2:45 PM - 3:00 PM
[HTT18-05] Fluvial transport of suspended solids from the Hayakawa and the Amehata Rivers to the Fuji River estimated by Sr isotopes
Keywords:Sr isotopes, suspended solids, Fuji River, rare earth elements, turbidity
The Fuji River is one of the steepest rivers in Japan, which originates from high mountainous area of Nagano and Yamanashi prefectures and flows into the Suruga Bay of Shizuoka prefecture. Because the turbidity of the Fuji River has recently been increased, it was pointed out that the increased suspended solids (SS) as a major cause of both the habitat degradation of aquatic organisms such as sweetfish in the middle and lower reaches of the Fuji River and a continuous decline of fishery catch of Sakura-Ebi shrimp (spotted shrimp) off the Suruga Bay. Although strong turbidity is often observed in the Hayakawa River, a major tributary of the Fuji River system, various possibilities have been pointed out as sources of the turbid water, such as landslides in tectonic areas, sediment leakage from a hydroelectric dam (i.e., Amehata Dam in the Amehata River of the Hayakawa River tributary basin), and industrial waste soil illegally dumped in the basin. However, it has not been clarified yet. The purpose of this study, therefore, is to identify (1) the source of the turbidity observed in the Fuji River and (2) the spatial extent of downstream transport of SS derived from each possible source.
The field survey was conducted at up to 26 sampling points in the middle and lower reaches of the Fuji River system (including the Hayakawa and the Amehata Rivers). The sampling was made irregularly once or twice a month in 2021. We measured Sr isotope ratio (87Sr/86Sr), Sr concentration, and rare earth elements (REE) of each river water, SS, and fluvial sediments (<1mm) collected at each sampling point. The results showed that the 87Sr/86Sr of river water was distinct among the major river systems. The average 87Sr/86Sr values of river water were 0.7098, 0.7088, and 0.7058 in the Amehata River, the Hayakawa River, and the Fuji River (before the confluence of the Hayakawa River), respectively, and those values were almost consistent during the study period. On the 87Sr/86Sr-1/Sr concentration diagram, all river water samples were distributed between the Amehata River and the Fuji River. The Sr isotope mass-balance model revealed that the river water derived from the Hayakawa River system contributed to approximately 15% of the Fuji River water during low-flow conditions. In contrast, the contribution of the Hayakawa River water increased to 24-61% during high-flow conditions, although the value became the lowest with 5% on July 2nd when the river level was the highest due to severe flooding. The proportions of the Amehata River water to the Hayakawa River were estimated to be approximately <25% and >40% during low-flow and high-flow conditions, respectively.
The 87Sr/86Sr of riverine SS also showed distinct values among the Amehata River (0.7148), the Hayakawa River (0.7098), and the Fuji River (0.7063). On the 87Sr/86Sr-1/Sr concentration diagram, all samples of the riverine SS were distributed between the Amehata and the Fuji Rivers SS values (before the confluence). The Sr isotopic mixing model showed that during high water level conditions, the proportion of SS derived from the Hayakawa River in the SS of the Fuji River was estimated to be high (56-65%). Moreover, the contribution of SS derived from the Amehata River to the Hayakawa River SS was also high (51-59%). On the other hand, the transport of riverine SS from the Hayakawa and the Amehata Rivers systems to the Fuji River was quite small during low-flow conditions. The composition of REE in fluvial sediments at the lower reaches of the Fuji River also showed similar characteristics of sediment transport from the Hayakawa and Amehata Rivers systems with that observed in the riverine SS. These results imply a possibility that both SS and riverbed sediments in the Fuji River were largely derived from the Hayakawa and the Amehata Rivers systems.
The field survey was conducted at up to 26 sampling points in the middle and lower reaches of the Fuji River system (including the Hayakawa and the Amehata Rivers). The sampling was made irregularly once or twice a month in 2021. We measured Sr isotope ratio (87Sr/86Sr), Sr concentration, and rare earth elements (REE) of each river water, SS, and fluvial sediments (<1mm) collected at each sampling point. The results showed that the 87Sr/86Sr of river water was distinct among the major river systems. The average 87Sr/86Sr values of river water were 0.7098, 0.7088, and 0.7058 in the Amehata River, the Hayakawa River, and the Fuji River (before the confluence of the Hayakawa River), respectively, and those values were almost consistent during the study period. On the 87Sr/86Sr-1/Sr concentration diagram, all river water samples were distributed between the Amehata River and the Fuji River. The Sr isotope mass-balance model revealed that the river water derived from the Hayakawa River system contributed to approximately 15% of the Fuji River water during low-flow conditions. In contrast, the contribution of the Hayakawa River water increased to 24-61% during high-flow conditions, although the value became the lowest with 5% on July 2nd when the river level was the highest due to severe flooding. The proportions of the Amehata River water to the Hayakawa River were estimated to be approximately <25% and >40% during low-flow and high-flow conditions, respectively.
The 87Sr/86Sr of riverine SS also showed distinct values among the Amehata River (0.7148), the Hayakawa River (0.7098), and the Fuji River (0.7063). On the 87Sr/86Sr-1/Sr concentration diagram, all samples of the riverine SS were distributed between the Amehata and the Fuji Rivers SS values (before the confluence). The Sr isotopic mixing model showed that during high water level conditions, the proportion of SS derived from the Hayakawa River in the SS of the Fuji River was estimated to be high (56-65%). Moreover, the contribution of SS derived from the Amehata River to the Hayakawa River SS was also high (51-59%). On the other hand, the transport of riverine SS from the Hayakawa and the Amehata Rivers systems to the Fuji River was quite small during low-flow conditions. The composition of REE in fluvial sediments at the lower reaches of the Fuji River also showed similar characteristics of sediment transport from the Hayakawa and Amehata Rivers systems with that observed in the riverine SS. These results imply a possibility that both SS and riverbed sediments in the Fuji River were largely derived from the Hayakawa and the Amehata Rivers systems.