HR: 1340h
AN: H43D-0395    [Abstracts]
TI: Groundwater study using drill holes in the Abukuma granitic province, NE Japan: the multi-isotopic approach to evaluate crack water stability
AU: * Takahashi, H A
EM: h.a.takahashi@aist.go.jp
AF: Research Center for Deep Geological Environments, Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567 Japan
AU: Yasuhara, M
AF: Research Center for Deep Geological Environments, Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567 Japan
AU: Tsukamoto, H
AF: Research Center for Deep Geological Environments, Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567 Japan
AU: Kazahaya, K
AF: Research Center for Deep Geological Environments, Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567 Japan
AU: Inamura, A
AF: Research Center for Deep Geological Environments, Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567 Japan
AU: Morikawa, N
AF: Research Center for Deep Geological Environments, Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567 Japan
AU: Takahashi, M
AF: Research Center for Deep Geological Environments, Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567 Japan
AU: Nakamura, T
AF: Center for Chronological Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602 Japan
AU: Nagao, K
AF: Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Sumino, H
AF: Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AB: Chemical nature or origin of groundwater in cracks has been poor understood because of difficulties on collection of water samples preserving its natural conditions. Little is known on the quantification on stability and mean residence time of crack water also. We conducted a study for flow parameters and processes of groundwater in cracks by drilling two bore holes, and analysis using the multi-isotope approach. The drill sites have set in a granitic province called Abukuma in Fukushima prefecture, and drill holes 140m- and 180m-deep were made. In situ sampling of waters at 6 depths for each drill holes are done with the single and double packer methods. Chemical type of groundwater has a variety with depth, the shallower groundwater is categorized as Ca-HCO$_{3}$$^{-}$ type with slight NO$_{3}$ contamination whereas deeper groundwater has Na-HCO$_{3}$$^{-}$ type. Bicarbonate concentration becomes greater in deeper level suggesting that older groundwater is placed in deeper cracks. Stable isotope composition of water showed that all the sample water is of meteoric origin. However, the $__delta$D at deeper level has significantly low values (20$__permil$ lower than the present one) obviously indicating that the groundwater does not originate from the present one. The $__delta$$^{13}$C values of total dissolved carbon from two sites, Shirasawa and Miharu, show different profiles vertically. The $__delta$$^{13}$C profiles indicate that carbon at the Shirasawa site is derived from biogenic source, but that at the Miharu site is influenced from other sources, such as _gcrustal fluid_h upwelling from a deep geologic environment. The contribution of crustal fluid can be canceled using the carbon isotopic mass balance, and the $__delta$$^{14}$C value of crack water excluding deep source contribution was evaluated. The apparent $^{14}$C date calculated using the evaluated $__delta$$^{14}$C value is getting older to deeper depth at both sites. At the Miharu site, the tritium is detected even at depth of 180m, indicating that relatively young water has invaded into the crack water. As the vertical profiles of $__delta$D and $__delta$$^{18}$O show relatively small changes, the vertical mixing of crack water, shallow surface water and deep crustal fluid likely occurs. However, the result beyond the 10000 yrBP in the apparent $^{14}$C date suggests that the carbon mixing does not frequently occurred. As for the Shirasawa site, the deeper the crack water, the lower the $__delta$D and $__delta$$^{18}$O values are represented. Groundwater with very low $__delta$D and $__delta$$^{18}$O values is likely recharged in an ice age consistent with the apparent $^{14}$C date showing the older age of carbon in the crack water than 10000 or 20000 yrBP. The evidence shows that crack water can be trapped for a very long period even at a shallow depth (80-180m).
DE: 1829 Groundwater hydrology
DE: 1894 Instruments and techniques
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting