论文部分内容阅读
Abstract [Objectives]The research was conducted to study characteristics of bioavailable levels pollution of heavy metals in the surface soil of the leadzinc mine in Changhua.
[Methods]A total of 56 surface samples were collected from the 3 study zones （peripheral zone， reclamation zone and tailing zone） located at the leadzine mine in Changhua. Based on total and bioavailable heavy metals （Pb， Zn， Cu and Cd） in soils， the correlation of total content with bioavailable content was analyzed， and their pollution characteristics were assessed by methods of single factor pollution index， Nemerow pollution index， geological accumulation index and potential ecological risk index.
[Results]（i） The surface soils were polluted by Pb， Zn， Cu， Cd in different degrees， and the overall trend of the contents of the 4 heavy metals was in order of Cd > Pb > Zn > Cu. The average excess multiple of heavy metals of total and bioavailable content in soils were in the order of peripheral zone > tailing zone > reclaimed zone， and their pollution degrees were tailing zone > peripheral zone > reclaimed zone. （ii） In general， the total and bioavailable contents of Pb， Zn， Cu， Cd showed a significant correlation， but were not correlated in each study zone. （iii） Pollution degrees of the study zones were in the order of tailing zone > peripheral zone > reclamation zone. The pollutions of heavy metals Pb and Cd were more serious， especially Cd reached high levels of pollution degree， and the pollution degrees of Zn， Cu were lighter. Contributors of ecological risk were mainly Cd， Pb and Cu， and the contribution of Cd achieved more than 90 %， making it the main source of pollution.
[Conclusions]The results not only revealed the pollution status of heavy metals in the surface soil of the abandoned coal mine， but also could provide scientific guidance for reasonable utilization and ecological recovery of the land.
Key words LeadZinc mine； Heavy metal； Bioavailable form； Analysis and assessment
In recent years， with the rapid development of industry， the plunder and exploitation of mineral resources have become increasingly serious. As one of the important mineral resources， the leadzinc mine has the characteristics of long mining time， wide coverage， and large area of pollution. As one of the leadzinc deposits in Hainan Province， Changhua leadzinc mine is characterized by its early mining time， high energy levels and associated multiple heavy metal elements[1]. Due to the historically backward mining capacity， there was no professional smelting processes adopted for quite a long period， and during the smelting process， heavy metals were discharged into the atmosphere in the form of gas or smoke， which entered the soil through atmospheric deposition， rainfall， etc. Meanwhile， without recycling， the waste rock， waste residue， and waste water of the leadzinc mine result in the continuous accumulation and enrichment of heavy metal pollutants in soil， causing pollution to the topsoil and bringing about strong ecological risks to the animals and plants in the polluted area[2]. Heavy metals are important pollutants with potential hazards， which have caused special attention because of the characteristics of longlasting environmental hazards， geochemical cycling and ecological risks[3]. Due to the complex dynamic interaction between heavy metals， soils and organisms， some of the soil heavy metals that can be absorbed and used by the organisms are called the bioavailable form. However， most of the previous research results are based on the assumption that the total amount of heavy metals in the soil can be absorbed and utilized by the organisms， which makes it difficult to accurately assess the risk of soil heavy metal pollution. In heavy metalcontaminated soils， the bioavailable amount is better than the total amount in reflecting the degree of damage to the crop caused by the pollutants[4]， and the bioavailable form is a more active stage of heavy metals in soil， which determines the bioavailability and environmental risk degree， making it an important measure to soil pollution risk[5-7]. Studying the correlation and pollution degree of the total and bioavailable contents of heavy metals is helpful to grasp the pollution degree and potential ecological damage of the heavy metals in the research zone and to provide a reference for the soil remediation. In this paper， the surface soil around the abandoned leadzinc mine in Changhua was taken as the research object， and the geostatistics method was used to analyze the correlations and evaluation methods of bioavailable Pb， Zn， Cu， and Cd in soil， with the aim to provide scientific basis for the rational use and restoration of land in the region.
Overview of the Study zone
The study zone is located in the leadzinc mine and the surrounding abandoned polluted land in the northwest of Changhua Town， Changjiang Li Autonomous County， Hainan Province. The surrounding soil is dominated by sandy land and bare land. There is no soil available for farming. The vegetation is dominated by shrubs， and arbor trees are sporadically scattered in abandoned land. The area is one of the typical representatives of the leadzinc deposits in the leadzinc metallogenic area of Hainan Province. It began mining in 1943， and in 1991， the mine was closed due to the depletion of ore sources， and the history of zinc smelting resulted in serious soil contamination in the area （Fig. 1）.
Fig. 1 Distribution of sampling points
The terrain in the study zone is dominated by plains， with high terrain in the northwest and low in the southeast with an area of about 19 km2. The area is located in the region with tropical marine monsoon climate， with a typhoon rainy season from May to November each year and a dry season from December to April of the following year. The area has an annual average temperature of 24.3 ℃， and the annual precipitation of 902-1 805 mm[8]. There is a river in the area down the mining area and the provincial road. Since the tailings slag is abandoned at the abandoned leadzinc mine area， the river often stops flowing. Caused by rainwater erosion and human activities， waste slag is brought into the farmland. The surrounding farmland has been heavily polluted by heavy metals， and no crops have been planted.
