Takeo Tsuchihara

Development of a numerical model for nitrates in groundwater in the reservoir area of the Komesu subsurface dam, Okinawa, Japan

A numerical model consisting of simplified equations was developed to simulate nitrate concentrations in groundwater in a reservoir area of a subsurface dam in a Quaternary limestone region. The model was composed of a water balance sub-model and a nitrogen balance sub-model; the water balance sub-model was built from tank models which can express the quick dilution near caves and the effect of dam construction; and the nitrogen balance sub-model was made to represent changes in nitrogen forms and movement of nitrogen in the soil and aquifer zones. The model was calibrated and verified by observed data before and after the dam construction and then applied to a predictive simulation under a simple assumption that rainfall descends gradually. The model seemed applicable to long-term prediction of changes in NO3-N in the reservoir area.

Integrated field- and model-based study of hydro-environmental aspects of a small, endangered wetland in eastern Hokkaido, Japan

The hydro-environmental aspects of a small, endangered wetland (Totsuru wetland), located in eastern Hokkaido, Japan, are investigated, and the efficacy of a proposed engineering strategy for restoring the wetland is examined. Taking 222Rn as a target tracer and building a static 222Rn mass balance equation, the share of groundwater-genetic surface water in yielding the entire surface water in the wetland is appraised. The share is also appraised by using a different approach based on a non-static, site-specific water balance model. Both approaches yielded nearly the same percentage share, and therefore, it is concluded that 70% or more of the surface water entering the Totsuru swamp through the natural rivers is groundwater genetic. Using the water balance model, it is deduced that the reverse flow from the Uenbetsu drainage river is not less than 30% of the entire surface water flow entering the swamp, and therefore contributes considerably to mitigating the shrinkage of the swamp. Finally, through numerical experiments, it is shown that raising the water level of the Uenbetsu drainage river with a constructed weir to enhance the reverse flow is efficacious for restoring the wetland.

Improvements in a simple harmonic analysis of groundwater time series based on error analysis on simulated data of specified lengths

Tidal response method is an efficient technique for investigating hydraulic properties of an aquifer in insular and coastal areas of highly permeable geological settings. The present study extends a simple and straightforward harmonic-analysis technique recently introduced as part of a tidal response method applied to a freshwater-lens aquifer. This simple analysis technique was examined with artificially synthesized time series composed of multiple realistic tidal components. The analysis outputs of major diurnal and semidiurnal components are sufficiently accurate if the analyzed time-series length is appropriately restricted. Limitations of the simple harmonic analysis in the applicable time-series length arise from tidal-component interference that occurs in analyses over a finite length. Definitively recommended simple harmonic-analysis technique with appropriate combinations of time-series lengths and extractable tidal components are convenient for investigating hydraulic properties of an aquifer, such as on a remote island where the freshwater lens is the only freshwater resource.