As is well known, main elements of groundwater recharge include rainfall, irrigation water to paddy field and river water. Among them, the former two elements play as a spatial source, whereas the latter one is a linear source.
The purpose of the present paper is to make clear the influence of river water on groundwater about water quality and quantity in the Kurobe Alluvial Fan. Observations were conducted three times from February 2003 to August 2003, and 47 observation wells were established in order to ascertain the effect of Kurobe river on groundwater. In order to delineate a relationship between groundwater and river water, the investigation was conducted in river discharge, groundwater level, water temperature and quality (pH, electric conductivity, chemical composition) as a tracer.
Judging from groundwater table maps in August, October and December, 2002, it was pointed out that groundwater has been recharged by river water in the middle part of the fan. As the result of observation in winter, it is pointed out that the temperature of groundwater has a tendency to rise with increasing distance from river channel in the right bank of the Kurobe River. Judging from the fact that the temperature of river water is quite lower than that of groundwater in winter season, such a peculiar distribution suggests a supply from river water to groundwater. Furthermore, a result of measurements on pH, electric conductivity and chemical composition supports the hypothesis as mentioned above. In the lower reaches of the Kurobe river, the amount of seepage is especially large.
The analysis of groundwater level was performed in order to make clear the relation between the Kurobe river and groundwater level at three wells where distance from the Kurobe river is different. In the Kurobe alluvial fan, groundwater level increases from April to May every year. From the result of this analysis at the observation well near the Kurobe river in the middle part of a fan, it was calculated that average contribution ratio of the Kurobe river to the rise of ground water level at irrigation period of five times was 73.9%. In that area, it was presumed that river water is the main source for the contribution to the rise of groundwater level. Moreover, it was clarified also that the influence of the Kurobe river reaches even groundwater that distance of groundwater flow line from Kurobe river is about 7200 meters.
It is concluded that river water plays an important role in the recharge of groundwater in the Kurobe alluvial fan.
