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Abstract Classical statistics， Inverse Distance Weighted and Hakasnson potential ecological index were used to study the distribution characteristics of 8 kinds of heavy metals （Cr， Ni， Cu， Zn， As， Cd， Pb and Hg） as well as their potential ecological risks in the topsoil （0-20 cm） of cultivated land in Donghai County， a typical agricultural area along the Southeast Coast of China. The results showed that the average content of heavy metals in the cultivated soil of Donghai County was not over the risk screening values set in the Environmental Quality Standard for Soils. However， it was worth noting that in some of the sampling points， the content of Cr exceeded the risk screening value， and the standard exceeding points accounted for 24.74% of the total. In addition， the average content of Cr， Ni， As， Cd， Pb and Hg exceeded the soil background values of Jiangsu Province， and values for Cd， Cr and Pb were more than 1.5 times of the soil background value of Jiangsu Province. The heavy metals were at the medium integrated potential ecological risk level， and the potential ecological risk indices were high in the east and low in the west， and distributed in interlaced island shape. The potential ecological risk indices of the plains in the east were higher than those of the central gentle slope area and the western hillock area. The potential ecological risk was at a high level in Niushan Town which was situated the county center and in some regions with high urbanization levels.
Key words Heavy metal contamination; Cultivated land; Ecological risk assessment; Farming area; Donghai County
The accumulation of heavy metals in cultivated land not only reduces the yield and quality of crops[1-2]， but also seriously threatens the safety of ecosystems and humans. Anthropogenic sources are the main source of heavy metal pollution in soil， mainly including fertilization， animal wastes and biosolids application， sewage irrigation， atmospheric deposition and so on[3-5]. Industrial development and urbanization have increased the accumulation of heavy metals in the soil， resulting in heavy metal pollution in the soil， and the pollution area has expanded year by year[6-8]. The National Soil Pollution Survey Bulletin， jointly issued by the Ministry of Land and Resources and the Ministry of Environmental Protection of the Peoples Republic of China on April 17， 2014， shows that the overall state of soil environment in China is not promising， soil pollution is serious in some regions and the cultivated soil environment quality is very low， where the exceeding standard rates of 8 kinds of heavy metals （Cd， Hg， As， Cu， Pb， Cr， Zn and Ni） are 7.0%， 1.6%， 2.7%， 2.1%， 1.5%， 1.1%， 0.9%， 4.8%， respectively. Moreover， 19.4% of the investigated sites in cultivated land exceed the maximum allowable heavy metal contents， and the proportions of slight， light， medium and heavy pollution are 13.7%， 2.8%， 1.8% and 1.1%， respectively. Therefore， it is the basis for agricultural ecological security to strengthen the investigation and evaluation of heavy metal pollution in cultivated land[9]. The eastern coastal areas of China are dominated by plains， with good quality of cultivated land， flat and deep topsoil， and high levels of agricultural intensification. These areas have high level of urbanization and industrialization， thereby causing serious risk of agricultural land pollution. Therefore， in this paper， the distribution characteristics of soil heavy metals （Cr， Ni， Cu， Zn， As， Cd， Pb and Hg） in the topsoil （0-20 cm） of cultivated land were investigated with Donghai County， a typical agricultural county along the eastern coast of China， as an example， and the potential ecological risks were evaluated using the soil heavy metal accumulation index， with the aim to provide scientific bases and guidance for soil environmental quality assessment and land use planning in agricultural areas along the eastern coast of China.
