Mixing of magmatic CO 2 into volcano groundwater flow at Aso volcano assessed combining carbon and water stable isotopes

Abstract

To understand deep groundwater flow systems and their interaction with CO 2 emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ 13C DIC) and DIC concentrations of the sampled waters show that magma-derived CO 2 mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ 13C DIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ 18O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ 13C DIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO 2 is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO 2 mixes into deeper groundwater flowing nearer the magma conduit or chamber.