To clarify the determining processes of stable isotope ratios of hydrogen (δD) and oxygen (δ¹⁸O) in paddy water and the factors affecting them, we collected irrigated water, drain water, and ponded water in mid-May, early in June, and at the beginning of August in 2002 from a paddy field, where intensive measurement of water balance was performed. The values of δD and δ¹⁸O gradually increased along the flow direction of paddy water. In a δ¹⁸O vs. δD diagram, these data were plotted linearly for each month, and the slopes ranged from 4.3 to 5.8, showing kinetic isotope fractionation during evaporation. In August, the increase of δ values was much less than that in May and June, while the slope of evaporation line (S_EL) was largest. Considering evaporative fractionation based on the Craig-Gordon model, the large S_EL in August resulted from limited kinetic fractionation due to high humidity under a grown rice canopy. The grown canopy would also lower the increase of δ values by restricting evaporation. At the beginning of August, however, rainfall from a thunderstorm and the change of irrigation management also influenced δ values, so the lowered δ-increase was not attributed only to the restriction of evaporation. The result of the study shows the possibility to separate evaporation and transpiration using isotope hydrological approaches, which is usually impossible by micrometeorological direct measurements.

The Kanto Plain is the largest groundwater basin and the largest groundwater used region in Japan. There is a groundwater area with high Cl concentration (from 10 to 150 mg/l) in the depth of GL-100m to -300m of the central part of the plain. This groundwater area is thought to be made by regional groundwater flow, from the viewpoint of three-dimensional observations of groundwater quality, stable isotopes, and subsurface temperature. On the other hand, it has been pointed out the reduction of the high Cl groundwater area since 1960’s. However, the tendency and the process of the reduction have not been clarified yet. In this study, secular change of high Cl groundwater area and distribution of Cl concentration in 40 years was studied. Also, process of the change was considered by comparison with secular change of distribution of hydraulic head and land subsidence area.

From the result, reduction of the high Cl groundwater area and decrease of Cl concentration in the groundwater area was clarified, and these changes were remarkable in the southern part of the high Cl groundwater area. The reduction and the decrease was caused by secular change of groundwater flow according to depression of hydraulic head in depths of GL-100m to -300m, and supply of low Cl groundwater from the shallower part, the deeper part, and the surroundings to the high Cl groundwater area.

Analyses of chemical compositions and hydrogen and oxygen isotopic ratios were carried out for 167 hot and cold spring waters from Abukuma area, northeasten part of Ibaraki, eastern part of Fukushima and southeastern part of Miyagi Prefectures, northeast Japan.

As a result, followings were concluded.

(1) Heat sources of all hot springs were able to be explained by conductive heat flow in this area, except those in the western side of the Abukuma River, near the volcanic front of northeast Japan.

(2) Vertical variation of geochemical characteristics of hot spring waters in this area was Cl type at the lowermost part, followed by Cl-SO₄ and Cl-HCO₃ types at the lower part, Cl/SO₄/HCO₃ types at the upper part, and SO₄ type and SO₄/HCO₃ types at the uppermost part.

(3) Hot spring waters in this area were categorized into two types by their origins. The first type of hot spring waters were originated from interstitial water or oil field brine which had very high chloride ion concentration and very low sulfate ion concentration. The second type of hot spring waters were originated from meteoric water. Chloride and sulfate ions in them were leached out from minerals in layers which had both high chloride and sulfate ion concentration, although chloride ion in them had a possibility for the origin from brine in layers.