Materials and Methods
Study area
Located in the northeastern part of Jiangsu Province， China with the coordinates of 34°11′-34°44′N， 118°23′-119°10′E， Donghai County borders the Yellow Sea on the east. With a humid monsoon climate， Donghai has an average annual temperature of 13.7 ℃ and an average precipitation of 912.3 mm. The county has 11 towns， 6 townships， 2 street offices， 2 stateowned farms， and 1 provincial farm. The total land area of the county is 200 981.02 hm 2， in which there is 155 642.97 hm 2 of agricultural land， accounting for 77.44% of the total land area; 36 770.72 hm 2 of construction land， accounting for 18.30% of the total land area; 8 583.33 hm 2 of unused land， accounting for 4.26% of the total land area. Among the agricultural land， the cultivated land area is 122 482.29 hm 2， accounting for 60.95% of the total land area. The soil of cultivated land is dominated by brown soil and Shajiang black soil， accounting for 46.38% and 39.52% of the cultivated land area of the county， respectively. Donghai County has a large ratio of agricultural land and is rich in cultivated land resources， making it suitable for planting rice， wheat， corn and other crops. It also has a long history of agriculture production， and longterm intensive cultivation has greatly improved the land limitation factors， so the land productivity is relatively high.
Sample collection and analysis
From the end of March to the beginning of April 2018， a total of 92 sampling points were set up in Donghai County. Considering the landform types and administrative divisions， 4-6 sampling points were arranged in each township. In order to avoid the influence of the surrounding environment， the sampling points were selected from the continuous cultivated land areas， which were located in the center of the cultivated land away from the rivers with the polygon area of greater than 6.67 hm 2 and a distance over 50 m from the roads and ditches （Fig.1）. The soil samples were collected using the 5spot method from the topsoil （0-20 cm） of each sampling point. For each sampling point， soil samples were collected from 5 spots on a diagonal of 10 m， and then the soil samples from the 5 spots were fully mixed. Then， 1 kg of soil was taken by the quartering method， which was then brought back to the laboratory in a plastic bag. In the meantime， the basic information of the sampling point was recorded， including the latitude and longitude， farming method， slope， and aspect. After removing the plant roots and rocks， the collected soil samples were airdried in the laboratory， and then the samples were ground and crushed， and screened through a 100mesh nylon sieve for determination of heavy metals in the soil. The contents of the 8 kinds of heavy metals， Cr， Ni， Cu， Zn， As， Cd， Pb and Hg， were determined according to the methods provided in the Guidance on Longterm Soil Monitoring in Natural Ecosystem （GB/T 32740-2016）. Data Analysis
The classical statistical parameters such as mean， maximum， minimum， standard deviation and coefficient of variation were used to describe the overall characteristics of heavy metals in cultivated land in the study area. Among them， the standard deviation was the average distance of each data deviating from the average， which could reflect the dispersion degree of soil heavy metal data. The coefficient of variation （CV） could reflect the spatial variability of soil heavy metals， and CV≤10% indicated weak variability， CV of 10%-100% indicated moderate variability， and CV≥100% was intensity variability[10]. Classic statistical analysis was performed using PASW Statistics 18.0， and relevant statistical maps were drawn using SigmaPlot 12.5. The spatial distribution of heavy metals in soil was obtained by using the inverse distance weighted interpolation of ArcGIS 10.6.
The potential ecological risks of heavy metals in soil were evaluated using the potential ecological risk index （PERI） proposed by Hakanson in 1980[11]. The calculation formula was as follows：
C if=Ci/Bi（1）
E ir=T ir×C if（2）
RI=∑mi=1E ir（3）
Where， Ci is a measured concentration of metal i in the soil sample; Bi is the background value of same metal i in the soil， and in this study， the background values （total amount） of soil elements in Jiangsu Province were taken as the reference values[12]， namely， Cr of 75.6 mg/kg， Ni of 32.8 mg/kg， Cu of 23.4 mg/kg， Zn of 64.8 mg/kg， As of 9.4 mg/kg， Cd of 0.085 mg/kg， Pb of 22 mg/kg， and Hg of 0.025 ng/g）; T ri is the toxicresponse factor of metal i according to the toxicity of heavy metals and the response of the environment， respectively Cr of 2， Ni of 5 ， Cu of 5， Zn of 1， As of 10， Cd of 30， Pb of 30 and Hg of 40[13]; C if is the single contamination factor; E ir is the monomial ecological risk factor of metal I; RI is potential ecological risk index， which characterizes the potential ecological risk degree of heavy metals in soil. The single contamination factor index and potential ecological risk index of Cr， Ni， Cu， Zn， As， Cd， Pb and Hg are classified according to the relevant literature[14-15].
