Chemosphere-2007-67-2148-2155

Survey of heavy metal pollution and assessment of agricultural soilin Yangzhong district,Jiangsu Province,ChinaS.S.Huang *,Q.L.Liao,M.Hua,X.M.Wu,K.S.Bi,C.Y.Yan,B.Chen,X.Y.ZhangGeological Survey of Jiangsu Province,Nanjing 210018,ChinaReceived 10January 2006;received in revised form 10November 2006;accepted 11December 2006Available online 2February 2007AbstractWe investigated concentrations of Hg,Cd,Pb,Zn,Cu,As,Ni,and Cr in samples of soil,cereal,and vegetables from Yangzhong district,pared to subsoils,the sampled topsoils are enriched in Hg,Cd,Cu,Pb,Zn,and As.High levels of Cd and Hg are observed in most agricultural soils.Concentrations of Cr and Ni show little spatial variation,and high Cu,Pb,and Zn contents cor-respond well to areas of urban development.High As contents are primarily recorded at the two ends of the sampled alluvion.The con-tents of Cd,Hg,and total organic carbon (TOC)increase gradually to maximum values in the upper parts of soil profiles,while Cr and Ni occur in low concentrations within sampled profiles.As,Pb,Cu,and Zn show patterns of slight enrichment within the surface pared to data obtained in 1990,Cd and Hg show increased concentrations in 2005;this is attributed to the long-term use of agro-chemicals.Cr and Ni contents remained steady over this interval because they are derived from the weathering of parent material and subsequent pedogenesis.The measured As,Cu,Pb,and Zn contents show slight increases over time due to atmospheric deposition of material sourced from urban anthropogenic activity.Low concentrations of heavy metals are recorded in vegetables and cereals because the subalkaline environment of the soil limits their mobility.Although the heavy metal concentrations measured in this study do not pose a serious health risk,they do affect the quality of agricultural products.Ó2006Elsevier Ltd.All rights reserved.Keywords:Distribution;Heavy metals;Vegetables;Yangzhong district1.IntroductionAgricultural soil has both direct and indirect influences on public health via food production;it is therefore of great importance to protect this resource and ensure its sustain-ability.The rapid development of industry and increasing release of agrochemicals into the environment has led to growing public concern over the potential accumulation of heavy metals within agricultural soil (Alloway,1990;Wong et al.,2002).Recent rapid economic expansion in China means that heavy metal contamination of agricul-tural soils has become an increasingly serious problem (Li et al.,1997a,b;Chen et al.,1999).Vegetables take upheavy metals by absorbing them from contaminated soils or from atmospheric deposition from polluted air.Chronic low-level intake of heavy metals has a negative effect on the health of humans and other animals,and no known medi-cal treatment is able to reverse these health effects.The Yangzhong region,in southern Jiangsu Province,China,is the second-largest alluvion along the Yangtze River (Fig.1),with a total area of 332km 2and a popula-tion of 0.28million.The region is situated between lati-tudes 30°000N and 32°190N and longitudes 119°420E and 119°580E,at an elevation of 4.5m above sea level.The area has a warm and humid subtropical climate with an average annual temperature and rainfall of 17.2°C and 1203mm,respectively;this makes the study area an important pro-duction base of foodstuffs and vegetables in Jiangsu Prov-ince.The soil in the study area is a typical paddy soil formed on calcareous deposits of the Yangtze River.0045-6535/$-see front matter Ó2006Elsevier Ltd.All rights reserved.doi:10.1016/j.chemosphere.2006.12.043*Corresponding author.Tel.:+862584368733.E-mail address:shunshengh@ (S.S.Huang)./locate/chemosphereChemosphere 67(2007)2148–2155In the past three decades,the use of agrochemicals in this region has increased in an effort to improve soil fertil-ity.According to the Agriculture Technique Organization,the use of chemical fertilizers in Yangzhong increased from 1.2T ha À1in 1972to 3.2T ha À1in 1995,and the use of pes-ticide increased from 80to 165kg ha À1over the same per-iod.As some of these fertilizers and pesticides contain heavy metals such as Cd,Hg,Pb,and Zn (Kabata-Pendias and Pendias,1992;Shi,1992),the continuous application of these agrochemicals and other soil amendments poten-tially exacerbates the accumulation of heavy metals in agri-cultural soil.Yangzhong is one of the most rapidly developing regions in China,and has experienced accelerated industri-alization and urbanization in recent years.In 1997,the region began a rapid transition from a traditionally agricul-tural-based economy to an industrial economy.This involved the establishment of many different industries,such as