Evaluation of Heavy Metal Pollution in Dazhai Mellow-soil Field

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College of Geographical Science, Shanxi Normal University, Linfen 041004, China

EvaluationofHeavyMetalPollutioninDazhaiMellow-soilField

JingjingZHANG,YongqingZHANG*,HuaZHANG,WenjuanZHANG

College of Geographical Science, Shanxi Normal University, Linfen 041004, China

In order to determine the pollution condition of heavy metal in Dazhai mellow-soil field, the content of Zn, Cr, Cu, Ni, Pb, As, Cd and Hg in this area were analyzed, the content of heavy metals in soils inShanxiSoilEnvironmentQualityStandardand the secondary standard ofNationalSoilEnvironmentQualityStandardwere used as evaluation standard. Based on the single factor index method, the result showed that the content of 8 heavy metals in soil didn’t exceed the secondary standard ofNationalSoilEnvironmentQualityStandard, while the content of As, Hg, Ni, Pb exceeded Shanxi Soil Environment Quality Standard. The soil in the study areas reached high pollution of As, Hg, Pb, and the pollution index of Cu, Ni, Zn, Cd reached a high level. The potential ecological risk index order was Hg﹥Cd﹥As﹥Pb﹥Ni﹥Cu﹥Zn﹥Cr. The pollution level risk intensity was mild. The correlation analysis indicated that Cr, Ni, Zn, Cu, Cd and As were probably from the same source, while Hg and other heavy metals were not correlated. Heavy metals with high levels of pollution were closely related to agricultural fertilization, tourism, transportation, construction, daily life and production activities.

Dazhai mellow-soil field, Soil, Heavy metal pollution, Ecological risk

1 Introduction

Soil bears about 90 percent of domestic wastewater, solid waste, pesticides, fertilizers and atmospheric dust and other contaminants from the environment, which is bound to affect the soil quality and the quality of human existence[1]. Soil pollution has become a global environmental issue, and soil heavy metal pollution is one of the most serious problems[2]. As a persistent toxic pollutant, heavy metal can not be broken down by microbe after entering into the soil environment; instead, it can go through human body and accumulate by sucking food, inhalation and skin contact and other means, causing direct harm to human health, especially children[3]. Some biologically toxic heavy metals such as Pb, Cd, Cr, As and Hg have attracted more and more attention of scholars[4-12]. Dazhai mellow-soil field is the terraced basic farmland built by Dazhai people in the 1960s-1970s, with the features of thick living soil, good structure, rich nutrients, active microorganism and high and stable yield, but after 40 years, it is not certain about whether large quantities of chemical fertilizers, and especially the "three wastes" from the surrounding plant and emerging tourism in recent years will cause pollution to mellow-soil field or not. There is not yet any relevant research about this recently. Therefore, this paper uses single pollution index, Nemerow comprehensive pollution index and potential ecological risk index to assess the soil heavy metal pollution and potential ecological risk of Dazhai mellow-soil field, with a view to provide a scientific basis pollution control of mellow-soil field and agricultural zoning

2 Study materials and methods

2.1OverviewofthestudyareaDazhai Village is located in the east of Xiyang County in Shanxi Province, with a total area of about two square kilometers. The elevation is about 1000 meters, the annual frost-free period is about 150 days, and the annual average temperature is 9.1℃. The annual rainfall is 500-700 mm, mostly concentrated in July, August and September. It has dry and windy springs and rainy summers. Formerly, it was a rocky mountain area with many natural disasters every year[13]. This study takes Langwozhang mellow-soil terrace in Dazhai as the study area, where corns, soybeans walnuts and other crops are planted.

2.2SamplecollectionandanalysisBased on the actual survey and study in Dazhai, we select Langwozhang mellow-soil terrace as the sampling area. The sampling points are evenly distributed in each terrace according to its area, and GPS is used for accurate positioning. Using serpentine sampling method, 0-20 cm of topsoil is collected as test sample, and each soil sample consists of five sampling points. Using sample quartering method, about 1 kg of soil is taken as one sample. The soil samples collected are naturally dried indoors, and screened by 20-mesh, 60-mesh and 100-mesh nylon sieves respectively after removing large stones, plant roots and other impurities. Then the samples are labeled for determining the physical and chemical properties and heavy metal content of soil. HNO3-HCIO3is used for digestion of Hg and As, and AFS-820 dual channel atomic fluorescence spectrometer is used to measure the content[14]; HNO3-HCIO3is used for digestion of Cd, Cr, Zn, Ni, Cu and Pb, and nov-AA400 flame-graphite furnace atomic absorption spectrometry is used to measure the content[15]. It is repeated three times and averaged.

1.3EvaluationofheavymetalpollutionUsing single factor pollution index and Nemerow comprehensive pollution index, we evaluate the heavy metal pollution, and the soil pollution grading standard is based on GB15618-1995. Using potential ecological risk index, we evaluate the potential ecological risk of Dazhai mellow-soil field, and it is conducted based on the method of[18-21]. The ecological risk coefficient of heavy metal pollution and ecological risk index classification standard can be shown in Table 1.