Materials and Methods
Sample collection and determination
The samples were collected from abandoned land in June-August 2013. To ensure the uniformity and representativeness of the collected samples， 5point sampling method was used to collect the samples of topsoil at the sampling depth of 0-20 cm， and a total of 56 soil samples were collected， including 15 soil samples from the tailing zone， 13 from the reclaimed zone and 28 from the peripheral zone. After collection， the samples were put in sealed storage in the polyethylene plastic bag， and the sampling points were numbered. The collected soil samples were placed in the shade for air drying， and then stones， dead remains and plant debris were removed from the samples. After grinding， the samples were sieved by the 100mesh nylon sieve for later use. The total contents of heavy metals Pb， Zn， Cu and Cd were first digested using HNO3HFHClO4 method and then measured by ICPMS （inductively coupled plasma mass spectrometry）. The bioavailable heavy metals Pb， Zn， Cu and Cd were extracted by DTPA and determined by ICPAES[9]. Data analysis
The correlation analysis and evaluation method of the bioavailable content and the total amount of heavy metal elements were performed using SPSS12.0 and Excel， and the graphics were drawn using Corel DRAW 10.
Results and Analysis
Total and bioavailable contents of heavy metals in surface soil
As shown in Table 1， the total amount of heavy metals Pb， Zn， Cu， and Cd ranged from 89.96-16 176.72， 120.65-10 225.56， 1.53-824.26， and 1.28-85， respectively. In addition to the Zn content from the reclaimed zone， all other samples had contents of heavy metals exceeding the standard. The average excess multiples of Pb， Zn， Cu， Cd in the tailing zone， reclaimed zone and peripheral zone ranged from 26.08-214.3， 12.73-83.03， 6.45-50.3， 136.01-496.78； the bioavailable contents of the heavy metal elements Pb， Zn， Cu， Cd ranged from 4.68-2 668.6， 2.07-419.84， 0.05-109.68， 0.04-14.58. The excess rate of Cd was 100% only in the tailing zone and peripheral zone. Although the Pb， Zn， Cu， Cd contents in the reclaimed zone and the Pb， Zn and Cu contents in the peripheral zone did not completely exceed the standard， the average excess multiples were all greater than 1. The results showed that only 2 samples collected from the reclaimed zone had the total amount of heavy metals in surface soil lower than the soil background value of Hainan， while all other samples exceeded the standard. Especially the associated element Cd had the total content range of 1.28-85 mg/kg， which was much higher than the background value 0.05 mg/kg of Cd content in the soil of Hainan， and the average excess multiple could reach up to 459.76 at the maximum， which was seriously beyond the standard. In the 3 study zones， the bioavailable contents of Ph， Zn， Cu and Cd accounted for no more than 20% of the total contents in the soil of the abandoned leadzinc mine. Although the proportions of bioavailable contents of heavy metals were low， the bioavailable forms of heavy metals were relatively active， so the bioavailable contents determined the degree of bioavailability and environmental risk， which could better reflect the damage degree of pollutants to animals and plants than the total contents of heavy metals.
Average proportion of the total volume∥%10010010010013.210.6913.3314.63
Hainan soil background value22.3435.114.950.0522.3435.114.950.05
The sample number of the tailing zone is n1= 15， the sample of reclaimed zone is n2 = 13， and the sample of peripheral zone is n3 = 28. The calculation of the excess rate and excess multiple use the total background value of soil heavy metals in Hainan as the reference standard. For the different study zones， the average excess multiples of the total and bioavailable contents of soil heavy metals were in the order of tailing zone > peripheral zone > reclaimed zone， indicating that there was a certain rule for the pollution degree of total and available heavy metals in soil， and the pollution degrees were in the order of tailing zone > peripheral zone > reclaimed zone. The bioavailable contents of heavy metals were controlled by the total contents of heavy metals， but the order of total contents and bioavailable contents was not exactly in the onetoone correspondence. Therefore， the order of total contents could not completely determine the pollution of heavy metals. In the 3 study zones， the appearance points of the extreme values of the bioavailable contents of the 4 heavy metals were not exactly the same as the appearance points of the extreme values of total contents of heavy metals. The extreme values of available Cd content appeared at the same points of the extreme total Cd content. The minimum value of available Cd content appeared in the reclaimed zone， while the maximum value appeared in the peripheral zone. The minimum available Pb content appeared at the same point as the minimum total Pb content in the reclaimed zone， while the maximum available content appeared at different points from the maximum total Pb content in the reclaimed zone. The appearance points of the extreme values of available Zn content were completely different from that of the total Zn content. The minimum available Cu content appeared at the same point as the minimum total Cu content in the reclaimed zone， while the maximum available content appeared at different points from the maximum total Cu content in the reclaimed zone.
