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.

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 (³H, ⁸⁵Kr, 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.

Assessing factors that influence groundwater levels such as land use and pumping strategy, is essential to adequately manage groundwater resources. A transient numerical model for groundwater flow with infiltration was developed for the Tedori River alluvial fan (140 km²), Japan. The main water input into the groundwater body in this area is irrigation water, which is significantly influenced by land use, namely paddy and upland fields. The proposed model consists of two models, a one-dimensional (1-D) unsaturated-zone water flow model (HYDRUS-1D) for estimating groundwater recharge and a 3-D groundwater flow model (MODFLOW). Numerical simulation of groundwater flow from October 1975 to November 2009 was performed to validate the model. Simulation revealed seasonal groundwater level fluctuations, affected by paddy irrigation management. However, computational accuracy was limited by the spatiotemporal data resolution of the groundwater use. Both annual groundwater levels and recharge during the irrigation periods from 1975 to 2009 showed long-term decreasing trends. With the decline in rice-planted paddy field area, groundwater recharge cumulatively decreased to 61 % of the peak in 1977. A paddy-upland crop-rotation system could decrease groundwater recharge to 73–98 % relative to no crop rotation.

To estimate the groundwater flow around a borehole heat exchanger (BHE), thermal properties of geological core samples were measured and a thermal response test (TRT) was performed in the Tsukuba upland, Japan. The thermal properties were measured at 57 points along a 50-m-long geological core, consisting predominantly of sand, silt, and clay, drilled near the BHE. In this TRT, the vertical temperature in the BHE was also monitored during and after the test. Results for the thermal properties of the core samples and from the monitoring indicated that groundwater flow enhanced thermal transfers, especially at shallow depths. The groundwater velocities around the BHE were estimated using a two-dimensional numerical model with monitoring data on temperature changes. According to the results, the estimated groundwater velocity was generally consistent with hydrogeological data from previous studies, except for the data collected at shallow depths consisting of a clay layer. The reasons for this discrepancy at shallow depths were predicted to be preferential flow and the occurrence of vertical flow through the BHE grout, induced by the hydrogeological conditions.

The Pleistocene Kimitsu aquifer was selected for examination of the relationship between groundwater age and chemical evolution of Ca(HCO₃)₂-type groundwater. For the most part, the aquifer is confined and composed mainly of quartz and feldspar with a small amount of calcite. The groundwater ages calculated by ¹⁴C were adjusted by using a carbon mass-balance method and corrected for effects of ¹⁴C diffusion. Groundwater ages in the Kimitsu aquifer vary from modern (upgradient) to approximately 2,400 years at 4.4 km from the edge of the recharge area. The ¹⁴C age was verified by groundwater velocity calculated from the hydraulic gradient and hydraulic conductivity. The confined groundwater evolved to Ca(HCO₃)₂-type around 50 years after recharge and this has been maintained for more than 8,300 years due to low chemical reactivity, derived from equilibrium with calcite, kaolinite and Ca-montmorillonite. In addition, high pH prevents the dissolution of Fe and Mn. Consequently, the rate of increase in electrical conductivity ranges from 10 to 30 μS/cm per 1,000 years. On the other hand, leakage from the deep region, which is recognized from high Cl– levels, causes remarkable increases in CH₄ and HCO₃ – concentrations, resulting in an apparent sulfidic zone at 500-m depth in most downgradient regions.

Groundwater movement through the slope area of Mt. Fuji to the coastal area of Suruga Bay (central Japan) was investigated using spatially dense geochemical data, as a case study for elucidating the groundwater flow system in a stratovolcano adjacent to the coast. Spatial distributions of the hydrogen stable isotope ratio, vanadium concentration, and water temperature in the groundwater showed anomalies at the coastal area of Suruga Bay. The anomalies were characterized as depleted isotope ratio, high vanadium concentration, and low water temperature relative to surroundings. This can be explained as a regional deep groundwater flow from the slope of Mr. Fuji to the coastal area of Suruga Bay because groundwater recharged at higher elevation has a depleted isotope ratio caused by the altitude effect and high vanadium concentration as a result of dissolution from the basaltic aquifer. These characteristics also imply a hierarchical flow system, which is incorporated into a hydrogeological model of the coastal aquifer.

Assessment of the level of activity of advective transport through faults and fractures is essential for guiding the geological disposal of radioactive waste. In this study, the advective flow (active, inactive) of meteoric water through fractures is assessed by comparing stable isotopes (δD and δ¹⁸O) between fracture and pore waters obtained from four boreholes in marine deposits in the Horonobe area, Japan. At 27–83-m depth in one borehole and 28–250 m in another, the isotopic compositions of pore and fracture water reflect mixing with meteoric water, with stronger meteoric-water signatures being observed in the fracture water than in pore water of the rock matrix. At greater depths in these boreholes and at all sampling depths in the other two studied boreholes, the isotopic compositions of fracture and pore waters are comparable. These results suggest that the advective flow of meteoric water is active at shallow depths where fossil seawater is highly diluted in the two boreholes. This interpretation is compatible with the occurrence of present or paleo meteoric waters and tritium, whereby present meteoric water and tritium are limited to those depths in the two boreholes. This difference in the level of activity of advective flow is probably because of the glacial–interglacial difference in hydraulic gradients resulting from sea-level change. Although fractures are hydraulically connected to the surface through the sedimentary rock, advective flow through them is inferred to remain inactive so long as sea level does not fall substantially.

The isotopic compositions (D and ¹⁸O) of 177 precipitation samples collected at seven observation stations in Toyama Prefecture and one station in Gifu Prefecture in the northern part of central Japan were determined. The source and characteristics of the isotopes were clarified and their contribution to the groundwater flow systems of three major alluvial fans in the same area were investigated. The δD and δ¹⁸O values ranged from −113.3 to −26.7‰ and − 16.4 to −4.2‰, respectively. Precipitation samples collected from May to September (summer) and November to March (winter) plotted along two meteoric water lines, with d-excess = 10 and 30, respectively. Conversely, precipitation samples collected in April and October, and some samples in November to March, plotted between the two meteoric water lines. The contribution of precipitation to the groundwater systems was modelled based on the assumption that groundwater is a mixture of major river water and precipitation. According to the observed δ¹⁸O values for the precipitation, river water, and groundwater samples, the contribution of local precipitation to groundwater reservoirs ranged from 5 to 100%. Groundwater flows near the rivers did not always originate from 100% river runoff; however, the contribution of river runoff to groundwater decreased with increasing distance from the rivers, and groundwater flows far from the river were generated mainly by precipitation. The possibility of using groundwater for a ground-source heat pump system, for air conditioning in houses and to melt the snow on roads, is also discussed.