engineering,electronics,metallurgy,and chemicals,all of which produce a variety of wastes and pollutants.This study of Yangzhong district provides information that will be valuable for other regions in China,and the findings may also be useful in the formulation of strategic sustainable agriculture in China and other rapidly develop-ing regions in the future.Thus,the primary purpose of this study is to examine the accumulation of heavy metals in agricultural soils,their potential sources,and to evaluate the uptake of heavy metals by plants.2.Materials and methods 2.1.Sampling and pretreatmentA total of 100samples,consisting of 76samples of top-soil,21of subsoil,and 3core samples,were collected from the study area (Fig.1).The density of topsoil sampling was one sample every 4km 2;the region was divided into 2·2km plots,and from each plot a composite of two subsam-ples was taken from the upper 20cm,which represents the ploughed layer.These subsamples were then mixed to obtain a bulk sample that provided a representative esti-mate of concentrations at that site.Three continuous 2-m cores were extracted at approximately 0.1-m intervals using a 0.04-m diameter stainless steel manual corer or a core driller.Samples of subsoil were collected at a density of one sample for every 16km 2.Samples were collected between depths of 150and 200cm,representing the back-ground level that is free from anthropogenic influence.Rhi-zosphere soil was separated from root tissue by gently shaking the soil attached to the roots.All soil samples were collected using a hand auger and stored in polyethylene bags.The collected soil samples were air-dried and sieved through a 2-mm polyethylene sieve to remove large debris,stones,and pebbles.The samples were then ground in a mechanical agate grinder until fine particles (<0.074mm)were obtained.A total of 37plant samples,consisting of 10wheat sam-ples,5horsebean,5cabbage,and 17artemisia,were col-lected from croplands by hand-picking using vinyl gloves and carefully packed into polyethylene bags and weighed in situ.Samples were cleaned (soil removed)using a dry pre-cleaned vinyl brush.The cereal and horsebean were threshed,weighed,washed with distilled water to eliminate air-borne pollutants,dried in an oven to a constant weight at 45°C,re-weighed to determine the water content,and ground into a power for analysis.2.2.Physical and chemical analysisSoil pH was measured in a 1:5soil-to-water suspension after stirring for 2h.The TOC content of the soil was determined by loss on ignition at 600°C (Nelson and Som-mers,1982).The cation exchange capacity (CEC)wasFig.1.Sampling locations of soils in Yangzhong district.S.S.Huang et al./Chemosphere 67(2007)2148–21552149determined in a10%Ba(CH3COO)x H2O and0.1M MgSO4extracting solutions(Gazzetta,1992).The Ni,Co,Cr,Zn,Pb,Fe,K,and Mg contents of the soil were determined by X-rayfluorescence spectrometry using pressed boric-acid-backed pellets of bulk sample. X-ray counts were converted into concentrations using a computer program based on the matrix correction method according to Franzini et al.(1975).The accuracy of deter-minations was checked using certified internal reference materials.The analytical precision,measured as relative standard deviations(RSDs),was below5%for Co,Cr, Zn,and Pb,below10%for Ni,and below12%for Fe, K,and Mg.One gram of soil was placed in a Teflon bomb and trea-ted with3ml of HCl,9ml of HNO3,2ml of HF,and8ml of HClO4in that order.Cd concentrations were measured using a graphite furnace,and As and Hg concentrations were determined using cold-vapor atomicfluorescence spectrometry.Calibration was performed every10sets of samples using prepared internal standards via the stan-dards curve approach.care was taken in preparing and analyzing samples to minimize contamination from air, glassware,and reagents.Replicated measures of internal reference material,reagent blanks,and duplicated soil sam-ples randomly selected from the set of available samples were used to assess contamination and precision.The ana-lytical precision(RSDs)was as follows:Cd:4%,As:1%, Hg:6%.One gram of vegetable sample was digested by15ml HCl and5ml H2SO4.Cr,Ni,Cu,Pb,Zn,and Cd concentrations were measured by atomic absorption spectrometry coupled with a graphite furnace,and As and Hg concentrations were determined by cold-vapor atomicfluorescence spectrome-try.The RSDs of the determinations ranged from0.7% (Pb)to11.5%(Hg).The accuracy of the method was assessed via the analysis of a reference plant material,yield-ing the following values(mg kgÀ1):As:0.33(certified value: 0.37±0.09),Cd:0.31(certified value:0.32±0.07),Cr:0.60 (certified value:0.55±0.07),Cu:9.19(certified value: 9.30±1.0),Hg:0.029(certified value:0.026±0.003), Ni:1.87(certified