Abstract
The Rokugo alluvial fan in Akita Prefecture has abundant groundwater that is discharged from a belt of springs situated at its toe. In order to examine the groundwater flow system in the fan, a monthly water sampling and chemical analyses program was conducted for the spring and river water around the fan covering the period between irrigation and non-irrigation.
A seasonal variability of isotopic ratios was not observed in either the river or spring water samples. However, a seasonal variability was seen in the chemistry of the river water. The results of present and previous studies show that the groundwater discharged in the belt of the springs at the toe of the fan is recharged by rainwater, irrigation water, and underflow water from the apex of fan. From observations of water chemistry and isotopic ratios in spring water samples, these studies also suggest that the multiple sources of recharge water are mixing within the fan. Additionally, it is suggested that this groundwater flow system is controlled by the differences in grain size in the fan deposit.
The groundwater flow system for three springs located at the edge of the fan is found to be different from that of others, which suggests that these specific springs might be affected by recharge from bedrock groundwater.
