Hydrogen and oxygen isotopic ratios of precipitations, river waters and groundwaters were analyzed to obtain the fundamental data for studying the process of groundwater flowing into Lake Biwa. Monthly precipitations were collected from March 1994 to August 1995 and water samples of river and groundwaters were also taken in March and August 1995 in the eastern area of the Hiei mountains. The isotopic ratios of precipitations show seasonal variations having a sawtoothed shape with a minimum in July and a maximum in November. The weighted means of δD and δ18O values for precipitation are -52.6‰ and -8.6‰ near the summit, and -50.4‰, -7.7‰ at the low land. Alutitude isotopic effect per 100m elevation is -0.4‰ for δD and -0.2‰ for δ18O. The meteoric water lines are expressed as δD=8δ18O+16 near the summit and as δD=8δ18O+18 at the low land.
Hydrogen and oxygen isotopic ratios in river and groundwater samples show no remarkable changes with time. Altitudinal effect for river and groundwater as well as that of precipitation having a slight difference between the mountain area (-0.1‰/100m for δ18O) and the low land area ( -0.3‰/100m).
