Hydrochemical characteristics and formation mechanism of the sediment-water interface diffusion boundary layer in the coastal reservoir
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Abstract:
Coastal reservoirs often face water salinization problems, which not only affect their function but also cause severe obstacles to the regional ecosystem and substantial waste of manpower and material resources. To improve the utilization efficiency of reservoirs, several researches have suggested the use of diffuse boundary layer at the sediment-water interface of the water body, as a reaction to hot zone of hydrogeochemical action, which is a key area for controlling the degree of salinization and the type of water quality of the water body. Dissolved oxygen, an important indicator in the evaluation of water quality, is also involved in the transformation of many substances by changing the redox environment. The current study focuses on the spatial and temporal change rules of salt release and the influence of external disturbing forces on the thickness of the diffusion boundary layer. Beidagang Reservoir in Tianjin City was selected as the study area, and field sampling and indoor simulation experiments were combined to investigate the hydrochemical evolution patterns and causative mechanisms of coastal salinized water bodies within the diffuse boundary layer. The results show that there is a diffusion boundary layer of total dissolved solids (TDS) and dissolved oxygen (DO) in the water body during the storage process, and within the diffusion boundary layer there is a high TDS mass concentration zone and a low DO mass concentration zone. The the process of increasing the thickness of the diffusion boundary layer of TDS before and after the test is more stable than that of DO. During the test, the oxygen flux of the water body in the diffuse boundary layer was generally negative, and the salt release flux was mostly positive, while the oxygen depletion reaction and the salt release phenomenon in the diffuse boundary layer in the early stage of the test were the most obvious. The phenomenon of increasing oxygen content appeared in the middle and late stages, at the same time the salts in the diffuse boundary layer were also released to the upper layer of the water body under the effect of the difference in mass concentration, meanwhile the oxygen flux and the salt release flux were finally presented as dynamic and stable. The percentage of water chemical constituents at the upper and lower boundaries of the diffusion boundary layer (DBL) showed obvious clustering, and the relative contents of Na+, Mg2+,SO2-4,and HCO-3, which had obvious stratification phenomena, showed consistency with time.Na+ and SO2-4 showed SWI greater than the DBL,while HCO-3 and Mg2+ showed SWI less than the DBL. Although the upper and lower boundaries of water chemistry of the diffuse boundary layer at each point showed diversified characteristics, Cl?, Mg2+ and Na+ were always the dominant ions in the water chemistry composition. The hydrogeochemistry at the SWI of the water body is more complex as compared to the DBL, and is more ignificantly affected by sulfate dissolution and silicate dissolution . This experimental design is based on the operation of the Beidagang Reservoir and simulates the changes in the hydrochemical characteristics of reservoir at the location of the Diffusion Boundary Layer (DBL) during the impoundment stage. It was found that the interior of the boundary layer is the main reaction area for oxygen depletion and salinity accumulation processes. The DBL serves as a buffer against the overall salinization of the water body. Furthermore, there is a discrepancy in hydrogeochemical roles between the SWI and the DBL. The results of this experiment can provide a reference basis for exploring the material exchange processes within the diffusion boundary layer of coastal salinized water bodies as well as the rational evaluation and prediction of water quality.
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