Assessing the hydrothermal influence on natural waters in the Kirishima Volcanic Complex: Insights from rivers, springs and groundwater with varying residence time

Abstract

Rivers and groundwaters that drain active volcanic areas are characterized by relatively high nutrient concentrations and dissolved loads. The high dissolved load generally results from the fluid–rock interactions with high temperature fluids containing voclanic gases. In this study, the chemistry of spring and river waters from the Kirishima volcanic region (Kyushu, Japan) is analysed to determine the hydrothermal influence and how their chemical composition varies with respect to the groundwater transit time (residence time). The hydrothermal component is distinguished by combining spring water residence times with stable isotopic composition of ${\delta }^{13}{C}_{DIC}$, ${\delta }^{34}{S}_{sulphate}$, ${\delta }^{18}{O}_{sulphate}$, ${\delta }^{18}{O}_{water}$, ${\delta }^2{H}_{water}$ and the selenium to sulphate molar ratio. We found that the DIC and SO₄^2- in the waters draining the Kirishima volcanic complex arose for more than 50 % from volcanic CO₂ and SO₂. Consistently, the groundwaters flowing through longer and deeper flow paths accumulate more volcanic CO_2(g) and SO_2(g). The results from this study highlight the relevance of regional studies of active volcanic settings in understanding the chemical evolution of groundwaters and its implication for the carbon and sulphur budgets in a volcanic edifice.