Assessment of the level of activity of advective transport through fractures and faults in marine deposits by comparison between stable isotope compositions of fracture and pore waters

Mochizuki, Akihito; Ishii, Eiichi

Assessment of the level of activity of advective transport through fractures and faults in marine deposits by comparison between stable isotope compositions of fracture and pore waters | Évaluation du niveau d′activité du transport advectif à travers les fractures et les failles des dépôts marins en utilisant une comparaison entre les compositions en isotopes stables des eaux de fracture et celles des eaux interstitielles Evaluación del nivel de actividad del transporte advectivo a través de fracturas y fallas en depósitos marinos por comparación entre las composiciones de isótopos estables de las aguas de fractura y de poro 通过对比裂隙水和孔隙水稳定同位素成分评估海洋沉积物中裂隙和断层的对流传输水平 Avaliação do nível de atividade do transporte advectivo através de fraturas e falhas em depósitos marinhos pela comparação entre a composição de isótopos estáveis da água de fraturas e poros

2022

Mochizuki, Akihito | Ishii, Eiichi

Assessment of the level of activity of advective transport through faults and fractures is essential for guiding the geological disposal of radioactive waste. In this study, the advective flow (active, inactive) of meteoric water through fractures is assessed by comparing stable isotopes (δD and δ¹⁸O) between fracture and pore waters obtained from four boreholes in marine deposits in the Horonobe area, Japan. At 27–83-m depth in one borehole and 28–250 m in another, the isotopic compositions of pore and fracture water reflect mixing with meteoric water, with stronger meteoric-water signatures being observed in the fracture water than in pore water of the rock matrix. At greater depths in these boreholes and at all sampling depths in the other two studied boreholes, the isotopic compositions of fracture and pore waters are comparable. These results suggest that the advective flow of meteoric water is active at shallow depths where fossil seawater is highly diluted in the two boreholes. This interpretation is compatible with the occurrence of present or paleo meteoric waters and tritium, whereby present meteoric water and tritium are limited to those depths in the two boreholes. This difference in the level of activity of advective flow is probably because of the glacial–interglacial difference in hydraulic gradients resulting from sea-level change. Although fractures are hydraulically connected to the surface through the sedimentary rock, advective flow through them is inferred to remain inactive so long as sea level does not fall substantially.

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