深層地下水におけるストラクチャー・コントロール
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概要
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The hydrological data obtained from 800 deep wells bored in the Tertiary strata distributed in Miyagi Prefecture, Northeast Japan, are the basis for interpreting the relationship between the underground water contained in those strata and the geologicl features. The water qualities and especially the chlor-ion content were studied. The chlor-ion content in the underground water observed in the Tertiary formations distributed in Miyagi Prefecture ranges from 10 to 20 ppm. However, a high chlor-ion content ranging from about 50 to 3000 ppm was also found. The high chlor-ion content of 58, 950 ppm was found in the deep well drilled in the Tertiary Tatsunokuchi Formation of Iwanuma-cho, Miyagi Prefecture. Also the lower content ranging from 50 to 3000 ppm was studied. And the relationship between such lower content of chlor-ion and the geological features was investigated. By the geological examinations, it was discovered that the water-bearing strata of these choir-ion-rich wells are all related with the water-bearing strata of marine facies, and that these wells are located at the bottom of synclinal or basin structures. They are also closely related with igneous sediments like agglomerate. From the above it can be said that the surrounding structures of these wells are of enclosed type. According to these data, it was inferred that the chlor-ion content originates in the marine water in which the Tertiary strata were deposited. Namely, this shows that the quality of underground water is restricted and controlled by the sedimentary environment of the water-bearing strata. The data given so far can be summarized as stated below. The fossil water was captured and settled in the stratum, then it began to flow downward, and the chlor-ion content decreased by the contact, mixing and diluting with the secondary circulating fresh water. When the geological structure developed into the open type as homoclinic, anticlinal and so on, the chlor-ion-rich fossil water migrated to the lower part of the geological structure and finally the secondary fresh water replaced the fossil water. But when the geological structure developed to the enclosed type as syncline, basin, structure and when the enclosed structure was formed by igneous activity, the chlor-ion-rich fossil water migrated downward to the lower part of these structures and may have been enclosed in the lowest bottom part of such structures. Due to the development of the geological structure, one fossil water mass separated into more than two parts. Such separation is the differentiation of underground water. When the enclosure occurs in a local area, the fossil water begins to contact gradually with the secondary fresh water from the upper part of the enclosed fossil water mass, leading to the mixing and diluting of each kind of water. Due to the dilution, the quality is considered as the evolution of underground water. As mentioned above, the underground water is not only so closely related to the lithological facies and/or geological structure, but it is influenced, restricted and controlled by the sedimentary environment of the water-bearing formation and geo-historical development of the geological structure. Due to these processes, the underground water becomes subjected to differentiation and evolution. These phenomena are named "the structure-control of the underground water". The fossil water mentioned above is different from the marine water or the so-called brine-water discovered in the oil or natural gas field especially in the chlor-ion content, but it can be considered that the fossil water is a diluted relic of the primary captured marine water.
- 1972-03-10