value:1.9±0.3),Pb:1.49(certified value: 1.5±0.3),and Zn:35.2(certified value:37±3).Reproduc-ibility was tested by reanalyzing5%of the samples and recovery was good(92–102%).2.3.Statistical analysesStatistical analyses were performed using SPSS V11.0 statistical software on a personal computer.Shapiro–Wilk statistical tests were employed to evaluate the normality of the data.The data for soils and vegetables show normal distributions,and the degree of confidence in the proce-dures was a<0.05.3.Results and discussion3.1.Physico-chemical parametersThe main physico-chemical parameters determined for topsoils from Yangzhong district are as follows:(i)TOC contents are within the range of0.8–2.91%,with most val-ues being approximately2%;(ii)values of pH fall in a nar-row range(7.6–8.2),indicating subalkaline conditions for all the sampled topsoils;(iii)values of CEC show little vari-ation(12.6–18.5cmol kgÀ1),with a mean value of15.6 cmol kgÀ1.3.2.Heavy metal concentrationsHeavy metal concentrations within the sampled topsoils and subsoils are presented in Table1.In topsoils,Pb,Cu, and Ni have arithmetical mean concentrations of around 30mg kgÀ1,with As recording around10mg kgÀ1,Cd around0.3mg kgÀ1,and Cr around80mg kgÀ1.Hg has the lowest mean concentration(0.2mg kgÀ1),while the highest contents were recorded for Zn(98.1mg kgÀ1).In topsoils,the arithmetical mean contents of Hg,Cd,Pb, Cu,Ni,and Zn are higher than those in subsoils,while the mean contents of As and Cr in topsoils are lower than those in subsoils.According to the Environmental Quality Standard for Soils(National Environmental Protection Agency of China,1995),these measured heavy metal con-centrations within topsoils are generally below the thresh-old values of nationwide natural background levels.Table1Concentrations(mg kgÀ1)of heavy metals in topsoils and subsoilsSubgroup As Cd Cr Cu Hg Ni Pb Zn Topsoils(n=76)Mean10.20.377.233.90.238.535.798.1 Median9.30.3176.432.50.1739.636.397.5Range6–160.22–0.7165.2–9123–52.10.08–0.3529–4728.3–7177.2–133 Subsoils(n=21)Mean12.00.280.932.10.133.426.779.4 Median10.50.282.731.40.0735.125.982.3Range9.2–14.30.17–0.2974.1–90.424.9–40.20.05–0.1229.3–45.119.9–35.762.8–101 Threshold of naturalbackground in China a150.290350.24035100a National Environmental Protection Agency of China(1995).2150S.S.Huang et al./Chemosphere67(2007)2148–21553.3.Spatial distribution of heavy metal concentrations The spatial distribution of heavy metal concentrations is shown in Fig.2.High concentrations of Cd and Hg are observed in most samples of agricultural land.Samples from Yangzhong City shows no anomalous concentrations of these elements.The spatial distributions of Cr and Ni contents are remarkably similar over a large area.These contents are especially elevated in urban areas,close to fac-tories.The enrichment of these metals around Yangzhong City is probably caused by metals released in association with human activity.High levels of Cu,Pb,and Zn corre-spond well to the extent of urban areas:a decreasing trend exists away from the center of Yangzhong City.In con-trast,measurements of As do not show this decreasing trend;high concentrations are recorded at both ends of the alluvion.3.4.Geochemical associationsA multivariate statistical treatment was used to identify geochemical associations within the sampled agricultural soils (Fig.3).The analyzed elements are grouped into three associations,as follows.3.4.1.Group I:Hg–Cd–TOCThe Cd,Hg,and TOC contents in the upper parts of the profiles increase gradually but significantly,and eventually attain concentrations that are greater than those in lower sections by a factor of 2–3.This implies that the fluxes of these elements have been progressively augmented over time.The fact that these elements are most stronglyenriched in the topsoil suggests that the agricultural soil has been subjected to a high input of anthropogenic metals,most likely related to the application of agrochemicals used to improve production and quality.The main source of heavy metal pollutants is the use of various agrochemicals,fertilizers,and pesticides that con-tain heavy metals such as Cd and Hg.For example,Cd is found predominantly in phosphatic fertilizers because Cd is commonly present as an impurity in phosphatic rocks.Yang et al.