Table1Thepotentialecologicalriskindexclassificationstandardforheavymetalpollution

EiEcologicalhazarddegreeRIEcologicalhazarddegreeEi<40MildRI<150Mild40≤Ei<80Medium150≤RI<300Medium80≤Ei<160Strong300≤RI<600Strong160≤Ei<320VerystrongRI≥600VerystrongEi≥320Extremelystrong

3 Results and analysis

3.1DistributionofheavymetalsinsoilTable 2 shows that Cr, Cu, Zn and Cd in Dazhai mellow-soil field do not exceedShanxiSoilEnvironmentQualityStandard, but As, Hg, Ni and Pb are over the standard. In all soil samples, the standard exceeding rate of heavy metals is in the order of As (76%)> Hg (69%)> Pb (67%)> Cd (53%)> Ni (52%)> Cu (44%)> Zn (35%)> Cr (0%). It demonstrates that there is serious accumulation for Hg, As and Pb, while there is a certain degree of enrichment for Cd, Ni, Cu and Zn in some sampling points. However, withNationalSoilEnvironmentQualityStandard(GB 15618-1995) as a reference, the average and maximum content of eight kinds of heavy metals in Dazhai mellow-soil field does not exceed the secondary standard ofNationalSoilEnvironmentQualityStandard. The coefficient of variation is a measure of the degree of variation in samples, and if the coefficient of variation is large, it indicates that the soil is significantly subject to outside disturbance, the spatial differentiation is obvious, and the soil pollution exists in the form of complex pollution[22]. CV≤10% is weak variation; 10% Ni> Cd> Pb> Zn> Hg> Cr> As.

Table2StatisticaldescriptionofheavymetalsinDazhaimellow-soilfield

ElementMinimummg/kgMaximummg/kgAveragemg/kgStandarddeviationmg/kgCoefficientofvariationCV∥%Backgroundvalue1Valuemg/kgStandardexceedingrate∥%Backgroundvalue2Valuemg/kgStandardexceedingrate∥%As3.38819.44911.1133.72633.529.1076200Cr2.27745.72627.61810.39337.6361.802500Cu0.01443.67921.81412.74558.4326.9441000Hg0.0010.4830.0280.01138.250.0236910Ni2.47459.83332.59915.73848.283252600Zn18.752142.69071.97630.66042.6075.5353000Cd0.0080.2470.1210.05445.070.128530.60Pb0.54330.88419.0148.12842.7515.8673500

Note: Background value 1 is the background value of soil elements in Shanxi Province; background value 2 is the secondary standard ofNationalSoilEnvironmentQualityStandard.

3.2CorrelationbetweenheavymetalelementsinDazhaimellow-soilfieldThe correlation between elements is significant or extremely significant, indicating that there is the same source or complex pollution relationship between elements[23]. From Table 3, it is found that As is significantly correlated with Cr, Ni and Zn; Cr is significantly correlated with As and Zn; Cu is significantly correlated with Ni and Cd; Ni is significantly correlated with Cd, As and Cu; Zn is significantly correlated with Cr, As and Pb; Cd is significantly correlated with Ni and Cu. It indicates that there is a close relationship between these heavy metals, and it can be inferred that these heavy metal elements are from the same pollution source or associated. Hg is not significantly correlated with the other seven kinds of metal elements, indicating that there is a heterologous relationship between Hg and other metal elements.

Table3ThecorrelationcoefficientofheavymetalcontentofDazhaimellow-soilfield

AsCrCuHgNiZnCdPbAs1Cr0.430**1Cu-0.087-0.232*1Hg0.0840.019-0.0631Ni0.324**0.0230.510**0.0661Zn0.220**0.281**-0.597**0.157-0.202*1Cd0.099-0.0090.306**0.0900.493**-0.0501Pb-0.093-0.324**-0.1090.098-0.0620.362**-0.0131

Note: ** denotes significant correlation at the 0.01 level; * denotes significant correlation at the 0.05 level.

3.3EvaluationofheavymetalpollutioninDazhaimellow-soilfieldFrom Table 4, it can be found that the pollution index of Cr is in the security domain; the pollution index of Cu, Ni, Zn and Cd is at a dangerous level; As, Hg and Pb are at a moderate pollution level. The pollution level of eight kinds of heavy metals is in the order of As=Pb=Hg>Ni=Zn>Cd>Cu>Cr. The evaluation results of comprehensive pollution index show that the comprehensive pollution index of Dazhai mellow-soil field is 1.4, at a moderate pollution level.

