| Heavy metal pollutants in farmland soils not only disturb crop growth and development,decrease their quality and yield,but also could be accumulated in the edible parts of crops,and then be introduced into target organs of human body via food chain,which is very harmful to human health.For the past few years,the Chinese government and the public have paid more and more attentions to environmental conservation,and gradually increase financial investment on pollution control.In July 2015,the Ministry of Finance and Ministry of Environmental Protection of the People's Republic of China decided to preferentially support the comprehensive prevention and control of heavy metal contamination in 30 regions nationwide,among which Baiyin district of Gansu province ranked the first due to its extremely severe pollution.This study surveyed the pollution status of several major heavy metals(Cd,Cu,Pb,Zn,Cr,Ni and Mn)in the soils of the agricultural area of the Dongdagou watershed,Baiyin district,Gansu province,NW China.On this basis,we delineated the spatial distribution maps of these tested elements,analyzed the effects of land uses on heavy metal levels,and further investigated their potential sources.According to heavy metal bioavailability in soils and their bioaccessibility in grains,we elucidated the transfer mechanism of heavy metals in the soil-crop-human system with the aid of linear regression analysis,and also evaluated the associated health risks of heavy metals to local adult male inhabitants via grain consumption.The results showed that:1.In the agricultural area of the Dongdagou watershed,Cd was the priority pollutant in soils,whose concentration was far more than its background in Gansu soils and the sated safe limit in Chinese soil quality standard of GB15618-1995.In addition,the concentrations of soil Cu,Pb and Zn also exceed the respective backgrounds,but below the relevant safe limits,while the concentrations of soil Cr,Ni and Mn were not obviously higher than their respective backgrounds.2.The levels of heavy metal concentrations for Cd,Cu,Pb and Zn in farmland soils in this region gradually decreased with the distance away from Dongdagou stream.Land uses had a remarkable effect on the levels of soil heavy metals,which were relatively higher in farmland soils than non-farmland soils(p < 0.05).This indicated that long-term agricultural activities could elevate the concentrations of soil heavy metals.3.Within the Dongdagou watershed,irrigation with sewage from Dongdagou stream was the primary source for Cd,Cu,Pb and Zn in farmland soils.With the strengthen of environment awareness and the development of pollution control,this type of sewage irrigation had been prohibitted,and irrigation with water from the Yellow River became the major source of heavy metal pollutants in soils,followed by agrochemical utilization.Besides,atmospheric deposition especially contributed to the accumulation of Cd and Zn in the soils.4.Within the Dongdagou watershed,due to geographically closer to local industrial zone,the soil-spring wheat system was more severely contaminated with heavy metals than the soil-maize and soil-rice systems(p < 0.05).For the tested elements,the bioavailability in soils and the bioaccessibility in grains were relative to their pollution degrees,where the heavier the pollution were,the higher levels the bioavailability and bioaccessibility were.5.In the soils of the Dongdagou watershed,the bioavailable concentrations of Cd,Cu,Pb and Zn were significantly and positively correlated with the total ones(p < 0.001).According to this study,DTPA(Diethylenetriaminepentacetic acid)-extracted Cu,CH3COOH-extracted Cd and Pb,and HCl-extracted Zn and Ni were the best indicators to predict their uptake by grains(R2 = 0.549-0.971,p < 0.001).6.According to the results of RIVM(Rijksinstituut voor Volksgezondheid en Milieu)in vitro simulated digestion,the bioaccessible concentrations of heavy metals in grains were relatively low in the studied area.Significant linear correlations were found between the bioaccessible and total metal concentrations for Cd,Cu,Zn and Ni(R2 = 0.518-0.666,p < 0.001).Thus,internal exposure dose of these elements in local residents via grain consumption could be evaluated based on the total metal concentrations in grains.7.Results of human health risk assessment with bioaccessible metal concentrations indicted that long-term consumption of spring wheat grains might exert negative effects on local residents,and special attentions should be given to Pb and Cd in spring wheat grains.All in all,the current study made clear the pollution status of several major heavy metals in the topsoils of farmland soils within the Dongdagou watershed,Baiyin district,Gansu province,and also shed lights on their spatial distribution patterns and potential pollution sources,which would provide local government with reliable foundation for the comprehensive prevention and control of heavy metal pollution.More importantly,this study elucidated the transfer mechanism of heavy metals in the soil-crop-human system,which verified that the potential bioaccumulation of heavy metals in human body and the associated risks could be well predicted by the regression functions derived from heavy metals in soils and grains.These above conclusions could be further used to guide local grain production and protect human health. |
PDF Full Text Request