Abstract
Natural springs containing volcanic and magmatic components occur in association with these activities. However, features of deeply originated fluids and solutes were less documented from fields, where active volcanic and magmatic activities are not distributed. To characterize the presence of deep components and identify their major pathways 28 groundwater samples (~ 1230 m deep) were collected from hot spring sites located at western coast of Kumamoto, southwestern Japan, where the typical subduction related magmatisms are absent. The samples were measured for dissolved ion concentrations and stable isotope ratios (δ2HH2O, δ18OH2O, δ13CDIC and δ34SSO4) that were compared with data of 33 water samples from vicinity surface systems. The groundwaters were classified into three types based on major hydrochemistry: high Cl− fluid, low concentration fluid, and high HCO3− fluid. Our data set suggests that the high Cl− fluid was formed by saline water mixing with aquifer waters of meteoric origin and subsequently evolved by reverse cation exchange. The low concentration fluid is identical to regional aquifer water of meteoric origin that was subjected to cation exchange. The high HCO3− fluid showed the highest HCO3− concentrations (~ 3,888 mg/l) with the highest δ13CDIC (up to − 1.9‰). Based on our carbon mixing model and observed δ2HH2O and δ18OH2O shifts, it is suggested that dissolved carbon of mantle origin and small fraction of fluids generated in deep crust were transported towards surface through structural weakness under open tectonic setting. These deeply derived components were then mixed with waters in the surface systems and diluted. Their impacts on surface hydrological systems were limited in space except few locations, where deeply connected pathways are anticipated along active structural deformations.
Highlights
Artesian springs with high temperature and/or elevated concentrations occur in major active volcanic and seismotectonic regions globally
The surface water samples are broadly plotted on compositional field between Ca–HCO3− type with pristine signature and modern sea water, suggesting presence of sea water mixing into fresh surface waters with lower dissolved solutes
The groundwater samples from hot spring sites are exclusively plotted along two different trends with increased concentrations, high Cl− with variable Ca2+ ratio and high Na+ with variable HCO3− ratio (Fig. 2b)
Summary
Artesian springs with high temperature and/or elevated concentrations occur in major active volcanic and seismotectonic regions globally. The groundwater samples categorized by the former trend are relatively more enriched in C l− and Na+ and we classified this type of waters as High-Cl− fluid (Fig. 2b, c, Additional file 1: Table S1).
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