Ecological Risk Assessment of Combined Pollution of Heavy Metals and Pesticides

Abstract. Urban soil is the main component of urban ecological systems and the key risk receptor from urbanization. Heavy metal and green pesticide pollutions in urban soils have been widely reported with the expanding of urbanization. Since urban soil pollution comes from various resources， application of integrated thinking and methods is needed in ecological risk assessment of urban soil pollution. This paper synthetically reviewed the combined pollution of heavy metals and pesticide， and ecological risk assessment， and then proposed some research trends and areas in the future that are required to carry out intensively according to the present situation of environmental pollution and international research fronts.

Keywords： Urban soil， Heavy metal， Pesticide， Combined pollution， Integrated eco-risk assessment

1. Introduction

Cities are the material， energy and information highly concentrated places on the earth's surface， and are the main regional space of the masses of the human living and activities. The city ecological system is the special ecological system， taking the humans as the center combined with the nature， economy and society. The soil of the city is an important part of the city ecosystem， and the soil degradation is a key environmental problem of the ecological environment problems in the city. In the ecological environment in the city， the main ecological service function of the soil of the city is embodied in the transformation， the absorption， and the retention of the pollutants， which plays the purge function to the pollutants before the chemistry saturation， and is the reservoir and purifier of the pollutants of the city. The second is to provide the habitat and the energy for the green plants， soil animals and the microorganisms [1]. Therefore， the soil of the city plays an important function of the ecological service in the city ecosystem， which is of great significance to the sustainable development of the city.

With the development of the urban， more and more pollutants enter the environment and coexist， which lead to a variety of pollutants coexist in the same environment and interact and form the effect of the environmental pollution. Many environmental effects cannot be expressed by the mechanism of action of a single pollutant. The evaluation criteria formulated relying on the single effect cannot reflect the requirements of the environmental quality， so the researches on the combined pollution have been paying more and more attention to. He Yongtian et al [2] thinks that the combined pollution refers to two kinds or more than two kinds of the pollutants of the different natures， or the different sources of the same pollutants， or the phenomenon of the environmental pollution formed by the coexistence of two and more than two kinds of the different types of the pollution in the same environment. Chen Huaiman et al [3] defined the compound pollution as the concurrent pollution of a variety of the elements or a variety of the chemicals， namely a variety of pollutants on the same medium （the soil， water， atmosphere， and creatures）. Recently， the definition of the complex pollution has been extended， which can define the combined pollution in different media， such as the soil compound pollution， the air and water compound pollution etc. Zhou Meidong et al [4] defined the combined pollution of the soil as two or more than two kinds of the pollutants coexist simultaneously in the soil， and the concentration of each pollutant exceeded the national soil environmental quality standards or the soil pollution has reached the level affecting the environmental quality of the soil. The combined pollution of the soil is various in the forms， including the compound pollution of the heavy metals， the heavy metals and nutrients， the compound pollution of the heavy metals and the organic pollutants and the organic compound pollution etc. Ye Haibo et al [5] discussed the ecological effects of the pollution of the heavy metals， and from the chemistry， physiology， and cytology and discussed the mechanism of the combined pollution. Shen Guoqing et al [6] reported the respective behaviors and the ways of generation of the heavy metals of the soil environment and the polycyclic aromatic hydrocarbons in the soil environment， and the combined effects of the pollutants in the combined pollution systems. Zheng Zhenhua et al [7] discussed the environmental effects of the complex pollution from the inorganic compound pollution， the inorganic and the organic composite pollution and the organic compound pollution.

