Distributions of tritium in the coastal waters off Aomori and Iwate Prefectures

Since 1991 radioactivity concentration of tritium (3H) in seawater has been monitored in the coastal waters off Rokkasho, Aomori Prefecture, where a commercial nuclear fuel reprocessing plant was in intermittent test operation beginning from 2006 and ending in 2008. Average 3H radioactivity concentrations, mainly derived from global fallout of atmospheric nuclear weapons tests in the 1950′s and early 1960′s, decreased from 0.28 Bq/L in 1991 to 0.13 Bq/L in 2006 due to water mixing and radioactive decay. The concentrations in the southern part of the monitoring area increased to the maximum value of 1.3 Bq/L during the active test period, 2007 to 2008. Since 2009, the concentrations have been in the same declining trend as before the active test period; hence, 3H inventory in the monitoring area has been decreasing with minor impact from the active test for a brief period (