13:45 〜 15:15
[HTT15-P09] Chromium behavior and isotopic signature in the estuarine mixing zone
キーワード：河川、汽水域、クロム、化学種、安定同位体
Chromium (Cr) is a redox-sensitive element with two stable oxidation states, Cr(III) and Cr(VI), under Earth surface conditions. Because Cr(VI) is a carcinogenic toxic element, numerous studies have been carried out on the behavior of Cr(VI) in rivers and groundwaters, while few studies have been conducted in estuaries. In estuarine areas, sedimentation due to the flocculation of suspended particles with increasing salinity and increased biological activity due to abundant nutrient supply may influence the behavior of trace elements. Since many trace elements, including Cr, are transported to the ocean through rivers, it is important to understand their behavior in estuaries in order to elucidate their cycling in the surface environment. The objective of this study is to understand the geochemical behavior of Cr in the estuary from Cr concentrations, speciation, and Cr stable isotope compositions in river water and riverbed sediment porewater.
Two rivers located in Kochi Prefecture were studied. The Kuma River flows from north to south through Kochi City, upstream passes through the serpentinite distributed area, while downstream passes through the center of the city. The Sakura River is located in Susaki City and it flows through a limestone area upstream and agricultural area in the lower reaches. River water and sediment samples were collected in September 2022. The porewater was extracted by centrifugation of riverbed sediment. The total Cr concentration was determined by ICPMS, and Cr(VI) and Cr(III) concentrations were measured by HPLC-ICPMS. For Cr stable isotope analysis, dissolved Cr in river water samples was pre-concentrated by the Fe(II) co-precipitation method. After adding of 50Cr-54Cr double spike and ion exchange purification, Cr isotope ratios (53Cr/52Cr) were determined by thermal ionization mass spectrometry.
In the upper reaches of the Kuma River, pH and Mg concentrations increased downstream, and the water quality was classified as Mg-HCO3 type. The total Cr concentrations showed a maximum value of 2.3 ppb just downstream of the serpentinite zone, followed by a decreasing trend. These suggest that water quality is controlled by the underlying lithology and that Cr is released into the river by the weathering of serpentinite. In the lower reaches, the water quality changed to Na-Cl type, and Cr(III)/Cr(VI) ratio increased. The δ53Cr values ranged from +1.32 to +1.96‰ and tended to be higher downstream as Cr concentrations decreased. These results indicate that Cr(III), which is partially reduced from Cr(VI) and has a lighter isotopic ratio, was removed from the river water by adsorption on suspended solids and/or riverbed sediment. A weak positive correlation was observed between Cr(VI) and DO concentrations in the Kuma River, suggesting that organic matter may role as a reducing agent. In the Sakura River, water quality was classified as Ca-HCO3 type upstream and Na-Cl type downstream. Total Cr and Cr(III) concentrations increased slightly downstream from 0.5 ppb to 1.3 ppb and δ53Cr values increased from +1.83 to +2.00‰. These results may suggest that seawater with a higher Cr concentration than river water and a δ53Cr value similar to that of river water is mixed with the river. Vertical profiles of total and Cr(VI) concentrations in porewater at both sites show changes in concentrations. The Cr(III) concentrations tend to decrease as the porewater becomes more reducing, showing that Cr(III) has been removed from the porewater by precipitation and adsorption on sedimentary particles. On the other hand, the sum of the concentrations of Cr(VI) and Cr(III) is much smaller than the total Cr concentration, indicating that the organic complexes and colloidal fractions of Cr(III) may be present in non-negligible amounts. Further studies of Cr species and isotopic ratios in river sediments, porewater, and suspended particles are expected to provide more detailed information on Cr behavior in estuarine systems.
Two rivers located in Kochi Prefecture were studied. The Kuma River flows from north to south through Kochi City, upstream passes through the serpentinite distributed area, while downstream passes through the center of the city. The Sakura River is located in Susaki City and it flows through a limestone area upstream and agricultural area in the lower reaches. River water and sediment samples were collected in September 2022. The porewater was extracted by centrifugation of riverbed sediment. The total Cr concentration was determined by ICPMS, and Cr(VI) and Cr(III) concentrations were measured by HPLC-ICPMS. For Cr stable isotope analysis, dissolved Cr in river water samples was pre-concentrated by the Fe(II) co-precipitation method. After adding of 50Cr-54Cr double spike and ion exchange purification, Cr isotope ratios (53Cr/52Cr) were determined by thermal ionization mass spectrometry.
In the upper reaches of the Kuma River, pH and Mg concentrations increased downstream, and the water quality was classified as Mg-HCO3 type. The total Cr concentrations showed a maximum value of 2.3 ppb just downstream of the serpentinite zone, followed by a decreasing trend. These suggest that water quality is controlled by the underlying lithology and that Cr is released into the river by the weathering of serpentinite. In the lower reaches, the water quality changed to Na-Cl type, and Cr(III)/Cr(VI) ratio increased. The δ53Cr values ranged from +1.32 to +1.96‰ and tended to be higher downstream as Cr concentrations decreased. These results indicate that Cr(III), which is partially reduced from Cr(VI) and has a lighter isotopic ratio, was removed from the river water by adsorption on suspended solids and/or riverbed sediment. A weak positive correlation was observed between Cr(VI) and DO concentrations in the Kuma River, suggesting that organic matter may role as a reducing agent. In the Sakura River, water quality was classified as Ca-HCO3 type upstream and Na-Cl type downstream. Total Cr and Cr(III) concentrations increased slightly downstream from 0.5 ppb to 1.3 ppb and δ53Cr values increased from +1.83 to +2.00‰. These results may suggest that seawater with a higher Cr concentration than river water and a δ53Cr value similar to that of river water is mixed with the river. Vertical profiles of total and Cr(VI) concentrations in porewater at both sites show changes in concentrations. The Cr(III) concentrations tend to decrease as the porewater becomes more reducing, showing that Cr(III) has been removed from the porewater by precipitation and adsorption on sedimentary particles. On the other hand, the sum of the concentrations of Cr(VI) and Cr(III) is much smaller than the total Cr concentration, indicating that the organic complexes and colloidal fractions of Cr(III) may be present in non-negligible amounts. Further studies of Cr species and isotopic ratios in river sediments, porewater, and suspended particles are expected to provide more detailed information on Cr behavior in estuarine systems.