Results and Analysis
Mean heavy metal content in topsoil of cultivated land
The statistical characteristics of heavy metal content in the topsoil soil of cultivated land in Donghai County were shown in Table 1. On the whole， the average contents of various heavy metals in cultivated land did not exceed the risk screening values set in the Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land， but it is worth noting that in some of the sampling points， the contents of Cr， Ni， Cu and Cd exceeded the risk screening values， especially Cr had the most standard exceeding points， which reached 20， accounting for 24.74% of the total， while the standard exceeding points for Ni and Cu were 3， and 1 for Cd. The overstandard content of Cr may be caused by metal smelting， openair waste incineration， livestock wastes， and farmland irrigation[16-17]. At the same time， the average contents of heavy metals of Cr， Ni， As， Cd， Pb and Hg exceeded the soil background values of Jiangsu Province， and the average contents of Cd， Cr and Pb were more than 1.5 times of the soil background value of Jiangsu Province. In addition， the coefficients of variation of heavy metals at each sampling point were between 26.13% and 72.13%， which indicated moderate variability， and the variation from high to low was in the order of Hg > Ni > Cu > Cr > As > Pb > Cd > Zn. Among them， Hg had the largest coefficient of variation， indicating that it was most affected by exogenous factors. Distribution of heavy metals content in topsoil of cultivated land
The spatial distribution of various heavy metals in the topsoil of cultivated land in Donghai County was shown in Fig.2. The spatial distribution characteristics of the 8 kinds of heavy metals were significantly different. Specifically， the content of Cr was between 62.37 and 279.09 mg/kg， which was obviously higher in the western hilly area， and the high values were found in Linian Township and Shuangdian Town. The content of Ni was between 16.99 and 153.56 mg/kg， which was higher in the eastern plain area than the central gentle slope area， and the highest content was found at the junction of Linian Township and Shuangzuokou Township. The content of Cu was between 11.21 and 56.47 mg/kg， and the highest content was located in Taolin Town and Shilianghe Town. The content of Zn was between 33.93 and 99.55 mg/kg， which was higher in the east and lower in the middle and west， and highest value was found in Pingming Town and Zhangwan Town. The content of As was between 5.33 and 27.06 mg/kg， and the distribution characteristics were consistent with the spatial trend of elevation， showing a trend of low in the west and high in the east. The content of Cd was between 0.083 and 0.35 mg/kg， and the highest content was located in Taolin Town. The content of Pb was between 19.64 and 78.78 mg/kg， and the areas with relatively higher content were mainly distributed in Taolin Town in the west and Niushan Town and Baitabu Town in the central part. The content of Hg was between 7.90 and 159.33 ng/g， and the content of Hg in the eastern plain was higher， mainly concentrated in Pingming Town and Shilianghe Town.
Assessment of potential ecological risk of heavy metals
The average single contamination factors （Ci） of the 8 kinds of heavy metals in cultivated land in Donghai County were calculated from the soil background values of Jiangsu Province as Cd of 1.89 > Cr of 1.65 > Pb of 1.53 > As of 1.19 > Hg of 1.14 > Ni of 1.12 > Cu of 0.97 > Zn of 0.88. According to PERI classification[14-15]， the heavy metals such as Cd， Cr， As， Hg and Ni in cultivated land were in the moderate pollution levels （1-3）， and Cu and Zn were in the light pollution levels （<1）.
