1999 Volume 29 Issue 2 Pages 81-92
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.