Table4HeavymetalpollutionindexofDazhaimellow-soilfield

AsCrCuHgNiZnCdPbComprehensivepollutionindexComprehensivepollutionindex1.20.40.81.21.01.00.91.21.4

3.4PotentialecologicalriskindexofheavymetalsinDazhaimellow-soilfieldFrom Table 5, it can be found that the potential ecological risk is in the order of Hg (49.39)> Cd (28.26)> As (12.21)> Pb (6.02)> Ni (5.09)> Cu (4.05)> Zn (0.95)> Cr (0.89). The potential ecological risk index of Hg is at the middle level, while the potential ecological risk index of other heavy metal elements is low. From the frequency statistics of potential hazards corresponding to the coefficient of potential hazards of heavy metals in soil, it is found that the potential ecological risk of As, Cr, Cu, Ni, Zn and Pb is at a low hazard level, and not going to cause harm to agricultural production. For Hg, there is strong risk of harm for 1%, there is moderate risk of harm for 68%, and there is slight risk of harm for only 31%. For Cd, there is moderate risk of harm for 16%, and there is slight risk of harm for 84%. Obviously, the ecological risk of Hg and Cd is most serious, and makes the maximum contribution to total potential ecological risk index. It is calculated that the potential ecological risk index of heavy metals in Dazhai mellow-soil field is 106.88, indicating that there is a slight risk of harm in Dazhai mellow-soil field on the whole.

Table5PotentialecologicalriskcoefficientofheavymetalsinDazhaimellow-soilfield

ElementEiProportion∥%MildMediumStrongVerystrongExtremelystrongAs12.211000000Cr0.891000000Cu4.051000000Hg49.393168100Ni5.091000000Zn0.951000000Cd28.268416000Pb6.021000000

3.5SpatialdistributionofmoderatepollutionelementsandsourceanalysisAs can be seen from Fig. 1, there are some similarities in the spatial distribution of As, Hg, Pb and Cd in Dazhai mellow-soil field (mainly in the central area), possibly because these sampling points are near roads and they are greatly affected by exhaust emissions and loss of parts of traveling vehicles to Dazhai. Several high-value points are scattered in the northern area, mainly because it is near Dazhai residential areas and there are construction sites nearby, having a large degree of influence on the soil. For Pb, the content value is also very high in the northwestern and southwestern areas. The high content in the northwestern area is related to the past coal mines lying in the northwest of sampling area, while the high content in the southwestern area is due to the fact that it is adjacent to road and greatly influenced by motor vehicle. Corn was previously grown in the southwestern area, and fertilization was conducted 3-4 times a year. Long-term use of livestock manure and fertilizer may be the cause of high content of Pb in the region.

Fig.1 Spatial distribution of As, Hg, Cd and Pb in Dazhai mellow-soil field

4 Conclusions and discussions

In this study, we use single factor pollution index, Nemerow comprehensive pollution index and potential ecological risk index to evaluate the heavy metal pollution in Dazhai mellow-soil field. The results show that the three methods reflect the light pollution levels of Dazhai mellow-soil field as a whole, demonstrating the consistency of the three methods, but because of the different evaluation methods, the main pollution elements have different manifestation. The main reason may be that the single factor pollution index makes the evaluation parameters unconnected, and can only reflect the soil pollution; in the evaluation of soil quality, Nemerow index makes comprehensive evaluation of a large number of soil samples; potential ecological risk index takes into account the difference in the toxicity of various heavy metal elements and the regional background of heavy metals, and can fully reflect the effect of heavy metals on the environment. This is consistent with the results of Dong Miao, Zheng Hailong, Yan Jiaoetal.[14, 24, 25], which fully shows that in the future evaluation of soil heavy metal, it is necessary to combine a variety of evaluation methods to evaluate soil heavy metal, in order to having a comprehensive understanding of soil quality in the region, and provide scientific basis for soil pollution prevention and control. Both the average and maximum of eight heavy metals in Dazhai mellow-soil field do not exceed the secondary standard ofNationalSoilEnvironmentQualityStandard, while the average and maximum of As, Hg, Ni and Pb exceedShanxiSoilEnvironmentQualityStandard. From the coefficient of variation, the variation of eight kinds of heavy metals is moderate, and overall, the content of each metal element in the soil is still relatively stable. From the correlation, it can found that there is a high degree of correlation among As, Cr, Ni, Zn, Cu and Cd; Hg is not correlated with other heavy metals. With Shanxi soil environment as background value, it is concluded that the pollution index of Cr is at the safe level; the pollution index of Cu, Ni, Zn and Cd is at the dangerous level; As, Hg and Pb are at the light pollution level; the comprehensive pollution index is at the light pollution level. According to the potential ecological risk index, there is a slight risk of harm in Dazhai mellow-soil field on the whole. The potential ecological risk index of Hg is at the middle level and the ecological risk of Hg is most serious, while the potential ecological risk index of other heavy metal elements is low.

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September 20, 2015 Accepted: November 22, 2015

Supported by National Natural Science Foundation of China (30871483).

*Corresponding author. E-mail: yqzhang208@163.com