Hydrogeochemical approach to estimate residence time of the groundwater in the freshwater lens of the Ryukyu limestone aquifer: Application of sulfur hexafluoride and nitrate isotopes
2016
Yoshimoto, S. (National Institute for Rural Engineering, National Agriculture and Food Research Organization, Tsukuba, Ibaraki (Japan)) | Asai, K. | Tsuchihara, T. | Shirahata, K. | Ishida, S.
Water and solute movements in the freshwater lens of Tarama Island, Okinawa Prefecture, Japan, were investigated with application of environmental tracers. Nitrate in the groundwater originated from multiple nitrogen sources such as chemical fertilizer, livestock manure and domestic wastewater. Composition of major ions in the groundwater showed that a part of the freshwater lens was affected by seawater. Values of δsup(15)N and vertical profiles of dissolved oxygen showed that lateral flow along the transition zone of freshwater and seawater occurred, bringing oxygen-rich and low-δsup(15)N groundwater. Averaged residence time of the groundwater was estimated to be approximately less than 14 years by the SFsub(6) analysis with application of a piston-flow model. Shallow groundwater at the periphery of the freshwater lens appeared to be recharged in relatively recent ages.
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