(2005)measured Cd and Hg contents in fertilizers of 0.009–2.58and 0.005–0.69mg kg À1,respec-tively.It is known that Cd and Hg pollute agriculture land in China because of the use of agrochemicals;the extent of this pollution is severe in the study area.Heavy doses of phosphate fertilizers have been applied to many farmsforFig.2.Spatial distribution of heavy metal contents in topsoils in Yangzhongdistrict.Fig.3.Dendrogram of heavy metal elements in Yangzhong agricultural soils.S.S.Huang et al./Chemosphere 67(2007)2148–21552151over thirty years,resulting in the accumulation of heavy metals within agricultural soils.Our results are in agree-ment with previous surveys that report significant increases in Cd concentrations within fertilized soils compared to unfertilized soils(Taylor,1997;Mann et al.,2002).TOC is an important soil component that reflects the fertility of agricultural soil.Long-term fertilization improves the fertility of the soil and thus increases the TOC content of the surface layer(Fig.4).Similar trends were observed by Xu and Shen(2000),who found that long-term fertilization acts to enhance the TOC content of agricultural land.3.4.2.Group II:Fe–Co–Ni–Cr–K–MgThis element group,which comprises primary elements within rocks and soils,show similar contents and distribu-tion patterns in the sampled soil profiles(Fig.4),with con-centrations being appreciably higher in the upper parts of the cores.Minor variations in elemental concentrations along the profiles are probably related to variations in the contents of clay/sand or organic matter,which appear to influence the concentrations of these elements(Vaselli et al.,1997).Where present in natural soils,these elements are derived from the weathering of parent material and subsequent pedogenesis.3.4.3.Group III:As–Pb–Cu–ZnThese elements show similar distribution patterns in the sampled soil profiles(Fig.4):slight enrichment at the sur-face or subsurface and decreasing with depth,although less pronounced than the trend recorded for Group I.Agricul-tural land in China is contaminated by heavy metals derived from atmospheric deposition(Chen et al.,1999;Zhang, 2001).Previous studies have found that atmospheric inputs of heavy metals to agricultural systems can be a significant contributor to Pb,Zn,Cu,and As loading in agricultural soils(Berthelsen et al.,1995;Alloway,1999;Gray et al., 2003).The most important sources of heavy metals in the atmosphere are energy production,mining,metal smelting and refining,manufacturing processes,traffic,and waste incineration,which unfortunately are spread throughout the study area.The contamination of agricultural soils by2152S.S.Huang et al./Chemosphere67(2007)2148–2155atmospheric heavy metals largely occurs by windblown fine granule material that originates from industrial areas.The study area is an alluvion along the Yangtze River,and the specific natural conditions of this area may be conducive to atmospheric deposition onto agricultural land.Metals deposited on the soil surface are then gradually incorpo-rated into the soil,thereby contributing to overall soil concentrations.3.5.Temporal variations in heavy metal concentrations The total concentrations of eight heavy metals measured in topsoils during 1990and 2005are given in Table 2.The Cd and Hg concentrations in 1990were 0.19and 0.09mg kg À1,respectively,with the equivalent concentrations in 2005being 0.30and 0.20mg kg À1.This represents a remarkable increase in the concentrations of the two ele-ments (by factors of 1.6and 2.2,respectively)between 1990and 2005.This increase reflects the long-term use of fertilizers and pesticides,which has led to significant accu-mulations of Cd and Hg in topsoils.In contrast,Ni concen-trations have remained unchanged over the past fifteen years,and Cr concentrations record a slight decrease.Ni and Cr in the topsoil are derived from the weathering of parent material and subsequent pedogenesis.The concen-trations of As,Cu,Pb,and Zn show slight increases over the past fifteen years,reflecting atmospheric deposition from urban anthropogenic activity.3.6.Heavy metal contents in staple food productsThe mean contents of heavy metals in vegetables and cereals are given in Table 3.The levels of heavy metals inanalyzed vegetables and cereals are generally lower than the amounts recorded in the soils in which the crops were grown.This indicates that only limited quantities of metals are absorbed into the upper parts of the plants,probably because the roots act as a barrier to the translocation of metals within the plant (Davies and White,1981).This may also reflect the small percentage of soluble metals in the soil because of the subalkaline environment.The aver-age Zn contents in wheat and horsebean,22.4and 22.9mg kg À1respectively,are much higher than those in cabbage and artemisia (2.0and 6.1mg kg À1,respectively).The mean Pb and Cd concentrations in artemisia are higher than those in wheat,horsebean,or cabbage.The mean Hg,Cu,and As concentrations in wheat and vegetable are relatively low,and the mean Cd,Ni,and Pb concentra-tions in artemisia are slightly higher than the threshold lev-els of the Food Quality Standard (National Environmental Protection