For sustainable use of groundwater, it is important to know the past and current water balance and the impact of changes in land and water use for working with stakeholder collectively. In order to understand these issues, a distributed hydrological model that includes the key processes of the regional hydrological system is considered to be a powerful tool, as it enables us to understand the impact of human activities at any given site. In the Kumamoto region, which is almost 100% dependent on groundwater for drinking water, there have been attempts to understand groundwater flow and water balance qualitatively and quantitatively. For example, groundwater levels have been monitored for about 30 years or more, mainly by the local government, to understand the current status of groundwater in the Kumamoto area. Based on these data, a multi-stakeholder group including government, academia and the private sector has developed an integrated surface-subsurface model to reproduce long-term changes in groundwater levels (Kawasaki et al., 2023). This presentation will present the results of several simulations using this model of possible future scenarios in the Kumamoto region, which identify key factors for sustainable groundwater use in the Kumamoto region.

The 2016 Kumamoto earthquake caused significant geological changes and triggered various hydrological phenomena. This study investigates the long-term abnormal rise in groundwater level (GWL) induced by the earthquake, with particular focus on seasonal co-variations, recovery processes, and duration. Using long-term time series data from before and after the earthquake, we applied a tank model and statistical methods to analyze these changes. Results revealed a two-stage mechanism behind the GWL rise: an initial phase of mountainside water release due to earthquake-induced permeability enhancements, followed by increased rainfall recharge through new ruptures via bypass or preferential flow pathways in the Aso mountain area. The abnormal GWL rise peaked about 3 months after the main shocks and gradually returned to baseline over 5.5 years, showing seasonal fluctuations throughout the recovery processes. These findings provide new insights into earthquake hydrology and are vital for the sustainable management and protection of regional groundwater resources.

熊本市の水前寺・江津湖地域において，2016年4月14日，16日に発生した熊本地震前後の地下水湧出量の変化を明らかにした。研究地域を4つのゾーンに分けて，各湧出量の正の値だけ合計した結果，地震前の2015年5月から2016年4月までの平均値は約490,000 m3/dayだったが，地震後の2016年5月から2018年4月までの平均値は約620,000 m3/dayと大きく増加した。この原因は，上江津湖ゾーンと中江津湖ゾーンで地下水湧出量が大きく増加したためである。反対に，下江津湖ゾーンでは湖水から地下への浸透量が大きく増加した。その後，2018年5月から2020年4月までの全湧出量の平均値は約510,000 m3/dayまで減少し，2020年5月以降は約550,000 m3/dayとなった。ただし，地震後に観測された河川流量は，主に上江津湖の復旧工事による影響も考えられる。

This study presented the first attempt to detect precursory changes in groundwater level before the 2016 Kumamoto Earthquake. This detection was achieved by accurately determining the relationship between long-term groundwater level fluctuation and crustal deformation over 16 years through analysis of groundwater level time-series data acquired at 17 sites within the study area. Here, we show that the observed groundwater levels were lower than the modelled levels in aquifers composed of porous strata (Togawa lava and part of the pre-Aso volcanic rocks), and that there were larger differences until 2014, which diminished until the occurrence of the Kumamoto Earthquake. The initial reduction in the modelled groundwater level and the latter recovery were most likely caused by crustal strain relaxation associated with the large 2011 earthquake off the Pacific coast of Tohoku (Mw 9.0) and the strain accumulation prior to the 2016 Kumamoto Earthquake.

Integrated watershed modeling techniques have been applied in recent years to examine surface and subsurface interactions. Model performance is often evaluated by best fit of the hydrograph, which alone cannot explicitly explain whole catchment dynamics. To overcome this problem, this study incorporated multiple tracers ( 3 H, 85 Kr, and groundwater temperature) into a physically-based fully distributed modeling framework for characterizing regional-scale hydrological processes in Kumamoto, southern Japan. First, a simulation performed by a hydrometrically calibrated model showed satisfactory performance for river discharge and groundwater level. However, this model showed poor fitting for isotopic composition and temperature due to the structural uncertainty of the model. A new model was established reflecting recent deep bore log data and incorporating tracer data showed acceptable accuracy for hydrographs and tracers. Thus, more reliable estimates of groundwater storage, groundwater age and water flow paths were depicted over the regional catchment. Comparisons between the two models indicate that the model structure of an area with an uncertain lower boundary can be addressed by incorporating multiple tracer data. Tracer-aided models could be applied for a holistic understanding of contaminant transport dynamics besides flow simulation.

