武蔵野台地における深層地下水の動態

Following the groundwater discharge control which was enforced in 1961 in the urban area of Tokyo Metropolis, the Metropolitan Government prohibited, from 1972 on, the drilling of deep wells in the suburban area (Santama district) and gave guidance in reducing the discharge of the existing wells.<BR>As a result, the groundwater level has been showing a gradual recovery. This tendency is not a local one but is recognized throughout the Santama district.<BR>Such a phenomenon cannot be readily judged whether it is merely a temporary state or it indicates a net rise of water level due to supply from rivers or from unconfined groundwater, but it will provide a good opportunity for knowing the behavior of confined groundwater.<BR>The present paper is made up of a part of the results of the investigation in the Musashino district and in the urban area of Tokyo Metropolis. This research was carried out by the request from the Environmental Pollution Bureau of the Metropolitan Government.<BR>In part I of this paper the author described an outline of hydrogeological condition of the Musashino district as a background of the study.<BR>In part II he mentioned the characteristics of annual change of groundwater level due to artificial impact and he showed some interpretations about groundwater recharge mechanism in short term.<BR>In III he clarified large scale recharge mechanism which is induced by long term discharge.<BR>In part W he showed two kinds of piezometric surface contour map based on the geological horizon of aquifer (one is Tokyo group and the other is Kazusa group), and then he clarified that water level difference between the upper and lower parts increases throughout the district with the increase of groundwater utilization.<BR>As the result, he emphasized the following points.<BR>1. In the early stage of groundwater utilization, recharge for confined groundwater proceeds in the form of compensation for the “elastic strain” in the aquifer which is caused by discharge.<BR>2. In the stage where the groundwater utilization is fairly advanced, several processes of recharge ndchanism are considered.<BR>Namely, the recharge in a relatively short period is related chiefly to the vertical movement of flow (in the form of squeeze and leakage), while the recharge in a long period is made in the form of compensation by the flow from the surrounding areas according to hydraulic gradient.