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Chemical and isotopic studies including analyses of noble gases were comprehensively conducted on the groundwater of the entire Kii Peninsula, which is located in the fore-arc region of southwest Japan. Groundwater of Na–Cl–HCO3, Na–HCO3–Cl, and Na–Cl types was shown to be distributed across the whole area. Groundwater in the inland central part of the peninsula shows relatively low salinity, whereas groundwater from the area along the ENE-trending Median Tectonic Line (MTL), on the north side of the peninsula, shows high salinity (up to 18,800 mg/L of Cl−) and the presence of unusual heavy oxygen isotopes. This trend is similar to that documented in saline waters from the Arima region (the so-called “Arima-type thermal water”). High 3He/4He ratios relative to the atmospheric value (up to 6.7 Ra) were recorded throughout the Kii Peninsula, covering a wider area than documented previously. The saline groundwater is also strongly depleted in 20Ne and heavy noble gases. From the wide distribution of high 3He/4He values and the associated 20Ne and Cl− concentrations, we infer that aqueous fluids derived from dehydration of the subducting slab are present at depth beneath almost the entire Kii Peninsula. These aqueous fluids may ascend along the major north-dipping boundary faults. The isotopic composition of groundwater from the southern part of the peninsula suggests that the contribution from these dehydration-derived fluids is relatively small in this region. However, volatile components (e.g., noble gases and CO2) in the groundwater of this area may originate from the dehydration-derived fluids. Upwelling of Arima-type thermal water of the Na–Cl–HCO3 type is expected to undergo a phase separation of volatile species due to decompression as the fluid ascends. The variety of water types documented may be due to this water–gas separation and the subsequent incorporation of gaseous species into shallow meteoric groundwater. The observed high 3He/4He ratios in the absence of a mantle wedge below the southern part of the Kii Peninsula may reflect the oblique ascent of these fluids along north-dipping boundary faults.

A geochemical investigation was undertaken at Mt. Etna Volcano to better define groundwater characteristics of its aquifers. Results indicate that the Na?Mg ± Ca-HCO3− ± (SO42− or Cl−) type accounts for more than 80% of the groundwater composition in the volcano. The remaining 20% is characterized by elevated Ca2+. Waters along coastal areas are enriched in SO42− or Cl−, mainly due to mixing with seawater and anthropogenic effects. The majority of the samples showed values between −4% to −9% for δ18O and −19? to −53% for δ2H, suggesting that precipitation is the predominant source of recharge to the aquifers, especially in the west of the study area. The analysis of δ13C and pCO2 shows values 1 to 3 times higher than those expected for waters in equilibrium with the atmosphere, suggesting a partial gas contribution from deep sources. The diffusion of gasses is likely to be controlled by tectonic structures in the volcano. The ascent of deep brines is also reflected in the CO2 enrichment (up to 2.2 bars) and enriched δ2H/δ18O compositions observed in the salt mounts of Paternò. Fil: Bellia, Carmelo . National Institute of Advanced Industrial Science and Technology. Geological Survey of Japan; Japón Fil: Gallardo, Adrian Hugo. Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Departamento de Geologia; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Yasuhara, Masaya . National Institute of Advanced Industrial Science and Technology. Geological Survey of Japan; Japón Fil: Kazahaya, Kohei . National Institute of Advanced Industrial Science and Technology. Geological Survey of Japan; Japón

