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

Highlights: • Average ³H radioactivity concentrations decreased from 0.28 Bq/L in 1991 to 0.13 Bq/L in 2006. • These decreases were due to water mixing and radioactive decay. • The sampling south area concentrations were the maximum 1.3 Bq/L in the active test. • Temporal change of inventory has been decreasing without impact from the active test. 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 (3H in surface water.