1:45 PM - 3:15 PM
[HTT15-P01] Understanding the 87Sr/86Sr isotope ratios of rock and soil near the coastal area of Fukushima and Abukuma mountains
Keywords:coastal area of Fukushima Prefecture, Abukuma mountains, 87Sr/86Sr, rock, soil, tritium (3H)
Objectives
The tsunami, caused by the 2011 off the Pacific coast of Tohoku Earthquake, flooded large area near the coastal area from Kanto district to Tohoku district, and water salinization occurred in many wells at those areas. And also, groundwater pollution occurred in some areas due to radioactive materials caused by the Fukushima Daiichi Nuclear Power Station (FDNPP) accident (Yabusaki and Asai, 2023). It is important to determine the water quality and groundwater flow system for grasp the situations of radioactive materials and salinization of groundwater and also sustainable groundwater use. The objectives of this study are to understand the characteristics of water quality, groundwater recharge area, and groundwater flow systems near the coastal area of Fukushima prefecture. Because 87Sr/86Sr values are different depend on the geological conditions; therefore, the strontium isotope ratio (87Sr/86Sr) is useful to understand the recharge area of groundwater and spring water. In last year (JpGU 2022), we reported the characteristics of 87Sr/86Sr ratio in groundwater, spring water, etc. in Abukuma mountains and the coastal area of Fukushima. In this presentation, we will report about the characteristics of 87Sr/86Sr ratio of soil/rock samples which were sampled at Abukuma mountains and coastal area of Fukushima and result of comparing 87Sr/86Sr ration between water samples and soil/rock samples.
Methodology
The observation has been started since April 2014 near the coastal area and Abukuma mountains area of Fukushima Prefecture, and the EC, pH, water temperature, ORP, water level, and discharge rate were measured in the field, and water for chemical analysis were sampled at each site. The dissolved inorganic ions, trace elements, δ18O, δ2H, and Sr isotope ratio (87Sr/86Sr) were analyzed for all samples, and CFCs and SF6 for estimating of water residence time were measured in some sites. The geological condition in this study area, a granitic rock is mainly distributed near the Abukuma mountains, and sedimentary rocks are deposited in the plateaus near the coast. The soil/rock samples, which is river sediment and soil near the spring, were taken at 15 sites in Iitate Village, Shinchi Town, Soma City, Minamisoma City, and Okuma Town on August, 2022. After preprocessing for analysis of water samples and soil/rock samples, measuring 87Sr/86Sr ratios with MC-ICP-MS (NEPTUNE PLUS) was carried out.
Results and discussion
87Sr/86Sr ratio of groundwater, spring water, river water was from 0.70545 to 0.71191, and that of hot spring water was relatively high; however, 87Sr/86Sr ratio in other sites was almost below 0.70918 (that is equal to sea water isotope ratio). According to the 87Sr/86Sr ratio, water samples of this study site were divided into two groups; one is 87Sr/86Sr = from 0.708 to 0.709, and another is 87Sr/86Sr < 0.708. The former almost corresponded to artesian wells and deep wells (confined groundwater), and the latter almost corresponded to spring water and shallow wells (unconfined groundwater). As a result of observation, it was estimated that the deep groundwater and artesian well water were recharged in the inland area (Abukuma mountains) and shallow groundwater and spring water were recharged in the upland area near the coast. Because the granitic rocks were distributed in large area of the Abukuma mountains, the 87Sr/86Sr values of water which was recharged at Abukuma mountain area is likely from 0.708 to 0.709. On the other hand, because the sedimentary rocks are deposited in the plateaus near the coast; therefore, the 87Sr/86Sr values of water which was recharged at the plateaus is likely relatively low as approximately 0.708 or less.
The 87Sr/86Sr ratios of soil/rock sampled at Abukuma mountains indicated approximately from 0.708 to 0.711, and that of soil/rock sampled neat the coastal area indicated approximately from 0.706 to 0.707. Form this result also suggested the recharge area of deep groundwater and artesian well water might be Abukuma mountains area, and that of shallow groundwater spring water might be the plateaus near the coast. This result agreed with the result of estimation by using the water quality and water isotope ratios. In future study, we will accumulate the 87Sr/86Sr ratio of soil/rock samples more, and also analyze the 87Sr/86Sr ratio of river water samples at large area in Fukushima.
