Quantitative Analysis Of Inflow Load Of Soil Heavy Metal Pollution With Different Land Use Based On Cssi-Real-Time Monitoring Technology

The change in land use type intensifies the accumulation of heavy metals in farmland soil.Heavy metals and other pollutants in the soil carried in eroded sediment are transported in different forms to surface water by rainfall and runoff.Which poses a serious threat to aquatic ecosystem and drinking water quality for human and livestock.At present,there are many studies on the relationship between land use change and soil heavy metal pollution,but studies that link heavy metal pollution in soil to that in rivers and lakes are scarce.Therefore,it is of great significance to understand soil heavy metal pollution status of different land use types in the watershed,quantitatively analyze their inflow loadsfor rational land planning and prevention of non-point source heavy metals pollution in water.The Nala watershed,located in the water source area of Kelan Reservoir in Guangxi,was selected as the research object for this study.The watershed was divided into three sub-watersheds for monitoring:one in downstream section（S₁）,and the other two in upstream（S₂ and S₃）;Land use types and crop coverage were obtained by using Unmanned Aerial Vehicle（UAV）.The amounts of heavy metals in surface soil of different land use types were determined by high-precision portable X-ray fluorescence spectrometry（HDXRF）,and the Kriging interpolation method was used to make the spatial distribution map of heavy metals in the watershed.Based on the real-time monitoring and sampling system of runoff and sediment,runoff parameters and sediment samples at the outlet of each sub-watershed were obtained to calculate the inflow load of sediment and the selected heavy metals.Combined application of CSSI and real-time monitoring technology to determine the inflow load of heavy metal pollutants of different land use types was employed.The key factors affecting heavy metal pollution load into the river was determined by Pearson correlation analysis.The main conclusions were as follows:（1）From the perspective of the whole watershed,the order of soil heavy metal concentration in different land use types was eucalyptus>road>river bank>sugarcane.The concentrations of Cd,Cr and Ni are significantly higher than the standard values,in order of 75%,91.7%and 100%,respectively.The existing environmental capacity of soils of different land use types in the sub-watersheds has been overloaded,and the pollution degree has exceeded the risk benchmark value.According to the classification of Nemerow comprehensive pollution index,road and eucalyptus belong to heavy and medium pollution,respectively,while river bank and sugarcane belong to light pollution.（2）The rainfall and runoff in 2019 were 878 mm and 160.5 mm respectively,which were 692 mm and 101.0 mm in 2020,and the greater the rainfall and runoff,the greater the heavy metal inflow load;The heavy metal inflow load in 2019 was 4.33 times higher than that in 2020.Cr,Ni and Zn were the uppermost heavy metal pollutants in the sediment inflow into river of small watershed in 2019,accounting for 77.8%of the total inflow load of 6 typical heavy metals.Cr,Ni and Cu were the most important pollutants entering the river in 2020,accounting for 81.5%of the total load.The inflow load of heavy metal pollution has distinct temporal and spatial variation characteristics.Temporally,heavy metal pollution inflow into river mainly occurred at the stages of Tillering and Grand growth of sugarcane.Spatially,the inflow load of heavy metals in upstream watershed was significantly higher than that of downstream.（3）Based on the CSSI traceability results,it was found that sugarcane, river bank and road were the main sources of inflow pollution contributions of Cr,Ni,Cu,Zn,As and Cd.The total inflow load of soil heavy metals of different land use types in 2020 showed a downward trend compared with 2019,with eucalyptus,sugarcane,river bank and road decreasing by 90.3%,84.4%,76.4%and 51.4%,respectively.（4）There was a very significant positive correlation between rainfall and runoff and the inflow load of six heavy metals（P<0.01）,which explained68%～74%and 77%～85%of the change of pollution inflow load respectively.There was a significant positive correlation between the amount of nitrogen and phosphorus application and the inflow load of six heavy metals（P<0.05）,which explained 31%～87%and 36%～87%of the change of pollution inflow load respectively.There was a significant negative correlation between coverage and the inflow load of heavy metals（P<0.05）,which explained 23%～42%of the change of pollution inflow load.