Summary Groundwater residence times in Nakano-shima Island, Oki-Dozen, Japan, which is situated on the continental shelf, are discussed based on groundwater-age indices (CFCs, 3 H, and 14 C), stable isotopic ratios of hydrogen (δD), oxygen (δ 18 O), carbon (δ 13 C), and noble gases ( 3 He/ 4 He and 4 He/ 20 Ne). Samples were taken from springs, shallow wells (up to 5 m deep), water-supply wells (about 10–100 m deep), and the hot spring well (866 m deep). δD and δ 18 O of all the samples plot along meteoric water lines, indicating a meteoric water-origin. Isotope values of the hot spring water were slightly lower than other samples. Most samples from springs, shallow wells, and the water-supply wells contained higher CFCs and 3 H while samples from two water-supply wells (W4 and W7) and the hot spring well showed much lower CFCs concentrations. This result indicates that mixing ratios of old groundwater that is free of CFCs and 3 H were higher for these samples. 14 C concentrations of samples with lower CFCs were measured, and corrected for addition of 14 C-free dissolved inorganic carbon. The corrected 14 C concentration of the hot spring water was lower than that of the water-supply well, indicating mixing of much older groundwater in the hot spring water. Because of lower corrected 14 C concentration and lower δD and δ 18 O values, groundwater from the hot spring well is considered to be recharged in a colder climate than present day. Because the island is situated on the continental shelf, the seafloor around the island was most likely widely exposed during the last glacial period. One possible explanation for the existence of groundwater showing lower δD, δ 18 O, and 14 C concentrations, such as the hot spring water, is that fresh groundwater, recharged when sea level was lower, still remains after transgression without being replaced by salt water.

Subsurface fluid flow processes have been studied for over 150 years, starting from securing water resources for drinking and agricultural purposes, salt production from brines, and exploration and production of energy resources such as petroleum and natural gas. Infiltration theory based on Darcy’s law (Darcy, 1856) has been used in analyses. Hubbert (1956) conducted a thorough review of this equation and showed that it can be understood as a microscopic momentum equation averaged over macroscopic scales, even though the equation was originally developed as an empirical one. Hubbert (1956) also stated that Darcy’s law can be used under the condition that viscous resistance is dominant compared to inertial resistance. The laminar flow condition is met with a Reynolds number of 600 to 700 in porous media (Dudgeon, 1966; Kyle and Perrine, 1971); however, according to Hubbert (1956)’s discussion, Darcy’s law does not hold when the Reynolds number is larger than unity in porous media with the reference length set to be the mean diameter of solid grains. The Darcy flux of 1 (cm /s /cm) corresponds to this condition when the mean grain size of the medium is 10-2 cm. Bear (1972) also presented similar results and showed that Darcy’s law is applicable when the Reynolds number is smaller than 1 or 10. As explained above, subsurface fluid flow is known to be slow. Recently problems related to subsurface fluid flow processes have been increasing, therefore, slow subsurface processes have become more and more important. Expectations are high today for the appropriate use of stable subsurface environments to solve waste disposal problems. For example, concepts for the geological disposal of radioactive wastes and subsurface sequestration of carbon dioxide (CO2) have been developed based on the fact that there are places where subsurface fluid flow is extremely slow and that we can expect possible very slow mass transport conditions in the subsurface if we design procedures for disposal/sequestration properly. Also, studies on the petroleum system include migration, accumulation, preservation, and destruction of petroleum and natural gas reservoirs over geological time scales. Here, an extremely slow subsurface fluid flow is the issue. Land subsidence can also be understood as a

We investigated oxygen isotope and Sr/Ca, Ba/Ca and Mg/Ca ratios in shells of a commercially cultured freshwater pearl mussel ( Hyriopsis sp., Unionidae) and ambient water. The first-order fluctuations of oxygen isotope ratios of the outer shell layer along the maximum growth axis recorded the mussels' annual growth from 2002 to 2007. Clear annual Sr/Ca cycles suggest that Sr was incorporated into the shells at a high rate during warm seasons. All three examined shells showed a clear decrease in annual average Sr/Ca with age. Inter- and intra-annual trends suggest that Sr incorporation was higher during periods of faster growth. The Ba concentration and the Ba/Ca ratio of ambient water showed neither an annual periodicity nor a seasonal trend. In contrast to the findings of an inorganic aragonite precipitation experiment, Ba/Ca was positively correlated with ambient temperature. In addition, Ba/Ca and Sr/Ca ratios were positively correlated with each other. These results suggest that not thermodynamic equilibrium partitioning but the shell growth rate controls Ba incorporation into Hyriopsis shells. The Mg/Ca profiles of the three shells suggest that complex factors control the incorporation of Mg into shell aragonite. Water temperature and annual growth rates did not significantly influence Mg incorporation, and substantial interspecimen differences suggested that physicochemical factors do not exert any systematic control on Mg/Ca ratios in Hyriopsis . Instead, the Mg content of shell aragonite might reflect biological contamination with organic material or adsorption phenomena during crystal precipitation rather than regulated substitution into the aragonite crystal lattice. Our results suggest that trace element profiles in aragonitic bivalve shells could be interpreted as the result of strong biological controls over-riding environmental influences on trace element incorporation.

