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To understand deep groundwater flow systems and their interaction with CO2 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 (δ13CDIC) and DIC concentrations of the sampled waters show that magma-derived CO2 mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ13CDIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ18O) 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 δ13CDIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO2 is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO2 mixes into deeper groundwater flowing nearer the magma conduit or chamber.

The formation of imminent hydrochemical precursors of earthquakes is investigated by the simulation for water–rock reaction in a brittle aquifer. Sixty-one soaking experiments were carried out with granodiorite and trachyandesite grains of different sizes and three chemically-distinct waters for 6 to 168 h. The experimental data demonstrate that water–rock reaction can result in both measurable increases and decreases of ion concentrations in short times and that the extents of hydrochemical variations are controlled by the grain size, dissolution and secondary mineral precipitation, as well as the chemistry of the rock and groundwater. The results indicate that water–rock reactions in brittle aquifers and aquitards may be an important genetic mechanism of hydrochemical seismic precursors when the aquifers and aquitards are fractured in response to tectonic stress.

One feature of volcanic lakes influenced by subaqueous fumaroles existing at lake bottoms (called active crater lakes) is the remarkable color of their waters: turquoise or emerald green. The active crater lake named Yudamari at Mt. Nakadake of Aso volcano, Japan, takes on a milky pale blue-green. The particular blue component of the lake water color results from Rayleigh scattering of sunlight by very fine aqueous colloidal sulfur particles; the green component is attributable to absorption of sunlight by dissolved ferrous ions. An objective color observation conducted during 2000–2007 revealed that the lake water color changed from blue-green to solid green. The disappearance of the blue ingredient of the water color will result in diminution of aqueous colloidal sulfur from chemical analyses of lake waters sampled simultaneously. The aqueous sulfur is produced by the reaction of sulfur dioxide and hydrogen sulfide supplied from subaqueous fumaroles. However, its production efficiency decreases by domination of sulfur dioxide in the subaqueous fumarolic sulfur gas species with increasing subaqueous fumarolic temperature. The disappearance of blue ingredients from the blue-green color of the lake water may be attributed to activation of subaqueous fumarole activity.

Fluid migration in subduction zones is one of the key phenomena to understand the global mass transfer system. While active volcanoes provide the most recognizable conduits for fluid flow in active margins, the existence of a large number of active fluid seepages demonstrates that other forms of fluid release are also important in subduction zone settings. The authors collected fluid samples from springs and wells across the forearc area in Kyushu, a southwestern island of Japan, covering hot spring activities associated with active volcanism and the Median Tectonic Line (MTL), a major fault system present in the southwestern part of Japan. In order to determine sources of these fluids, halogen concentrations as well as 129 I/I and 36 Cl/Cl ratios were measured in samples from several locations. While Cl concentrations of the forearc fluids in Kyushu range between seawater and meteoric water value, I concentrations are considerably higher than seawater value. Fluids in the Miyazaki area are much higher in I, and somewhat higher in Br, than waters in the Oita area, which is closely associated with the MTL. The differences between those two areas are also pronounced in 129 I/I ratios, which range between 800 and 900 × 10 −15 in the Oita area and between 100 and 360 × 10 −15 in the Miyazaki area. The 129 I/I ratios obtained from the Oita area are compatible with an I derivation from subducting marine sediments, similar to findings from an earlier investigation of fluids collected from Satsuma-Iwojima, an active volcano south of Kyushu Island. In the Miyazaki area, on the other hand, I ages are too old to be derived from currently subducting marine sediments and point to a derivation from old organic-rich materials in the upper plate of the forearc region. The results demonstrate the presence of very different fluid systems in the forearc area of Kyushu: old CH 4 -rich fluids dominate in the seaward side of the forearc, while fluids close to the MTL and the Quaternary Volcanic Front demonstrate derivations from subducting marine sediments. The latter fluids in the MTL area probably are transported through the fractures associated with the fault activities, suggesting that this fault system reaches the transition zone between upper and lower plates in this region.

On 11 October 1995, Kuju Volcano (Central Kyushu, Japan) erupted steam and ash from new vents opened south of the active Kuju–Iwoyama fumarole area. The He/Ar atomic ratio of fumarolic gas, which is a possible indicator of the ratio of magmatic gas to air, suddenly decreased from 0.12–0.22 to 0.04–0.06 at the beginning of the 1995 eruption. The flux of magmatic He soon after the eruption is estimated to be almost 12 times larger than that before the eruption: values before and after the eruption are 57×10 −3 and 694×10 −3 kg/day, respectively. In contrast, the flux of air-derived He increased after the eruption by nearly 40 times than before the event: values before and soon after the eruption are 0.2×10 −3 and 7.6×10 −3 kg/day, respectively. The sudden decrease in the He/Ar ratio suggests that air has been suctioned into the flow path of uprising volcanic fluid through the surrounding formation of altered rocks piled loosely. The volume of air entrained is estimated to be nearly 1×10 4 m 3 /day.