Evaluation And Source Apportionment Of Heavy Metal Pollution In Cultivated Soil Around A Coal Gangue Accumulation Area

Chongqing is rich in coal resources and has numerous large and small coal mines,which play an important role in promoting the economic development of Chongqing.However,a large amount of coal gangue remains in the process of coal mining,which continuously affects the surrounding environment.Heavy metal elements in coal gangue are released after long-term weathering and leaching,and accumulate continuously in the surrounding soil through rainfall erosion and surface runoff,resulting in heavy metal pollution in soil and gradually transferred to crops,which further causes greater harm to human beings through the food chain.It has become an important ecological and environmental problem restricting the ecological civilization construction and green development of Chongqing.In this study,the cultivated soil around a gangue accumulation area in Chongqing was selected as the research object to investigate the contents of eight heavy metals in soil and crops.Multiple statistical analysis,receptor models,isotope tracer and other analysis methods were used to systematically explore the characteristics,source apportionment and health risks of heavy metals in soil and crops,in order to provide scientific basis for the prevention and control of heavy metal pollution sources and safe utilization of cultivated soil.The main results are as follows:（1）The average values of Cd,Cr,Ni,Cu,Zn,As,Pb and Hg in the soil of the study area were 1.19,57.69,30.46,23.56,85.77,9.27,30.23 and 0.21 mg·kg^-1,respectively.The average values of Cd,Zn,As,Pb and Hg were 10.82,1.07,1.85,1.16 and 3.50 times higher than the background values of the soil in Chongqing city,with significant enrichment of Cd and Hg.Compared with the risk screening and control values in the“Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land”（GB15618-2018）,the excessive rates of Cd were 100%and 4%,respectively,indicating the most serious pollution.（2）The three crops of rice,corn and sweet potato in the research area had a significant enrichment effect on Cd,Cu and Zn.Among them,37.50%of the corn Cd exceeded the corresponding standard of the“National Food Safety Standards Limits of Pollutants for Foods”（GB2762-2022）,while the heavy metals in other crops did not exceed the standard.Therefore,in the prevention and control of soil heavy metal pollution,the accumulation characteristics of heavy metals in crops should be fully considered,and Cd pollution should be paid special attention to and the key control should be carried out.（3）The spatial and vertical distribution of heavy metals in soil indicated that as the distance from the coal gangue accumulation area increased,the overall heavy metal content in the soil showed a decreasing trend.The heavy metal content in the surface soil was generally higher than that in the bottom layer,and the heavy metals in the study area were more significantly affected by human activities.Based on a variety of pollution assessment methods,it can be concluded that the heavy metal in the soil of the study area was generally at a severe pollution level and had a strong ecological risk.Among them,Cd pollution was the most severe and was the main contributing factor to the comprehensive ecological risk.It should be a priority to control pollutants in the study area and carry out corresponding control measures.（4）APCS-MLR and PMF models both identified three same pollution sources,namely coal gangue leachate input,agricultural natural mixed source,and traffic source.In the APCS-MLR model,the average contribution rates of the three pollution sources were 46.43%,24.47%and 29.10%,respectively.In the PMF model,the average contribution rates of the three pollution sources were 40.92%,32.23%and 26.85%,respectively.The two models can complement and verify each other,providing theoretical support for later management departments to develop soil heavy metal source control measures.The Cd isotope tracer results were consistent with the source apportionment results,indicating that the source of Cd pollution in surface soil was mainly related to the input of coal gangue leachate,and it could penetrate into deep soil.（5）The results of soil heavy metal health risk assessment indicated that oral ingestion was the most important exposure route to human health risks.The non-carcinogenic risks and carcinogenic risks of adults and children under the three exposure routes（oral ingestion,skin contact and respiratory inhalation）were all lower than the corresponding threshold,and the impact of heavy metals in soil on the health risk of local residents was relatively small.Coal gangue leachate input and agricultural and natural mixed sources contributed significantly to the non-carcinogenic and carcinogenic risks,accounting for70.81%and 74.85%respectively.They were the main sources of health risks for local residents and should be taken as key risk control pollution sources.（6）The results of crop health risk assessment showed that the consumption of rice was the main pathway of crop intake leading to health risks for residents in the study area.The total non-carcinogenic risks for adults and children were 10.92 and 23.35,respectively,and the total carcinogenic risks were 2.58×10^-3 and 1.41×10^-3,respectively,both exceeding the corresponding thresholds.Therefore,measures such as planting structure adjustment and crop variety improvement should be taken to reduce the health risks of food crops in the study area.In summary,this study conducted a systematic discussion on the pollution characteristics,source apportionment and health risks of heavy metals in soil and crops,aiming at comprehensively mastering the level of heavy metal pollution in soil,and further revealing the sources of heavy metal pollution in soil and the health risks to local residents,in order to provide a scientific basis for the prevention and control of the source and safe utilization of heavy metal pollution in cultivated soil in such areas.