10:45 〜 12:15
[AHW18-P24] Estimation of Groundwater Residence Time in Ryukyu limestone aquifer
キーワード：地下水、地下水年代、SF6、地下水滞留時間
Groundwater age, or residence time, is a critical parameter in determining the sequence of flow patterns from groundwater recharge to discharge. It also plays a crucial role in evaluating the quality and quantity of water resources. In recent times, sulfur hexafluoride (SF6) has emerged as a promising dating tracer for young groundwater. The study area under consideration, located in the southern part of Okinawa Island, is characterized by limestone of the Ryukyu Group, and the groundwater contained therein is believed to flow at a relatively rapid rate. To cater to the requirements of agricultural and water supply purposes, an underground dam has been constructed in the study area. However, the chronological information on groundwater in the study area is limited despite the considerable research conducted on groundwater flow in Ryukyu limestone.
This study aims to estimate the residence time of groundwater in the Ryukyu limestone of the southern Okinawa Island area using age tracers such as SF6. Water sampling was carried out at eight sites in the study area from 30 November 2022 to 22 December 2022. The water samples were collected in glass bottles using SF6-free nylon tubes and stainless-steel cylinders in adherence to the water sampling methods specified by the International Atomic Energy Agency (IAEA) to avoid any contact with the atmosphere. The SF6 concentrations in the samples were measured using a pretreatment device to remove components other than SF6 (Nakaya et al., 2015), a gas chromatograph (Shimadzu GC-8A) with an electron capture detector (ECD) and a data processing device (Shimadzu Chromatopack C-R8A).
The measured SF6 concentrations in the groundwater were converted to atmospheric concentrations using Henry's law of equilibrium to estimate the groundwater age information. The SF6 measurements in the air samples taken at the study site were used to verify the results obtained. The SF6 concentration levels ranged from 12.759 to 13.168 pptv, which were consistent with the current SF6 concentration in the atmosphere of approximately 12 pptv. The residence time of groundwater in the study area varied from 1 to 32 years, with a median of 15.9 years. The shortest residence times were observed at sites slightly off the path of concentrated groundwater flow in the downstream area, while the longest residence time was observed at a site located midway between the upstream and downstream areas.
The planar distribution of groundwater residence time was estimated to be about 8 years in the upper reaches of the basin, approximately 15 years in the middle reaches, and about 18 years in the lower reaches. The residence time of groundwater was found to be shorter near the groundwater surface and near the basement rock than in the central part of the aquifer. However, the shorter the time near the groundwater surface near the underground dam embankment, the longer the time near the groundwater surface at a point slightly out of the main flow path in the upstream area. The results obtained from this study are considered reasonable from the viewpoint of groundwater flow systems in karst areas, and the residence time of groundwater in the study area could be estimated by using the SF6 dating tracer.
This study aims to estimate the residence time of groundwater in the Ryukyu limestone of the southern Okinawa Island area using age tracers such as SF6. Water sampling was carried out at eight sites in the study area from 30 November 2022 to 22 December 2022. The water samples were collected in glass bottles using SF6-free nylon tubes and stainless-steel cylinders in adherence to the water sampling methods specified by the International Atomic Energy Agency (IAEA) to avoid any contact with the atmosphere. The SF6 concentrations in the samples were measured using a pretreatment device to remove components other than SF6 (Nakaya et al., 2015), a gas chromatograph (Shimadzu GC-8A) with an electron capture detector (ECD) and a data processing device (Shimadzu Chromatopack C-R8A).
The measured SF6 concentrations in the groundwater were converted to atmospheric concentrations using Henry's law of equilibrium to estimate the groundwater age information. The SF6 measurements in the air samples taken at the study site were used to verify the results obtained. The SF6 concentration levels ranged from 12.759 to 13.168 pptv, which were consistent with the current SF6 concentration in the atmosphere of approximately 12 pptv. The residence time of groundwater in the study area varied from 1 to 32 years, with a median of 15.9 years. The shortest residence times were observed at sites slightly off the path of concentrated groundwater flow in the downstream area, while the longest residence time was observed at a site located midway between the upstream and downstream areas.
The planar distribution of groundwater residence time was estimated to be about 8 years in the upper reaches of the basin, approximately 15 years in the middle reaches, and about 18 years in the lower reaches. The residence time of groundwater was found to be shorter near the groundwater surface and near the basement rock than in the central part of the aquifer. However, the shorter the time near the groundwater surface near the underground dam embankment, the longer the time near the groundwater surface at a point slightly out of the main flow path in the upstream area. The results obtained from this study are considered reasonable from the viewpoint of groundwater flow systems in karst areas, and the residence time of groundwater in the study area could be estimated by using the SF6 dating tracer.