10:15 〜 10:30
[HTT16-06] Geochemical investigation of water quality formation at Ono City, Fukui Prefecture
キーワード：大野市、地下水、河川水、地球化学
Ono City, Fukui prefecture has been selected as one of the 100 best water towns in Japan. However, the city's water supply coverage rate as of March 2023 is only about 15% (37.5% including small scale public water supply). Many households rely on groundwater such as well water and spring water, thus making it important to elucidate the origin of groundwater and the factors that are shaping the water quality.
Since 2018, our group has been investigating the geochemical characteristics of the groundwater and river water from Ono city. More than 200 water samples from 180 sites were collected during the course of this study. The pH, electric conductivity, major dissolved constituents, trace element concentrations, and H-O-S-Sr isotopic signatures of the water samples were determined. Using these data, we have successfully constructed a high-resolution geochemical map of the surface water and have shown that the geochemical properties of the river and groundwater is geographically variable. For example, the Sr isotope ratio of the river and groundwater varies in an east-west direction, ranging from 0.7059 to 0.7167. This variation exceeds the isotopic variation of precipitation in the area which ranges from 0.7089 to 0.7092. We have also demonstrated that the Sr isotopic signature of water is close to that of rocks exposed in the vicinity of the sampling site. These observations suggest that the major source of Sr in the water is rocks exposed in the catchment area. We have further extended our study and determined the Sr isotopic signature of plants from 15 sampling sites with distinct Sr isotopic signature. The Sr isotopic signature of the plants showed strong correlation with the water from the sampling site, indicating that the major source of Sr in the plants is the river water.
The concentrations of Cl, NO3 and P are high in the densely populated northwest region of the city, implying that agricultural and other human activities are likely responsible for the elevated signatures. The sulfur isotopic signature also shows geographical variation. The δ34S tends to converge to 0 ‰ as the concentration of SO4 increases. Since the δ34S of fertilizers used in the area ranges from -5.9 to +2.4‰, the correlation between the SO4 concentration and δ34S may be related to the use of fertilizers in the agricultural activities (e.g. Nakano et al. 2008).
We have also assessed long-term fluctuation of water geochemistry by analyzing samples collected several times during 2018-2024 from 28 selected sampling sites. Our data show that the concentrations of most major dissolved constituents and trace elements show small variations. However, elevation in Ca concentration was apparent along the Kuzuryu river during 2020-2021. This coincides with the timing of major highway construction in the area and therefore the two may be related.
Since 2018, our group has been investigating the geochemical characteristics of the groundwater and river water from Ono city. More than 200 water samples from 180 sites were collected during the course of this study. The pH, electric conductivity, major dissolved constituents, trace element concentrations, and H-O-S-Sr isotopic signatures of the water samples were determined. Using these data, we have successfully constructed a high-resolution geochemical map of the surface water and have shown that the geochemical properties of the river and groundwater is geographically variable. For example, the Sr isotope ratio of the river and groundwater varies in an east-west direction, ranging from 0.7059 to 0.7167. This variation exceeds the isotopic variation of precipitation in the area which ranges from 0.7089 to 0.7092. We have also demonstrated that the Sr isotopic signature of water is close to that of rocks exposed in the vicinity of the sampling site. These observations suggest that the major source of Sr in the water is rocks exposed in the catchment area. We have further extended our study and determined the Sr isotopic signature of plants from 15 sampling sites with distinct Sr isotopic signature. The Sr isotopic signature of the plants showed strong correlation with the water from the sampling site, indicating that the major source of Sr in the plants is the river water.
The concentrations of Cl, NO3 and P are high in the densely populated northwest region of the city, implying that agricultural and other human activities are likely responsible for the elevated signatures. The sulfur isotopic signature also shows geographical variation. The δ34S tends to converge to 0 ‰ as the concentration of SO4 increases. Since the δ34S of fertilizers used in the area ranges from -5.9 to +2.4‰, the correlation between the SO4 concentration and δ34S may be related to the use of fertilizers in the agricultural activities (e.g. Nakano et al. 2008).
We have also assessed long-term fluctuation of water geochemistry by analyzing samples collected several times during 2018-2024 from 28 selected sampling sites. Our data show that the concentrations of most major dissolved constituents and trace elements show small variations. However, elevation in Ca concentration was apparent along the Kuzuryu river during 2020-2021. This coincides with the timing of major highway construction in the area and therefore the two may be related.