Agency of China,2001).The mean Ni and Zn concentrations in horsebean are much higher than the threshold levels of the Food Quality Standard,but the heavy metal concentrations in wheat and cabbage are below the threshold levels.The average Cd and Hg concen-trations in artemisia grown in soil that is contaminated with heavy metals,0.072and 0.0011mg kg À1respectively,are slightly higher than values from uncontaminated soil (0.048and 0.0006mg kg À1).The average levels of Pb,Cu,Zn,Ni,Cr,and As in analyzed plant samples are similar in the two areas.The BCF values for heavy metals in the sampled vegeta-bles and wheat are below 1.0pH;CEC and organic matter play an important role in the retention of heavy metals in the soil (Laura et al.,2003).The pH values and TOC con-tents of topsoils in Yangzhong district are high for agricul-tural soils,whereas CEC shows typical levels.These trends indicate that the topsoils adsorb significant amounts of heavy metals (especially Cd).The narrow range of pH (7.6–8.2)measured in the sam-pled topsoils limits the mobility of heavy metals because of the subalkaline environment;thus,the concentrations of heavy metals in the sampled plants are low,leading to low BCF values.In comparison,the BCF value for Cd in artemisia is higher than that for other plants analyzedTable 3Concentrations (mg kg À1)of heavy metals in vegetables,cereals and their BCF values in Yangzhong district Heavy metals Mean content Bioconcentration factors Threshold invegetables a Threshold in cereal a Wheat Horseban Cabbage Artemisia Wheat Horseban Cabbage Artemisia As 0.150.120.120.190.0150.0130.0120.0140.50.7Hg 0.00070.0001LOD 0.00050.0040.0010.0040.010.02Cd 0.060.020.030.150.2140.0590.1070.3060.050.2Pb 0.210.160.080.300.0050.0050.0020.0080.20.5Cr 0.240.150.050.210.0030.0020.0010.0020.5 1.0Cu 4.10.590.53 2.550.0120.0170.0130.05210.010.0Ni 0.150.760.040.310.0040.0180.0010.0070.30.4Zn22.422.92.06.10.2320.2300.0190.05420.050.0aNational Environmental Protection Agency of China (2001).LOD-below the limit of detection.Table 2Comparison of mean concentrations (mg kg À1)of heavy metals in topsoil for 1990,2005Year As Cd Cr Cu Hg Ni Pb Zn 1990a 9.800.1991.031.90.0938.134.983.9200510.20.3077.233.90.2038.535.798.1aJiangsu Environment Monitoring Center (1990).S.S.Huang et al./Chemosphere 67(2007)2148–21552153in this study.This indicates that Cd is more easily absorbed by artemisia than by other plants.Xue et al. (2005)also reported that Cd had maximal BCF values among Cd,Zn,Cu,Pb,and Cr.In most cases,the BCF val-ues decreases with increasing metal concentration in the soil.4.ConclusionsThe present study examined the distribution of heavy metal contents in topsoils and subsoils of the Yangzhong district,Jiangsu Province,pared to subsoils, topsoils are enriched in Cd,Hg,Pb,Cu,Zn,and As.High levels of Cd and Hg are observed in most of the sampled agricultural soils,although not in areas around Yangzhong City.Concentrations of Cr and Ni show only minor spatial variations;only around urban areas do they show anoma-lously high concentrations.The spatial distribution of high Cu,Pb,and Zn concentrations correspond well to the loca-tions of urban areas,with a decreasing trend away from the center of Yangzhong City.High levels of As are recorded at both two ends of the alluvion.A dendrogram was used to examine the effect of surface geology and anthropogenic activity on elemental concen-trations,with elements grouped into three associations. The Cd–Hg–TOC association shows concentrations that increase upward to a maximum in the upper part of the profile,possibly reflecting the long-term use of heavy doses of agrochemicals.The Cr–Ni–Fe–Co–K–Mg association is characterized by relatively low elemental concentrations throughout the profiles;these elements originate from the weathering of parent material and subsequent pedogenesis. The As–Pb–Cu–Zn association shows similar distribution patterns in the measured soil profiles,with slight enrich-ment in the surface layer.These elements may be derived from atmospheric deposition associated with anthropo-genic activity.Chen et al.(2003)found that the concentra-tions of Zn,Cu,Pb,and As in atmospheric deposition at Chendou were758,151,192,and22mg kgÀ1,respectively. Given that the present study area is more highly developed than Chendou,it can be assumed that Zn,Cu,Pb,and As concentrations are even higher in atmospheric deposition within Yangzhong district,thereby leading to their enrich-ment in topsoils.Compared to data for1990,Cd and Hg concentrations in topsoils are much higher in2005.The contents of As,Cu,Pb,and Zn showed a slight increase over this per-iod,while Cr and Ni contents remained largely unchanged. 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