MEXT conducted emergency sea area monitoring around Fukushima Dai-ichi NPP from March 23, 2011 to investigate the distribution of radioactive substances released by the Fukushima Dai-ichi NPP accident following the great east Japan earthquake. In the beginning, observation was mainly conducted in the offshore of Fukushima Prefecture. Upon the initiation of "Enhancement of Sea Area Monitoring (April 22, 2011, Headquarters against Nuclear Disasters)", wider area monitoring including offshore of Miyagi Prefecture and Ibaraki Prefecture was conducted from May 2011. At each sampling point, 500 mL - 60 L seawater samples were collected for analyses from outer layer (1- 5 m from sea surface), middle layer (half of sea depth at costal and offshore area from April to July, and 100 m layer and 200 m layer from July), and lower layer (10 - 20 m above sea bed). The data here was created based on the result of analyses.
Publicly available data collected from the website of the relevant organizations and converted into CSV, XML, and KML formats that were easy to use.