17:15 〜 18:45
[HTT16-P04] Investigation of water quality formation at Ono City, Fukui Prefecture, using geochemical methods
キーワード：大野市、地下水、河川水、水質マップ
The water quality of groundwater and river water varies due to the influence of the surrounding natural environment and human activities. River water originates mainly from groundwater that has percolated underground. Since groundwater discharge into river water and river water infiltration into the underground, within its watershed, both groundwater and river water are closely related in terms of their respective water qualities. Ono City, Fukui prefecture, which is the target site of this study, has been selected as one of the 100 best water towns in Japan. According to Ono City, the city's water supply coverage rate as of March 31, 2023, was only 14.29% (37.52% including small scale public water supply), many households are using groundwater such as well water and spring water. While Ono City’s abundant groundwater resources are supporting the daily lives of residents, in the past, it has suffered from drainage of wells and water pollution. Therefore, it is important to elucidate the origins of groundwater and the factors that are shaping water quality.
Four river basins exist in Ono City, from the east: the Kuzuryu River, Mana River, Kiyotaki River, and Akane River. The urban areas of the city are located inside the Ono basin in the northwest, and all rivers flow from south to north. Earlier studies have focused on the Ono basin and revealed the geochemical characteristics of groundwater and river water there. However, the mountainous areas in the upper reaches, which are the sources of groundwater and river water of the Ono basin, had previously not been investigated. In order to quantitatively evaluate the water circulation processes of the city, it is important to identify the water quality in those areas. Therefore, this study will focus on water samples in upper reach areas, and a detailed geochemical and isotopic study will be conducted based on different forms of water e.g. groundwater, river water, paddy water, and precipitation.
Water and rock samples were collected from 2018 to 2023. Although the analyses for pH, electric conductivity, major dissolved constituents, trace elements, and H-O-S-Sr isotopes were conducted, this study will focus mainly on the Sr isotopes. The Sr isotope ratios of river and groundwater vary in an east-west direction with a measured value ranging from 0.7059 to 0.7167. This variation is wider than the Sr isotopic signature of precipitation in the area which ranges from 0.7089 to 0.7092. The isotope ratios of water and rocks in the vicinity showed a close relationship, implying that the surrounding rocks are an important source of Sr in the river and groundwater. Mass balance calculations in the upper reach areas revealed that 60.9~92.7% of Sr in the recharge area originated from rocks and soils, while the remaining 7.3~39.1% are from precipitation.
Four river basins exist in Ono City, from the east: the Kuzuryu River, Mana River, Kiyotaki River, and Akane River. The urban areas of the city are located inside the Ono basin in the northwest, and all rivers flow from south to north. Earlier studies have focused on the Ono basin and revealed the geochemical characteristics of groundwater and river water there. However, the mountainous areas in the upper reaches, which are the sources of groundwater and river water of the Ono basin, had previously not been investigated. In order to quantitatively evaluate the water circulation processes of the city, it is important to identify the water quality in those areas. Therefore, this study will focus on water samples in upper reach areas, and a detailed geochemical and isotopic study will be conducted based on different forms of water e.g. groundwater, river water, paddy water, and precipitation.
Water and rock samples were collected from 2018 to 2023. Although the analyses for pH, electric conductivity, major dissolved constituents, trace elements, and H-O-S-Sr isotopes were conducted, this study will focus mainly on the Sr isotopes. The Sr isotope ratios of river and groundwater vary in an east-west direction with a measured value ranging from 0.7059 to 0.7167. This variation is wider than the Sr isotopic signature of precipitation in the area which ranges from 0.7089 to 0.7092. The isotope ratios of water and rocks in the vicinity showed a close relationship, implying that the surrounding rocks are an important source of Sr in the river and groundwater. Mass balance calculations in the upper reach areas revealed that 60.9~92.7% of Sr in the recharge area originated from rocks and soils, while the remaining 7.3~39.1% are from precipitation.