HR: 0800h
AN: H51A-0342    [Abstracts]
TI: Analysis of stable chlorine isotopic ratios and saline water diffusion model in sedimentary formation, Yatsushiro Bay, Japan
AU: * Kimura, Y
EM: tt46533@mail.ecc.u-tokyo.ac.jp
AF: Department of Geosystem Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan
AU: Tokunaga, T
EM: tokunaga@k.u-tokyo.ac.jp
AF: Institute of Environmental Studies, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan
AU: Inoue, D
EM: d-inoue@es.sci.kumamoto-u.ac.jp
AF: Department of Systems in Natural Environment, Kumamoto University, Kurokami, Kumamoto-shi, 860-8555 Japan
AU: Shimada, J
EM: jshimada@sci.kumamoto-u.ac.jp
AF: Department of Earth Sciences, Kumamoto University, Kurokami, Kumamoto-shi, 860-8555 Japan
AU: Hase, Y
EM: geohase@gpo.kumamoto-u.ac.jp
AF: Department of Earth Sciences, Kumamoto University, Kurokami, Kumamoto-shi, 860-8555 Japan
AB: Distribution of saline and brackish groundwater at many coastal areas is known to show very complicated spatial patterns. At the same time, the mechanism of solute transport has become important to better understand the water and mass movement at the coastal area._@In this study, we have attempted to evaluate long-term behavior of saline groundwater by analyzing chloride concentration, chlorine isotopic ratio, and by applying diffusion-sedimentation model at Yatsushiro Bay, southwestern part of Japan. Differences in diffusion coefficients caused by the difference in molecular mass of the isotopes create isotope fractionations. Thus, fractionation of chlorine isotopic ratio caused by diffusion process can be measurable and be useful as a direct evidence of diffusion. At the studied location, tuff breccia of later Pliocene to early Pleistocene is distributed from 3.8 meters below sea floor (mbsf) to the bottom of the borehole (50 m). Marine clay deposited after the last glacial maximum covers the tuff breccia. The hydraulic conductivity of the tuff breccia was estimated to be 1.15*10-3 cm/s at 7.0 to 9.4 mbsf. Pore waters were extracted from core samples by squeezing method. Chloride concentrations and stable chlorine isotopic ratios were measured on 12 pore water samples. Groundwater with chloride concentrations higher than 16,400 mg/l was found at shallower than 1.5 mbsf. Chloride concentration decreases downwards gradually, and it becomes lower than 250 mg/l below 7.7 mbsf,. The stable chlorine isotopic ratio showed minimum value of -1.27 per mil at 5.5mbsf, and those from other depths showed minor fluctuation, from -0.45 per mil to 0.11 per mil. From the depth profile of the chloride concentration and stable chlorine isotopic ratios, the diffusion is considered to be the dominant process for the transport of chloride in Quaternary marine clay deposits. The diffusion-sedimentation model at least qualitatively describes both the changes of chloride concentration and chlorine isotopic ratios. Based on our modeling results, an effective diffusion coefficient at this site was estimated to be 3*10-11 to 5*10-11 m2/s.
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: Fall Meeting 2005