Groundwater flow system estimated by stable isotopes of oxygen and hydrogen in an alluvial fan of the Echi River, Japan

摘要

The groundwater flow systems are variously influenced by geological structure of alluvial fans. In the alluvial fans area, dynamic exchange between surface water and groundwater is observed in investigations of infiltrations and springs. The groundwater environment is also affected by groundwater use and artificial land alteration in accordance with these water circulation characteristics of alluvial fans. In recent years, the gravels for building materials have been collected from the Echi River bed and waste material and sludge buried in the sites left from this digging. On landsides in the Echi River alluvial fan, the groundwater is efficiently utilized as sources of tap water and agricultural water, making environmental contaminations of groundwater a great concern. Understanding the groundwater flow system in the alluvial fans is an important task in understanding the effects of waste materials and contaminants on groundwater quality and to develop antipollution measures. To understand the groundwater flow systems in alluvial fan regions, stable isotopes of hydrogen (D) and oxygen (~(18)O) are the effective tracers and can be used to analyze the source of groundwater (Mizutani et al. 2001, Wakui & Yamanaka 2006). Horino et al. (1995) and the Kinki Regional Agricultural Administration Office (1979) reported water quality and flow systems of the groundwater in alluvial fan regions of the Echi River. However, they investigated the groundwater flow systems in the middle and/or lower part of the fan, excluding the upper part of the area important to understanding the groundwater flow system in the alluvial fan. The aim of our study was to clarify the groundwater flow system in the Echi River alluvial fan including the areas of upper, middle, and lower parts, from results of oxygen and hydrogen isotopes analyses and water level observations of groundwaters. We describe the flow routes and sources of shallow groundwater.