In terms of the coefficients of variation， the variation coefficients of the total contents of the 4 heavy metals in the 3 study zones ranged from 41.36% to 82.37%， 87.79% to 166.64%， 87.92% to 108.67%， and the variation coefficients of available contents in the 3 study zones ranged from 56.22% to 89.05%， 26.78% to 150%， 93.55 to 121.61%. Comparatively speaking， in addition to the variation coefficients of bioavailable contents of Cu， Cd in the tailing zone and Cu in the reclaimed zone， which were smaller than those of the total contents， the variation coefficients of all other bioavailable contents were greater than those of the total contents. The results showed that the coefficients of variation of total and bioavailable contents of heavy metals in the 3 study zones were in the order of reclaimed zone > peripheral zone > tailing zone. The variation coefficients of heavy metals in the reclaimed zone and peripheral zone were all highintensity variations with great differences in spatial distribution， while the variation coefficients of tailing zone were of moderate variation intensity with some spatial distribution. In addition to the variation coefficients of bioavailable contents of Zn Cu in the tailing zone and Cu in the reclaimed zone， which were smaller than those of the total contents， the variation coefficients of bioavailable contents of Pb， Zn， Cu， Cd were greater than those of the total contents. The greater variation coefficients for the bioavailable contents of Pb， Zn， Cu， Cd indicated that the contents and compositional proportions of the bioavailable Pb， Zn， Cu， and Cd were different in different forms （exchangeable form， carbonate bounded form， ironmanganese oxide bounded form and organic matter bounded form）. The heavy metals in the soil solution mainly exist in the form of simple ions， organic or inorganic complex ions， and the favorable hydrothermal conditions in Hainan are conducive to the formation of soil humic acid. The fulvic acid in the humic acid， which has relatively low molecular weight and relatively strong chemical activity， is prone to combine with heavy metals Pb， Zn， Cu and Cd， and it is easily oxidatively decomposed itself[10]， releasing free Pb， Zn， and Cu. Cd ions， which are easily utilized by the organism. The high content of ironmanganese oxides in Hainan soil adsorbs a large amount of bioavailable， exchangeable and ionized Pb， Zn， Cu， and Cd ions in the soil solution. Therefore， the changes in the contents of manganese oxide bounded and loose organic bounded forms also have the most significant influence on the variability of Pb， Zn， Cu and Cd in soil. It is due to the fact that the different forms of Pb， Zn， Cu， and Cd have a large variation in each soil sample that results in the greater variation coefficients of bioavailable contents than that of total contents of Pb， Zn， Cu， Cd in soil samples[10]. Correlation analysis of bioavailable and total amount of heavy metals in surface soil
As shown in Table 2， there was no significant correlation between the bioavailable contents and total contents of Pb， Zn， Cu， and Cd in the soil of the tailing zone， which may be related to the fact that most of the tailing zone was affected by the washing of ore acid， and the acid infected the soil， making the surface soil have a strong acidity， thereby affecting the availability of plant nutrient elements and the activity of polluting elements. There was extremely significant correlation between the bioavailable contents and total contents of Zn， Cd in the reclaimed zone （P<0. 01）， while the correlation between the total and bioavailable contents of Pb and Cu was not significant. The reason was that the reclaimed zone was planted with the plants which could enrich heavy metals Fimbristylis dichotoma， Imperata cylindrical， Vitex rotundifolia， Actinidia polygama Eucalyptus tereticornis and Casuarina equisetifolia， and the dominant plants had high accumulation amounts of Zn， Cd. E. tereticornis had the highest accumulation amount of Zn， with an average reaching up to 596 mg/kg， and V. rotundifolia had the highest accumulation amount of Cd， with the average reaching 25 mg/kg. However， the accumulation amounts of Pb， Cd were much lower. The plant with the highest accumulation of Pb was I. cylindrical with an average of 71.13 mg/kg， and the plant with the highest accumulation of Cu was A. polygama with an average of 16.8 mg/kg[11]. There was a very significant correlation between the total contents and bioavailable contents of Pb， Zn， Cu and Cd in the peripheral zone. The main reason was that tailings slag was discarded at the leadzinc mine waste disposal site， and the peripheral zone was in relatively low terrain compared to the tailing zone， so the tailings slag could be easily transported to the peripheral zone under the scouring of rainwater， causing the river to stop flowing frequently. Moreover， the heavy metals were easily activated and leaked out under the rain. The results show that although the heavy metal contents in the soil can not accurately assess the heavy metal environmental effects and ecobioavailability of soil， in general， there is a very significant positive correlation between the total contents of heavy metals and the bioavailable contents of heavy metals in the abandoned leadzinc mine in Changhua. The results are not quite consistent to the results of relevant research， which show that there is a significant positive correlation between the bioavailable contents and total contents of heavy metals[12-13]. Although the content of bioavailable heavy metals in soil is controlled by the total contents of soil heavy metals to a large extent， the total contents of heavy metals in the soil can also reflect the bioavailable contents to a certain extent. However， the total contents of heavy metals in soil are not significantly correlated with the bioavailable contents. Therefore， the study of soil pollution status of the bioavailable heavy metals must be combined with the correlation analysis and evaluation of the total contents of soil heavy metals. Evaluation of Heavy Metal Pollution in Surface Soil
Mining time， natural conditions， existing soil activity and vegetation growth capacity of Changhua leadzinc mine
In this paper， firstly， single factor pollution index method was used to analyze the pollution degree of each sampling point； then， the Nemerow comprehensive index method was used to evaluate the comprehensive pollution index of leadzinc mine area[14]， and the pollution degrees were classified according to the Nemerow comprehensive pollution indexes. And then， the geological accumulation pollution index was used to calculate the pollution caused by natural diagenesis and human activities， and the pollution degrees were classified accordingly. Finally， the potential ecological risk index method was used to carry out ecological risk assessment of the region. Heavy metals are potentially important pollutants. Only through the evaluation of heavy metals in soil can make it possible to monitor soil environmental quality， and take appropriate management and effective technical measures for soil environmental governance and restoration[15-22].
