Geochemistry and stable isotopic variations of water and dissolved sulfate from landslide areas in Niigata and Nagano Prefectures, Japan. A case study

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The chemical and isotopic characteristics of waters from typical landslide areas in Niigata and Nagano Prefectures, Japan, are described in this article. The study areas are Mushigame, Nigorisawa, Yomogihira (Niigata Pref.) and Kanayamazawa (Nagano Pref.) where landslides have often occurred. We present the chemical composition and δD-δ¹⁸O values of surface and groundwaters from the areas in order to characterize a geochemical nature of the waters involved in the landslides. The waters were of meteoric origin as indicated by the δD-δ¹⁸O values. The Mushigame waters were enriched in NaCl, suggesting contribution of fossil seawater trapped in oil-producing mudstone strata in the area. The Nigorisawa and Yomogihira waters are less in Cl^– and relatively more enriched in HCO₃^– and SO₄^2–. The Na-K-Mg relationship of all the waters except the Kanayamazawa waters indicated close equilibration with some minerals such as K-feldspar, K-mica, chlorite and silica in the rocks, suggesting water-rock interaction at deeper depths, although the waters were greatly diluted by surface groundwater on the way to the surface as indicated by their δD-δ¹⁸O signatures. Dissolved sulfate is a major anion and was originally produced by oxidation of sulfides in volcanic rocks. Variable δ³⁴S and δ¹⁸O values of sulfate suggest reduction of such sulfate to sulfide probably at shallow levels under different reducing conditions followed by re-oxidation, inducing the variability in the isotopic values. Thus, multiple oxidation-reduction processes may be common in the areas.