Evaluation of change in subsurface thermal environment due to groundwater flow in the Tokyo Lowland,Japan

Information on the distribution of subsurface temperature and hydraulic heads at 24 observation wells in and around the Tokyo Lowland, the eastern part of the Tokyo Metropolitan area, were examined to make clear the relationship between groundwater and the subsurface thermal environment in the urban area. Minimums in temperature–depth logs due to subsurface temperature increasing at shallow parts were recognized in 21 wells. This fact shows subsurface temperature is affected by ground surface warming in almost all of this area. Deeper than minimums, where the effects of surface warming became relatively small, regional variation is observed as follows: high temperatures are shown in the central part to the southern part, and low temperatures shown in the inland to eastern part. The high temperature area corresponds to an area where the lower boundary of groundwater flow is relatively shallow. This area corresponds also to an area with severe land subsidence resulting from excessive groundwater pumping. It is considered that this high temperature area is formed by the effects of upward groundwater flow affected by hydrogeological conditions and pumping. On the other hand, a comparison between past data (1956–1967) and present data (2001–2003) revealed widespread decreasing temperature in the inland area. This is explained by downward groundwater flow based on an analysis of temperature–depth logs. This fact suggests that subsurface temperature is not only increasing from the effects of surface warming but also decreasing from the effects of groundwater environment change due to pumping.     

Coastal aquifer system in late Pleistocene to Holocene deposits at Horonobe in Hokkaido,Japan

The groundwater flow systems and chemistry in the deep part of the coastal area of Japan have attracted attention over recent decades due to government projects such as geological disposal of radioactive waste. However, the continuous groundwater flow system moving from the shallow to deep parts of the sedimentary soft rock has not yet been characterized. Therefore, the Cl^–, δD and δ¹⁸O values of the pore water in the Horonobe coastal area in Hokkaido, Japan, were measured to 1,000 m below the ground surface, and a vertical profile of the pore-water chemistry was constructed to assist in elucidating groundwater circulation patterns in the coastal area. The results show that the groundwater flow regime may be divided into five categories based on groundwater age and origin: (1) fresh groundwater recharged by modern rainwater, (2) fresh groundwater recharged by paleo rainwater during the last glacial age, (3) low-salinity groundwater recharged during the last interglacial period, (4) mixed water in a diffusion zone, and (5) connate water consisting of paleo seawater. These results suggest that the appearance of hydrological units is not controlled by the boundaries of geological formations and that paleo seawater is stored in younger Quaternary sediments.     

Landslides in Sado Island of Japan: Part I. Case studies, monitoring techniques and environmental considerations

A sufficient knowledge on the kinematics and development of landslides helps to adopt proper measures that can be used to protect slopes and the environment in general. This can be achieved by adequate monitoring programs. This paper presents the findings of intensive monitoring activities carried out on Shiidomari and Katanoo landslides found in Sado Island of Japan. More than one year of observation of the two landslides allowed defining some peculiar futures of their kinematics and style of development. The problem of slope instability in the two areas is generally accredited to various factors. But, both landslides were triggered by heavy rainfalls and snowmelt. Because of the outline of the area and the presence of relict topographic features, the Shiidomari landslide is considered to be a large-scale reactivation of old slope failures. The Katanoo landslide is, however, a first-time case. Geophysical investigations and drilling activities in Shiidomari indicated the presence of two slip planes. The deepest (80–100 m) of these is controlled by existing lineaments. Monitoring data suggests that the body of the landslide has subsided as much as 1.16 m just below the main scarp, but a centimeter in the central region. The toe sector also experienced a significant amount of subsidence, but this was counter-balanced by an uplift on the opposite side of the landslide. Hence, the landslide seems not any more active along the deepest slip surface, although it may extend upward and define a series of shallow shear planes around the crown. In the case of Katanoo, the landform characteristics, differential weathering, the road cut and groundwater fluctuations appeared to contribute much to determine the exact location of the landslide. Extensional cracks that preceded the landslide can be related to heavy rainfalls and the cold and warm cycles thereafter. Subsurface investigations and monitoring works indicated that the landslide has two slide blocks with different slip planes. During the observation period, the upper part of the landslide responded more effectively to rainfall and snowmelt than the middle and lower sections. The corresponding movements, however, appeared to settle about three months after failure. There were also little strain transmissions in boreholes and no significant change in the characteristics of the landslide. The kinematics of deformation of many of the slopes in Sado Island resembles that of Shiidomari landslide. But mass movements along highways and mountain roads are usually similar to Katanoo. Landslides of the type like Shiidomari may not show sudden and drastic failures, but are usually long lasting and can reactivate repeatedly along new, shallow shear planes. Monitoring works and long-term supervisions in these types of landslides are useful to identify impending failures and take the right measures before they brought about large-scale destruction to the environment.     

