Oxygen Stable Isotopic Ratios of Precipitations and River Waters in Niigata Prefecture in Recent Years

Oxygen stable isotopic ratios (δ18O) of the precipitation and some river water samples in Niigata prefecture were measured from 1999 to 2002. Isotopic measurements were performed by a stable isotope ratio mass spectrometer. Consequently, the following matters were found. (1) The mean δ18O value of precipitations was -9‰, and the variation of ±7‰ was shown throughout the year. (2) All of 9 observation points, the seasonal behavior of isotopic ratio was similar to each other. (3) In June and January, δ18O value o f precipitations was small, and was large in August. (4) In January, the relationship between the temperature of observation points and δ18O values in precipitation was most remarkably found throughout the year. (5) The variation of δ18O values in precipitations in winter is smaller than that in the other seasons. (6) The mean δ18O value of river waters in Niigata prefecture was about -11‰ to -10‰, and the variation was not so large (±2‰) throughout the year. (7) δ18O values of river waters may vary when the melted snow or the precipitation flow into the rivers. (8) The variation of δ18O values in groundwater was smaller than that in river waters.