人工的構造物による琉球石灰岩地域の地下水収支の変化--那覇市首里の琉球石灰岩ブロックを例として

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Pleistocene Ryukyu Limestone, unconformably over lying the Miocene to Lower Pleistocene Shimajiri Group, forms primary aquifers in Ryukyu Islands. The groundwater reserved in Ryukyu Limestone has been often investigated both in Miyako-jima and southeastern part of Okinawa-jima with a part of the projects for agricultural water supply. In those areas, it is clarified that the distribution of groundwater in Ryukyu Limestone is controlled by the erosion surface structure of impermeable basement consisting mainly of the Shimajiri Group. And the groundwater balance is nearly explained based on the following major three factors; the ratio of direct runoff, groundwater recharge and evapotranspiration to the rainfall of these areas, respectively.While many hydrogeological studies on Ryukyu Limestone have been carried out in agricultural areas, they were not done in such urbanized area as Naha City. Generally speaking, the ground surface of the urbanized area is broadly covered with artificial constructions (such as buildings, asphalt roads etc.), and so it is expected that the recharge system in such area is different from that in agricultural areas.The aim of this study is to clarify the groundwater balance in Shuri Area, one of the type localities of Ryukyu Limestone in Naha City.From the results of electric soundings, pumping tests and long-term observations of water level fluctuation carried out in Shuri Limestone Block, the hydrogeology in this area is summarized as follows;(1) The groundwater basins in Shuri Area correspond to buried valley formed on the erosion surface of Shimajiri Group, as same as in the other areas, Miyako-jima and southeastern part of Okinawa-jima. The major axes of the basins in Shuri Area show nearly NW-SE direction.(2)The flow direction of the groundwater reserved in Ryukyu Limestone is parallel to the inclination of the erosion surface structure of impermeable basement, Shimajiri Group.(3)The analysis on some data from pumping tests indicates that the Ryukyu Limestone in Shuri Area has average permeability coeffident of 5.6 x 10^<-3> cm/sec, and the effective porosity, represented by coefficient of rainfall and penetration, indicates the value of 0.1.(4)The ratio of direct runoff to the rainfall obtained in the Gibo Groundwater Basin shows high percentage of 42.As mentioned above, there are no remarkable differences between Shuri and agricultural areas with regard to the erosion surface structure of impermeable basement and aquifer properties indicated by permeability coefficient and effective porosity. Furthermore, it is also known that the annual rainfall and evapotraspiration in Shuri Area are nearly equal to those in Miyako-jima and southeastern part of Okinawa-jima. On the contrary, however the present study shows that the ratio of direct runoff to the rainfall obtained in Shuri Area indicates high percentage of 42 as compaired with 3% obtained in agricultural area, such as Miyako-jima.Considering the similalities in the erosion surface structure of basement, aquifer properties and climatic conditions between Shuri and Miyako-jima, it is suggested that direct runoff in Shuri Area would be increased by the constructions broadly covering ground surface throughout the area, and that the groundwater recharge would be decreased in Shuri Area, according to increasing direct runoff.
琉球大学理学部の論文
1989-03-00

著者

黒田登美雄
琉球大学農学部

人工的構造物による琉球石灰岩地域の地下水収支の変化--那覇市首里の琉球石灰岩ブロックを例として

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