| In order to explore the migration and transformation characteristics of iron and manganese in Jinpen reservoir and the mechanisms of its in-situ control by water-lifting aerators(WLAs),the distribution and variation of iron and manganese in the water column and sediment of Jinpen reservoir were analyzed from July in 2018 to July in 2019.The main conclusions are as follows:(1)The Jinpen reservoir is a stable seasonal stratified reservoir.Depletion of dissolved oxygen at the bottom water caused by thermal stratification is an important incentive for internal release of iron and manganese.During April and May 2019,the bottom water entered seasonal hypoxia,and the highest concentration of total manganese in the water column was 0.109 mg/L.Due to the redox potential of iron and manganese,iron remained in particulate forms,while manganese was maintained in dissolved form.Variation of concentration and forms of iron and manganese in the water column were affected by the combination of internal release of sediment and external runoff.(2)The variation of iron and manganese concentrations are basically consistent with the runoff process.When the runoff was an underflow,the maximum concentration of iron and manganese was at the bottom water.As the depth of the river increased,the runoff turned into an interflow,the maximum concentration of iron and manganese was at the middle of water column.In the runoff water,iron remained in particulate forms,while manganese was maintained in dissolved form.During the storm runoff,a large amount of iron and manganese pollutants flowed into the main reservoir area,and a lot of iron mainly in the particulate forms was deposited into the surface sediment,and most of the manganese remained in the water column as dissolved form.The iron and manganese flowed into main reservoir area and further participated in the redox cycle of the iron and manganese in the main reservoir area,which influenced on the water quality in the long-term.(3)The main forms of iron in the sediments of the Jinpen Reservoir were the residue state(RES)and the Fe-Mn oxide bound states(ERO);the content ratios of exchangeable state(EXC),carbonates bound state(CARB),Fe-Mn oxide bound state(ERO)and residue state(RES)of the manganese in the sediment were relatively high and closer.In the long-term deposition process,the iron and manganese in the sediments tended to migrate to the surface sediments and eventually formed a vertical distribution of “upper high and lower low”.Compared with iron,manganese in the sediments had stronger environmental sensitivity,release risk,and biological toxicity,while iron also had a certain release risk during seasonal hypoxia.(4)The concentration distribution of iron and manganese in the pore-water showed that the dissolved manganese was more easily released into the overlying water than the iron due to the redox potential.The dissolved manganese constantly accumulated in upper sediments and overlying water under anaerobic conditions.However,the release of dissolved iron was not only suppressed by dissolved oxygen but also by other oxidants such as manganese oxide.(5)The water-lifting aerators had a significant effect on removing dissolved iron and manganese in water column.The average reduction rates of dissolved iron and manganese during WLAs operations in 2018 were 44.4% and 33.3%,respectively.WLAs effectively increased the dissolved oxygen concentration at the sediment-water interface.The dissolved iron was continuously oxidized and deposited when it diffused to the surface of the sediment,while the dissolved manganese continuously accumulated under the sediment-water interface.Diffusion flux of manganese increased after the WLAs operated.After the deactivation of WLAs,the dissolved oxygen concentration in the bottom water declined and the dissolved manganese concentration at the sediment-water interface increased,resulting in the decrease of diffusion flux of dissolved manganese. |
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