Many geochemical anomalies associated with earthquakes have been reported after World War II. Such seismo-geochemical observations have been carried out mainly in Japan, the former Soviet Union, China, and the United States. Major components measured in the observations are radon, hydrogen, and helium as well as dissolved ion species which are issued from underground. Radon has been widely monitored in the observations because it is easily detected by radioactivity. Multiple chemical species in spring gas and groundwater can be easily analyzed by gas chromatography and ion chromatography, respectively. The mechanism causing the geochemical anomalies has not been fully clarified yet, but some results of geochemical observations correspond with those of geophysical ones. More extensive seismo-geochemical observations should help us to develop earthquake prediction research.

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We have developed a new system for continuous measurements of elemental and isotopic ratios of gases dissolved in groundwater using a quadrupole mass spectrometer and a gas-permeable membrane. Gases dissolved in water are extracted from the liquid phase using a gas exchange module made with silicone membrane tubes. Appropriate amounts of extracted gases are introduced into a quadrupole mass spectrometer through a variable leak valve and an orifice of 50μm in diameter. Selected gas species up to twelve different mass numbers can be repeatedly measured with a time interval as short as one second. This operation is fully automated using a personal computer. Temperature dependence of the gas extraction rate from tap water was examined using a constant temperature water bath, which demonstrated that the gas extraction rate through the silicone membrane is strongly dependent on temperature. We show some preliminary results of the continuous measurements of dissolved gas composition in the Ontake Myojin hot spring water, central Japan. CO₂/³⁶Ar and O₂/³⁶Ar ratios showed clear daily variations, which are probably explained by the difference in temperature dependence of gas extraction rate through the silicone membrane. In contract, daily variations in ⁴He/³⁶Ar, ⁴⁰Ar/³⁶Ar, and ⁸⁴Kr/³⁶Ar ratios were much less obvious, suggesting that relative gas exchange rates among noble gas species are smaller than those among different chemical species. We are preparing a comprehensive continuous observations at the Ontake Myojin hot spring, including data telemetry system, simultaneous continuous measurements of radon concentration and environmental parameters such as water temperature, barometric pressure, rain fall in addition to the system developed in the present study.

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Discharge rates of many springs in Awaji Island increased markedly just after the 1995 Kobe earthquake. For the following one year, the discharge rates at the six springs had gradually decreased to less than 50% of the rates measured in May 1995. On the other hand, major chemical composition did not change except for one spring located in Nojima-ezaki, the northern end of the island. The change in Nojima-ezaki is a long term fluctuation of anion composition. The concentration of bicarbonate ion decreased gradually, and the concentration of nitrate ion increased simultaneously. In Awaji Island, bicarbonate and nitrate ions characterize deep and shallow groundwaters, respectively. Therefore, the observed geochemical fluctuation is thought to be caused by a change of mixing ratio of groundwaters at different depths.

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In general, the apparent equilibrium temperatures calculated for several reactions among gases species in high temperature fumarolic gases does not agree each other. The disagreement has been attributed to the result of some disturbances after a hypothetical equilibrium state of gas. The equilibrium state can be disturbed due to, for example, (1) the difference in reaction rate of SO₂+3H₂=H₂S+2H₂O (reaction-1) and CO₂+H₂=CO+H₂O (reaction-2), (2) the addition of an external water or loss of water from gas, (3) the decrease of CO by a reaction with strong alkaline solution used for gas sampling, and (4) the decrease of gas pressure. Under the assumption (1) or (2), two independent equations of mass reaction can be held corresponding to the reactions-1 and -2. Each the two equations involve one unknown parameter and the unknown equilibrium temperature and known concentration of chemical species. Two different unknown parameters corresponding to the assumption (1) and (2) can not be involved into one set of two equations. Therefore the assumption (1) or (2) must be selected prior to the estimation of equilibrium temperature. In this context, an arbitrariness has been left. If a third reaction is found, the two parameters would be evaluated simultaneously. However such a reaction has not been obtained yet. The equilibrium temperatures are calculated under the assumption of (1) and (2) for the previous results of high temperature fumarolic gases from ten volcanoes. The calculated equilibrium temperatures are similar to or higher than the fumarolic temperature in many cases. However, some gases shows the equilibrium temperature lower than the fumarolic temperature, suggesting the requirement of additional reasons for the disequilibrium state of gas other than (1) and (2). The (3) and (4) may be possible to the the additional reasons. The oxygen fugacity of high temperature gases are expected to be controlled by lava, therefore the gases would show specific values for individual volcanoes. The variation of the oxygen fugacity obtained from gas composition of each volcano can be used to infer the cause for disequilibrium state of gas. The equilibrium oxygen fugacities for gases from individual volcanoes converged well under the assumption (1) rather than (2), suggesting that (1) is the dominant factor disturbing the equilibrium state of gases. The overall trend of equilibrium state for gases is the high oxygen fugacity at low temperature and low oxygen fugacity at high temperature. The trend is consistent to the "gas buffering". A relatively oxidized low temperature gas can be made due to the cooling of a reduced high temperature gas with sufficient amount of SO₂ and H₂S. A gas with limited concentration of SO₂ and H₂S such as the gases from Showa-Shinzan does not show the oxidizing trend at low temperature. The oxygen fugacity of such a gas is thought to reflect the fugacity at the formation of the gas from magma.

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We have developed a new continuous monitoring system for volcanic gas chemistry and set up this system at 3 km north from the summit of Izu-Oshima volcano. For continuous monitoring of dry gas, we use three types of dehydration methods to remove water vapor. CO₂ and SO₂ contents were measured by IR sensors, O₂ by a zirconia sensor and H₂ by a semiconductor sensor. Our system has been in operation more than 6 months without significant troubles. The dry volcanic gas is a mixture of CO₂ of magmatic origin and air component. Temporal variation in chemical composition of the volcanic gas is related with that in atmospheric pressure. The CO₂ content is inversely-correlated and O₂ is correlated to atmospheric pressure with 8 hours' delay. In contrast, H₂ content increases intermittently up to 1000ppm 1 hour after sudden drop of atmospheric pressure. The N₂/O₂ ratio is higher than the air ratio (3.8), and changes in a similar manner to CO₂. This suggests that a part of N₂ is of magmatic origin like CO₂.

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Geochemical study on Yugama, a crater lake at Kusatsu-Shirane volcano, has been conducted since 1966. Amounts of various cationic species in Yugama water started increasing around 1981, slightly before the phreatic eruptions in 1982-1983, and kept increasing until 1985. In 1986, they turned to decrease and at present restore their former levels before the eruptions. The concentration of sulfate ion showed a secular change similar to those of cationic species, but no such variation was observed for chloride ion even during the 1976 and 1982-1983 eruptions. However, it started increasing in 1989. A high correlation between the concentrations of chloride and hydrogen ions suggests an increasing influx of hydrogen chloride from the deep volcanic systems under the lake. A Cl^--SO₄^2- is an excellent monitor of the variation in volcanic activity at Kusatsu-Shirane volcano; all of the three past activities since 1966, i.e., the high-level subsurface activity in 1968, the eruption in 1976 and the eruptions in 1982-1983, showed a reverse secular change with time in the Cl^--SO₄^2- plot. This could be attributable to the function of Yugama water as a condenser of volatiles released underground.

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