Review: Deep groundwater research with focus on Germany

While research focuses mainly on the intensively used shallower aquifers, only a little research has addressed groundwater movement in deeper aquifers. This is mainly because of the negligible relevance of deep groundwater for daily usage and the great efforts and high costs associated with its access. In the last few decades, the discussion about deep geological final repositories for radioactive waste has generated strong demand for the investigation and characterization of deep-lying aquifers. Other utilizations of the deeper underground have been added to the discussion: the use of geothermal energy, potential CO₂ storage, and sources of potable water as an alternative to the geogenic or anthropogenic contaminated shallow aquifers. As a consequence, the fast growing requirement for knowledge and understanding of these dynamic systems has spurred the research on deep groundwater systems and accordingly the development of suitable test methods, which currently show considerable limitations. This review provides an overview of the history of deep groundwater research. Deep groundwater flow and research in the main hydrogeological units is presented based on six projects and the methods used. The study focuses on Germany and two other locations in Europe.     

Modeling the dynamics of the freshwater-saltwater interface in response to construction activities at a coastal site

A numerical model was developed to evaluate the response of groundwater flow and the fresh-saltwater interface in relation to the construction of a particle accelerator at the coastal plain of Tokaimura, Japan. Undisturbed conditions were initially simulated and validated against field observations as a prerequisite for the analysis of predictive scenarios. Groundwater heads and the shape of the saltwater interface were appropriately described by the model, although it tended to underestimate salinity concentrations. Saltwater penetrated up to 250 m inland during predevelopment conditions, reaching more than 400 m at the dewatering phase. Flushing of entrapped saline groundwater might occur in addition to seawater intrusion. In depth, multiple saltwater fronts develop in response to the hydraulic properties of the sediments. Groundwater discharges offshore through the sandy aquifers, but salinity fronts prevail in the relatively impermeable layers. Routes for freshwater outflow turned into pathways of seawater intrusion during the pumping phase. The equilibrium would be reestablished within 2 years from the end of the stress, with no evidence of a permanent deterioration of neighbor residential wells. Nonetheless, after construction the accelerator forms a barrier that leads to a sharp rise in piezometric levels and creates a new and long-term disequilibrium in the saltwater wedge. Despite further work is still necessary to test many of the ideas proposed, the present study makes a new contribution to enhance the understanding of the processes occurring in coastal aquifers subjected to anthropogenic influence.     

Landslides in Sado Island of Japan: Part II. GIS-based susceptibility mapping with comparisons of results from two methods and verifications

Slope instability research and susceptibility mapping is a fundamental component of hazard management and an important basis for provision of measures aimed at decreasing the risk of living with landslides. On this basis, this paper presents the result of a comprehensive study on slope stability analyses and landslide susceptibility mapping carried out in part of Sado Island of Japan. Various types of landslides occurred in the island throughout history. Little is known about the triggering factors and severity of old landslides, but for many of the recent slope failures, the slope characteristics and stratigraphy are such that ground surfaces retain water perennially and landslides occur when additional moisture is induced during rainfall and snowmelt. A range of methods are available in literature for preparation of landslide susceptibility maps. In this study we used two methods namely, the analytical hierarchy process (AHP) and logistic regression, to produce and later compare two susceptibility maps. AHP is a semi-qualitative method, which involves a matrix-based pair-wise comparison of the contribution of different factors for landsliding. Logistic regression on the other hand promotes a multivariate statistical analysis with an objective to find the best-fitting model that describes the relationship between the presence or absence of landslides (dependent variable) and a set of causal factors (independent parameters). Elevation, lithology and slope gradient were casual factors in this study. The determinations of factor weights by AHP and logistic regression were preceded by the calculation of class weights (landslide densities) based on bivariate statistical analyses (BSA). The differences between the AHP derived susceptibility map and the logistic regression counterpart are relatively minor when broad-based classifications are considered. However, with an increase in the number of susceptibility classes, the logistic regression map gave more details but the one derived by AHP failed to do so. The reason is that the majority of pixels in the AHP map have high values, and an increase in the number of classes gives little change in the spatial distribution of susceptibility zones in the middle. To verify the practicality of the two susceptibility maps, both of them were compared with a landslide activity map containing 18 active landslide zones. The outcome was that the active landslide zones do not completely fit into the very high susceptibility class of both maps for various reasons. But 70% of these landslide zones fall into the high and very high susceptibility zones of the AHP map while this is 63% in the case of logistic regression. This indicates that despite the skewed distribution of susceptibility indices, the AHP map was better to capture the reality on the ground than the logistic regression equivalent.     

Submarine groundwater discharge: an outlook of recent advances and current knowledge

Driven by the hydraulic gradient, terrestrial groundwater can be discharged directly into the coastal ocean where heads are above sea level, a phenomenon known as submarine groundwater discharge (SGD). Although long overlooked, in recent years SGD has received increasing attention due largely to its potential importance in the transport of chemical constituents to the sea. Indeed, SGD occurrence and control mechanisms are still poorly understood. Submarine discharge often represents only a minor component of regional total water budgets, but because estimated fluxes are highly variable and quantification remains challenging, the process deserves to be carefully evaluated when investigating marine ecosystems and/or the safety of geological repositories in coastal areas, notably for the underground storage of CO₂. In view of the focus the topic is gaining, reflected in the large amount of information continuously being published in this field, here we present a selected review of our current knowledge of key mechanisms determining submarine groundwater discharge, and an updated compilation of most recent advances in research both in Japan and other regions of the world. In addition, we identify some potential pitfalls that need to be borne in mind in future work on SGD. Particularly in view of globally increasing urbanization and climate change, this contribution should prove useful also to local governmental authorities and scientists involved in groundwater coastal ecosystem management.