Evaluation of single factor pollution index and Nemerow comprehensive pollution index
As shown in Table 3， the evaluation results of single factor pollution index （Pi） showed that the indexes of pollutants from large to small was in the order of Cd > Pb > Cu > Zn， and the level of pollution reached severe level. As for different study zones， the pollution levels were different. The contents of Pb， Zn， Cu， Cd all reached the severe pollution levels in the tailing zone and peripheral zone， while in the reclaimed zone， only Cd and Pb reached the severe pollution levels， and Zn， Cu were of light pollution levels. Comparatively speaking， for the 3 study zones， the pollution in the tailing zone and peripheral zone was more serious， while the pollution in the reclaimed zone was lighter. The evaluation results of Nemerow comprehensive pollution index （PN） showed that the pollution in the tailing zone， reclaimed zone and peripheral zone all reached severe levels， and in the order of tailing zone > peripheral zone > reclaimed zone. The pollution level of the reclaimed zone suggests that the reclamation of heavy metalcontaminated soil is helpful to reduce the degree of pollution， and is conducive to the restoration of abandoned land in the mining area. However， there are still different degrees of pollution in the reclaimed zone， so it still could not grow crops， so as to avoid heavy metals entering the body through the food chain， which can cause harm to human health. Evaluation of geological accumulation index
The geological accumulation index is mainly used to evaluate the degree of heavy metal pollution in sediments， and it is also applicable to the evaluation of heavy metal pollution in contemporary soils. Since the leadzinc mine in Changhua has been mined for a long time， it is necessary to carry out geological accumulation index evaluation to the heavy metals in the mine. As shown in Table 4， in terms of the study zones， the pollution degrees were in the order of tailing zone > peripheral zone > reclaimed zone. In terms of the pollution levels of heavy metals， the pollution was in the order of Cd > Pb > Zn > Cu. The pollution of Zn and Cu was of lightmedium levels， but the pollution of Zn and Cu in the tailing zone and peripheral zone was of medium levels， while there was no Zn and Cu pollution in the reclaimed zone. The results indicated that among the 4 heavy metal elements， the pollution of Pb and Cd was much more serious， which were the major pollutants in soil. However， the overall evaluation of heavy metals could not truly reflect the pollution of various study zones. Therefore， it is necessary to make a correct evaluation of each study zone in order to formulate reasonable soil remediation measures.
Evaluation of potential ecological risk index
Heavy metals are potentially harmful important pollutants. Heavy metals that enter the natural environment are not easily dissolved. They can enter the plants and animals through many natural and artificial pathways， and they may eventually enter and accumulate in the human body， and cause harm to human health. Therefore， it is necessary to classify the potential ecological risk degree of the toxicity coefficients of heavy metals quantitatively to carry out objective evaluation on the pollution status of heavy metals in soil. As shown in Table 5， the overall ecological risk indexes of heavy metals in the 3 study zones were in the order of Cd > Pb > Cu > Zn. In the tailing zone， the potential ecological risk index of Cd was 2 488.2， which exceeded the threshold of serious risk degree of 320， and the pollution degree reached the serious risk level； the potential ecological risk index of Pb was 151.95， and the pollution degree also reached the serious risk level； the potential risk indexes of Zn and Cu were less than 40， belonging to light pollution. In the reclaimed zone， the potential ecological risk index of Cd was 406.28， which exceeded the threshold of serious risk degree of 320， and the pollution degree reached the serious risk level； the potential ecological risk indexes Pb， Zn and Cu were all less than 40， and all were light pollution. In the peripheral zone， the potential ecological risk index of Cd was 2 218.85， which exceeded the threshold of serious risk degree of 320， and the pollution degree reached the serious risk level； the potential ecological risk index of Pb was 121.35， and the pollution was heavy， while the potential risk indexes of both Zn and Cu were less than 40， belonging to light pollution. The contribution rates of the 4 kinds of heavy metals in the 3 study zones to the compound ecological risk indexes were as follows： tailing zone （5.67%， 0.18%， 1.35%， 92.80%）， reclaimed zone （3.73%， 0.46%， 1.29%， 94.52%）， peripheral zone （5.47%， 0.30%， 1.7%， 93.6%）. The results showed that the potential ecological pollution of Pb was heavy in the abandoned leadzinc mine， and the potential ecological pollution of Cd was serious. Moreover， the major contributors for ecological risk were Cd， Pb and Cu， in which the contribution rate of Cd was the largest， reaching 90%， so Cd was the main source of pollution. Conclusion
Through the analysis， correlation analysis of the total and bioavailable contents as well as the pollution evaluation of Pb， Zn， Cu， Cd in the surface soil of the abandoned leadzinc mine in Changhua， the following conclusions are obtained.
First， the general situation of heavy metal pollution in surface soil. The tailing zone， reclaimed zone and peripheral zone are damaged by Pb， Zn， Cu and Cd to different degrees in the abandoned leadzinc mine. The average excess multiples of heavy metals of total and bioavailable contents in soils are in the order of peripheral zone > tailing zone > reclaimed zone， and their pollution degrees are tailing zone > peripheral zone > reclaimed zone. Compared with the soil background value of Hainan， the pollution of Cd is the most serious， followed by Pb， Zn， and the pollution of Cu is the lightest.
Second， the correlation between total and bioavailable contents of heavy metals in the surface soil. In general， the total and bioavailable contents of Pb， Zn， Cu， Cd show an extremely significant positive correlation， but are not correlated in each study zone. Although the total contents of heavy metals are the major controlling factors for the bioavailable contents， the order of total contents and bioavailable contents is not exactly in the onetoone correspondence.
Third， Evaluation of heavy metal pollution in surface soil. The evaluation results of heavy metal pollution in the surface soil of the abandoned leadzinc mine in Changhua using the methods of single factor index method， Nemerow pollution index， geological accumulation index and potential ecological risk index show that the pollution degrees of the study zones are in the order of tailing zone > peripheral zone > reclamation zone. The pollution levels of heavy metals are in the order of Cd > Pb > Du > Zn， and the pollution of Pb， Cd is serious， especially Cd reaches the serious pollution level， while the pollution degrees of Zn and Cu are much lighter. The evaluation of potential ecological risk shows that the major contributors for ecological risk areCd， Pb and Cu， in which the contribution rate of Cd is the largest， reaching 90%， so Cd is the main source of pollution.
