Risk Assessment And Process Simulation Of Watershed Nonpoint Source Pollution Based On Spatial Attributes

Water quality is one of the most serious water issues in watershed management around the world,and presents a formidable challenge to global water security.In 2015,China issued the "Water Pollution Control Action Plan",which proposed to reduce industrial,agricultural and municipal pollutant emission,and strengthen pollution load control.For a long time,China has focused on point source pollution prevention,while the control of nonpoint source pollution has not been in the scope of pollution load management,which resulted in relatively insufficient nonpoint source pollution research.Exploring the response of water quality to natural and anthropic processes,evaluating the spatial nonpoint source pollution risk,analyzing the generation and migration processes of nonpoint source pollutants,and further formulating strategy of reducing nonpoint source pollutants,are important means to deal with increasingly serious nonpoint source pollution.With the highly disturbed Huai River Basin as the study area,the study was carried out on the basis of environmental hydrological theory,and made use of several research methods including multivariate statistical,spatial analysis and water quality simulation model.Thus,the study was to detect the spatio-temporal variation of watershed water quality and its correlation with land use change,analysis the spatial distribution of nonpoint source pollution and identify the critical source areas,analysis the generation,terrestrial and instream migration processes of pollutants by constructed water quality simulation model.The main work and achievements are as follows:(1)Combined with water pollution control status in China and abroad,the importance of watershed environmental hydrological process was elaborated.Meanwhile,the research status of land use impacts on water environment,nonpoint source pollution risk analysis,and watershed water pollution process analysis were described.(2)The internal mechanisms of watershed environmental hydrological process and influence of human activities on water environment were clarified.Moreover,the study framework including water environment evolution characteristic analysis based on watershed spatial attributes and its response relationship to natural and anthropic processes were put forward,along with the supporting methods of watershed water environment research.The Moran's I index was used to detect the spatial autocorrelation and corresponding type of water quality parameters.Cluster analysis was adopted to analysis the spatial distribution of water pollution characteristic.Linear mixed effects model was applied to analysis the relationship between water quality and land use and its seasonal and spatial scale dependence.The spatial distribution of nonpoint source pollution risk and critical source areas in Huai River Basin was assessed and identified by nonpoint source pollution risk index model based on multi-criteria decision method.Large-scale water quality simulation model based on watershed spatial attributes was constructed to simulate the generation,terrestrial and instream migration processes of pollutants from various sources.(3)The spatio-temporal water quality evolution during 2005?2010 was evaluated with monitoring water quality data,and the result showed that:all water quality parameters except DO showed significant positive spatial autocorrelation,and were mainly characterized by"high-high" and "low-low" type.The water quality condition at all monitoring stations could be classified as three types including high,moderate and low degrees of pollution,which exhibited obvious regional characteristic.The impacts of land use on water quality exhibited significant seasonal and spatial scale dependence.Specifically,DO and NH3-N were more vulnerable to land use change at subwatershed scale,while TP and Fluoride were better explained by land use within fine buffer zones.In particular,the spatial scale with better explanation of land use for COD and BOD varied with seasons.The peak concentrations of COD,NH3-N and Fluoride in highly urbanized regions easily occurred in dry season,while the peak concentrations of COD,TP and NH3-N in less urbanized regions tended to occur in wet season.(4)Construct nonpoint source pollution risk index model constructed by land use,soil erosion,runoff capacity and riverine distance indices was adopted to analysis the spatial distribution of nonpoint source pollution risk.The results showed that the western and southern headwater regions in Huai River Basin generally exhibited low risk value,while the risk value in downstream plain regions showed a decreasing trend from north to south on the whole.Besides,as the riverine distance to the same reach increased,the risk value showed an obvious descending trend.The subzone classification of spatial nonpoint source pollution risk with subwatershed as identifying unit showed that the subzones with high and serious pollution risk for COD,TP and NH3-N covered area percentages of 37.4%,40.4%and 25%,respectively in the Huai River Basin.In particular,the pollutants in subzones with high pollution risk mainly sourced from rural residential area,while in subzones with serious pollution risk mainly sourced from agricultural and rural lands.(5)Water quality simulation model based on watershed spatial attribute data was constructed to simulate the generation,terrestrial and instream migration processes of pollutants from various sources.The results showed that instream COD was vulnerable to point and livestock breeding source pollutants,while NH3-N was significantly influenced by point and agricultural source pollutants.The pollutants significantly influenced instream TP were from point,agricultural and rural sources.All landscape variables including soil erosion,runoff capacity and riverine distance were found to significantly influence the terrestrial migration processes of COD and TP,while only runoff capacity and riverine distance significantly influenced the terrestrial migration process of NH3-N.Reaches with concentrations of COD,TP and NH3-N exceeding class III level of national water quality standard account for 6.3%,49.2%and 60.5%of all reaches in length,respectively.COD load in most watersheds were dominated by livestock breeding source pollutants,while the composing of TP and NH3-N sources varied greatly with regions.Accordingly,the composing of TP and NH3-N fluxes showed obvious regional difference.With watershed outlet as the target end,the percentage of pollution load at subwatersheds transported to the target end ranged between 25%?100%for COD,17%?100%for TP,and 3%?100%for NH3-N.Thus,the rank of instream migration capacity was COD>TP>NH3-N.