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.

As well as reducing groundwater recharge by precipitation, urbanization creates a new important source of water for groundwater recharge, that is mains leakage. Although it has been realized by a water balance method that urban groundwater recharge is often as high, or higher, than pre-urbanization rates owing to the downward percolation of mains leakage, the increases have rarely been quantified in a direct manner. In this study, contribution of mains leakage to the discharge of springs in the Kurome River basin and Shakujii River basin, both in Tokyo, were evaluated through a comparison among the δD and δ18O of spring water and mains leakage (tap water). In the moderately-urbanized Kurome River basin, all the springs represented almost 100% precipitation contribution. This is also the case in the more urbanized upper reaches of the Shakujii River basin, where precipitation alone could account for 85-100% of the discharge of springs. Precipitation was found to play a less important role in spring discharge in the highly-urbanized lower reaches of the Shakujii River basin (impermeable surface ratio: some 0.8 and more), were some springs contained substantial proportions of mains leakage of as much as 30-70%. On the other hand, it is worthy of notice here that, even in central Tokyo, the isotopic composition of most spring waters was indicative of predominant contribution of precipitation, which is contrary to the results of the previous studies on the basis of a water balance method.

Environmental tracers become a common tool for the groundwater study and a number of methods have been presented in order to understand groundwater flow processes, water budget, origins, chemical reaction processes and retention time. Tracers often used are selected and reviewed for their various methods and advantages as follows; 1) stable 18O, D in water, 2) stable 13C and radioactive 14C in DIC, 3) noble gases such as He, Ne, Ar, Kr, Xe and their isotopes, 4) radioactive 36Cl in dissolved chloride and some heavier isotopes, and 5) inert gaseous species such as CFCs.If they are less reactive species, they likely preserve information at the time of recharge or their origin. Use of D, 18O and the d-value of water is the powerful tool to determine the recharge area because recharged meteoric water have their inherent isotopic ratios correlated with the recharge elevation, distance from the coast, or the local topography. Carbon-bearing species are more reactive though, use of stable isotopes of DIC leads to identify its origin and helps to analyze the chemical reaction between minerals and water or gas addition processes during the groundwater flow in aquifers. Radioactive 14C has been used to estimate groundwater age however special attention should be paid for, i.e., the origin of DIC, before applying the method. Noble gas tracers are the useful species to presume recharge temperature from their concentrations in water using their temperature dependence of solubilities. Radiogenic 4He concentration can be used for the very long-term groundwater dating since the 4He is produced in the crust and is accumulated in the deep aquifers, if the local accumulation rate of 4He is known. Radioactive 36Cl has been used to determine the age of very old saline waters up to million years. This isotope will also be convenient for the dating of very younger waters, by the use of bomb-produced 36Cl resulted from surface nuclear experiments near the seawater in the 1950s. Chlorofluorocarbons (CFCs) are the gas species produced by the recent human activity and dissolve in water during the recharge, therefore, the affected younger groundwater will have equivalent CFCs concentrations with the atmospheric CFCs concentrations at the time of the recharge. As these species are easy to detect with very high sensitivity, this tracer has now been applied not only for the age determination but for the mixing or contamination of shallow young water to a deep old groundwater. As an individual method listed above is valid only for the very simple flow system, appropriate assumptions or coupling of using different tracers is necessary to understand natural complex groundwater flow system where mixing of groundwaters of different origin or age occurs. Combination of tracers helps us simulating the complex system in detail and is being a growing trend in groundwater study.