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Editor： Na LI Proofreader： Xinxiu ZHU
[Methods]A total of 56 surface samples were collected from the 3 study zones （peripheral zone， reclamation zone and tailing zone） located at the leadzine mine in Changhua. Based on total and bioavailable heavy metals （Pb， Zn， Cu and Cd） in soils， the correlation of total content with bioavailable content was analyzed， and their pollution characteristics were assessed by methods of single factor pollution index， Nemerow pollution index， geological accumulation index and potential ecological risk index.
[Results]（i） The surface soils were polluted by Pb， Zn， Cu， Cd in different degrees， and the overall trend of the contents of the 4 heavy metals was in order of Cd > Pb > Zn > Cu. The average excess multiple of heavy metals of total and bioavailable content in soils were in the order of peripheral zone > tailing zone > reclaimed zone， and their pollution degrees were tailing zone > peripheral zone > reclaimed zone. （ii） In general， the total and bioavailable contents of Pb， Zn， Cu， Cd showed a significant correlation， but were not correlated in each study zone. （iii） Pollution degrees of the study zones were in the order of tailing zone > peripheral zone > reclamation zone. The pollutions of heavy metals Pb and Cd were more serious， especially Cd reached high levels of pollution degree， and the pollution degrees of Zn， Cu were lighter. Contributors of ecological risk were mainly Cd， Pb and Cu， and the contribution of Cd achieved more than 90 %， making it the main source of pollution.
[Conclusions]The results not only revealed the pollution status of heavy metals in the surface soil of the abandoned coal mine， but also could provide scientific guidance for reasonable utilization and ecological recovery of the land.
Key words LeadZinc mine； Heavy metal； Bioavailable form； Analysis and assessment
In recent years， with the rapid development of industry， the plunder and exploitation of mineral resources have become increasingly serious. As one of the important mineral resources， the leadzinc mine has the characteristics of long mining time， wide coverage， and large area of pollution. As one of the leadzinc deposits in Hainan Province， Changhua leadzinc mine is characterized by its early mining time， high energy levels and associated multiple heavy metal elements[1]. Due to the historically backward mining capacity， there was no professional smelting processes adopted for quite a long period， and during the smelting process， heavy metals were discharged into the atmosphere in the form of gas or smoke， which entered the soil through atmospheric deposition， rainfall， etc. Meanwhile， without recycling， the waste rock， waste residue， and waste water of the leadzinc mine result in the continuous accumulation and enrichment of heavy metal pollutants in soil， causing pollution to the topsoil and bringing about strong ecological risks to the animals and plants in the polluted area[2]. Heavy metals are important pollutants with potential hazards， which have caused special attention because of the characteristics of longlasting environmental hazards， geochemical cycling and ecological risks[3]. Due to the complex dynamic interaction between heavy metals， soils and organisms， some of the soil heavy metals that can be absorbed and used by the organisms are called the bioavailable form. However， most of the previous research results are based on the assumption that the total amount of heavy metals in the soil can be absorbed and utilized by the organisms， which makes it difficult to accurately assess the risk of soil heavy metal pollution. In heavy metalcontaminated soils， the bioavailable amount is better than the total amount in reflecting the degree of damage to the crop caused by the pollutants[4]， and the bioavailable form is a more active stage of heavy metals in soil， which determines the bioavailability and environmental risk degree， making it an important measure to soil pollution risk[5-7]. Studying the correlation and pollution degree of the total and bioavailable contents of heavy metals is helpful to grasp the pollution degree and potential ecological damage of the heavy metals in the research zone and to provide a reference for the soil remediation. In this paper， the surface soil around the abandoned leadzinc mine in Changhua was taken as the research object， and the geostatistics method was used to analyze the correlations and evaluation methods of bioavailable Pb， Zn， Cu， and Cd in soil， with the aim to provide scientific basis for the rational use and restoration of land in the region.
Overview of the Study zone
The study zone is located in the leadzinc mine and the surrounding abandoned polluted land in the northwest of Changhua Town， Changjiang Li Autonomous County， Hainan Province. The surrounding soil is dominated by sandy land and bare land. There is no soil available for farming. The vegetation is dominated by shrubs， and arbor trees are sporadically scattered in abandoned land. The area is one of the typical representatives of the leadzinc deposits in the leadzinc metallogenic area of Hainan Province. It began mining in 1943， and in 1991， the mine was closed due to the depletion of ore sources， and the history of zinc smelting resulted in serious soil contamination in the area （Fig. 1）.
Fig. 1 Distribution of sampling points
The terrain in the study zone is dominated by plains， with high terrain in the northwest and low in the southeast with an area of about 19 km2. The area is located in the region with tropical marine monsoon climate， with a typhoon rainy season from May to November each year and a dry season from December to April of the following year. The area has an annual average temperature of 24.3 ℃， and the annual precipitation of 902-1 805 mm[8]. There is a river in the area down the mining area and the provincial road. Since the tailings slag is abandoned at the abandoned leadzinc mine area， the river often stops flowing. Caused by rainwater erosion and human activities， waste slag is brought into the farmland. The surrounding farmland has been heavily polluted by heavy metals， and no crops have been planted.
