Oxygen and hydrogen isotopic ratios and Cl- concentration of saline water in the Neogene siliceous sediments of Horonobe, Hokkaido, Japan

Abstract

Research and development on geological disposal of high-level radioactive waste have been carried out by Japan Atomic Energy Agency JAEA, previous JNC in Horonobe, the northern Hokkaido. The geology of Horonobe comprises the Neogene siliceous marine sediments which contain saline water. The origin of saline water is one of the most fundamental factors for evaluating geological environments. The synthesis of geo-scientific understanding contributes to develop a performance assessment methodology by providing a basis for setting parameters and constructing conceptual model. Geological environments are also important for exploration of oil fields. This study analysed the various data of JAEA reseach for hydro-geochemistry of Horonobe.The squeezed water and the lifted water from eight boreholes drilled in Horonobe are analyzed on oxygen and hydrogen isotopes and Cl concentration by JAEA. Based on the analytical data, the groundwater is subdivided into three groups derived from the mixtures of meteoric water and different saline waters, which distributions are consistent with the regional geologic situations. 18O, D and Cl of pore water during diagenesis of the siliceous sediments are calculated based on the assumption that water/rock interaction makes redistribution of the isotopes in rocks and pore water during the phase transformations from opal A through opal CT to quartz in open systems, then that water in biogenic silica is expelled to pore-space during the successive compaction in closed systems. The measured data points fall approximately on the mixing lines between the meteoric water and the pore waters calculated on the assumption of the phase transformations. The geological study on the diagenesis of the field shows that the silica phase transformations actually occur near the depth calculated from the porosity reduction, indicating that the transformation and the succeeding compaction contribute to the origin of the saline water.

Feasibility of 129I groundwater dating calibrated by both 81Kr and 4He for the assessment of deep geological repositories in Japan

“…S1 ), the detection of 81 Kr dissolved in the seawater that intruded into rock pores can be difficult as radioactive decay proceeded for 2 Ma. The presence of groundwater with a Cl concentration (17,000 mg L -1 ) lower than that of seawater could be due to groundwater dilution by water released from the dehydration of organic matter and minerals 25 . Alternatively, if brackish water from lagoons and/or rivers permeated into the Yuchi Formation approximately 0.15–1 Ma, this permeation might explain the lower Cl concentration relative to that of modern seawater (e.g., the salinity of Tokyo Bay is 22–34‰ with an average of approximately 32‰) 26 , 27 .…”

A Proposed Method to Estimate In Situ Dissolved Gas Concentrations in Gas‐Saturated Groundwater

“…At the sampling locations, the salinities of groundwater of fossil sea‐water origin (Hama et al ; piper diagram in Figure S2) have been lowered by the dehydration of opal in the diatomaceous sediments during diagenesis and by the intrusion of meteoric water (Kai and Maekawa ). The higher salinity at deeper locations (J3 and J5, Table ) reflects the reduced extent of intrusion of meteoric water.…”

Improvements in Drill-Core Headspace Gas Analysis for Samples from Microbially Active Depths

“…2 Geofluids [37]. The δD vs. δ 18 O plots for groundwater from the Wakkanai and Koetoi Formations indicate that it is a mixture of local meteoric water and altered seawater [38,39]. Methanogenic and methane-oxidizing microbial communities have played an important role in these formations [40][41][42], where secondary microbial gas with 13 C-enriched isotopic values (δ 13 C CH4 , −74‰ to −28‰; δ 13 C CO2 , −7‰ to +31‰) formed through CO 2 reduction after uplift of the area [28].…”