The average monomial ecological risk factor （E ri） of the soil heavy metals showed that the ecological risk factors of As （11.88）， Ni （5.58）， Cu （4.83）， Cr （3.30） and Zn （0.88） were all smaller than 30， indicating light potential risk， the ecological risk factors of Cd （56.69）， Pb （45.87） and Hg （45.69） were in the range of 30-60， indicating that the potential ecological risk was moderate， but it was worth noting that the ecological risk factor of Cd was close to the upper limit of the moderate risk span. The potential ecological risk index （RI） of heavy metals in the surface of cultivated land in Donghai County was between 107.39% and 383.76%， and the regional average was 174.73， indicating that the overall potential ecological risk level of heavy metals was moderate （110-220） in Donghai County. The spatial distribution of potential ecological risk index showed a gradual increasing trend from west to east. The potential ecological risk index of the plain area in the east was higher than that in the central gentle slope area and the western hill area. The reasons may be related to soil type， arable land elevation， proximity to urban areas， and agricultural production levels （use of agricultural inputs such as chemical fertilizers， pesticides， organic fertilizers， and sludge）[18-20]. In addition， the ecological risks were high in the regions with high urbanization levels like Niushan Town， Taolin Town， Baitabu Town， Pingming Town， Shilianghe Town and Shilianghe Town， which were in the highlevel risk area.
Conclusion
（1） The average content of heavy metals in the cultivated soil of Donghai County was not over the risk screening values set in the Environmental Quality Standard for Soils. However， it was worth noting that in some of the sampling points， the content of Cr exceeded the risk screening value， and the standard exceeding points accounted for 24.74% of the total. In addition， the average content of Cr， Ni， As， Cd， Pb and Hg exceeded the soil background values of Jiangsu Province， and values for Cd， Cr and Pb were more than 1.5 times of the soil background value of Jiangsu Province.
（2） The heavy metals were at the medium integrated potential ecological risk level， and the potential ecological risk indices were high in the east and low in the west， and distributed in interlaced island shape. The potential ecological risk indices of the plains in the east were higher than those of the central gentle slope area and the western hillock area. The potential ecological risk was at a high level in Niushan Town which was situated the county center and in some regions with high urbanization levels.
References
[1] CHEN HM， ZHENG CR， TU C， et al. Heavy Metal Pollution in Soils in China： Status and Countermeasures[J]. Ambio， 1999， 28（2）：130-134.
[2] RAGHUNATH R， TRIPATHI RM， KUMAR AV， et al. Assessment of Pb， Cd， Cu， and Zn exposures of 6 to 10yearold children in Mumbai[J]. Environmental Research， 1999， 80（3）：215. [3] JIAO WT， CHEN WP， CHANG AC， et al. Environmental risks of trace elements associated with longterm phosphate fertilizers applications： a review[J]. Environmental Pollution， 2012， 168（1）：44-53.
[4] SINGH J， LEE BK. Reduction of environmental availability and ecological risk of heavy metals in automobile shredder residues[J]. Ecological Engineering， 2015， 81： 76-81.
[5] LIU JL， WU H， FENG JX， et al. Heavy metal contamination and ecological risk assessments in the sediments and zoobenthos of selected mangrove ecosystems， South China[J]. Catena， 2014， 119： 136-142.
[6] YANG Y， JIN Q， FANG JM， et al. Spatial distribution， ecological risk assessment， and potential sources of heavy metal（loid）s in surface sediments from the Huai River within the Bengbu section， China[J]. Environmental Science and Pollution Research， 2017， 24（12）： 11360-11370.
[7] ISLAM MS， AHMED MK， RAKNUZZAMAN M， et al. Heavy metals in the industrial sludge and their ecological risk： A case study for a developing country[J]. Journal of Geochemical Exploration， 2017， 172： 41-49.
[8] KE X， GUI S， HUANG H， et al. Ecological risk assessment and source identification for heavy metals in surface sediment from the Liaohe River protected area， China[J]. Chemosphere， 2017， 175： 473-481.
[9] TIAN K， HUANG B， XING Z， et al. Geochemical baseline establishment and ecological risk evaluation of heavy metals in greenhouse soils from Dongtai， China[J]. Ecological indicators， 2017， 72： 510-520.
[10] WANG HM， XIE YZ， WANG K. Spatial heterogeneity of soil moisture in different artificial grasslands with finer scales[J]. Acta Agrestia Sinica， 2013， 21（6）：1052-1058.
