| At present,the research on water resources management mainly focuses on the management and supervision of large and medium-sized watersheds and lakes.The attention to the water environment issues in large quantities of small watersheds with more complex situation is still insufficient.Corresponding control of them is relatively inadequate.The practice and knowledge of Nature-based solutions(NBS)case are limited.This study takes Fengshan creek watershed,a coastal watershed in southeast China with significant urbanization process,as the research area.With the methods of field monitoring,multivariate statistical analysis,geographic information technology etc.,this paper focuses on the spatiotemporal variation characteristics of water quality in small watershed under the background of rapid urbanization,and analyzes the role of constructed wetland as a nature-based solution in improving water quality and ecological restoration of watershed.Here are the main results:Fengshan creek watershed has witnessed significant urban expansion in the past ten years.It was pointed out that the Build-up area increased more than 14 times.The intensity of land use change in 2015-2020 was significantly higher than that in 20102015.The increase of Built-up area tends to be active,which is mainly converted from cropland,grassland and woodland.Thereinto,the conversion of cropland to Built-up in 2015-2020 is the greatest.The loss of cropland is significant.The characteristics of rapid urbanization in Fengshan creek watershed are evident.The concentration of nitrogen and phosphorus index in the river of Fengshan creek watershed has markedly spatial and temporal variation characteristics.The phosphorus index concentration of Fengshan Creek showed a zigzagging trend with time with the discovered rule of TP>DTP>SRP>TPP.The trend of TN,NH4+-N,NO3--N and NO2--N concentrations change remains basically the same.The nitrogen index concentrations in the whole basin showed a zigzag decreasing trend with time.There are rules discovered that TN>NH4+-N>NO3--N>NO2--N.The main DIN component is NH4+N,followed by NO3--N.TN,NH4+-N and NO3--N keep decreasing trend from upstream to downstream.Nitrogen pollution in tributaries is greater than in the main stream,and phosphorus pollution in the main stream is greater than in the tributaries.Nitrogen and phosphorus pollution in the tributaries of the left bank is worse than that of the right bank.The correlation between the area proportion of every kind of land use type and nitrogen and phosphorus concentration was not strong.Only the proportion of water area showed a significant negative correlation with TN concentration,and a significant positive correlation with NO2--N concentration.Therefore,it is speculated that the impact of point source pollution on water body in this watershed has exceeded that of non-point source pollution.In terms of water quality improvement,the constructed wetland system in Fengshan creek watershed has a significant effect on the removal of SRP,TN and NH4+N.The removal rates were 54%,35%and 29%,respectively.In the aspect of ecological restoration,the biological oxidation pond can improve the water hypoxia condition through aeration,enhance the activity of aerobic microorganisms,and enhance the selfpurification ability of water.The introduction of aquatic plants and benthic organisms in the constructed wetland can increase the species and number of organisms in the ecosystem which can enhance the self-regulation ability and stability of the ecosystem.This study can provide practice and understanding of the specific case about nature-based solutions for the treatment of point source pollution and non-point source pollution in small watershed under the current background of rapid urbanization.The research results can also provide reference for the local interpretation and effective implementation of water-related policies such as the Ten-measure Action Plan to Tackle Water Pollution,Beautiful Village and River Manager. |
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