The hydraulic conductivity of an unconfined carbonate aquifer at the uplifted atoll of Minami-Daito, Japan, was evaluated by a combination of cross-spectral analysis, analytical solution, and density-dependent groundwater modeling based on observed groundwater levels in 15 wells and at sea level. The island area was divided into 10 subregions based on island morphology and on inland propagation of ocean tides. The hydraulic conductivity was obtained for each subregion using analytical solutions based on phase lags of M2 constituents of ocean tides at each well by assuming two aquifer thicknesses (300 and 1,800 m) and two effective porosities (0.1 and 0.3). The density-dependent groundwater model evaluated the hydraulic conductivity of the subregions by reproducing observed groundwater levels. The hydraulic conductivity in the subregions was estimated as 3.46 × 10−3 to 6.35 × 10−2 m/s for aquifer thickness of 300 m and effective porosity of 0.1, and as 1.73 × 10−3 to 3.17 × 10−2 m/s for aquifer thickness of 1,800 m and the effective porosity of 0.3. It was higher in southern and northern areas, and higher in interior lowland than in the western and eastern areas. Fissures and dolomite distributions on the island control differences of the omnidirectional ocean tidal propagation and cause these differences in hydraulic conductivity. The method used for this study may also be applicable to other small islands that have few or no data for hydraulic conductivity.

The 2016 Kumamoto earthquake had a significant impact on groundwater levels and quality. In some areas, the groundwater level increased significantly due to the release of groundwater from upstream mountainous regions. Conversely, the groundwater level in other areas greatly decreased due to the creation of new fracture networks by the earthquake. There were also significant changes in certain groundwater quality variables. In this study, we used clustering based SOM (self-organizing maps) analysis to improve the understanding of earthquake effects on groundwater quality. We were especially interested in effects on groundwater used for drinking purposes and in nitrate concentration. For this purpose, we studied groundwater nitrate (NO3− + NO2−–N) concentrations for the period 2012–2017. Nitrate concentration changes were classified into seven typical SOM clusters. The clusters were distributed in three representative geographical regions: a high concentration region (>4 mg/L), a low concentration region (<1.6 mg/L) with minimal anthropogenic loading area, and an intermediate concentration region (2–4 mg/L). Depending on these regions, the nitrate concentration changes just before and after the earthquake had both increasing and decreasing trends between 2015–2017. This points to complex physiographical relationships for release of stored upstream groundwater, promotion of infiltration of shallow soil water/groundwater, and nitrate concentration as affected by earthquakes. We present an analysis of these complex relationships and a discussion of causes of nitrate concentration changes due to earthquakes.

Study region: An uplifted atoll of Minami-Daito Island, Japan. Study focus: Major ions and stable isotopes (delta H-2 and delta O-18) of groundwater at fifteen observation wells, surface water at eight representative lakes and one seawater site were measured to unravel the dominant processes controlling the chemistry of water, its spatial distribution and to identify the salinization mechanism caused by long-term sea-level rise. New hydrological insights for the region: Rainfall is the main source for groundwater and lake water. Evaporation affects both the ion concentration and the stable isotopes of the lake water. Geochemical modeling suggests that freshwater-seawater mixing is the main process increasing concentrations of Na+, Cl-, Mg2+, and SO42-, whereas dissolution of calcite and dolomite increases concentrations of Ca2+, Mg2+, and HCO3- in groundwater. Fresh groundwater and lake water (i.e., Cl- < 500 mg/L) are largely distributed along a SW-NE direction, but they have been reduced since the 1970s. Sea-level rise causes an increase in the salinity of lake water by flowing through fractures being connected from lakes to the northern coast, then spreading to other lakes through the artificial channels built in the years.

The 2016 Kumamoto earthquakes comprised an Mw 6.2 foreshock and an Mw 7.0 mainshock that occurred at 21:26 JST on April 14 and at 01:25 JST on April 16, respectively. Ruptures and vibrations associated with these earthquakes induced effects on streams' discharge in Kumamoto and Oita prefectures; however, few studies have conducted comprehensive investigation of such changes in those areas. In this study, 174 measurements of river water level were carried out as reflecting increase, decrease, or no change after the foreshock or after the mainshock when compared with the level before the foreshock. Trends of increase and decrease after the foreshock (mainshock) were recognized at 6 and 19 (37 and 15) stream gauges, respectively. The ones that showed a trend of increase after the mainshock were located mostly in mountainous areas (elevation >300 m). A coseismic increase in river water level suggested increased groundwater supply from the mountains via enhanced permeability. The difference in river discharge between upstream and downstream gauges relative to the Suizenji fault zone revealed a huge volume (approx. 900,000 m3) of river water was lost from the river during the 12-h period following the mainshock. It is possible that the missing river water flowed into new fault zones formed by the earthquakes.

