| Dianchi Lake Basin is an important area for agricultural production in China.However,in recent decades,the population expansion and economic development in the basin have caused unreasonable land use,thereby aggravating soil erosion in the Dianchi Lake Basin.Lake plays a key role in maintaining human survival and development,their functions include water conservation,regulating regional climate and promote social development.Environmental proxy indicators in lake sediments are widely used to reconstruct the sedimentary characteristics,sediment sources,production capacity and nutritional status.To some extent,they reflect the historical changes of soil erosion in the basin.In addition,sediments in small watershed are more sensitive to environmental changes.Therefore,this paper takes the Da River small watershed in Dianchi Lake Basin as the research object,210Pb radioactive isotope was used to determine the river reservoir sediment age and deposition rates,combined with other physicochemical indicators（Particle size,nutrient,metal,C/N,C/P,heavy metal/Al,deposition fluxes,etc）of the sediment to determine sedimentary characteristics and soil erosion modulus of the basin,so as to construct the influence of natural succession and human activities on soil erosion of the basin.Finally,the contribution of different land use types to sediment in different periods was determined by composite fingerprint technique,and the sources of heavy metal pollution in the sediment were also traced.The results showed that:（1）The geological age of the 60 cm sediment core was determined by a 210Pb dating model,with a time span of about 1880–2018,and the sedimentary column of Da River Reservoir was divided into three stages（stageⅠ:1880–1985（–60～–38 cm）,stageⅡ:1986–1997（–37～–29 cm）,stageⅢ:1998–2018（–28～–1 cm））by cluster analysis.The results showed that the the physicochemical elements（particle size,nutrient,metal,heavy metal/Al,C/P,and C/N）in the sediment of stage II were significantly changed compared with other stages,indicating that human interference was the most serious in the basin.The average mass sedimentation rate and average depth sedimentation rate of the sedimentary column were（1.07±0.31）g/cm2/a and（0.95±0.59）cm/a,respectively.The deposition rates and deposition fluxes increased gradually with the decrease of deposition depth,and the deposition rates of stage II increased the most.In addition,the correlation analysis and redundancy analysis showed that the deposition rates of the sediment were positively correlated with fine particles,nutrient,metal,element ratio s,temperature,precipitation and social factors,indicating that climate factors and human activities led to the increase of soil erosion year by year.（2）Using 137Cs tracing technology to study the degree of soil erosion in the sediment source area of the Da River Basin,the results showed that the average erosion modulus of cultivated land,forestland and grassland were（2383.30±1040.71）,（642.89±498.20）and（366.82±145.91）t/（km2·a）,respectively.The degree of cultivated land and woodland was significantly greater than that of grassland,mainly because cultivated land was disturbed for a long time and the forest in the basin was seriously cut down.The composite fingerprint tracing technique also showed that the contribution rates of cultivated land,woodland and grassland to sediment were 76.37%,20.63%and 3.00%,respectively,which confirmed that the contribution rates of cultivated land and woodland to sediment were greater than that of grassland in the whole deposition stage.Therefore,it is necessary to control of soil erosion of cultivated land and forestland reasonably.（3）The principal component analysis and correlation analysis showed that there were two main sources of heavy metals in the sediment of Da River Reservoir.Ni,Zn,Cr,Cu,and Pb indicated that the sediments were mainly from industrial point source pollution and urban household source pollution,while As and Mn were mainly from agricultural non-point source pollution.（4）Single pollution index（Pi）,single potential ecological risk index（Er）,Nemerow index（PN）,pollution load index（PLI）and potential ecological risk index（RI）is used to assess the pollution levels of heavy metals of Da River Reservoir.Pi values showed that the reservoir was slightly polluted by heavy metals（except Pb and Ni）,and the Er values showed that the reservoir was slightly polluted by even heavy metals in the sediment,and the PN,PLI and RI values were 1.24,1.09,and 31.73,respectively,indicating the reservoir was slight pollution.Trophic status index of TN（TLI（TN））,trophic status index of TP（TLI（TP））,organic nitrogen index,pollution index（PI）and organic index were used to evaluate the levels of reservoir nutrient pollution,which were 53.94,59.01,0.23,1.96 and 0.37,respectively,indicating that the reservoir was in a state of pollution.Therefore,Da River Reservoir was in a long-term pollution level,so it was urgent to control the pollution of reservoir.Therefore,this study can accurately obtain the sediment characteristics and sediment sources of the Da River Reservoir,which provides scientific support for the implementation of water and soil conservation and hydrological management in the small basin,as well as guarantee for the safety of human drinking water. |