[11] HAKANSON L. An ecological risk index for aquatic pollution control： a sedimentological approach[J]. Water Research， 1980， 14（8）：975-1001.
[12] GB 15618—1995 Environmental Quality Standard for Soil. Beijing： Standards Press of China，1997.
[13] XU ZQ， NI SJ， TAO XG， et al. Calculation of Heavy Metals Toxicity Coefficient in the Evaluation of Potential Ecological Risk Index[J].Environmental Science & Technology， 2008， 31（2）：112-115.
[14] LI QL， WU Q， GAO JB， et al. Distribution of heavy metals in topsoils affected by land use patterns at a small watershed scale： a case study in the Bantou Reservoir watershed in Xiamen， China[J]. Acta Ecologica Sinica，2015，35（16）： 5486-5494.
[15] JU TN， WU X， SHI HD， et al. Heavy metal pollution and Ecological Risk assessment of arable land soil in Haigou small watershed[J]. Journal of Environmental Engineering Technology， 2018， 8（5）： 556-562. [16] LU SJ， WANG YY， TENG YG， et al. Heavy metal pollution and ecological risk assessment of the paddy soils near a zinclead mining area in Hunan[J]. Environmental monitoring and assessment， 2015， 187（10）： 627.
[17] BAI JH， CUI BS， CHEN B， et al. Spatial distribution and ecological risk assessment of heavy metals in surface sediments from a typical plateau lake wetland， China[J]. Ecological Modelling， 2011， 222（2）： 301-306.
[18] SUN H， DU Y， SONG YB， et al. Distribution and Evaluation of Contamination of Heavy Metals in Selected Farmland Soils in Donghai County of Jiangsu Province[J]. Journal of AgroEnvironment Science， 2006.
[19] BARKETT MO， AKUN E. Heavy metal contents of contaminated soils and ecological risk assessment in abandoned copper mine harbor in Yedidalga， Northern Cyprus[J]. Environmental Earth Sciences， 2018， 77（10）： 378.
[20] DING XG， YE SY， YUAN HM， et al. Spatial distribution and ecological risk assessment of heavy metals in coastal surface sediments in the Hebei Province offshore area， Bohai Sea， China[J]. Marine pollution bulletin， 2018， 131： 655-661.
Key words Heavy metal contamination; Cultivated land; Ecological risk assessment; Farming area; Donghai County
The accumulation of heavy metals in cultivated land not only reduces the yield and quality of crops[1-2]， but also seriously threatens the safety of ecosystems and humans. Anthropogenic sources are the main source of heavy metal pollution in soil， mainly including fertilization， animal wastes and biosolids application， sewage irrigation， atmospheric deposition and so on[3-5]. Industrial development and urbanization have increased the accumulation of heavy metals in the soil， resulting in heavy metal pollution in the soil， and the pollution area has expanded year by year[6-8]. The National Soil Pollution Survey Bulletin， jointly issued by the Ministry of Land and Resources and the Ministry of Environmental Protection of the Peoples Republic of China on April 17， 2014， shows that the overall state of soil environment in China is not promising， soil pollution is serious in some regions and the cultivated soil environment quality is very low， where the exceeding standard rates of 8 kinds of heavy metals （Cd， Hg， As， Cu， Pb， Cr， Zn and Ni） are 7.0%， 1.6%， 2.7%， 2.1%， 1.5%， 1.1%， 0.9%， 4.8%， respectively. Moreover， 19.4% of the investigated sites in cultivated land exceed the maximum allowable heavy metal contents， and the proportions of slight， light， medium and heavy pollution are 13.7%， 2.8%， 1.8% and 1.1%， respectively. Therefore， it is the basis for agricultural ecological security to strengthen the investigation and evaluation of heavy metal pollution in cultivated land[9]. The eastern coastal areas of China are dominated by plains， with good quality of cultivated land， flat and deep topsoil， and high levels of agricultural intensification. These areas have high level of urbanization and industrialization， thereby causing serious risk of agricultural land pollution. Therefore， in this paper， the distribution characteristics of soil heavy metals （Cr， Ni， Cu， Zn， As， Cd， Pb and Hg） in the topsoil （0-20 cm） of cultivated land were investigated with Donghai County， a typical agricultural county along the eastern coast of China， as an example， and the potential ecological risks were evaluated using the soil heavy metal accumulation index， with the aim to provide scientific bases and guidance for soil environmental quality assessment and land use planning in agricultural areas along the eastern coast of China.
