Distribution of the Fukushima-derived radionuclides in seawater in the Pacific off the coast of Miyagi, Fukushima, and Ibaraki Prefectures, Japan

摘要

The activities of artificial radionuclides in seawater samples collected offthe coast of Miyagi, Fukushima, and Ibaraki Prefectures were measured as partof a monitoring program initiated by the Japanese Ministry of Education,Sports, Science and Technology immediately after the Fukushima Dai-ichiNuclear Power Plant accident. The spatial and temporal distributions of thoseactivities are summarized herein. The activities of strontium-90, iodine-131,cesium-134 and -137 (i.e. 90Sr, 131I, 134Cs, and 137Cs)derived from the accident were detected in seawater samples taken from areasof the coastal ocean adjacent to the power plant. No 131I was detectedin surface waters (≤ 5 m depth) or in intermediate and bottom watersafter 30 April 2011. Strontium-90 was found in surface waters collected froma few sampling stations in mid-August 2011 to mid-December 2011. Temporalchanges of 90Sr activity in surface waters were evident, although the90Sr activity at a given time varied widely between sampling stations.The activity of 90Sr in surface waters decreased slowly over time, andby the end of December 2011 had reached background levels recorded before theaccident. Radiocesium, 134Cs and 137Cs, was found in seawatersamples immediately after the accident. There was a remarkable change inradiocesium activities in surface waters during the first 7 months (Marchthrough September 2011) after the accident; the activity reached a maximum inthe middle of April and thereafter decreased exponentially with time.Qualitatively, the distribution patterns in surface waters suggested that inearly May radiocesium-polluted water was advected northward; some of thewater then detached and was transported to the south. Two water cores withhigh 137Cs activity persisted at least until July 2011. In subsurfacewaters radiocesium activity was first detected in the beginning of April2011, and the water masses were characterized by σ_t (an indicatorof density) values of 25.5–26.5. From 9–14 May to 5–16 December 2011, thedepths of the water masses increased with time, an indication that deepeningof the isopycnal surfaces with time can be an important mechanism for thetransport of radiocesium downward in coastal waters. During 4–21 February2012, the water column became vertically homogeneous, probably because ofconvective mixing during the winter; the result was nearly constant values ofradiocesium activity throughout the water column from the surface to thebottom (~200 m depth) at each station.