[A-HW23] Isotope Hydrology 2021

[AHW23-08] Origin and hydro-chemical evolution of geothermal waters from Nozawa-Onsen hydrothermal springs in the northern Nagano Prefecture, central Japan

*Shunsuke Akazawa¹, Madhusoodhan Satish-Kumar², Naoki Watanabe³, Cheibany Ould-Elemine⁴, Masaki Yuhara⁵ (1.Graduate School of Science and Technology, Niigata University, Japan, 2.Department of Geology, Faculty of Science, Niigata University, Japan , 3.Research Institute for Natural Hazards and Disaster Recovery, Niigata University, Japan, 4.Ensero Solutions, Canada, 5.Department of Earth System Science, Faculty of Science, Fukuoka University, Japan)

Keywords:Nozawa-Onsen, Geothermal water, Isotope composition, Hydrological geochemistry, Fault

Nozawa-Onsen hydrothermal springs are located between the northern segment in the active fault zone along the western margin of the Nagano basin and the Quaternary Kenashi Volcano that was active during the period from 1.7 to 1.0 Ma at approximately 40 km northeast of Nagano City, central Japan. Geothermal waters have temperatures ranging from 85.4 to 26.4 degree centigrade, pH values from 8.1 to 9.2 and electric conductivity from 47.6 to 149.0 mS/m, which is correlated to the total ion concentration. They are categorized as sodium-sulfate type, even though respective waters have different temperatures and concentrations. In this study we have carried out hydro-chemical and isotopic analyses to better understand the origin and evolution of geothermal waters. The isotopic results of delta O-18 and delta D values reveals that these waters are meteoric in origin. In addition, delta S-34 values of sulfate indicate that volcanic fluids did not contribute to the formation of geothermal waters. Relationship between chloride concentration and water temperature suggest that several waters were diluted with low-temperature groundwater. Variations of sulfate and hydrogen sulfide concentration in these waters can be explained by the precipitation of native sulfur. It is likely that the calcite was removed as precipitates from some geothermal waters during transportation through faults on the basis of the relationship between Sr-87/Sr-86 and Rb-87/Sr-86 ratios. In conclusion, the evolution process of geothermal waters from Nozawa-Onsen hydrothermal springs are closely related to (1) the recharge of meteoric waters to deep zone, (2) dissolution of soluble minerals such as plagioclase, gypsum and pyrite in Neogene sediments, (3) removal of calcite from these waters during transportation through faults, (4) dilution with low-temperature groundwaters around shallow aquifers and (5) precipitation of the small amount of native sulfur near the earth surface.