Materials and Methods
Study area
Located in the northeastern part of Jiangsu Province， China with the coordinates of 34°11′-34°44′N， 118°23′-119°10′E， Donghai County borders the Yellow Sea on the east. With a humid monsoon climate， Donghai has an average annual temperature of 13.7 ℃ and an average precipitation of 912.3 mm. The county has 11 towns， 6 townships， 2 street offices， 2 stateowned farms， and 1 provincial farm. The total land area of the county is 200 981.02 hm 2， in which there is 155 642.97 hm 2 of agricultural land， accounting for 77.44% of the total land area; 36 770.72 hm 2 of construction land， accounting for 18.30% of the total land area; 8 583.33 hm 2 of unused land， accounting for 4.26% of the total land area. Among the agricultural land， the cultivated land area is 122 482.29 hm 2， accounting for 60.95% of the total land area. The soil of cultivated land is dominated by brown soil and Shajiang black soil， accounting for 46.38% and 39.52% of the cultivated land area of the county， respectively. Donghai County has a large ratio of agricultural land and is rich in cultivated land resources， making it suitable for planting rice， wheat， corn and other crops. It also has a long history of agriculture production， and longterm intensive cultivation has greatly improved the land limitation factors， so the land productivity is relatively high.
Sample collection and analysis
From the end of March to the beginning of April 2018， a total of 92 sampling points were set up in Donghai County. Considering the landform types and administrative divisions， 4-6 sampling points were arranged in each township. In order to avoid the influence of the surrounding environment， the sampling points were selected from the continuous cultivated land areas， which were located in the center of the cultivated land away from the rivers with the polygon area of greater than 6.67 hm 2 and a distance over 50 m from the roads and ditches （Fig.1）. The soil samples were collected using the 5spot method from the topsoil （0-20 cm） of each sampling point. For each sampling point， soil samples were collected from 5 spots on a diagonal of 10 m， and then the soil samples from the 5 spots were fully mixed. Then， 1 kg of soil was taken by the quartering method， which was then brought back to the laboratory in a plastic bag. In the meantime， the basic information of the sampling point was recorded， including the latitude and longitude， farming method， slope， and aspect. After removing the plant roots and rocks， the collected soil samples were airdried in the laboratory， and then the samples were ground and crushed， and screened through a 100mesh nylon sieve for determination of heavy metals in the soil. The contents of the 8 kinds of heavy metals， Cr， Ni， Cu， Zn， As， Cd， Pb and Hg， were determined according to the methods provided in the Guidance on Longterm Soil Monitoring in Natural Ecosystem （GB/T 32740-2016）. Data Analysis
The classical statistical parameters such as mean， maximum， minimum， standard deviation and coefficient of variation were used to describe the overall characteristics of heavy metals in cultivated land in the study area. Among them， the standard deviation was the average distance of each data deviating from the average， which could reflect the dispersion degree of soil heavy metal data. The coefficient of variation （CV） could reflect the spatial variability of soil heavy metals， and CV≤10% indicated weak variability， CV of 10%-100% indicated moderate variability， and CV≥100% was intensity variability[10]. Classic statistical analysis was performed using PASW Statistics 18.0， and relevant statistical maps were drawn using SigmaPlot 12.5. The spatial distribution of heavy metals in soil was obtained by using the inverse distance weighted interpolation of ArcGIS 10.6.
