Water geochemistry and quantitative estimation of its potential sources in 15 representative rivers, Sado Island, Niigata Prefecture, Japan

The contributions of potential sources to river water geochemistry have rarely been studied in Japan. To characterize the geochemistry of river water on Sado Island, Niigata Prefecture, Japan, water sampling and subsequent chemical analyses were conducted during high- and low-water seasons in 15 representative rivers. The contribution rates of potential sources to the water geochemistry were quantitatively estimated using a straightforward method based on the mass budget equation. The contribution of atmospheric inputs as a source of river water cations was relatively high, ranging from 24.9 to 65.9% and averaging 39.2%. Contributions from rock and mineral weathering, namely gypsum and anhydrite dissolution, silicate weathering and carbonate weathering were estimated to be up to 12.7% (average: 5.8%), 6.1–48.3% (average: 27.4%) and 4.1–52.8% (average: 27.6%), respectively. Although the distribution of carbonates was more limited than that of silicates, their contributions were nearly equal because of the large difference in their weathering rates. This indicates that the water geochemistry of the 15 rivers on Sado Island was mainly controlled by atmospheric inputs and silicate and carbonate weathering.