天然水中の^<228>Ra/^<226>Ra放射能比に関する水文学的研究(博士論文要録)

概要

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A variation of the ^<228>Ra/^<226>Ra activity ratio of underground water suggested a monotonous decrease along with an increase in the residence time within an aquifer. Observation with hot-spring water samples showed that the ^<228>Ra/^<226>Ra activity ratios in water from deeper aquifers with longer residence times tended to be smaller than those in river waters with a shorter residence time. The residence time of Kakita-gawa river water, estimated from the ^<228>Ra/^<226>Ra activity ratios of river water and relevant rock, agree well with the ^3H-^3He age, suggesting that the ^<228>Ra/^<226>Ra activity ratio of underground water could be used as a tool for a residence-time estimation of underground water. The ^<228>Ra/^<226>Ra activity ratios in the river-water sample showed that the activity ratios tended to decrease after heavier rainfall. The decrease suggested that heavy rainfall forced deeper underground water with a long residence time to discharge into river. The variation of the monthly averaged concentration of ^<222>Rn in underground water was similar to that of precipitation. The variation pattern suggested that precipitation may give rise to the mixing of underground water components of different origins, which is consistent with the consideration that heavy rainfall forced the deeper underground water to discharge into the river.
2006-11-05