The potential ecological risks of heavy metals in soil were evaluated using the potential ecological risk index （PERI） proposed by Hakanson in 1980[11]. The calculation formula was as follows：
C if=Ci/Bi（1）
E ir=T ir×C if（2）
RI=∑mi=1E ir（3）
Where， Ci is a measured concentration of metal i in the soil sample; Bi is the background value of same metal i in the soil， and in this study， the background values （total amount） of soil elements in Jiangsu Province were taken as the reference values[12]， namely， Cr of 75.6 mg/kg， Ni of 32.8 mg/kg， Cu of 23.4 mg/kg， Zn of 64.8 mg/kg， As of 9.4 mg/kg， Cd of 0.085 mg/kg， Pb of 22 mg/kg， and Hg of 0.025 ng/g）; T ri is the toxicresponse factor of metal i according to the toxicity of heavy metals and the response of the environment， respectively Cr of 2， Ni of 5 ， Cu of 5， Zn of 1， As of 10， Cd of 30， Pb of 30 and Hg of 40[13]; C if is the single contamination factor; E ir is the monomial ecological risk factor of metal I; RI is potential ecological risk index， which characterizes the potential ecological risk degree of heavy metals in soil. The single contamination factor index and potential ecological risk index of Cr， Ni， Cu， Zn， As， Cd， Pb and Hg are classified according to the relevant literature[14-15].
Results and Analysis
Mean heavy metal content in topsoil of cultivated land
The statistical characteristics of heavy metal content in the topsoil soil of cultivated land in Donghai County were shown in Table 1. On the whole， the average contents of various heavy metals in cultivated land did not exceed the risk screening values set in the Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land， but it is worth noting that in some of the sampling points， the contents of Cr， Ni， Cu and Cd exceeded the risk screening values， especially Cr had the most standard exceeding points， which reached 20， accounting for 24.74% of the total， while the standard exceeding points for Ni and Cu were 3， and 1 for Cd. The overstandard content of Cr may be caused by metal smelting， openair waste incineration， livestock wastes， and farmland irrigation[16-17]. At the same time， the average contents of heavy metals of Cr， Ni， As， Cd， Pb and Hg exceeded the soil background values of Jiangsu Province， and the average contents of Cd， Cr and Pb were more than 1.5 times of the soil background value of Jiangsu Province. In addition， the coefficients of variation of heavy metals at each sampling point were between 26.13% and 72.13%， which indicated moderate variability， and the variation from high to low was in the order of Hg > Ni > Cu > Cr > As > Pb > Cd > Zn. Among them， Hg had the largest coefficient of variation， indicating that it was most affected by exogenous factors. Distribution of heavy metals content in topsoil of cultivated land
The spatial distribution of various heavy metals in the topsoil of cultivated land in Donghai County was shown in Fig.2. The spatial distribution characteristics of the 8 kinds of heavy metals were significantly different. Specifically， the content of Cr was between 62.37 and 279.09 mg/kg， which was obviously higher in the western hilly area， and the high values were found in Linian Township and Shuangdian Town. The content of Ni was between 16.99 and 153.56 mg/kg， which was higher in the eastern plain area than the central gentle slope area， and the highest content was found at the junction of Linian Township and Shuangzuokou Township. The content of Cu was between 11.21 and 56.47 mg/kg， and the highest content was located in Taolin Town and Shilianghe Town. The content of Zn was between 33.93 and 99.55 mg/kg， which was higher in the east and lower in the middle and west， and highest value was found in Pingming Town and Zhangwan Town. The content of As was between 5.33 and 27.06 mg/kg， and the distribution characteristics were consistent with the spatial trend of elevation， showing a trend of low in the west and high in the east. The content of Cd was between 0.083 and 0.35 mg/kg， and the highest content was located in Taolin Town. The content of Pb was between 19.64 and 78.78 mg/kg， and the areas with relatively higher content were mainly distributed in Taolin Town in the west and Niushan Town and Baitabu Town in the central part. The content of Hg was between 7.90 and 159.33 ng/g， and the content of Hg in the eastern plain was higher， mainly concentrated in Pingming Town and Shilianghe Town.