The ecological risk assessment can determine the relationship between the risk sources and the ecological effect， judge the probability that the toxic and harmful substances will generate the significant harm to the ecological system， and provide the basis for the environmental management and the decision making [8]. The soil pollution and ecological risk assessment are based on the researches on the laws of the changes of the soil environment quality， according to the certain principles， standards and methods， and assess the degrees of the soil pollution， or evaluate the suitability of the soil on the human health. The purpose is to improve the quality of the soil， and put forward the countermeasures and measures to control and mitigate the adverse changes in the soil environment [9]. In the process of the urbanization， because the original agricultural ecosystem is replaced by the urban ecological system， there will be some negative effects not conducive to the sustainable development of the rural urbanization， resulting in the ecological risks of the city. The emission of all kinds of the wastes and pollutants generated by the human activities is one of the city ecological risk sources which have the greatest concern， and the soil of the city is the main risk receptor in the city ecological risks [10， 11]. The main characteristics of the researches on the ecological risk assessment of the contaminated soils of the city is that the pollutant sources have both the point sources and the surface sources， and the multiple pollutants coexist， which have a strong temporal and spatial characteristic， and are greatly affected by the historical factors. In addition， in the aspect of the risk receptor， because of the unique ecosystem service function of the soil of the city， the selection of the key points of the evaluation and the distribution of the weights have the very big difference from the researches on the ecological risks of the farmland and the soil pollution. The risk assessment of the soil pollution in the city of the scholars of our country started in the 1990s， which mainly relate to the risks and evaluation of the city water， atmospheric environment risks， ecological risks and other areas of our country [9]. Since the new century， the researches on the risk assessment of the soil pollution in the cities of our country have entered a period of the rapid development. Zheng Yuanming et al [12] taking Beijing city as an example， carried out large-scale researches on the regional soil environment quality assessment and the pollution risks of the provincial units in China. Zhao Qinna et al [13] taking the development of a regional land as an example， using the aromatic health risk assessment model， conducted the evaluation of the health risks brought by the soil polycyclic （PAHs） pollution to the future crowds. Through the studies of the ecological effects and its ecological risks of the soil pollution in the cities， we can provide the theoretical basis and the research methods for the study of the city ecological risk assessment， which has the scientific significance and the social significance.

The city green space is a very important part of the city ecosystem， and is an important symbol of the city modernization and the civilization degree [14]. In the recent years， the city green area has been increasing in China. By the end of 2011， in China， the city green area has reached 2.243 million hectares [15]. The rapid growth of the city green areas in the cities beautifies the city environment， and provides more recreational places for the people， and at the same time， in the curing process， because using a large number of the chemical fertilizers and pesticides， the problem of the pesticide surface source pollution to the water and soil and atmosphere become prominent [16]. The heavy metal pollution of the soil is one of the important characteristics of the soil of the city green space [17， 18]. How to deal with some problems in the process of the application of the green space system in the city is the most important project worth our careful exploration and solution in the large-scale city ecological construction [19]. With the heavy metals and the typical pesticides as the representatives， this paper reports the progress of the researches on the combined pollution of the heavy metals and the typical pesticides， and carries on the further evaluation and prediction of the ecological risks of the compound pollution of the heavy metals and the typical pesticides， and finally prospects the scientific problems that this field needs the further researches， in order to promote the research progresses of the complex pollution of the soil of the city in China.

2. Researches on the heavy metal and pesticide pollution of the soil of the city

The main sources of the heavy metal pollution in the soil of the cities are the pollution source of the industrial production， the pollution source of the transportation， the pollution source of the daily life， the city cast off and the input of the parent materials of the original soil. Through the results of the investigations and researches of the distribution of the heavy metals in many domestic and foreign cities， we can find that the distribution of the heavy metals in urban soils showed a certain pattern， which is manifested in that on the both sides of the traffic roads， in the downtown areas and squares where the human activities are intensive， in the old industrial areas and the old residential districts， the heavy metal pollution is serious [20-22]. Lin et al [23] found in the results of the researches that the soil Cd， Cr， Cu， Ni and Zn and the landscape indexes of the urbanization and industrialization showed a significant correlation. Chen Tongbin et al [24] found in the results of the investigations of heavy metal contents in the soils of 30 parks in Beijing city that the Cu and Pb contents were higher in the soils of the parks in the city center and the parks of a few hundred years of history. Therefore， the phenomenon of the heavy metal pollution in the soil of the city is very common.

So far， the studies of eco-environmental effects of pesticides on the city green fields are mostly focused on the environmental risks of the groundwater. Starret et al [25] found in the studies of the herbicide 2，4-D that the irrigation frequency of the green lawn has the important influence on the downward migration of the herbicide， and the irrigation frequency often easily leads to the downward leaching of the 2，4-D. Hixson et al [26] showed in the studies of the herbicide Simajine of the green lawn that in the green， ecological environment， Simajine will lead to a very small leaching ratio due to the strong biodegradation and the close combination of the soil organic matters. The herbicide siduron of the green lawn is the best herbicide after sowing before germination in the cold season， which is widely used in the northern cities， and the preparation is 50% of the wettable powder， without the adverse effects on the germination of the seeds， and can effectively control the green bristle grass， Digitaria Ischaemum， Brome and barnyard grass. The early results of the researchers showed that the leaching of the sojourn is small， and will be accumulated more on the surface of the soil， and the degradation rate is medium， and has the inhibitory effect on the growth of some soil bacteria， such as Azotobacter sip and Chlorella vulgaris [27]. And there are fewer reports on the toxic effects of other ecological effects and the environmental behaviors of the siduron on the soil animals， and the effects of the soil properties on the adsorption and the degradation.

