To provide a quantitative background for evaluation of a complex interaction between surface and groundwater in the Kanto Plain, spatial and temporal distributions of oxygen and hydrogen isotopic ratios of river waters in the Kanto Plain and surrounding mountainous regions were investigated. Temporal variations of δD values for river waters sampled at the points where mountain rivers flow into the Kanto Plain were small comparison of spatial variations. The hydrogen isotopic ratios of river waters tend to have lower values for the upstream of Tone river basin, and higher values for the eastern part of the Kanto Plain. The hydrogen isotopic altitude effects of river waters were calculated at -2.4‰ δD/100m. Although each river basin has similar values of the isotopic altitude effects, δD values of the Ara and Tone river basin were 5-10‰ smaller than the Naka and Kuji, Kinu and Kokai and Watarase river basin at the same altitude.
The Kanto Plain has the largest groundwater basin. In this study, regional groundwater flow system in the major part of the plains are estimated from the three dimensional distribution of major dissolved ions, stable isotopes, and hydraulic heads.
Hydraulic heads gradually decrease from the surrounding part to the central part, in recent years. In the central part, minimal head zone is formed in depths from GL-100m to -300m. It is estimated the minimal head zone is made by the effect of groundwater pumping.
The distribution of hydraulic heads suggests the groundwater flows to the minimal head zone from the surrounding part.
On the other hands, from the distribution of chemical characteristics, groundwater zone with relatively low isotopic ratio is distributed in the central part, between Kazo lowland and Tokyo bay area. In the groundwater zone with low isotopic ratio, concentration of dissolved ions, especially Na+, HCO3- and Cl-, are higher than surrounding part. Low isotopic ratio zone has not found in the distribution of meteoric water, such as spring water and shallow groundwater collected from open well. Therefore, it is estimated the groundwater zone with low isotopic ratio had made by regional groundwater flow, not by recharge of meteoric water.
Groundwater with low isotopic ratio is distributed above the minimal head zone. This result shows the groundwater flow estimated from the distribution of recent hydraulic heads cannot make the groundwater zone with low isotopic ratio in the shallow depths of the central part. Therefore, the regional groundwater flow estimated from the distribution of stable isotopes represent the groundwater flow which has much natural state than the groundwater flow estimated from hydraulic heads.
Temperature-depth profiles and hydraulic heads were measured on 117 observation wells in the Kanto Plain. Subsurface temperature distribution in the Kanto Plain is assumed that it is strongly affected by heat advection due to groundwater flow. The low temperature area is distributed in highlands such as hills and uplands located in the surroundings of the plain. On the other hand, the high temperature area is located in lowlands around the central part of the plain and the coastal part of the Tokyo Bay. Considering the observed distribution of subsurface temperatures and hydraulic heads, we estimated that there are two groundwater flow systems which discharge to the central part of the plain and the coastal part of the Tokyo Bay.
Kanto Plain is the largest plain and also it is the largest groundwater basin in Japan. It has been believed that the deep groundwater is recharged by meteoric water (Hayashi, 2003). However, the basin is underlain by marine deposits that contain fossil water, as well as, in some parts, natural gas and fossil seawater. This is a review work about deep groundwater mainly from two previous studies (Marui et al., 2001 and Seki et al., 2001).
Deep groundwaters from deep drill holes were analyzed to determine chemical characteristics and modes of occurrences, and to understand deep groundwater environment. Deep groundwater in the Kanto Plain was divided into two types: one with low EC and of Na-HCO3 type, and another with high EC and of Na-Cl type, from the analyseis of dissolved ions and isotopic compositions. It was concluded that the former was groundwater originated from meteoric water, and the latter formed by mixing of meteoric water and fossil seawater from the viewpoint of the isotopic compositions. At the depths of more than 1,000 m in coastal areas, the Cl- concentrations of groundwater were lower than those of seawater. This suggests existence of upward deep groundwater flow system.
Groundwater, soil water, precipitation and flooded water were collected periodically at paddy, upland and forest fields in the National Institute for Agro-Environmental Sciences (NIAES) from April to December, 1999 to study whether the oxygen and hydrogen isotopic ratios of soil water and groundwater can be useful as tracer to estimate the soil water movement. To consider how the flooded water have effects on the river water, river, flooded and irrigation water samples were also taken in the Kinu River basin at four times. δ18O values of them were analyzed for all samples, and δD were also analyzed for precipitation water samples taken in NIAES, and river, flooded and irrigation water samples in the Kinu River basin.
δ18O values of soil water at paddy field showed that the flooded water percolated to the upper part of the Joso clay layer. δ18O values of soil and groundwater at upland and grove fields are different from those of paddy field. It is suggested that the estimation of the soil water movement could be possible by using the oxygen and hydrogen isotopic ratios of soil water.
The altitude effect was found for the Kinu and Ta rivers. Each rivers, in addition, have their own isotopic compositions in some places, thus isotopic compositions of river water can be used for estimation of the groundwater flow and presumption of the recharge area of groundwater.
On premise of GSJ in Tsukuba, we have 4 wells for continuous ground water level observation. The depth of wells are 60m,150m,300m and 600m. The 600m well had been drilled in 1998. At that time, hydrological and geological research using drilling hole had been done.
By the results of those researches, it is observed that the depth of basement is 573m. There are several obvious aquifers at this point, and pumping tests and water quality analysis had been carried out at 5 aquifers. Ground waters in shallow aquifers are originate in precipitation, and deeper ones are influenced by old sea water.
Groundwater flow in confined aquifers of mainly Tokyo-Saitama alluvial area was assessed by examining horizontal and cross-sectional hydraulic potential distribution, change in head, and taking areal pumping discharge into account. Existence of low-head zone near east and north-east of Saitama, and Chiba Tokatsu area were observed in the confined aquifers with more than 100-m depth, while high-head zones existed around the low-head region. The low-head in the area was the result of groundwater pumping and limited supply from nearby high water potential zone.
The pattern in groundwater head change in the deep confined aquifers from low-head area to Tokyo alluvial area has been classified into three groups: (a) with direct influence of pumping in low-head area, (b) with indirect influence of pumping in low-head area, and (c) with direct influence of local pumping. Pumping regulation has resulted to decreased pumping discharge and increase in groundwater head in all the observation wells.
In the Metropolitan area, it has been pointed out that the rise of the groundwater level after the pumping restraint measured against the land subsidence has an adverse affect on many subsurface infrastructures. In order to solve those problems, it is considered to be essential to clarify the groundwater recharge and/or discharge system in the Kanto Groundwater Basin. In this context, we evaluated budget of the hydrologic cycle in the Tama River Basin and it’s neighboring several small river basins locating at the Southwestern Kanto Plain. In the study, we utilized the spatial analysis capability of GIS to handle the large amount of the hydrological data. As a result, it was confirmed that considerable amount of groundwater is flowing out continuously from the Tama River Basin to the neighboring several small river basins and/or subsurface aquifers. In this paper, we will demonstrate that the water budget study is sufficiently effective to bring out the groundwater flow system over a large area such as the whole Kanto Plain.