17:15 〜 18:45
[AHW22-P21] Estimating Groundwater Residence Times in the Hija River Basin, Okinawa Using SF6 and CFCs Tracers
キーワード：地下水、地下水年代、SF6、CFCs
Groundwater age information (residence time) is crucial for understanding the flow patterns from recharge to discharge areas, assessing the quality and quantity of water resources, and promoting sustainable development. Sulfur hexafluoride (SF6) and chlorofluorocarbons (CFCs) are valuable age tracers, particularly effective for dating younger groundwater. However, in the Hija River basin on the main island of Okinawa (hereafter referred to as "the study area"), where contamination by organofluorine compounds such as PFOS is a concern, groundwater dating data have been notably absent. The primary objective of this study was to estimate the residence time of groundwater in the Hija River basin using age tracers such as SF6 and CFCs.
Sampling was conducted from November 8, 2023, to January 11, 2024, at 10 sites within the study area. Samples were meticulously collected in accordance with International Atomic Energy Agency (IAEA) guidelines, employing nylon or stainless steel tubes to prevent atmospheric contact. A pretreatment system, based on Nakaya et al. (2015), was utilized to remove non-SF6 and CFCs components from each sample. The concentrations of SF6 and CFCs were determined using gas chromatography with an electron capture detector (ECD), employing a Shimadzu GC-8A for SF6 and a GC-14B for CFCs. Data processing was conducted with a Shimadzu Chromatopack C-R8A. Measured concentrations were converted to atmospheric concentrations via Henry's law of equilibrium to estimate groundwater age information, correlating recharge times with atmospheric SF6 and CFCs concentration records.
The measured SF6 concentrations in the groundwater ranged from 5.429 to 13.245 pptv, aligning with the expected range of atmospheric concentration records and, notably, were unlikely to be influenced by human activities in the urban area. However, measured CFCs concentrations in the groundwater were exceedingly high at most sites and could not be reconciled with aging records. It was determined that CFCs in the groundwater of the study area were significantly affected by urban areas and U.S. military bases, rendering them unsuitable for groundwater dating. The groundwater residence time, as estimated by SF6, varied from 1 to 17 years across the study area, with a median of 9 years. The residence time was shortest in the upstream areas and longest in the downstream areas. When examining residence time by depth, most observed wells indicated a trend of longer residence times at deeper sites compared to shallower ones. Overall, this study illuminates the groundwater dynamics within the Hija River watershed, offering valuable insights for resource management and environmental restoration efforts.
Sampling was conducted from November 8, 2023, to January 11, 2024, at 10 sites within the study area. Samples were meticulously collected in accordance with International Atomic Energy Agency (IAEA) guidelines, employing nylon or stainless steel tubes to prevent atmospheric contact. A pretreatment system, based on Nakaya et al. (2015), was utilized to remove non-SF6 and CFCs components from each sample. The concentrations of SF6 and CFCs were determined using gas chromatography with an electron capture detector (ECD), employing a Shimadzu GC-8A for SF6 and a GC-14B for CFCs. Data processing was conducted with a Shimadzu Chromatopack C-R8A. Measured concentrations were converted to atmospheric concentrations via Henry's law of equilibrium to estimate groundwater age information, correlating recharge times with atmospheric SF6 and CFCs concentration records.
The measured SF6 concentrations in the groundwater ranged from 5.429 to 13.245 pptv, aligning with the expected range of atmospheric concentration records and, notably, were unlikely to be influenced by human activities in the urban area. However, measured CFCs concentrations in the groundwater were exceedingly high at most sites and could not be reconciled with aging records. It was determined that CFCs in the groundwater of the study area were significantly affected by urban areas and U.S. military bases, rendering them unsuitable for groundwater dating. The groundwater residence time, as estimated by SF6, varied from 1 to 17 years across the study area, with a median of 9 years. The residence time was shortest in the upstream areas and longest in the downstream areas. When examining residence time by depth, most observed wells indicated a trend of longer residence times at deeper sites compared to shallower ones. Overall, this study illuminates the groundwater dynamics within the Hija River watershed, offering valuable insights for resource management and environmental restoration efforts.