Hydrogen and oxygen isotope measurements on river waters collected from the Kurobegawa, Joganjigawa and Shogawa Fans, and their surrounding areas in Toyama Pref., Japan, have been made to examine their utility as an index of the source of groundwaters in aquifers beneath the fan.
Water samples were monthly collected from 7 major and 5 minor rivers at the fixed sampling point on or by the fan for the period from 6 to 39 monthes. Although the river waters show a tendency of slightly decreasing their δD and δ¹⁸O values during the summer, the range of variations in their δD and δ¹⁸O values are found to be small at each sampling point throughout the year (1σ : ±2-5‰ δD, ±0.2-0.3‰ ¹⁸O).
The average δD and ¹⁸O values of river waters are inversely correlative to the average altitude(h, m) of their catchment area;
δD=-(0.0193±0.017)h-(48.4±2.3)
δ¹⁸O=-(0.00236±0.00016)h-(8.68±0.22)
By using these relationships, the δD and δ¹⁸O values of river waters from the catchment area with an average altitude of 0 to 100m are estimated to be -48 to -50‰ and -8.7 to -8.9‰, respectively. These estimates agree well, respectively, withthe δD and ¹⁸O values of the groundwater (spring water) of local precipitation origin, collected at the foot of the hill with an altitude lower than 100m in Toyama Prefecture.