Background Concentration of 14C in Aquatic Samples from Brackish Lake Obuchi, Rokkasho, Japan, Adjacent to Nuclear Fuel Reprocessing Facilities

Abstract

The brackish Lake Obuchi in Rokkasho, Japan, is adjacent to the first Japanese commercial nuclear fuel reprocessing facilities, which are now undergoing performance testing, with commercial operation scheduled to start in 2007. Preparatory surveys were made by measuring the background levels of radiocarbon for water, aquatic biota, and sediment samples using accelerator mass spectrometry (AMS) in order to study the potential effects of 14C released by the plant to the 14C concentration in aquatic samples. Concentrations of 14C in Futamata River in 2004 ranged from 102 ± 0.5 to 109 ± 0.6 pMC (average 106 ± 0.6 pMC), while 14C concentrations in brackish water from Lake Obuchi and in seawater were 89 ± 0.5 to 104 ± 0.4 pMC (average 98 ± 0.5 pMC) and 82 ± 0.6 to 102 ± 0.4 pMC (average 93 ± 0.5 pMC), respectively. The relationship between 14C concentration and salinity showed a negative correlation (r = 0.68, P < 0.01, n = 20). 14C concentration in selected aquatic biota (i.e. fish, benthos, and seagrass) from 2003 to 2004 ranged from 105 ± 0.7 to 107 ± 0.6 pMC and in zooplankton and phytoplankton was 103 ± 2.4 to 105 ± 1.7 pMC. The depth profile of 14C in 3 core sediment samples from Lake Obuchi showed maximum concentrations from 103 ± 0.5 to 106 ± 0.5 pMC at 5–20 cm depth. The vertical profile of 14C concentration in the sediment did not follow global atmospheric 14C fallout. We confirmed that the background level of 14C concentration in aquatic samples in brackish Lake Obuchi before operation of the reprocessing plant was similar to the concentration (∼106 pMC) in the recent atmosphere.