3. Studies on the combined ecological effects of the combined pollution of the soil of the city

The soil is the source and the sink of all the environmental pollutants， and the combined pollution system where a variety of the pollutants exist at the same time. Therefore， the researches on the combined ecological toxic effects of the compound pollution of the soil have been paid much attention to. However， so far， most of the researches are limited to the laboratory simulation， so the contacts between the research results obtained and the actual field situations are controversial. The past ecological risk assessment of the soil pollution， mainly adopted the point-to-point evaluation method of the single pollution source and the single risk receptor. However， the soil of the city system， as a complex ecosystem， has the multi-risk sources and the multi-risk receptors in the contaminated ecological risk assessment， so it is necessary to adopt the integrated overall risk evaluation ideas and the system of methods [28-30].

At present， there are more researches on the joint toxicity effects of the combined pollution of the quantitative assessment of the application model， and the two kinds of the evaluation models that are widely applied are respectively the concentration addition model （CA） and the independent action （IA）. In addition， there are the Toxic Unit （TU） and the Toxicity Index of additive joint action of toxicants in a mixture. The databases of these models are the experimental results of the indoor microcosm toxic effects， and the applicability of these models in the field sites is very difficult to verify. On the other hand， using the soil of the compound contaminated fields， which are the researches on the joint toxicity effects of the compound pollution based on the matrix rather than the chemical substances， also received much attention [31-33]. In their studies， the researched soil is the contaminated soil of the heavy metals that the distance from the point sources is different， and the concentration of the heavy metals in the soil shows the gradient variation， and characterizes the ecological independent effect of the composite pollution through the parameters of the ecological functions of soil microorganism. Among them， Teng Yin et al [32] through the statistical analysis carried out the relative quantitative assessment of the combined ecological effects of the four kinds of the compound pollution of the heavy metals in the actual polluted soil on the soil enzyme kinetics. Because the above researches take the soil of the contaminated fields with heavy metal concentration as the research objects， in the samples obtained， the gradient changes of the heavy metal concentration are more obvious， and therefore， in the absence of the laboratory simulation， we can also get better quantitative results. However， for the soil of the city whose the concentration of the pollutants is relatively low and the scope of the changes is relatively small， there is greater difficulty in the direct application of the field contaminated soil to carry on the assessment of the combined ecological effects. In addition， the ecological effects of the actual contaminated soils in the fields will hide the generation mechanism of the ecological effects of the pollutants because of many of the interference factors， and in the experimental simulation experiments， because of the control conditions， it is often easy for the researches on the generation mechanism of the composite effects [11]. Therefore， we also need to take the results of the laboratory researches as the guidance， make contact the laboratory results and the field survey results through some mathematical methods， and finally reach the evaluation of the combined ecological effects of the compound pollution of the field soil.

The combined ecological effects of the compound pollution of the soil are not only performed in the joint toxic effects on the organisms， but also are reflected the interaction between the ecological processes of the pollutants， especially the effects of the heavy metals on the degradation of the organic pollutants in the soil. Huang Xin and Lu Yitong [34] found through the results of the researches that the existence of the heavy metal Cd of high concentrations can reduce the degradation of the butachlor， while Wang Jinhua [35] found through the results of the researches that the Cd of low concentration could induce the degrading enzyme activity of the degrading bacteria REI of the butachlor， and thus enhance the degradation ability. This may be because the heavy metals and the combined pollution of the herbicides affect the structures and functions of the microbial communities of the soil [36， 37].

4. The overall evaluation methods of the ecological risks of the soil pollution in the cities

The end of the comprehensive evaluation has become a research hotspot of the ecological risk assessment of the soil pollution. In the traditional ecological risk assessment of the contaminated soils， we usually should use the species sensitivity distributions （SSDs） to determine the safety levels of the single， and the compound pollutants in the soil environment. The data base of the SSDs method is the eco-toxicological data of a single species that the multiple microcosm tests show. However， alone SSDs is not enough to evaluate the ecological risks of the soil ecosystem. