Soil Contamination by Heavy Metals and Evaluations on Potential Ecological Risks in an Antimony Mine

Wei ZHANG,Jiantuan GE

1.Ministry of Environmental Protection of Heshui County,Qingyang City,Gansu Province,Qingyang 745400,China;2.School of Geography and Environmental Science,Northwest Normal University,Lanzhou 730070,China

Heavy metal contamination in mining areas and ecological remediation is a hot issue at home and abroad[1-5].During the process of mining,mineral powders would enter environment and accumulate in surrounding soils,even transform to toxicchemicalcompounds,posing threats to people through food chain.In the research,caving mining method was used for mining with forced ventilation installation and by the technique of crushing-ore grinding-flotation.Furthermore,the activator was Pb(NO)2.Based on sampling and analyzing of soils in the mining area,the contamination degree of heavy metal was researched and assessment was made on potential ecological risks,in order to provide references for evaluation on eco-environment and ecological remediation in the mining area.

Materials and Methods

Collection and treatment of soil samples

The antimony mine was located at 104°43′59″-104°50′00″N and 33°47′59″-33°50′59″E.In accordance with SoilEnvironment Monitoring Technology Standard,soil samples were in a farmland 1 km away from a tailings pond (1#),a farmland 150 m away from a tailings pond (2#),farmlands in a forestland of mines(3#)and 100 m away from a downstream area of mines(4#).In addition,sample areas were distributed in a plum blossom shape and samples were collected as per quartering.Every mixed sample was 1-2 kg and soils were measured after drying and sieving[6].

Measurement methods

As of soils was measured as per determination of total arsenic-Silver diethyldithiocarbamate spectrophotometry[7];Hg was as per determination of total mercury-cold atomic absorption spectrohotometry[8];Cr was by flame atomia absorption spectrophotometric method[9];Cu and Zn were by flame atomic absorption spectrophotometric method[10];Ni was by flame atomia absorption spectrophotometric method[11];Pb and Cd were by KIMIBK flame atomic absorption spectrophotometry[12];Sb was by atomic fluorescence spectrometry[13].

Evaluation method

On basis of single factor contaminant index,the pollution degree and potential ecological damages of heavy metals were evaluated as per N.L.Nemerow’s index and potential ecological risk index[14].

Single factor contaminant indexIt is a popular method at home and abroad for evaluating contamination degrees of a specific pollutant,as follows:

wherePiis single factor contaminant index;Ciis measured concentration value of an index;Sirefers to evaluation standard of a pollutant.WhenPi＜1,it suggests the samples are not polluted and whenPi＞1,it suggests that the samples are polluted.The bigger the value,the more serious the pollution.Except of Sb,evaluation on rest elements are conducted according to GB 15618—1995 II standard[15](Table 1)and Sb was evaluated in accordance with the reference[16](Sb≤3.5 mg/kg).

N.L.Nemerow indexIt is an index highlighting the damages of the most serious pollutanton environment,based on reflection of average pollution of individual pollutant in soils,as follows:

wherePrepresents N.L.Nemerow index;Piaverepresents averages of individual pollution index;Pimaxrefers to maximum of individual pollution index.

In accordance with single factor pollution index and N.L.Nemerow index,soil pollution by heavy metals can be classified into 5 grades and the evaluation standards keep the same(Table 2).

Potential ecological risk indexIt is proposed by Hakanson[16],a Sweden scientist,for evaluating heavy metals in soils and sediments from the perspective of sedimentology according to property and characters of heavy metals in environment.Firstly,the contents of heavy metals in soils should be measured to conclude single factor contaminant index through background values of heavy metals.Secondly,toxic response coefficient should be introduced to obtain single coefficient of potential ecological risk.Finally,potential ecological risk index of heavy metals in the region should be weighed,as follows:

whereRIrefers to comprehensive potential ecological risk index of different heavy metals;Eirrefers to potential risk coefficient of a heavy metal;Tiris toxic response coefficient of a heavy metal.According to standard toxic coefficeint of heavy metals formulated by Hakanson[16];Tiris the determined reference[17];potential ecological risks ofEirandRIwere shown in Table 3(18-19).

Results and Analysis

Contents of heavy metals in soils of research regions

Based on analysis of soil samples from 4 sample sites,the minoring results of heavy metals were shown(Table 4).It can be concluded that except of Hg,Zn and As,the contents of rest elements reached the peaks in the farmland 100 m away from downstream region of a mine(4#).

Evaluation results

Evaluation results of single factor contaminant coefficient and N.L.Nemerow indexAs shown in Table 5,single factorcontaminantindex of heavy metals in 4 sample sites tended to be volatile in the range from 0.039 0-0.867 3,of which the index of Ni in sample site 4#was grade 2,and the rest kept at grade 1.Furthermore,N.L.Nemerow indices of the 4 sample sites were 0.498 8,0.540 2,2.576 1,and 0.682 9,respectively,which indicated that soil quality of different sample sites is clean,and safe.

Evaluation results of potential ecological risk indexAs shown in Table 6,potential risk indices of heavy metals were of 0.231 3-14.120 0,which incorporated that potential risk maintained slight of heavy metals.Furthermore,the comprehensive potential ecological risk indices were 16.718 1,29.262 6,19.387 0,and 39.307 0,respectively.In general,the pollution degree of heavy metals is lower and operation of the antimony mine has small effects·soil pollution.

Conclusions

The monitoring values of Cd,Cr,Cu,Zn,Pb,Hg,As and Ni were all loweraccording to Environmental quality standard for soils(GB 15618-1995)and the value of Sb was lower than 3.5 mg/kg.It can be concluded from single factor contaminant index that Ni pollution was grade 2 in sample site 4#,and the rest were all grade 1.In accordance with N.L.Nemerow index,the 4 sample sites all kept safe and soil quality from high to low was the farmland 1 km away from downstream region of a tailings pond(1#),a farmland 150 m away from a tailings pond(2#),farmlands in a forestland of mines (3#)and 100 m away from a downstream area of mines(4#).

Table 1 Environmental quality standard for soils mg/kg

Table 2 Classification of pollution standards in soils by heavy metals

Table 3 Grading of potential ecological risks

Table 4 Monitoring results of heavy metals in soils in the mine mg/kg

Table 5 Single factor contaminant coefficient and N.L.Nemerow index

Table 6 Evaluation on potential ecological risks in soils of the research region

The evaluations as per potential ecological risk index indicated that both of single and comprehensive potential ecological risks kept light of the 4 sample sites and the 4 sample sites all kept safe and soil quality from high to low was the farmland 1 km away from downstream region of a tailings pond (1#),a farmland 150 m away from a tailings pond(2#),farmlands in a forestland of mines(3#)and 100 m away from a downstream area of mines(4#).

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