Isotopic compositions were analyzed to grasp the altitude effect observed in and the discharge characteristics of river water in Yakushima Island, southwestern Japan. The isotopic characteristics of the river water are similar to those of precipitation in summer; a regression line for the river water is δ D = 7.68 δ18O + 10.06 and d values are in a range of 6.5 ∼ 16.1‰. Moreover, the altitude effects are -0.17‰/100 m for δ18O and-1.3‰/100 m for δ D irrespective of river drainage size. These altitude effects are well concordant with the values in the previous report in Japan, strongly indicating that the river water in Yakushima Island is scarcely affected by the processes of evaporation and mixing with groundwater having long residence time. This idea is not contradict with verifications using mass ratio of potential evapotranspiration in precipitation as an index. Such discharge characteristics of the river might be attributed to hydrological setting of Yakushima Island, i.e., lower permeability of granite and high amounts of precipitation up to 10,000 mm/yr in the mountainous area. Some river water samples in central-northern and western parts of the Island have apparently light isotopic compositions against the average altitudes of drainage basin. Considering landforms of Yakushima Island and meteoric conditions, such isotopic data would be caused by rain shadow effect in which vapor mass as a source of these river water samples has been brought across a ridge by seasonal southern wind.
To understand hydro-chemical characteristics of river water and NO3--N load toward Toyama bay through the base flow. The hydrological field survey and analysis of inorganic ion chemistry, stable isotopic ratios of hydrogen and oxygen, and tritium activity in water samples were conducted for 25 rivers around Toyama bay. The isotopic ratio of river water in Toyama prefecture shows different content in comparison with that of Ishikawa prefecture, and water chemistry of small catchment rivers in western part of Toyama prefecture and Noto peninsula, except Kumabuchi and Sakiyama rivers, shows Na+-HCO3- type due to wind-borne chloride. As for the NO3--N load toward Toyama bay, the contribution of rivers with large catchment and the flow through the residential area is relatively large in both prefectures. The results suggested that the isotopic and geochemical characteristics of river water were related to the site of catchment, topographic feature, local geology, and major land use in the catchment area. The estimated annual total NO3--N load toward the bay through the base flow was about 4,200 tons.