 2011. However, the 129 I/ 137 Cs ratios of the surface riverbank sediments were all low, indicating that radionuclides transported from the mountainous area were deposited on the riverbank in the plain. The vertical distribution of the 129 I/ 137 Cs ratio in the riverbank sediments indicated that some 129 I and 137 Cs deposited during the accident remained in the lower layers, but most were eroded immediately after the accident. Based on the 129 I/ 137 Cs ratios of sediments deposited on the riverbank, which remained constant until 2015 after the accident, the amount of 129 I discharged to the ocean was determined from the previously evaluated 137 Cs discharge. It was calculated that 1.8 x 10 5 Bq and 1.2 x 10 7 Bq of 129 I were discharged with sediment from the studied watershed and the contaminated river watersheds (Abukuma River and Fukushima coastal rivers, including the study river), respectively. This amount of 129 I was 0.3% of the 129 I released from the Fukushima Dai-ichi Nuclear Power Plant into the ocean immediately after the accident. Furthermore, a comparison of the 129 I/ 137 Cs ratio showed that the continuous 129 I and 137 Cs discharge from the river contribute little to their amount in the seafloor sediments along the Fukushima coast.