Estimation of Groundwater and Spring Water Residence Times near the Coast of Fukushima, Japan

The massive Tohoku earthquake, colloquially known as "The 2011 Great East Japan Earthquake," occurred off the Pacific coast of Japan on March 11, 2011. The coastal area of the Tohoku region was severely affected by the tsunami, and the tsunami also caused severe damage to the Fukushima Daiichi Nuclear Power Plant (FDNPP) releasing a large amount of radioactive material into the atmosphere and environment. Determining the residence time of groundwater is important for evaluating how long radioactive materials are present after nuclear accidents such as FDNPP and multiple methods are needed to account for mixing between old/young water that can occur. The apparent residence times of spring water, groundwater, and artesian well water samples obtained during 2014 to 2018 from the northern coastal area of Fukushima Prefecture were estimated using tritium (³ H), chlorofluorocarbons (CFCs), and sulfur hexafluoride (SF₆ ) concentrations. The lowest ³ H concentrations were within the background range (1-5 TU) and were observed in artesian wells in Shinch, Soma and Minamisoma. The highest ³ H concentrations (∼8-15 TU) were observed near the FDNPP, and were probably affected by the accident following the 2011 earthquake. The average apparent residence time of spring water/shallow groundwater was 29 years based on the SF₆ concentration and exponential mixing model, whereas that of artesian well water was 62 years based on the CFC-12 concentration and piston flow model. Considering the relatively short apparent residence time of spring water/shallow groundwater, it is important to conduct continuous observations to understand the influence of the FDNPP accident.