泉と地下水

A spring is defined as a place where water flows naturally from a rock or soil onto land or into a body of surface water. Springs occur in many forms and have been classified according to cause, rock structure, discharge, temperature, and variability. As to causes, springs are divided into nongravitational and gravitational types. The former includes volcanic and fissuer springs, and such are usually thermal. The latter includes depression spring, contact spring, artesian spring, impervious rock spring, and tubular or fracture spring as illustrated in Figure 1. The large springs are usually associated with permeable aquifers like caveronous limestone, porous basalt, or sorted gravel. The discharge from a spring depends on the area contributing recharge to the aquifer and the rate of recharge, and the relation is expressed graphically in Figure 2.<BR>It is considered that groundwater is a basic source for water supply in the river basin, and springs often serve as useful sources of water. A rapid industrialization and urbanization after the Second World War has brought about many water problems such as the deterioration of hydrological environment. The overdevelopment of groundwater pumping caused a decline in the groundwater table, the drying of wells and springs, and land subsidence in several areas of Japan. Therefore, it is strongly required to maintain the permanent use and conservation of groundwater resources.<BR>In this paper, the author presents some results of case study on springs in the Takamatsu Plain, and attempts to discuss environmental problems relating to conservation of springs and groundwater.<BR>The Takamatsu Plain is located in the northern foot of the Asan Mountains has a large number of irrigation ponds (Figure 3). On the other hand, groundwater is also widely utilized by means of facility named desui, with ditch for collecting and transporting groundwater. In order to clarify the distribution, discharge and water quality of such springs, filed survey was conducted during the period of 1983 to 1988. About one handred of desui systems in the study area are classified into the perennial springs along river and the intermittent springs (Figure 4). During the period of irrigation the groundwater table has risen, and the discharge from desui increases remarkably. Jubging from the observed result of spring discharge as listed in Table 3, the total amount of yield from all springs is roughly estimated to be more than ten millions cubic meters during the irrigation period. Desui is a re-use system for collecting the percolating water from the paddy field as well as groundwater, and it constitutes still a large proportion of irrigation water source in the study area.<BR>The actual condition of desui water use for irrigation and domestic purposes has changed rapidly due to new irrigation water project, the spreading of municipal water supply system, and the water pollution caused by increasing effluence of domestic sewage. A questionnaire survey of 387 communities was made to obtain information on the recent changing process of spring water use for irrigation and domestic purposes. According to the result of questionnaire survey in Tables 4 and 6, about 40 % of rural communities have continuously used desui water for irrigation, and about 30 % for domestic purposes, such as washing of vegetables, clothes and farming tools.<BR>In the urbanized area, the flow and balance of water are significantly modified by changes in land surface condition. Extension of impermeable land surface, such as residential area and roads, causes the decreases of area contributing recharge to the aquifer and of recharge rate. With decreases in area and rate of groundwater recharge, the spring discharge decreases gradually. Conservation of springs is necessary in order to maintain the harmonious relationship between hydrological environment and human activity in the urban and rural areas.