MECHANISM OF UNCONFINED GROUND WATER RECHARGE AT HAKUSAN ON THE KONGÔ TERRACE, TÔKYÔ

Abstract

In this paper the author analyzes the mechanism of natural recharge of unconfined ground water at Hakusan on the Hongô terrace in Tôkyô putting emphasis on water balance. Special attention is paid to variations in soil moisture, whose amount has not exactly been observed though the important role of it in the hydrologic cycle is widely accepted. As a result of this study some characteristics are found concerning the mechanism of unconfined ground water recharge in urban area and redistribution of soil moisture in the Kantô Loam formations. The Hongô terrace, whose top is about 25m above sea level, is a Pleistocene upland (M 2), and is located in the eastern margin of the Musashino upland. This terrace consists of the volcanic ash called Kantô Loam (6 to 7m thick), Yamanote sand and gravel (4 to 5m thick, but its upper part of 1 to 2m in thickness is clay), and the sand stratum of Upper Tôkyô formation (more than 5m thick) in the descending order. The main uncon-fined aquifers of ground water studied are the Yamanote gravel bed and the sand stratum of Upper Tôkyô formation. Annual variation of ground water level in the aquifer is within the range of 6m to 12m below the surface. The studied area is located in an ordinary residen-tial area and the experimental garden (Tôkyô University Botanical Garden) is nearby. From April in 1962 to March in 1968 ground water level in the unconfined aquifer was measured daily in an observation well at Hakusan. At that place, the fluctuations of ground water level is affected by rainfall. Relation between rainfall and ground water level was in-vestigated, and the rise and fall of round water level in one period was found to be deter-mined by the total rainfall bserved in a 6-month period before that time. Contribution of the rainfall about 1 to 2 months before is the most prominent. However, the individual rises of ground water level correspond to individual rainfalls observed in the preceding 10 days. At Hakusan, hydrologic elements such as rainfall, fluctuations of soil moisture, the outflow of ground water and artificial recharge to the main aquifer were observed for one year from April in 1967 to March in 1968. Water balance analysis was applied and the relationship between the annual march of these water balance factors and that of ground water level was investigated.As for the annual total of water balance factors, about 1000mm of annual rainfall can be divided into following entries : the surface runoff is about 210mm, the amount of evapo-transpiration about 500mm, effective rainfall (amount of rainfall that is available for ground water recharge) for ground water recharge 290mm, of which 280mm is actually added to ground water storage. In addition to this, a 410mm of artificial recharge from leakage of waterworks pipelines is added and the total ground water recharge to the aquifer is 690 mm. The discharge from ground water reservoir is 670mm. It is worthy of note that unconfined ground water in urban area is balanced by a large quantity of artificial recharge (leakage from pipelines). The amount of evapotranspiration at Hakusan is about 50 to 60 percent of its average value in the Kanto Plain, which was calculated by the method of heat budget. Soil moisture measurement from the surface to the depth of 15m was made by a neutron soil moisture meter. The seasonal variation of soil moisture is large in a layer from the surface to 8m (Kantô Loam formation and Yamanote Clay bed), and the difference between the maximum in summer and the minimum in spring is more than 100mm. It is recognized that soil moisture changes greatly and it moves upward in the lower part of Kanto Loam formation, in which little variation of soil moisture was believed to exist, though existence of slow downward movement of soil moisture was reported.