Chemical and stable carbon isotopic analyses of dissolved inorganic carbon (DIC) were carried out for groundwater samples collected from cold springs and shallow wells in the Unzen volcanic region in 1999 and 2000. All of the data sets plotted on the carbon isotope ratio (δ13C) vs 1/DIC diagram can be explained by mixing of volcanic CO2 with DIC equilibrated with soil CO2. Groundwater DIC showing a high mixing ratio of volcanic CO2 appears to have a tendency to distribute along two major faults near the activity center of the 1990–1995 eruption. This suggests that these faults are escape routes of volcanic CO2 diffused into the volcanic edifice. The total flux of the volcanic DIC discharged from the cold springs is shown to be one to two orders of magnitude lower than the roughly estimated flux of volcanic CO2 discharged from the summit during the eruptive period.

[1] Magmatic volatile supply to the groundwater around Iwate volcano, which is an active volcano with weak fumarolic activity, is investigated using chemical and isotopic compositions of 180 groundwater samples from various depths. Magmatic 3He flux is estimated by an applied method using helium isotopes. The three-dimensional distribution of 3He flux can be obtained as well as that of the concentrations. High 3He flux with high concentration is found in shallow groundwaters at N and NE flanks, and in shallow and deep groundwater around the fault zone. Low 3He flux with high concentration in deep groundwater suggests the slow and long-term accumulation of magmatic volatiles. High 3He fluxes and concentrations around the fault zone indicate that the fault acts as an ascending path of magmatic volatiles. Distribution of flux indicates the three-dimensional anisotropy of magmatic volatile supply, which is due to the groundwater flow variation restricted by the structure of the volcanic body. Supply rates of other magmatic volatiles can be also obtained using 3He flux, and the total supply rates are estimated to be 2.3 × 10−3 (3He), 5.1 × 108 (magmatic C), 1.3 × 108 (Cl), and 1.3 × 108 (S) mol/y. Comparison of supply rate to groundwater with volcanic gas emission rate reveals that the magmatic volatiles are largely supplied to the groundwater. However, the total emission combining the groundwater and volcanic gas is not prominently large compared with other active volcanoes. We infer that the method combining the magmatic volatile concentration and flux to groundwater is available for the evaluation of passive activity of volcano.

[1] Because mixing of groundwater commonly occurs in geologically active regions such as subduction zones, sole use of a conventional method for groundwater dating is not applicable. Here we present a new model using dissolved 4He concentration and He isotopic ratio as well as crustal He flux to estimate the very long residence time of groundwater where mixing of waters of different origin occurs. We applied this model to the Kobe area, southwest Japan, where Arima-type thermal brine with a high 3He/4He (1 × 10−5) component is incorporated into meteoric water. A groundwater residence time of 25–230 kyr is estimated for the deep groundwater (up to 1500 m depth) aquifers. The flux of Arima-type brine is also estimated to be 2 × 105 m3 yr−1 in the studied area (20 km × 5 km). These attempts contribute to evaluating the long-term stability of geological formations over the time range of 104–105 years and to considering the deep-seated fluid flux to shallower aquifers.