Materials and Methods
Sample collection and determination
The samples were collected from abandoned land in June-August 2013. To ensure the uniformity and representativeness of the collected samples， 5point sampling method was used to collect the samples of topsoil at the sampling depth of 0-20 cm， and a total of 56 soil samples were collected， including 15 soil samples from the tailing zone， 13 from the reclaimed zone and 28 from the peripheral zone. After collection， the samples were put in sealed storage in the polyethylene plastic bag， and the sampling points were numbered. The collected soil samples were placed in the shade for air drying， and then stones， dead remains and plant debris were removed from the samples. After grinding， the samples were sieved by the 100mesh nylon sieve for later use. The total contents of heavy metals Pb， Zn， Cu and Cd were first digested using HNO3HFHClO4 method and then measured by ICPMS （inductively coupled plasma mass spectrometry）. The bioavailable heavy metals Pb， Zn， Cu and Cd were extracted by DTPA and determined by ICPAES[9]. Data analysis
The correlation analysis and evaluation method of the bioavailable content and the total amount of heavy metal elements were performed using SPSS12.0 and Excel， and the graphics were drawn using Corel DRAW 10.
Results and Analysis
Total and bioavailable contents of heavy metals in surface soil
As shown in Table 1， the total amount of heavy metals Pb， Zn， Cu， and Cd ranged from 89.96-16 176.72， 120.65-10 225.56， 1.53-824.26， and 1.28-85， respectively. In addition to the Zn content from the reclaimed zone， all other samples had contents of heavy metals exceeding the standard. The average excess multiples of Pb， Zn， Cu， Cd in the tailing zone， reclaimed zone and peripheral zone ranged from 26.08-214.3， 12.73-83.03， 6.45-50.3， 136.01-496.78； the bioavailable contents of the heavy metal elements Pb， Zn， Cu， Cd ranged from 4.68-2 668.6， 2.07-419.84， 0.05-109.68， 0.04-14.58. The excess rate of Cd was 100% only in the tailing zone and peripheral zone. Although the Pb， Zn， Cu， Cd contents in the reclaimed zone and the Pb， Zn and Cu contents in the peripheral zone did not completely exceed the standard， the average excess multiples were all greater than 1. The results showed that only 2 samples collected from the reclaimed zone had the total amount of heavy metals in surface soil lower than the soil background value of Hainan， while all other samples exceeded the standard. Especially the associated element Cd had the total content range of 1.28-85 mg/kg， which was much higher than the background value 0.05 mg/kg of Cd content in the soil of Hainan， and the average excess multiple could reach up to 459.76 at the maximum， which was seriously beyond the standard. In the 3 study zones， the bioavailable contents of Ph， Zn， Cu and Cd accounted for no more than 20% of the total contents in the soil of the abandoned leadzinc mine. Although the proportions of bioavailable contents of heavy metals were low， the bioavailable forms of heavy metals were relatively active， so the bioavailable contents determined the degree of bioavailability and environmental risk， which could better reflect the damage degree of pollutants to animals and plants than the total contents of heavy metals.
Average proportion of the total volume∥%10010010010013.210.6913.3314.63
Hainan soil background value22.3435.114.950.0522.3435.114.950.05
The sample number of the tailing zone is n1= 15， the sample of reclaimed zone is n2 = 13， and the sample of peripheral zone is n3 = 28. The calculation of the excess rate and excess multiple use the total background value of soil heavy metals in Hainan as the reference standard. For the different study zones， the average excess multiples of the total and bioavailable contents of soil heavy metals were in the order of tailing zone > peripheral zone > reclaimed zone， indicating that there was a certain rule for the pollution degree of total and available heavy metals in soil， and the pollution degrees were in the order of tailing zone > peripheral zone > reclaimed zone. The bioavailable contents of heavy metals were controlled by the total contents of heavy metals， but the order of total contents and bioavailable contents was not exactly in the onetoone correspondence. Therefore， the order of total contents could not completely determine the pollution of heavy metals. In the 3 study zones， the appearance points of the extreme values of the bioavailable contents of the 4 heavy metals were not exactly the same as the appearance points of the extreme values of total contents of heavy metals. The extreme values of available Cd content appeared at the same points of the extreme total Cd content. The minimum value of available Cd content appeared in the reclaimed zone， while the maximum value appeared in the peripheral zone. The minimum available Pb content appeared at the same point as the minimum total Pb content in the reclaimed zone， while the maximum available content appeared at different points from the maximum total Pb content in the reclaimed zone. The appearance points of the extreme values of available Zn content were completely different from that of the total Zn content. The minimum available Cu content appeared at the same point as the minimum total Cu content in the reclaimed zone， while the maximum available content appeared at different points from the maximum total Cu content in the reclaimed zone.