Large earthquakes change the hydrogeological properties of aquifer systems, such as permeability, and cause changes that impact groundwater flow. To understand these changes in mountain aquifer systems, we analyzed stable isotopic ratios of water molecular (delta D and delta O-18) of H2O and chlorofluorocarbons (CFCs) concentrations of natural spring waters and compared these values between before (2009) and after (2017) the 2016 Kumamoto crustal earthquake sequence in Kumamoto-Aso area, southern Japan. Stable isotope ratios were used to identify the spring source characteristics that reflected recharge elevations, whereas CFC age tracers were applied to evaluate the contribution of earthquake-induced additional waters from different pathways (shorter or longer) and/or CFC-enriched (contaminated) surface waters. In general, spring waters after the earthquake became more depleted in water isotopic compositions than those before the earthquake, suggesting an increased contribution of waters recharged from higher elevations across the area. In addition, changes observed in CFC-12 concentrations were classified into several increasing/decreasing patterns defined by the contribution of additional waters from different flow paths, such as older groundwater with longer flow paths, younger groundwater from shorter flow paths, and CFC-enriched (contaminated) water released from shallow aquifers and soils. These isotopic and chemical features, when combined with previously documented seismotectonic surface rupture distributions, demonstrated the occurrence of coseismic mountain water release due to enhanced permeability. These findings enabled us to document how groundwater flow changes in mountain aquifers. Although CFC age markers have rarely been applied as a tool to investigate coseismic hydrological changes, our study exemplifies their usefulness-in combination with stable isotope ratios-in such investigations.

In order to evaluate the behavior of a freshwater lens in the uplifted atoll island of Minami-Daito, Japan, groundwater level and electrical conductivity were simultaneously measured in six monitoring wells. The monitoring allowed determination of the position and variability of the top and bottom positions of the freshwater lens. The freshwater lens periodically oscillates with phase lags (delay time) every 3 h following sea tides. Recharge from local rainfall temporarily increases the volume of the freshwater lens but is disturbed by the low-permeability muddy sediments deposited on the central lowland of the island. Changes in the groundwater levels correlate well with rainfall, after first removing semi-diurnal, diurnal, and long-term components of the sea tides from the groundwater level data using a multiple regression analysis. Changes in the water electrical conductivity provide information on the temporal and spatial fluctuation of the freshwater lens. The monitoring scheme for this freshwater lens could be applied to other uplifted atolls, contributing to better evaluation of the potable groundwater resources and to making freshwater use sustainable on other islands.

Kumamoto is known as the largest groundwater city in Japan. Geochemical modeling (saturation indices and mineral stability diagram) was applied in this area for better understanding a hydrogeochemical evolution in volcanic aquifers in regional scale with additional constraint from stable isotopic dataset. Geochemical evolutionary model was interpreted along the water flow dynamics. In total 136 water samples were collected from wells, springs, and rivers in and around the major groundwater flow lines for geochemical analyses. Our results indicated that plagioclase is the major weathering reactant in aquifers with secondary important weathering minerals of pyroxenes. These reactions facilitate current hydrochemical signatures and produce secondary minerals of kaolinite or halloysite and smectite in later stage. Observed hydrogeochemical processes can be distinguished into two distinct criteria along flow regime. The first processes are material loads from the surface (mixing of contaminants and river waters) and initial stage of silicate weathering, resulting in the formation of Ca-HCO 3 , Ca-NO 3 –HCO 3 , and Ca–Mg–Na-HCO 3 type waters. These processes prevail in aquifers at the recharge to lateral flow zones with relatively shorter residence time of < ca. 40 years. The second processes are the precipitation of clay minerals, i.e., smectite, and cation exchange reaction of Ca 2+ and Mg 2+ to Na + in downslope aquifers (Na-HCO 3 type) with relatively longer groundwater residence time of > 55 years. Microbiological reduction reactions dominate over these areas and salinization occurs at the coast that changes aquifer waters to be Na-Cl type. Proposed models and approach shown in this study may be useful and applicable in interpreting systems in other volcanic aquifer systems at similar climate conditions and for sustainable water resources management.

Monthly precipitation sampling for stable isotopes （ δ 2 H, δ 18 O ） has been under-taken since 2005 in Miyakonojo City of southern Japan to investigate their seasonal variability and controlling factors. Long-term monthly mean precipitation values are higher than 250 mm in the rainy season （ June–September ） and lower than 200 mm in the non-rainy season （ October–May ） . Monthly mean δ 18 O values are lower than and higher than − 7 ‰ in rainy and non-rainy seasons, except for August. Likewise, monthly mean δ 2 H values are lower than and higher than − 40 ‰ in rainy and non-rainy seasons. Monthly mean d-excess values are lower than and higher than 15 ‰ in warm （ April–October ） and cold （ November–March ） seasons, show-ing a strong negative correlation with monthly mean temperature. A negative correlation occurs between monthly isotopic compositions and monthly precipitation amounts over the entire year, with effects on δ 18 O and δ 2 H of − 0.29 ‰ /100 mm and − 3.86 ‰ /100 mm. A positive correlation occurs between monthly isotopic compositions and monthly temperatures only in the non-rainy season, with effects on δ 18 O and δ 2 H of 0.42 ‰ /°C and 2.98 ‰ /°C. Slopes and intercepts of Local Meteoric Water Lines ranged from 8.30 to 8.67 and from 12.85 to 22.07, yielding slopes close the Global Meteoric Water Line, but with much higher intercepts.