15:30 〜 15:45
[HTT18-07] 岡山県・鳥取県一級水系の水質と長期変動
キーワード：岡山県、河川水、主要溶存成分、微量元素、同位体
In order to quantitatively evaluate the effects of various factors that control the geochemical properties of river water such as water-rock interactions and human activities, we have undertaken a detailed geochemical and isotopic study of major river systems of the Okayama and Tottori prefectures Japan. Over the past ~11 years, we have collected more than 850 samples from 545 locations. In some locations, samples were collected periodically to monitor the temporal variation of the water geochemistry. All samples were filtered with acetate cellulose disposable filter to remove the suspended particles prior to the geochemical and isotopic analyses. Samples were measured for major dissolved components, trace element concentrations and O-H-S-Sr isotopes. These data were used to construct high-resolution geochemical maps of the two prefectures.
The concentrations of elements such as Li, Si, V, Rb, and Cs are particularly high in the Daisen and Hiruzen areas of the Tottori Prefecture, where rhyolitic to quartz andesitic volcanic rocks are distributed. The concentration of Ca, on the other hand, is less than 10 ppm in most areas of the two prefectures but exceeds 20 ppm in areas where limestones are widely exposed. These observations suggest that the concentrations of these elements are strongly influenced by the chemical composition of the rocks exposed in the area.
In Okayama and eastern Tottori prefecture, the sulfur isotopic signature (δ34S) is correlated with the SO4concentration, and the δ34S converges to ~0‰ as the concentration of SO4 increases. This may imply that the breakdown of fertilizer with δ34S close to 0‰ may be an important factor that controls the SO4 concentration in the river water.
The long-term monitoring of water geochemistry has revealed a gradual increase in the Ca and SO4 concentrations at the downstream of the Yoshii River. This, however, was not accompanied by increase in the Cl and NO3 concentrations. Increase in the Ca and SO4 concentrations may be related to the use of fertilizer such as calcium sulfate, but further analyses of δ34S and 87Sr/86Sr is necessary to clarify this issue.
In addition to water samples, the 87Sr/86Sr ratio of fine-grained river sediments was determined. Three step acid dissolution (10% acetic acid, 6N HCl, and HF+HNO3) of the sediments showed that Sr adsorbed on the surface of the sediment has isotopic composition similar to the river water. The isotopic signature of oxides and silicates, on the other hand, has 87Sr/86Sr ratio similar if not higher than the river water from the same site. This may imply that Sr released into water is strongly controlled by the weathering of plagioclase which typically has low 87Sr/86Sr ratio compared to bulk rock.
The concentrations of elements such as Li, Si, V, Rb, and Cs are particularly high in the Daisen and Hiruzen areas of the Tottori Prefecture, where rhyolitic to quartz andesitic volcanic rocks are distributed. The concentration of Ca, on the other hand, is less than 10 ppm in most areas of the two prefectures but exceeds 20 ppm in areas where limestones are widely exposed. These observations suggest that the concentrations of these elements are strongly influenced by the chemical composition of the rocks exposed in the area.
In Okayama and eastern Tottori prefecture, the sulfur isotopic signature (δ34S) is correlated with the SO4concentration, and the δ34S converges to ~0‰ as the concentration of SO4 increases. This may imply that the breakdown of fertilizer with δ34S close to 0‰ may be an important factor that controls the SO4 concentration in the river water.
The long-term monitoring of water geochemistry has revealed a gradual increase in the Ca and SO4 concentrations at the downstream of the Yoshii River. This, however, was not accompanied by increase in the Cl and NO3 concentrations. Increase in the Ca and SO4 concentrations may be related to the use of fertilizer such as calcium sulfate, but further analyses of δ34S and 87Sr/86Sr is necessary to clarify this issue.
In addition to water samples, the 87Sr/86Sr ratio of fine-grained river sediments was determined. Three step acid dissolution (10% acetic acid, 6N HCl, and HF+HNO3) of the sediments showed that Sr adsorbed on the surface of the sediment has isotopic composition similar to the river water. The isotopic signature of oxides and silicates, on the other hand, has 87Sr/86Sr ratio similar if not higher than the river water from the same site. This may imply that Sr released into water is strongly controlled by the weathering of plagioclase which typically has low 87Sr/86Sr ratio compared to bulk rock.