Risk Assessment And Source Identification Of Heavy Metal Contamination Of Soil And Water In Agricultural Land In The Nianchu River Basin

As the "breadbasket of Tibet" and an important part of one river and two streams,the quality of the soil and water environment in the Nianchu River Basin has an important impact on food security and the health and safety of the surrounding residents.In this paper,the Nianchu River Basin was taken as the subject,and the temporal variation characteristics of the concentrations of eight heavy metals(Cu,Pb,Zn,Cr,Ni,Cd,As and Hg)in water and soil on agricultural land in the study area from 2019 to 2021 were analyzed.Based on the concentration data of heavy metals in water and soil and soil fertility indicators of agricultural land in the study area in2021,the spatial variation characteristics of soil heavy metals and soil fertility indicators(TOC,TN and TP)in the study area were explored,the pollution risk levels of heavy metals in water and soil in the study area were evaluated,the differences in concentrations of soil heavy metals and the differences in pollution risks under different agricultural land use methods were compared and analyzed,and the sources of heavy metals were explored,through the above study the following results were obtained:A time-scale analysis of the data on the concentrations of eight heavy metals in soil and water on agricultural land in the study area from 2019-2021 revealed that the mean concentrations of all heavy metals exceeded the background values from 2019-2021,except for the mean concentration of Pb,which did not exceed the background values of soil heavy metals in Tibet.In general,the accumulation of Pb gradually increased during the three years,while the accumulation of Cu,Zn,Cr and Ni decreased continuously,and the remaining heavy metals did not change much.Compared with the concentrations of heavy metals under different agricultural land use practices,the mean concentrations of all heavy metals in the shed were higher than those in cultivated soils,except for As,where Cr and Ni exceeded the maximum concentrations.The spatial distribution of Pb and Cd was similar,and the changes in the concentrations of both were related to the amount of traffic;the spatial distribution of Cr and Ni was similar,and the high values of both were found in the shed and the surrounding cultivated land in Shigatse;the concentrations of As in the lower reaches of the Nianchu River were mostly lower than the background values,while the rest of the sites were mostly above background values;Hg exceeded background in most of the sites,and was generally high in the shed sample sites.The study of soil fertility in the study area found that the mean values of TOC,TN and TP in the study soils were 17.00 g/kg,1.43 g/kg and 0.69 g/kg,respectively.the content of TN in the study area was mainly in level 4 to 2 and the content of TP was mainly in level 4 to 3.The overall content of TN in the cultivated soil was higher than that of the shed,and the TP was significantly lower than that of the shed soil.The spatial distribution of the three found that the content of TOC was more evenly distributed in the study area as a whole,with the content of TN gradually decreased along the river direction and the content of TP gradually increased along the river direction.A study of the characteristics of changes in heavy metal concentrations in water in the study area from2019-2021 found that there was little change,the concentration of Hg was always below the detection limit,the average values of Cu,Pb,Zn,Cd and As were all lower than the limits of surface water category I standards,and the average concentrations of the eight heavy metals met the water quality standard for irrigated agricultural land.Based on the 2021 soil and water heavy metal concentration data in the study area for pollution risk evaluation,the single-factor pollution index found that the degree of Hg contamination in the soil of the study area was greater.The pollution of the eight heavy metals in the shed and the cropland was similar,with Cu,Zn,Cr,Ni,Cd and As mostly in light pollution and below,and Pb mainly showed no pollution.Compared with the cropland,As in the shed sample sites was mainly no pollution and lower than the pollution level of As in the cropland,while Cr,Ni and Hg were more polluted in the shed.The single-factor pollution indices of the six heavy metals in the water showed that Cu,Pb,Zn,Cd and As were all nonpolluting,while Cr was relatively more polluting.From the Nemerow pollution index,the overall soil in the study area was mainly light to moderate pollution.The overall pollution level of the cultivated land was lower than the pollution level of the shed;the Nemerow pollution index in water showed that the study area was mainly in a safe state.Combined with the water quality pollution index to see the study area water WQI value were less than 1,the overall in a pollution-free state.The cumulative ground contamination index showed that the eight heavy metals were mainly in the clean state.The mean values of the ground cumulative pollution indices for all the heavy metals were generally higher in sheds than in cultivated land,with the exception of As.Hg generally showed a lower level of pollution in both agricultural land use methods,while Cr and Ni showed a higher level of pollution in sheds than in cultivated land.The potential ecological risk index showed that the ecological risk of Cu,Pb,Zn,Cr and As in the study area was low,while Cd showed a moderate risk in most sample sites and a mild risk in some sample sites,with an overall low risk.The overall risk of Hg was higher than that of the other seven heavy metals,but it was an acceptable risk.The study area was dominated by moderate or lower ecological risk from the combined potential ecological risk index,and the ecological risk level was low.The spatial distribution of RI values of soil heavy metals showed a gradual increase from southeast to northwest,and the trend was consistent with the direction of river flow.The ecological risk of cultivated land was lower than the ecological risk of shed,but both ecological risks were at moderate and below risk,and the riskiness was low.In this paper,the sampling field situation in the study area was combined with correlation analysis,principal component analysis and positive definite matrix factorization(PMF)to analyze the sources of heavy metals in soil and water in the study area.The analysis concluded that there were four main sources of heavy metals in soil,namely: traffic pollution sources,agricultural activity sources,soil parent material sources,and mixed agricultural and atmospheric transport sources,in which Cu,Zn,Pb and Cd were mainly from traffic pollution sources,Cr and Ni were mainly from agricultural activity sources,As was mainly from soil parent material sources,and Hg was mainly from mixed agricultural and atmospheric transport sources.There were three main sources of heavy metals in water:traffic pollution sources,agricultural activity sources,and natural recharge sources,with Cu and Pb mainly from traffic pollution sources,Cr mainly from agricultural activity sources,Zn mainly from agricultural activity and traffic pollution sources,and As and Ni mainly from natural recharge sources.