Spatio-Temporal Distribution Of Non-Point Source Pollution And Risk Prevention And Control Strategies In The Ashi River Watershed

Non-point source（NPS）pollution is the main pollution source affecting the quality of receiving water bodies.Due to the complexity of the generating process of NPS pollution,the research on NPS pollution has always been the focus of attention from all sectors of society,which affects not only the construction of the ecological environment but also the process of sustainable agricultural development.The Ashi River watershed is a typical small watershed in the cold region of Northern China.It is abundan t in agricultural and mineral resources,so agricultural production and mining industries are relatively developed.The high-intensity agricultural production mode over many years has led to the use of chemicals increasing year by year,and pollutants in t he process of mineral mining will cause harm to the surrounding ecological environment with the migration of runoff.Therefore,various pollutants will eventually cause damage to the environment under the action of various processes,and how to promote the coordination and unity of economic and environmental development in the Ashi River watershed is still the focus of attention.In this study,qualitative and quantitative analysis is systematically carried out in a layered manner from the perspectives of the spatiotemporal distribution characteristics and source analysis of NPS pollution,the identification and quantitative analysis of the driving factors of NPS pollution,and the ecological risk prevention and control measures and management strategies for NPS pollution in the Ashi River watershed.This provides a scientific reference for the research on NPS pollution in other similar small watersheds,and also provides a framework for systematic research on NPS pollution.According to data-driven models and field fixed-point sampling methods,the spatiotemporal distribution characteristics of different types of NPS pollution and the sources are quantitatively and qualitatively analyzed.This study uses the Integrated Assessment Model of Ecosystem Services a nd Tradeoffs（InVEST）to quantitatively calculate the total export of normal pollutants（TN and TP）in the Ash River watershed in 1995,2005,2015,and2020.In the time scale,the export results vary in the ranges of 3919.32-4047.4t and 283.57-289.22t,respectively.In terms of spatial distribution,the regions with high TN and TP export are characterized by being concentrated in the northwest of the watershed,mainly in the cultivated land area downstream,and the output is lower in the southeast of it,m ainly in the forest area upstream.The spatial distribution of particular pollutants（As,Cd,Cr,Cu,Pb,and Zn）has the heterogeneity,and the average concentrations of them are 2.35,16.16,1.00,0.90,0.41,and 1.72 times that of the soil background va lues in Heilongjiang Province,respectively.It indicates that As,Cd and Zn have the significant characteristics of accumulation in the time scale.The spatial distribution is characterized by the case that the closer the distance to the site for mineral resource exploitation,the higher the concentration of particular pollutants in the soil.As indicated by the calculation results of Export Coefficient Model（ECM）,the main sources of normal pollutants include land use pattern,livestock and poultry breed ing,and rural life,of which land use pattern accounts for 57.19%and 63.81%of the total load,respectively.It is the most important source.The particular pollutants are divided into three categories（Pb,Cu,Zn,Cd;As;Cr）by principal component analysis（PCA）and cluster analysis（CA）,and qualitative analysis shows that the major sources of the three kinds of particular pollutants are industrial sources,agricultural activity sources,and natural sources,respectively.To deeply analyze the factors affecting NPS pollution in the Ashi River watershed,a variety of mathematical statistical methods are used in this paper to quantify the relationships between NPS pollution and various types of factors.Geographic detector is used to reveal the key driver s for spatial heterogeneity in NPS pollution load,which are land use pattern and precipitation factors.Research findings on the spatiotemporal distribution characteristics of key drivers indicate that the area of cultivated land and forest from 1995 to 2 020decreases by 38.29km² and 21.07km²,respectively,and the construction land increases by 22.31km².Reservoir and urbanization construction are the dominant factors affecting the changes in land use pattern.The Ashi River watershed has obvious two-phase seasons of frozen period and non-frozen period.The results of path analysis show that temperature,air pressure and evaporation are the main factors affecting runoff in typical hydrological years,while humidity,sunshine duration and precipitation are the main factors affecting runoff in non-frozen periods of the flood year.Quantitative analysis of the impact of different types of factors on NPS pollution shows that according to redundancy analysis（RDA）,the single factor that has the greatest impact on normal pollutants is fertilizer,and its contribution rate varies in the range of65.10-76.00%;according to regression analysis,the influence of human factors on normal pollutants is greater than that of natural factors.RDA results show that the factors that have a greater impact on particular pollutants are K and C/N,and the interpretation rate of environmental factors for particular pollutants is more than 80%.The results of correlation analysis show that there is a significant positive correlation between particular pollutants（P<0.05）,the correlation coefficient varies from 0.455 to 0.958,and there is a negative correlation between soil bulk density and particular pollutants.Based on the analysis of the spatiotemporal distribution,and the driving and main factors of NPS pollution in the Ashi River watershed,this paper proposes the prevention and control measures of NPS pollution and the strategy of zoning and grading environmental management.The vegetation buffer zone is one of the effective measures to prevent and control NPS pollution.A total of184（TN）and 126（TP）trials are screened from 20 pieces of literature by Meta-analysis.The results from random-effect model show that compared with bare land,the removal efficiency of the veget ation buffer zone on normal pollutants is significant(TN,ln R₊₊=0.60;TP,ln R₊₊=0.55),and slope,buffer width,vegetation type,and initial concentration are the main factors affecting the comprehensive effect value.The effect of width on the removal efficiency of the vegetation buffer zone is stronger,and the relationship between them presents logarithmic function.In configuring the vegetation buffer zone,it is recommended that the slope should preferably be below 5°and the width should be at least 10m,but 20m achieves a better removal efficiency on pollutants.The selection of vegetation should be dominated by herbs.On the basis of the ecological function regionalization,with the main function as the orientation,regionalized management is carried out in a stepwis e manner.The MCR model is used to divide the ecological risk units of NPS pollution in the Ashi River watershed,which are the high risk zone,the medium risk zone,and the low risk zone.For normal pollutants,with cultivated land taken as the“source landscape”,the risk level in the downstream area is high,and f or particular pollutants,with the mine source taken as the“source landscape”,the risk level in the middle and upper reaches is high.Taking the resistance values of NPS pollutants and the economic factor as the dominant factors,and the administrative unit as the clustering unit,nine refined ecological units are divided.On this basis,priority control areas and general control areas are identified according to the characteristics of normal p ollutants and particular pollutants in administrative units.Finally,the administrative units are divided into four grades according to the superimposed zone results.The administrative units with the strongest superposition effect need to strengthen prot ection of natural forests,increase the coverage of forests,r educe soil erosion,strengthen the ecological environment management of mining enterprises,and reduce the damage to the surrounding soil environment,develop green mines,take measures to protect cultivated land,etc.