In terms of the coefficients of variation， the variation coefficients of the total contents of the 4 heavy metals in the 3 study zones ranged from 41.36% to 82.37%， 87.79% to 166.64%， 87.92% to 108.67%， and the variation coefficients of available contents in the 3 study zones ranged from 56.22% to 89.05%， 26.78% to 150%， 93.55 to 121.61%. Comparatively speaking， in addition to the variation coefficients of bioavailable contents of Cu， Cd in the tailing zone and Cu in the reclaimed zone， which were smaller than those of the total contents， the variation coefficients of all other bioavailable contents were greater than those of the total contents. The results showed that the coefficients of variation of total and bioavailable contents of heavy metals in the 3 study zones were in the order of reclaimed zone > peripheral zone > tailing zone. The variation coefficients of heavy metals in the reclaimed zone and peripheral zone were all highintensity variations with great differences in spatial distribution， while the variation coefficients of tailing zone were of moderate variation intensity with some spatial distribution. In addition to the variation coefficients of bioavailable contents of Zn Cu in the tailing zone and Cu in the reclaimed zone， which were smaller than those of the total contents， the variation coefficients of bioavailable contents of Pb， Zn， Cu， Cd were greater than those of the total contents. The greater variation coefficients for the bioavailable contents of Pb， Zn， Cu， Cd indicated that the contents and compositional proportions of the bioavailable Pb， Zn， Cu， and Cd were different in different forms （exchangeable form， carbonate bounded form， ironmanganese oxide bounded form and organic matter bounded form）. The heavy metals in the soil solution mainly exist in the form of simple ions， organic or inorganic complex ions， and the favorable hydrothermal conditions in Hainan are conducive to the formation of soil humic acid. The fulvic acid in the humic acid， which has relatively low molecular weight and relatively strong chemical activity， is prone to combine with heavy metals Pb， Zn， Cu and Cd， and it is easily oxidatively decomposed itself[10]， releasing free Pb， Zn， and Cu. Cd ions， which are easily utilized by the organism. The high content of ironmanganese oxides in Hainan soil adsorbs a large amount of bioavailable， exchangeable and ionized Pb， Zn， Cu， and Cd ions in the soil solution. Therefore， the changes in the contents of manganese oxide bounded and loose organic bounded forms also have the most significant influence on the variability of Pb， Zn， Cu and Cd in soil. It is due to the fact that the different forms of Pb， Zn， Cu， and Cd have a large variation in each soil sample that results in the greater variation coefficients of bioavailable contents than that of total contents of Pb， Zn， Cu， Cd in soil samples[10]. Correlation analysis of bioavailable and total amount of heavy metals in surface soil
As shown in Table 2， there was no significant correlation between the bioavailable contents and total contents of Pb， Zn， Cu， and Cd in the soil of the tailing zone， which may be related to the fact that most of the tailing zone was affected by the washing of ore acid， and the acid infected the soil， making the surface soil have a strong acidity， thereby affecting the availability of plant nutrient elements and the activity of polluting elements. There was extremely significant correlation between the bioavailable contents and total contents of Zn， Cd in the reclaimed zone （P<0. 01）， while the correlation between the total and bioavailable contents of Pb and Cu was not significant. The reason was that the reclaimed zone was planted with the plants which could enrich heavy metals Fimbristylis dichotoma， Imperata cylindrical， Vitex rotundifolia， Actinidia polygama Eucalyptus tereticornis and Casuarina equisetifolia， and the dominant plants had high accumulation amounts of Zn， Cd. E. tereticornis had the highest accumulation amount of Zn， with an average reaching up to 596 mg/kg， and V. rotundifolia had the highest accumulation amount of Cd， with the average reaching 25 mg/kg. However， the accumulation amounts of Pb， Cd were much lower. The plant with the highest accumulation of Pb was I. cylindrical with an average of 71.13 mg/kg， and the plant with the highest accumulation of Cu was A. polygama with an average of 16.8 mg/kg[11]. There was a very significant correlation between the total contents and bioavailable contents of Pb， Zn， Cu and Cd in the peripheral zone. The main reason was that tailings slag was discarded at the leadzinc mine waste disposal site， and the peripheral zone was in relatively low terrain compared to the tailing zone， so the tailings slag could be easily transported to the peripheral zone under the scouring of rainwater， causing the river to stop flowing frequently. Moreover， the heavy metals were easily activated and leaked out under the rain. The results show that although the heavy metal contents in the soil can not accurately assess the heavy metal environmental effects and ecobioavailability of soil， in general， there is a very significant positive correlation between the total contents of heavy metals and the bioavailable contents of heavy metals in the abandoned leadzinc mine in Changhua. The results are not quite consistent to the results of relevant research， which show that there is a significant positive correlation between the bioavailable contents and total contents of heavy metals[12-13]. Although the content of bioavailable heavy metals in soil is controlled by the total contents of soil heavy metals to a large extent， the total contents of heavy metals in the soil can also reflect the bioavailable contents to a certain extent. However， the total contents of heavy metals in soil are not significantly correlated with the bioavailable contents. Therefore， the study of soil pollution status of the bioavailable heavy metals must be combined with the correlation analysis and evaluation of the total contents of soil heavy metals. Evaluation of Heavy Metal Pollution in Surface Soil
Mining time， natural conditions， existing soil activity and vegetation growth capacity of Changhua leadzinc mine
In this paper， firstly， single factor pollution index method was used to analyze the pollution degree of each sampling point； then， the Nemerow comprehensive index method was used to evaluate the comprehensive pollution index of leadzinc mine area[14]， and the pollution degrees were classified according to the Nemerow comprehensive pollution indexes. And then， the geological accumulation pollution index was used to calculate the pollution caused by natural diagenesis and human activities， and the pollution degrees were classified accordingly. Finally， the potential ecological risk index method was used to carry out ecological risk assessment of the region. Heavy metals are potentially important pollutants. Only through the evaluation of heavy metals in soil can make it possible to monitor soil environmental quality， and take appropriate management and effective technical measures for soil environmental governance and restoration[15-22].