REFERENCES
Yabusaki, S., and Asai, K. (2023): Estimation of groundwater and spring water residence times near the coast of Fukushima, Japan. Groundwater. (in press). https://doi.org/10.1111/gwat.13288.
The tsunami, caused by the 2011 off the Pacific coast of Tohoku Earthquake, flooded large area near the coastal area from Kanto district to Tohoku district, and water salinization occurred in many wells at those areas. And also, groundwater pollution occurred in some areas due to radioactive materials caused by the Fukushima Daiichi Nuclear Power Station (FDNPP) accident (Yabusaki and Asai, 2023). It is important to determine the water quality and groundwater flow system for grasp the situations of radioactive materials and salinization of groundwater and also sustainable groundwater use. The objectives of this study are to understand the characteristics of water quality, groundwater recharge area, and groundwater flow systems near the coastal area of Fukushima prefecture. Because 87Sr/86Sr values are different depend on the geological conditions; therefore, the strontium isotope ratio (87Sr/86Sr) is useful to understand the recharge area of groundwater and spring water. In last year (JpGU 2022), we reported the characteristics of 87Sr/86Sr ratio in groundwater, spring water, etc. in Abukuma mountains and the coastal area of Fukushima. In this presentation, we will report about the characteristics of 87Sr/86Sr ratio of soil/rock samples which were sampled at Abukuma mountains and coastal area of Fukushima and result of comparing 87Sr/86Sr ration between water samples and soil/rock samples.
Methodology
The observation has been started since April 2014 near the coastal area and Abukuma mountains area of Fukushima Prefecture, and the EC, pH, water temperature, ORP, water level, and discharge rate were measured in the field, and water for chemical analysis were sampled at each site. The dissolved inorganic ions, trace elements, δ18O, δ2H, and Sr isotope ratio (87Sr/86Sr) were analyzed for all samples, and CFCs and SF6 for estimating of water residence time were measured in some sites. The geological condition in this study area, a granitic rock is mainly distributed near the Abukuma mountains, and sedimentary rocks are deposited in the plateaus near the coast. The soil/rock samples, which is river sediment and soil near the spring, were taken at 15 sites in Iitate Village, Shinchi Town, Soma City, Minamisoma City, and Okuma Town on August, 2022. After preprocessing for analysis of water samples and soil/rock samples, measuring 87Sr/86Sr ratios with MC-ICP-MS (NEPTUNE PLUS) was carried out.
Results and discussion
87Sr/86Sr ratio of groundwater, spring water, river water was from 0.70545 to 0.71191, and that of hot spring water was relatively high; however, 87Sr/86Sr ratio in other sites was almost below 0.70918 (that is equal to sea water isotope ratio). According to the 87Sr/86Sr ratio, water samples of this study site were divided into two groups; one is 87Sr/86Sr = from 0.708 to 0.709, and another is 87Sr/86Sr < 0.708. The former almost corresponded to artesian wells and deep wells (confined groundwater), and the latter almost corresponded to spring water and shallow wells (unconfined groundwater). As a result of observation, it was estimated that the deep groundwater and artesian well water were recharged in the inland area (Abukuma mountains) and shallow groundwater and spring water were recharged in the upland area near the coast. Because the granitic rocks were distributed in large area of the Abukuma mountains, the 87Sr/86Sr values of water which was recharged at Abukuma mountain area is likely from 0.708 to 0.709. On the other hand, because the sedimentary rocks are deposited in the plateaus near the coast; therefore, the 87Sr/86Sr values of water which was recharged at the plateaus is likely relatively low as approximately 0.708 or less.
The 87Sr/86Sr ratios of soil/rock sampled at Abukuma mountains indicated approximately from 0.708 to 0.711, and that of soil/rock sampled neat the coastal area indicated approximately from 0.706 to 0.707. Form this result also suggested the recharge area of deep groundwater and artesian well water might be Abukuma mountains area, and that of shallow groundwater spring water might be the plateaus near the coast. This result agreed with the result of estimation by using the water quality and water isotope ratios. In future study, we will accumulate the 87Sr/86Sr ratio of soil/rock samples more, and also analyze the 87Sr/86Sr ratio of river water samples at large area in Fukushima.
REFERENCES
Yabusaki, S., and Asai, K. (2023): Estimation of groundwater and spring water residence times near the coast of Fukushima, Japan. Groundwater. (in press). https://doi.org/10.1111/gwat.13288.