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[HCG24-P03] Pore water extraction from sediment cores in a sedimentary soft rock area: Toward obtaining water chemistry and stable isotope profiles
Keywords:sedimentary soft rock, pore water, squeezing, water chemistry, water stable isotopes
For intermediate depth disposal of radioactive waste, it is necessary to evaluate the regional groundwater flow including a potential repository site. We have developed a conceptual model of groundwater flow in the Kamikita Plain, Aomori Prefecture, based on water chemistry and stable isotope data obtained for existing wells. For the verification of the model, we have been conducting borehole drilling surveys since 2019. Here we report on the attempts of extracting pore water from core samples and obtaining water quality and stable isotope profiles for a 200-m sediment core.
In this study, we mainly used core samples cut every 12 cm in length, trimmed the outer section, and then extracted pore water using the squeezing method. Stepwise extraction of pore water was performed with the extraction pressure set to 2.5, 5, 10, 15 MPa, to observe changes in water chemistry and stable isotope composition.
First, the trimming area of the core sample was examined for each particle size to potentially reduce the influence of drilling fluid on the pore water samples. The mixing rate of drilling fluid was calculated from the concentration of rhodamine WT used as a tracer. For fine-grained deposits such as silt and mud, the mixing rate of drilling fluid was approximately several percent or less, when the outer section of the core ~5 mm thick was removed. On the other hand, in the case of unconsolidated coarse sediments (sand layer), some pore water samples showed a drilling fluid mixing rate of ~10% or more, therefore we investigated in more detail. Water extracted from the trimmed sample (approximately 10-20 mm thick) was compared with the pore water extracted from the central section of the core sample. As a result, there was no significant difference in the concentration of rhodamine WT between the outer and central parts of the core sample, therefore it would be difficult to reduce the contamination for sand layer. After excluding the pore water samples with a large contamination rate and correcting the contamination, we examined the water chemistry and stable isotope data. In the presentation, we will consider changes in water chemistry depending on the storage period of core samples, changes in isotopic composition in stepwise water extraction, and differences due to lithology.
Acknowledgement: The main part of this research project has been conducted as the regulatory supporting research funded by the Secretariat of Nuclear Regulation Authority (NRA), Japan.
In this study, we mainly used core samples cut every 12 cm in length, trimmed the outer section, and then extracted pore water using the squeezing method. Stepwise extraction of pore water was performed with the extraction pressure set to 2.5, 5, 10, 15 MPa, to observe changes in water chemistry and stable isotope composition.
First, the trimming area of the core sample was examined for each particle size to potentially reduce the influence of drilling fluid on the pore water samples. The mixing rate of drilling fluid was calculated from the concentration of rhodamine WT used as a tracer. For fine-grained deposits such as silt and mud, the mixing rate of drilling fluid was approximately several percent or less, when the outer section of the core ~5 mm thick was removed. On the other hand, in the case of unconsolidated coarse sediments (sand layer), some pore water samples showed a drilling fluid mixing rate of ~10% or more, therefore we investigated in more detail. Water extracted from the trimmed sample (approximately 10-20 mm thick) was compared with the pore water extracted from the central section of the core sample. As a result, there was no significant difference in the concentration of rhodamine WT between the outer and central parts of the core sample, therefore it would be difficult to reduce the contamination for sand layer. After excluding the pore water samples with a large contamination rate and correcting the contamination, we examined the water chemistry and stable isotope data. In the presentation, we will consider changes in water chemistry depending on the storage period of core samples, changes in isotopic composition in stepwise water extraction, and differences due to lithology.
Acknowledgement: The main part of this research project has been conducted as the regulatory supporting research funded by the Secretariat of Nuclear Regulation Authority (NRA), Japan.