1:45 PM - 3:15 PM
[HTT15-P04] Geochemical variation and long-term fluctuation of major river systems in the Okayama and Tottori Prefectures
Keywords:Okayama Prefecture, Tottori Prefecture, River water
We have undertaken a detailed geochemical and isotopic study of major river systems in the Okayama and Tottori prefectures in order to understand the processes responsible for the geochemical variation and long-term fluctuation of the water quality. More than 900 water samples were collected from >550 locations over the past 12 years. All samples were filtered with 0.2μm acetate cellulose disposable filter to remove the suspended particles prior to the analyses for major dissolved components, trace element concentrations, and O-H-Sr-S isotope ratios. These data were then used to create geochemical and isotopic maps that cover the entire region of the two prefectures.
The high-resolution geochemical maps created in this study show that the concentrations of some elements such as Ca, Si, V, Cr, Rb, and Cs strongly reflect the geochemical nature of rocks exposed in the area. For example, the concentrations of Si, V, Rb, Cs are high in the western Tottori prefecture near Mt. Daisen, where andesitic to rhyolitic rocks are widely exposed. The concentrations of Ca and Cr, on the other hand, are high in regions of western and central Okayama prefecture where limestone and serpentinite, respectively, are exposed.
Our geochemical maps also show that the concentrations of some major dissolved components reflect influence of human activity. For example, the concentration of SO4 is generally low in western to central Tottori prefecture where the population density is low compared to the Okayama prefecture and eastern Tottori prefecture. The δ34S of river water in the densely populated area converges to ~0‰ as SO4 concentration increases, suggesting that an anthropogenic input of SO4 with δ34S of ~0‰ is responsible for the elevation of SO4.
Because samples were collected over a period of ~12 years, we have also evaluated the long-term fluctuation of water geochemistry by analyzing samples collected once every 3 to 6 months at 7 sampling sites of the Asahi and Yoshii rivers. Analyses of samples collected between 2011 and 2022 have revealed that even at the downstream of the Asahi river, where a largest temporal variation was observed, the variation of the major dissolved components and trace element concentrations were smaller than ±30% of the average value. This is significantly smaller than the regional variation which can vary by an order of magnitude. Our long-term monitoring also revealed that in most sampling sites, the geochemical properties show no systematic change over the period of ~12 years. However, an exception to this was found in the midstream of the Yoshii river, where a small but systematic increase in the Ca and SO4 concentrations were observed. Since this change was accompanied by increase in the concentrations of trace elements such as Zn, this may be related to change in the water usage by industrial activities in this area.
The high-resolution geochemical maps created in this study show that the concentrations of some elements such as Ca, Si, V, Cr, Rb, and Cs strongly reflect the geochemical nature of rocks exposed in the area. For example, the concentrations of Si, V, Rb, Cs are high in the western Tottori prefecture near Mt. Daisen, where andesitic to rhyolitic rocks are widely exposed. The concentrations of Ca and Cr, on the other hand, are high in regions of western and central Okayama prefecture where limestone and serpentinite, respectively, are exposed.
Our geochemical maps also show that the concentrations of some major dissolved components reflect influence of human activity. For example, the concentration of SO4 is generally low in western to central Tottori prefecture where the population density is low compared to the Okayama prefecture and eastern Tottori prefecture. The δ34S of river water in the densely populated area converges to ~0‰ as SO4 concentration increases, suggesting that an anthropogenic input of SO4 with δ34S of ~0‰ is responsible for the elevation of SO4.
Because samples were collected over a period of ~12 years, we have also evaluated the long-term fluctuation of water geochemistry by analyzing samples collected once every 3 to 6 months at 7 sampling sites of the Asahi and Yoshii rivers. Analyses of samples collected between 2011 and 2022 have revealed that even at the downstream of the Asahi river, where a largest temporal variation was observed, the variation of the major dissolved components and trace element concentrations were smaller than ±30% of the average value. This is significantly smaller than the regional variation which can vary by an order of magnitude. Our long-term monitoring also revealed that in most sampling sites, the geochemical properties show no systematic change over the period of ~12 years. However, an exception to this was found in the midstream of the Yoshii river, where a small but systematic increase in the Ca and SO4 concentrations were observed. Since this change was accompanied by increase in the concentrations of trace elements such as Zn, this may be related to change in the water usage by industrial activities in this area.