The Kusatsu and Tamagawa hot springs contain both toxic elements and rare metals used in high-technology industries. High acidity and significant discharge of these springs have led to the construction of dams and neutralization of spring waters by lime addition. However, the same neutralization process causes differing effects in the Kusatsu and Tamagawa areas. This study focuses on the effects of neutralization on fractionation between arsenic (As) and indium (In) during river transport and precipitation of these elements on the lake beds.

The Yubatake thermal waters in the Kusatsu area are rapidly neutralized after addition of a lime suspension. After this neutralization, the dissolved As and In are nearly completely removed by sorption onto suspended hydrous ferric oxides (HFO), which then settle out and are added to sediments within a man-made lake without significant fractionation.

In contrast, the Obuki thermal waters in the Tamagawa area are neutralized by both addition of lime and by mixing with tributary streams, leading to a gradual downstream increase in pH. Arsenic tends to be sorbed by HFO in upstream areas, leading to wide dispersal of As across the entire watershed. In comparison, In is transported to the lake inlet as stable chloride and/or sulfate complexes without significant removal, with considerable portions being precipitated in a man-made lake. As a result, In is effectively concentrated within downstream lake sediments.

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