Assessment of potential ecological risk of heavy metals
The average single contamination factors （Ci） of the 8 kinds of heavy metals in cultivated land in Donghai County were calculated from the soil background values of Jiangsu Province as Cd of 1.89 > Cr of 1.65 > Pb of 1.53 > As of 1.19 > Hg of 1.14 > Ni of 1.12 > Cu of 0.97 > Zn of 0.88. According to PERI classification[14-15]， the heavy metals such as Cd， Cr， As， Hg and Ni in cultivated land were in the moderate pollution levels （1-3）， and Cu and Zn were in the light pollution levels （<1）.
The average monomial ecological risk factor （E ri） of the soil heavy metals showed that the ecological risk factors of As （11.88）， Ni （5.58）， Cu （4.83）， Cr （3.30） and Zn （0.88） were all smaller than 30， indicating light potential risk， the ecological risk factors of Cd （56.69）， Pb （45.87） and Hg （45.69） were in the range of 30-60， indicating that the potential ecological risk was moderate， but it was worth noting that the ecological risk factor of Cd was close to the upper limit of the moderate risk span. The potential ecological risk index （RI） of heavy metals in the surface of cultivated land in Donghai County was between 107.39% and 383.76%， and the regional average was 174.73， indicating that the overall potential ecological risk level of heavy metals was moderate （110-220） in Donghai County. The spatial distribution of potential ecological risk index showed a gradual increasing trend from west to east. The potential ecological risk index of the plain area in the east was higher than that in the central gentle slope area and the western hill area. The reasons may be related to soil type， arable land elevation， proximity to urban areas， and agricultural production levels （use of agricultural inputs such as chemical fertilizers， pesticides， organic fertilizers， and sludge）[18-20]. In addition， the ecological risks were high in the regions with high urbanization levels like Niushan Town， Taolin Town， Baitabu Town， Pingming Town， Shilianghe Town and Shilianghe Town， which were in the highlevel risk area.
Conclusion
（1） The average content of heavy metals in the cultivated soil of Donghai County was not over the risk screening values set in the Environmental Quality Standard for Soils. However， it was worth noting that in some of the sampling points， the content of Cr exceeded the risk screening value， and the standard exceeding points accounted for 24.74% of the total. In addition， the average content of Cr， Ni， As， Cd， Pb and Hg exceeded the soil background values of Jiangsu Province， and values for Cd， Cr and Pb were more than 1.5 times of the soil background value of Jiangsu Province.
（2） The heavy metals were at the medium integrated potential ecological risk level， and the potential ecological risk indices were high in the east and low in the west， and distributed in interlaced island shape. The potential ecological risk indices of the plains in the east were higher than those of the central gentle slope area and the western hillock area. The potential ecological risk was at a high level in Niushan Town which was situated the county center and in some regions with high urbanization levels.
References
[1] CHEN HM， ZHENG CR， TU C， et al. Heavy Metal Pollution in Soils in China： Status and Countermeasures[J]. Ambio， 1999， 28（2）：130-134.
[2] RAGHUNATH R， TRIPATHI RM， KUMAR AV， et al. Assessment of Pb， Cd， Cu， and Zn exposures of 6 to 10yearold children in Mumbai[J]. Environmental Research， 1999， 80（3）：215. [3] JIAO WT， CHEN WP， CHANG AC， et al. Environmental risks of trace elements associated with longterm phosphate fertilizers applications： a review[J]. Environmental Pollution， 2012， 168（1）：44-53.
[4] SINGH J， LEE BK. Reduction of environmental availability and ecological risk of heavy metals in automobile shredder residues[J]. Ecological Engineering， 2015， 81： 76-81.
[5] LIU JL， WU H， FENG JX， et al. Heavy metal contamination and ecological risk assessments in the sediments and zoobenthos of selected mangrove ecosystems， South China[J]. Catena， 2014， 119： 136-142.
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