Evaluation of single factor pollution index and Nemerow comprehensive pollution index
As shown in Table 3， the evaluation results of single factor pollution index （Pi） showed that the indexes of pollutants from large to small was in the order of Cd > Pb > Cu > Zn， and the level of pollution reached severe level. As for different study zones， the pollution levels were different. The contents of Pb， Zn， Cu， Cd all reached the severe pollution levels in the tailing zone and peripheral zone， while in the reclaimed zone， only Cd and Pb reached the severe pollution levels， and Zn， Cu were of light pollution levels. Comparatively speaking， for the 3 study zones， the pollution in the tailing zone and peripheral zone was more serious， while the pollution in the reclaimed zone was lighter. The evaluation results of Nemerow comprehensive pollution index （PN） showed that the pollution in the tailing zone， reclaimed zone and peripheral zone all reached severe levels， and in the order of tailing zone > peripheral zone > reclaimed zone. The pollution level of the reclaimed zone suggests that the reclamation of heavy metalcontaminated soil is helpful to reduce the degree of pollution， and is conducive to the restoration of abandoned land in the mining area. However， there are still different degrees of pollution in the reclaimed zone， so it still could not grow crops， so as to avoid heavy metals entering the body through the food chain， which can cause harm to human health. Evaluation of geological accumulation index
The geological accumulation index is mainly used to evaluate the degree of heavy metal pollution in sediments， and it is also applicable to the evaluation of heavy metal pollution in contemporary soils. Since the leadzinc mine in Changhua has been mined for a long time， it is necessary to carry out geological accumulation index evaluation to the heavy metals in the mine. As shown in Table 4， in terms of the study zones， the pollution degrees were in the order of tailing zone > peripheral zone > reclaimed zone. In terms of the pollution levels of heavy metals， the pollution was in the order of Cd > Pb > Zn > Cu. The pollution of Zn and Cu was of lightmedium levels， but the pollution of Zn and Cu in the tailing zone and peripheral zone was of medium levels， while there was no Zn and Cu pollution in the reclaimed zone. The results indicated that among the 4 heavy metal elements， the pollution of Pb and Cd was much more serious， which were the major pollutants in soil. However， the overall evaluation of heavy metals could not truly reflect the pollution of various study zones. Therefore， it is necessary to make a correct evaluation of each study zone in order to formulate reasonable soil remediation measures.
Evaluation of potential ecological risk index
Heavy metals are potentially harmful important pollutants. Heavy metals that enter the natural environment are not easily dissolved. They can enter the plants and animals through many natural and artificial pathways， and they may eventually enter and accumulate in the human body， and cause harm to human health. Therefore， it is necessary to classify the potential ecological risk degree of the toxicity coefficients of heavy metals quantitatively to carry out objective evaluation on the pollution status of heavy metals in soil. As shown in Table 5， the overall ecological risk indexes of heavy metals in the 3 study zones were in the order of Cd > Pb > Cu > Zn. In the tailing zone， the potential ecological risk index of Cd was 2 488.2， which exceeded the threshold of serious risk degree of 320， and the pollution degree reached the serious risk level； the potential ecological risk index of Pb was 151.95， and the pollution degree also reached the serious risk level； the potential risk indexes of Zn and Cu were less than 40， belonging to light pollution. In the reclaimed zone， the potential ecological risk index of Cd was 406.28， which exceeded the threshold of serious risk degree of 320， and the pollution degree reached the serious risk level； the potential ecological risk indexes Pb， Zn and Cu were all less than 40， and all were light pollution. In the peripheral zone， the potential ecological risk index of Cd was 2 218.85， which exceeded the threshold of serious risk degree of 320， and the pollution degree reached the serious risk level； the potential ecological risk index of Pb was 121.35， and the pollution was heavy， while the potential risk indexes of both Zn and Cu were less than 40， belonging to light pollution. The contribution rates of the 4 kinds of heavy metals in the 3 study zones to the compound ecological risk indexes were as follows： tailing zone （5.67%， 0.18%， 1.35%， 92.80%）， reclaimed zone （3.73%， 0.46%， 1.29%， 94.52%）， peripheral zone （5.47%， 0.30%， 1.7%， 93.6%）. The results showed that the potential ecological pollution of Pb was heavy in the abandoned leadzinc mine， and the potential ecological pollution of Cd was serious. Moreover， the major contributors for ecological risk were Cd， Pb and Cu， in which the contribution rate of Cd was the largest， reaching 90%， so Cd was the main source of pollution. Conclusion
Through the analysis， correlation analysis of the total and bioavailable contents as well as the pollution evaluation of Pb， Zn， Cu， Cd in the surface soil of the abandoned leadzinc mine in Changhua， the following conclusions are obtained.
First， the general situation of heavy metal pollution in surface soil. The tailing zone， reclaimed zone and peripheral zone are damaged by Pb， Zn， Cu and Cd to different degrees in the abandoned leadzinc mine. The average excess multiples of heavy metals of total and bioavailable contents in soils are in the order of peripheral zone > tailing zone > reclaimed zone， and their pollution degrees are tailing zone > peripheral zone > reclaimed zone. Compared with the soil background value of Hainan， the pollution of Cd is the most serious， followed by Pb， Zn， and the pollution of Cu is the lightest.
Second， the correlation between total and bioavailable contents of heavy metals in the surface soil. In general， the total and bioavailable contents of Pb， Zn， Cu， Cd show an extremely significant positive correlation， but are not correlated in each study zone. Although the total contents of heavy metals are the major controlling factors for the bioavailable contents， the order of total contents and bioavailable contents is not exactly in the onetoone correspondence.
Third， Evaluation of heavy metal pollution in surface soil. The evaluation results of heavy metal pollution in the surface soil of the abandoned leadzinc mine in Changhua using the methods of single factor index method， Nemerow pollution index， geological accumulation index and potential ecological risk index show that the pollution degrees of the study zones are in the order of tailing zone > peripheral zone > reclamation zone. The pollution levels of heavy metals are in the order of Cd > Pb > Du > Zn， and the pollution of Pb， Cd is serious， especially Cd reaches the serious pollution level， while the pollution degrees of Zn and Cu are much lighter. The evaluation of potential ecological risk shows that the major contributors for ecological risk areCd， Pb and Cu， in which the contribution rate of Cd is the largest， reaching 90%， so Cd is the main source of pollution.
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