The Nobi plain, 1300km2 in area, comprises thick Cenozoic and Neogen strata, and it makes one of the biggest artesian basin in Japan. A tremendous volume of groundwater is confined to the basin. Artesian wells in the plain have been extensively drilled. After the World War II, particularly, the large-scale exploitation of the groundwater for industries has inevitably lowered the water level in the artesian basin, giving rise to land subsidence; the most serious subsidence occurred around 1973. According to the regulation for the use of groundwater, the water level has been rising, although the subsided ground did not recover. This study is intended to describe the evolution of groundwater-qualities during the serious fluctuation of water level, in comparison with data obtained before the large-scale exploitation. A noticeable feature of the water quality during the period of lower water level is increase of chloride and sulfate ions. This feature can be attributed to vertical permeation of surface waters. Sulfur isotopic data show that, in an extreme case, atmospheric oxygen infiltrated into strata of marine clay where the oxygen oxidized marine sulfides in the strata; as a result, sulfate ion conspicuously appeared in the groundwater. The vertical filtration enhanced the concentration of chloride stemmed from human activities on the earth.
In the Hokusei area (the northern district of Mie Prefecture), a number of artesian wells dug by Kazusa-bori method have been used since the beginning of the Syowa era. The purpose of this study is to evaluate the flow system of artesian groundwater of this area from three-dimensional characteristics of water quality. According to the depth of the artesian wells, it is found that the major aquifer of artesian groundwater shows so-called basin-shaped.
To assemble the information regarding chemical evolution of artesian groundwater quality, the chemical evolutionary index was obtained by a principal component analysis.
Using the chemical evolutionary index and the contour map of the major aquifer, the flow system of artesian groundwater is characterized as follows : Artesian groundwater is recharged in the vicinity of the Tertiary hills of this area, and flows in the direction to the center of basin-shaped major aquifer of artesian groundwater and also to the Kuwana anticline.
Time variations of chemical and 222Rn concentrations and discharge of groundwater from a flowing well were investigated from 1984 to 1989 to clarify principally the hydrogeological characteristics of a confined groundwater in an alluvial fan at the western shore of Lake Biwa. Chemical concentrations of the flowing well waters show no seasonal and secular variations. 222Rn concentrations of the flowing water are 20～30% higher than those of spring water around the well. 222Rn concentrations of both the waters generally show periodical change accompanied by a distinct short-term fluctuations after the heavy rainfall. The level of high 222Rn concentrations of each water shows nearly constant ; 1600～1700pCi/l for flowing well water and 1300～1400pCi/l for spring water, through the year. There is week correlation between 222Rn concentrations and discharge of flowing well water. Those high levels seem to be a steady state 222Rn concentrations in each aquifer when the infiltrating recharge is minimum.
Discharge of the flowing well shows an apparent seasonal change having with its high level from April to August and low from September to January. There is a high correlation between discharge and the former 20-days precipitations.
These results indicate that the groundwater in the fan is recharged by infiltration of the precipitation, and also a peculiar 222Rn concentrations in the shallow and deep groundwaters seem to be influenced strongly by the infiltrating recharge.
The Ashigara Plain is divided into three areas based on its topography: the upper area consists of the alluvial fans of the Sakawa and Kawaoto Rivers, the intermediate area in the alluvial plain characterized by a number of springs and flowing wells and the lower area in the coastal plain. The groundwater system of these areas can be explained by Tolman’s classification of the fan. The upper zone of the plain is a recharge area. The groundwater system composed of one thick aquifer is mainly recharged by both continuous influent flow from the Sakawa and Kawaoto Rivers and summer irrigation water for paddy fields. These effects on the groundwater are recognized by tracing groundwater temperature and seasonal fluctuations of groundwater level. Tracing cold and warm water movements in the aquifer, the trace is closely accord with the groundwater flow system estimated by the distribution of groundwater heads. The groundwater system along the intermediate and lower zones, which is hydrogeologically composed of two gravel aquifers and one silty aquitard, is divided into a shallow and a deep aquifer system. These groundwaters are mostly supplied by subsurface flow from the upper zone of the plain and the area surrounding the Hakone volcano. Groundwater in the deep aquifer, which is confined by the silty aquitard, flows out of artesian wells. Their distribution narrowed in the intermediate and lower zones due to increase in groundwater pumping rates during the 1960s. Artesian wells and their recharge system are well explained by the profile of groundwater heads, and the distribution of tritium and δ18O. It is clarified that the small tritium content below 5TU and the low temperature below 16℃ in the deep aquifer of the intermediate and lower zones indicate that groundwater recharge is derived from the Hakone volcano.
The area of flowing well is widely recognized in the Nobi Plain, and flowing groundwater has been developed with the improvement of technology for drilling. As modern industry has become prosperous, a large quantity of groundwater has been utilized mainly for textile and chemical industries.
In the early 1930s, the artesian flowing of shallow wells has stopped its characteristic phenomena, and those areas have been enlarged. During the high economic growth period in 1960s, the artesian flowing of deep wells with a high pressure head also stopped in some areas of the Plain. As the result of reduction in the discharge spring in the alluvial fan called “Gama”, the number of local fresh water fish “Hariyo” living in the spring has also decreased.
It is pointed out from the present study that the artesian flowing well is recognized in the following four areas of the Nobi Plain : (1) North-eastern Ogaki City and eastern Ogaki City, (2) the terminal part of the Ibi River and the Kasu River alluvial fan in the northern Ogaki City, (3) the terminal part of the Ai River alluvial fan in the western Ogaki City and the south-eastern Tarui Town, and (4) the bank of the Makida River in the southern Ogaki City and Yoro Town. Because of the decrease in withdrawal of groundwater by the recent regulation of groundwater use, the area of artesian flowing shows little change as compared with previous conditions.