Field observation of water quality and investigation of the amount of groundwater use were carried out in the estuary of Naka River, Tokushima Prefecture, Japan. This region has an ongoing salinity problem of the confined groundwater. By using numerical analysis, the reproduction of the present state and prediction of future salinity were made. Though in this region the groundwater is utilized as drinking water, industrial water, fishery product water and agricultural water, the greater part is utilized as industrial water and fishery product water. The amount used for drinking water is 10-20%, and very little is used for agriculture. Most industrial water originates in river-bed water. In the past, the main factor in the salinity of the confined groundwater in this region was superfluous pumping for fishery product water. As the number of culture farms has decreased recently, if the current amount of pumping is maintained, the intrusion of salt water further inland is not expected to occur. However, presently there is a condition in which salt water has intruded about 6 km inland from the coast into a confined aquifer on the right bank of the Naka River. Moreover, in the analysis on the assumption of sea level rise with climatic change as a future prediction, salt water will intrude further inland than in the present state.
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.