| Recent years,there is a growing concern about heavy metals due to their adverse effects on soil ecology and human health.Generally,the origin of heavy metals in soil may be either geogenic source or anthropogenic input.So far,a large number of studies have investigated the farmland pollution of heavy metals caused by human activities,and results showed that human activities have caused serious pollution to farmland soil and ecological environment.However,for arable land pollution caused by heavy metals relating with the weathering of bedrock with high concentration of heavy metals,most of the current researches mainly focus on the ultramafic rock.As a type of mafic volcanic rock,basalt enriches transitional metal elements,and it is susceptable to weathering under surficial circumstances,so it is an important potential heavy metals source for agricultural land.Basalts are widely distributed on the Earth surface,covering about 5%of the continental area.Farmland is widely distributed on these basaltic areas,but the distribution characteristics,health risks,cumulative patterns,and controlling factors of heavy metals in these farmland soils and crops are still unclear.With the purpose of exploring the environmental effects of heavy metals in basaltic agricultural land,this study selected the farmland derived from Cenozoic basalt in eastern China,1)determined the content of heavy metals in basalt bedrock,farmland topsoil and crops(wheat and rice)plant,2)investigated the enrichment pattern and accumulation feature of heavy metals in basaltic farmland topsoil and crops plant,3)revealed controlling factors of Nickel in topsoil and their spatial variability and 4)estimated the human health risks of heavy metals in food crop grains.Combined with the bioavailability of Ni in topsoil,the mitigration behavior of Ni from soil to rice plant was analyzed.The applicability of Ni risk screening value in the newly issued national soil environmental quality:risk control standard for soil contamination of agricultural land(GB15618-2018)was discussed.The main results are listed as follows:1.Compared with the upper continental crust(UCC),the basaltic bedrock and topsoil are significantly enriched in Cr,Cu and Ni.Among them,Ni is the most enriched in the topsoil of the farmland in Xuyi County(the enrichment foctor is 2.33),while Cr is the most enriched in the Leiqiong area(the enrichment factor is 3.58).The relationship of heavy metal concentrations between basaltic bedrock and topsoil indicates that there is an inheritance of heavy metals from bedrock to topsoil in agricultural lands in the study area.2.The Ni concentration in basaltic agricultural land topsoil in Xuyi County is higher than the soil environmental quality:risk control standard for soil contamination of agricultural land(GB15618-2018)in more than 70%sampling sites,and the over-standard rates of Ni and Cr in Leiqiong area are as high as 80%and 76%,respectively.Except for Ni,there is a slight Cu over-standard situation in Xuyi County,and a certain degree of Cu and slight Zn exceeding the standard in Leiqiong area.3.According to the national food safety standard:limit of contaminants in food(GB2762-2017),the concentration of Cr and Pb in wheat grain is above the limit in less than 5%sampling sites.Similarly,the over-standard rate of Cd and Pb in rice grain is less than 5%sampling sites in Xuyi County.However,both wheat grain and rice grain showed high Ni concentrations(1.42 mg/Kg and 3.71 mg/Kg,respectively).Since the standard lacks the Ni limit,if set 1 mg/Kg as safety reference limit,then,the Ni concentration will be above the limit in more than 65%and 100%sampling sites for wheat and rice grain,respectively,in Xuyi County.In Leiqiong area,the concentration of Cr and Ni is above the limit in rice grain in more than 9%and 10%sampling sites,respectively,while that for Cd and Pb is less than 2%.4.The results of health risk assessment showed that there is a non-carcinogenic risk in rice grain due to heavy metals(non-carcinogenic factor HI=3)in Xuyi County.Among them,Cr,Ni and Cu are the largest contributors,and the total rate exceeds 60%.Both wheat and rice grains in the region show a carcinogenic risk(carcinogenic factors are one to three order of magnitude higher than the safety reference value),of which Cd is the largest contributor with a contribution rate of more than 70%.5.The enrichment of Ni in topsoil of basaltic agricultural land in Xuyi County is closely related to iron oxides,while in Leiqiong area,it is mainly aluminum oxides,followed by iron oxides.Soil pH is an important factor affecting the accumulation of Ni in crops in Xuyi County,but it does not seem significant in Leiqiong area.There is a significant linear correlation between the concentration of Ni in crop grains and 0.01 M CaCl2 extractable in topsoils in Xuyi County,but no significant correlation in Leiqiong area.The concentration of Ni in different parts of rice plant from high to low ranks as root>grain>shoot,which indicates that Ni tend to migrate to the top in rice plant.6.The relationships of topsoil pH,total Ni concentration with rice grains Ni concentration in study area indicate that the risk screening value of Ni in the newly issued soil environmental quality:soil pollution risk control standard for soil contamination of agricultural land(GB15618-2018)is too high and not applicable for here.It is necessary to revise the risk screening value of Ni in basaltic farmland in the study area. |
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