D,T, ^<18>Oから見た北陸地方の降水と河川水の水文学的特徴

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  • Environmental isotope hydrology of precipitations and river waters in the Hokuriku district, Japan

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To study the hydrology of the Hokuriku district, measurement was made of the D, T and 18O contents of precipitation samples collected at Toyama and Wajima during May, 1981 to October, 1983 and river water samples from the Kurobe River during April, 1980 to March, 1983 were measured. The δD and δ18O values of the precipitation samples were minimum in the summer and a maximum in winter. Precipitation during the spring to autumn, was δD=8δ18O+10 and for the winter, δD=8δ18O+30. The source of the water vapor of precipitation differed for the summer and winter seasons. The summer air mass generated on the southern Pacific Ocean or the East China Sea precipitated rain depleted in the heavy isotopes. A large value for the intercept with the δD axis, such as 30 in the case of winter precipitation, may imply the rapid evaporation of water vapor from the sea surface. The tritium content in the precipitation samples was rather constant (10-15 TU) except for the spring peak. The Kurobe River samples showed no seasonal isotopic variation having values close to, δD=8δ18O+20, suggesting the water from this river to be recharged by aquifers in which equal amounts of summer and winter precipitation are well mixed. The tritium content of the Kurobe River ranged from 25 to 30 TU, this being larger than that of the precipitation samples. This fact suggests that precipitation polluted by past thermonuclear tests is still present in aquifers and is being discharged to some extent in the river.

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