In May 8-11, 1997, a landslide occurred at the Sumikawa Spa, the northern flank and about 4 km northeast from the summit crater of Akita-Yakeyama Volcano, northeastern part of Akita Prefecture. The scale of the landslide was 400 m wide and 800 m long, the volume 2, 500, 000 m3, and the height of the mainscarp was 60 m. In May 11, when the main landslide mass movement occurred, the toe (front unit) of the landslide mass was transformed into the debris avalanche, 500, 000 m3 in volume. The debris avalanche rushed over the River Sumikawa, and to the Akagawa Spa, 1100 m northward and 190 m low of the Sumikawa Spa. The debris avalanche was finally transformed into the debris flow, that flowed about 800 m northward to the confluence with the River Kumazawa, one of the upper reaches of the River Yoneshiro. At the same time, when the main landslide and the debris avalanche occurred, the intermittent phreatic eruptions (hydrothermal explosions) broke out at the Sumikawa and Akagawa Spas. The smoke (fume) of these eruptions rose up to a height of>100 m at the Sumikawa Spa and up to a height of several decades of meter at the Akagawa Spa.Three times of observations of water temperatures and chemical concentrations were carried out for thermal waters and interstitial waters centrifuged from altered clay and ash samples obtained from at and around the Sumikawa and Akagawa Spas, to know the possibility of contribution of high temperature volcanic gas and deepseated hydrothermal water to the occurrence of the 1997 Sumikawa Landslide with phreatic eruptions (hydrothermal explosions).The results lead us to conclude the followings.1) The increase of ground water temperature and chemical concentrations, and the decrease of pH observed at the Sumikawa Spa were caused by the decrease of the rate of the intrusion of subsurface, meteoric and/or river water to the Sumikawa hot spring layer.2) The increase of chloride ion concentrations compared with other reference values observed at the Akagawa Spa were caused by the increase of the rate of the intrusion of interstitial water to the Akagawa hot spring layer, because of the movement of spring head, usually observed in hot spring area.These results obviously denied the possibility of contribution of high temperature volcanic gas and deepseated hydrothermal water to the occurrence of the 1997 Sumikawa Landslide with phreatic eruptions (hydrothermal explosions).

The research on the three-dimensional behavior of subsurface water during and after a rainstorm throughout a small forested basin (0.56 ha) was carried out in the Tama Hills. The basin is located in the northeastern part of Kanagawa Prefecture, Japan and composed of flat valley bottom (mean gradient of 10°) and steep side slopes (mean gradient of 25°). Twenty four borings were practiced to know soil profiles and depths to the Tertiary bed rock (mudstone) from the ground surface. As the results, soil profiles were classified as A-horizon, B-horizon, transported soil layer, weathered zone of bed rock and bed rock. Depths to the bed rock were about 3 to 4m in the valley bottom and 1.5 to 2m in the middle parts of the side slopes. The ancient V-shaped valley on the bed rock and its weathered zone were buried by the transported soil, and the present geomorphic feature was formed over. The transported soil is composed of loam and masses of weathered bed rock and assumed to be carried from upper parts of the side slopes. Fifty-six observation wells, being classified into Wells D and Wells S, were installed through-out the basin. The Wells D were dug to the top of the bed rock and used to observe the behavior of the “deep groundwater body”. On the other hand, the Wells S were dug to about 1m below the ground surface and used to observe the behavior of the “shallow ground-water body”. In order to characterize vertical distributions of soil-water pressure head in response to rainstorms, forty tensiometers were equipped at five sites. Three rainstorms with a total rainfall of more than 100mm occurred during an observation period from September 28 to October 21, 1979. The major results clarified by this research are as follows: 1. The impermeable layer, the top of which is generally about 1m below the ground surface, is found almost over the basin. This layer plays important roles in building up the temporal shallow groundwater body and confining the deep groundwater body during and shortly after the rainstorms. 2. The impermeable layer is not found both in a lower part of the valley bottom and in vicinity of the ridge top. A lack of the impermeable layer in vicinity of the ridge top may affect the behavior of the deep groundwater body during the rainstorms. The rainwater brought on this part is expected to cause a quick response of the piezometric surface of the deep groundwater body to the rainstorms and make the piezometric surface rise about 10cm above the ground surface in some wells. 3. A temporal saturated zone is formed in the A-horizon during and shortly after the rainstorms and discharges into the stream channels, making an important contribution to stormflow generation. There is a strong possibility of application of the results to other headwater basins in the Tama Hills, and therefore more similar researches must be carried out from viewpoints of disaster prevention and geomorphological evolution.