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Laboratory experiments and a numerical simulation were performed to investigate the behavior of residual saltwater trapped in a storage area for freshwater after installation of a cut-off wall. First, dispersivities were estimated by image analysis of the results of pulse and continuous injection experiments performed with tracers, and then the estimated dispersivities were validated by numerical analysis and by calculating the analytical solution. The ratio of longitudinal to transverse dispersivity was 20 in a red food dye tracer and 27 in a fluorescent tracer experiment. The saltwater intrusion and removal experiments were reproduced by a SEAWAT numerical model using an estimated dispersivity ratio of 27. Numerical analysis using a fixed longitudinal dispersivity showed that with a dispersivity ratio of 10, the resulting removal time of the residual saltwater was 1.48 and 1.97 times the removal time obtained with a ratio of 27 and 100, respectively. This result shows that transverse dispersion significantly affected the removal process.

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In Japan, the artificial forestation has been believed to increase the groundwater recharge and widely applied for the purpose of groundwater resources management. However, the quantitative evaluation of effect of the artificial forestation on the groundwater recharge rate is not well studied. In this study, to understand the effect of groundwater recharge rate by artificial forestation, groundwater recharge rate were compared by using the Displacement Flow Model for stable isotope ratio in rain and soil water of the selected forest and grassland catchments, the western foot of Mt. Aso, Kumamoto prefecture, south-west Japan. In both catchments, stable isotope ratio of soil water samples were plotted on the summer LMWL in a δ diagram. This means that the main recharge occurred during the summer season and the influence of isotopic fractionation was small.
The estimated mean annual recharge rates by DFM were 1493 mm/y and 1920 mm/y in forest and grassland catchments, respectively. As a result, this study shows the groundwater recharge rate is about 30 % larger for the grassland catchments than that for the forest catchment under the similar geological and hydrological condition. This result suggests the effect of seepage characteristics of the surface soil of the study area.

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An area of high groundwater temperature had been recognized beneath the Tatebayashi area in the northern Kanto Plain. Our previous study suggested that changes of subsurface temperature distribution were occurred. The purpose of this study is to understand the present distribution of high subsurface temperatures and to evaluate the changes in the temperature distribution and the factors leading to the changes.
Observation results indicated that the west and east sides of the Tatebayashi area had high temperature areas with a low temperature area between them. A comparison of results between the present subsurface temperature data (2010) and past data by the preceding studies (1950s to 1970s, 2000) shows general warming in the shallow part. This warming is interpreted to be due to the effects of warming of the ground surface and shallow groundwater flow by urbanization. Moreover, temperature decreases were found in the deep flow system beneath the Tatebayashi area. The areas of temperature decreases correspond with areas of low hydraulic heads. These results indicate that subsurface temperatures have been decreasing over the long term in the Tatebayashi area due to the effects of groundwater development.

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A novel watershed modeling technique of the fate and transport processes of nitrogen integrated into a surface and subsurface coupled fluid-flow simulation was developed. The developed technique enables to handle spatiotemporal dynamics of nitrogen loads, oxidation-reduction reactions including nitrification and denitrification by coupling transport processes of multiple chemical species and biogeochemical reaction kinetics in watershed scale. Different N-transformation processes and their interactions can be modeled depending on the site-specific characteristics and the available information. We applied the developed computer code to the actual fields with the different modeling approaches of fate and transport processes of nitrogen, and discussed the reproducibility of simulated results in surface and subsurface water environment. Results indicated that the model is successful in reproducing observed measurements of NO₃^－ concentrations by employing the modeling method appropriate for the site characteristics.

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On coral islands, Kikai and Miyako Islands in Japan, where intensive agriculture is developed and ammonium sulfate is extensively spread, we discussed the difference of carbonate dissolution and CO₂ emission between the forest and the agricultural areas using fertilizer, groundwater, and CO₂ data in spring sites. In the agricultural area, higher CO₂ concentrations were detected, which were thought to be mainly caused in the carbonate dissolution accelerated by H^＋ produced in nitrification of ammonia from ammonium sulfate and the decline of acid neutralizing capacity due to NO₃^－ and SO₄^2－ deriving from the fertilizer. Carbonate dissolution rate in the agricultural area was estimated to be 1.7 times in average and 2.3 times at maximum as compared to the forest area. Carbon emission in the agricultural area was also estimated to be 27.4 and 15.8 tC km^－2 y^－1 in Kikai and Miyako Islands, respectively.

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[in Japanese]

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[in Japanese]

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