| Mining and smelting activities were considered to be one of the most important sources of heavy metal pollution in soils,causing ecological pollution and posing a great threat to the human health of the surrounding residents.Yunnan Province is rich in mineral resources,but some areas along rivers in province where non-ferrous metal mining and smelting activities are concentrated have problems with excessive heavy metals in soil and watershed waters.In this study,the heavy metal mining area in a typical small watershed in Yunnan Province was selected as the research area.The concentration of heavy metal cadmium(Cd)was determined and analyzed in the soil samples from the surface and different depths of villages around the mining area,as well as the samples from crops and river sediment.The pollution degree of Cd in the two functional areas of the study area was evaluated by four heavy metal pollution evaluation methods: RAC morphological analysis,geo-accumulation index,single factor index and potential ecological risk assessment.Moreover,the horizontal and vertical distribution of Cd in soil was determined.Based on the heavy metal health risk assessment model recommended by the United States Environmental Protection Agency(US EPA),the risks of Cd in soil to human health through three exposure pathways of hand-mouth intake,respiratory inhalation and skin contact were evaluated,and the and the differences in exposure risks between children and adults were compared and analyzed.Combined with the topography of small watershed,the unique input pathways of soil erosion on sloping land were determined and the contribution of each pathway was accurately quantified.On this basis,the accumulation trend of Cd in farmland soil in the future was predicted by multi-scenario dynamic simulation,which provided a theoretical basis for the remediation of local Cd contaminated soil.The main research results obtained in this study are as follows:1.The Cd average concentration in the soils of the study area were much higher than the national and Yunnan soil background values,and it showed strong spatial variability due to the influence of external sources.With reference to the contamination concentration of Cd at different soil p H ranges,more than half of the soil samples exceeded the contamination risk screening and risk control values.The spatial distribution characteristics of Cd concentration showed a trend of gradually decreasing from upstream to downstream due to upstream mining activities,and the vertical distribution of Cd concentration trended towards decreasing with depth.2.The results of the RAC evaluation showed that the regional agricultural soils were at an overall medium risk.Both the agricultural soil and construction soil in the study area were polluted by Cd.As for the the two functional areas,the pollution levels of the geo-accumulation index were pollution-free to medium pollution and medium pollution levels,respectively.The pollution levels of the single factor pollution index have reached the level of heavy pollution,and the heavily polluted soil samples occupy the vast majority.The potential risk index has also reached a high potential risk level.Overall,the pollution of Cd in construction soil is more serious than that in agricultural soil.The results of the health risk assessment showed that the carcinogenic and non-carcinogenic risk indices for residents in the study area did not exceed the safety thresholds,and the children and adults were not yet exposed to health risks.However,the health risks of Cd exposure to children were higher than that of adults,so more attention should be paid to the management of children’s activities and habits outside the study area.3.The results of the input-output flux study showed that irrigation watersource was the main factor affecting the input flux of Cd in agricultural soils.Specifically,river irrigation accounting for over 75% of the total Cd input flux and slope erosion also contributing 10% to the Cd input flux.Cd outport fluxes through crop harvesting did not contribute significantly due to the low enrichment of Cd by crops in the study area.Output fluxes from leaching and runoff were limited and overall input fluxes were greater than output fluxes,so Cd was still accumulating in the study maize field soils.If the current input fluxes were maintained,straw retmoving to the field could significantly reduce the annual increase of Cd in the farm soils.If clean water is used instead of river water for irrigation,the concentration of Cd in farmland soil will be significantly reduced.In the future,crops yields will increase significantly,resulting in the output of crop harvest to Cd increasing year by year.The results of this study can provide theoretical data support and strategic decision guidance for the prevention and control of soil heavy metal pollution in typical small watershed metal mining areas in Yunnan Province. |
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