This paper deals with the problem of the groundwater pollution caused by both the seawater intrusion into phreatic and confined aquifers and the fertilizer dissolution into a phreatic aquifer in Gogo Island, Ehime Prefecture.
Groundwater levels, chloride ion concentrations and electrical conductivities in shallow wells were measured at five districts in this island during a period from August 1983 to December 1984. At Yura district, groundwater levels and chloride ion concentrations in deep wells as well as tide levels were measured.
The main results are as follows: 1) Chloride ion concentration profiles show the rapid increase of the concentration in the neighborhood of the deep well bottom.2) Using a modified Kishi & Inouchi's model, the variation of the groundwater level and fresh water-salt water interface in response to the tide was analysed, which is applicable to the confined aquifer with an exponentially oaring thickness. It was found that the thickness of the confined aquifer estimated from this analysis is in fairly good agreement with the one obtained by boring examination.3) Very high chloride ion concentrations far greater than 200 mg⋅ 1^-1, the standard value of city water, were found in coastal regions at Yura district.4) The concentration of total ions estimated by the electrical conductivity of groundwater minus the chloride ion- concentration increases at one to two months after fertilization.

抄録全体を表示

The purpose of this survey is to determine the chemical quality of the groundwaters in the Ooyanojima Island and its surrounding islands (Iwajima, Tobasejima, Yushima, and Nokamajima Island). These islands, a small island on the Ariake Sea and Yatsushiro Sea, is situated about fifty kilometers south west of Kumamoto City, Kumamoto Prefecture.
In this article,46 groundwater samples (38 shallow well waters and other samples) in this area were collected in September 1977 (partly during 1979 to 1983) and analyzed for the pH value, potassium permanganate consumed, total residue, major components and some ions dissolved in them.
Based on the analytical results of many dissolved components, it was shown that the concentrations of the major components were widely varied in each well (table 1). There were considerably a large amount of contents of Na⁺, Ca²⁺, HCO₃^-, Cl^-, SO₄^2-, and soluble silicate in the groundwaters in this area. The relations of the concentration of cations and anions in the groundwaters were Na⁺>Ca²⁺>Mg²⁺>K⁺and HCO₃^->C1^->SO₄^2-, respectively.
By use of the key diagram method, the chemical composition of the groundwaters in this area was mainly devided into two different composition in each (fig.2), namely noncarbonate hardness type (Ca-Cl⋅SO₄type) and carbonate hardness type (Ca⋅HCO₃type).
The concentrations of chemical components dissolved in the shallow well waters were generally larger than that of dissolved in the deep well waters, except for total iron and soluble silicate. The concentrations of nitrate nitrogen in the shallow well waters in this area were found to be in the range of 0.2∼39.1mg/1. Their arithmetic mean value (7.0mg/1) was markedly high and became about 10 times larger than that in the deep well waters, however, the source of that was not made to elucidate for this investigation. The concentration of soluble silicate in the groundwaters collected from the Cretaceos period strata was lower than that of the other strata; the Diluvium, Paleogene group, Pyroxene andesite and Basalt.

抄録全体を表示

The problem of the seawater intrusion into confined coastal aquifers is analysed with the use of two different models; a fresh water-salt water interface model and a hydrodynamic dispersion model. First, the outline is given of the fresh water-salt water interface model, describing the unsteady motion of the groundwater level and the interface through approximate equations, and of the three-dimensional hydrodynamic dispersion model, including the effect of pumping of groundwater. These models are then applied to the confined groundwater in the estuaries of the Naka and Kiki Rivers in Japan and phenomena of the seawater intrusion in both estuaries are simulated rather well by these two models. Finally, the two-dimensional model computations are presented in the vertical cross section of aquifer on the salt concentration distribution and velocity profile based on the dispersion model with both constant and velocity-dependent dispersion coefficients.

抄録全体を表示

We discussed basic characteristics on the groundwater bearing conditions of the Dejima area in our first report. The present report is concerned with the result of subsequent detailed research on groundwater flow systems in the experimental basin.
Table 1 summarizes the major arguments of our study. At first, we divided the groundwater flow systems of this area into three category from their spacial scale. That is to say, the partial scale, the local scale and the regional scale.
The dominating factors on the groundwater flow systems are different according to the above mentioned scale. In the case of the small scale, the groundwater phenomena, for instance“ concentrated flow of groundwater”, are more intensively affected by the geological factors rather than topographic conditions.
On the other hand in the case of the regional flow system of which scale covers one drainage basin, the topographic conditions come up as the dominating factor for groundwater flow systems.
Next, in this paper, we emphasized the human activities on groundwater. Especially, irrigation to paddy fields becomes so large that it can no more be disregarded as compared to natural factors.
The examples of such case are given below.
1) In the paddy fields which are irrigated in summer season, the groundwater level rises about 2-4m compare with the other areas.
2) Ephemeral groundwater mounds or artificial groundwater mounds appear in the paddy areas within summer season.
3) The direction and velosity of groundwater flow vary with the above mentioned conditions.

抄録全体を表示

Groundwater behavior is generally complex because of various influences of many factors which are not only natural phenomena but also artificial impacts. It is necessary for flow estimation of regional groundwater to clarify the relation between various scales of groundwater behavior and its governing factors.
In the present paper, characteristics of unconfined groundwater behavior recharged by rainfall infiltration are discussed. Field observation was carried out in the KOMAKI hill region. In order to investigate the effect of unsaturated zone to variation of groundwater level, physical characteristics of soil water were examined for soil samples.
Through this paper, it has been clarified that variation of the groundwater level in the hill region are consist of the behaviors with two different scales in time and space. The behavior during and just after rainfall is mainly governed by inhomogeneity of the geological characteristics in the region, and the behavior during long-term recession state by hydrogeological structure of aquifer. The phenomenological discussions on mechanism of vertical recharge to unconfined aquifer are also presented.

抄録全体を表示

Two numerical models for predicting production behavior of the water-dissolved gas field are developed. One model is constructed for the ordinary type gas reservoir that is composed of sandstone layer and the other for the Mobara type gas reservoir that is composed of the alternation of sandstone and mudstone. These models consist of the mathematical description of the two-phase (water and methane gas) flow in a horizontal, twodimensional system. In particular, the description of the Mobara type model considers the gas excudation at the face of the mudstone layer.
These models are used to discuss the problem of interference between two production wells. The result shows that the well interference causes a decrease in production gaswater ratio in the ordinary type, but a remarkable increase in the Mobara type.
An application of the Mobara type model to the practical water-dissolved gas field has revealed that the calculated value of average production gas-water ratio of all the wells is in a general agreement with the measured value.

抄録全体を表示