(a) The concentration of ¹³⁷Cs in seawater has been monitored (yellow dots) inside the 1F port since 2013 and the concentration data have been published nearly every day. The black dotted lines represent the borders of the Voronoi cells obtained by using Voronoi tessellation for a set of the monitoring points. (b) The estimated monthly discharge decreased to approximately 1/100000^th of that in the early stage of the accident. (c) Assuming that all ¹³⁷Cs from the 1F port in 2014 spread uniformly and remains in the coastal area, the inventory discharge is estimated to contribute an increase of 0.003 to 0.0045 Bq/L. Since the observed concentration in the sea area was increased by 0.012 Bq/L due to the accident, the contribution of the ¹³⁷Cs from the 1F port to the concentration rise is approximately 25% to 38%.