ABSTRACT
ABSTRACT: To understand the groundwater flow system in the alluvial fan of the River Echigawa (-gawa means river, here after Echi-gawa or the River), we conducted a hydrogeological survey with emphasis on the observations of the groundwater level, and sampling of groundwater and surface water for the analysis of water quality and stable and radioactive isotopes. Groundwater in the shallow aquifer originates from precipitation and irrigation water (river water or abstracted groundwater) which penetrates into the deeper layers and, except in the area at the centre of the fan on the left bank of the Echi-gawa where there is an area of flowing artesian conditions, generally flows in a north-west direction. Shallow groundwater also discharges into the river in the area around the apex of the fan and is recharged by river water in the area around the edge of the fan on the left bank, and flows in two directions near the residual hill in the downstream area. Groundwater in the shallow and intermediate aquifers in the fan actively circulates and mixes with water flowing vertically or laterally in the groundwater system. On the other hand, groundwater in the deep aquifer is recharged by groundwater around the intermountain area and flows towards the downstream area at an extremely low speed and is almost static. It is also noted that excessive groundwater abstraction from deep wells greatly affects the water level in the deep aquifer in the alluvial fan of the Echi-gawa.
Keywords: Alluvial fan, groundwater flow, water quality, stable and radioactive isotopes
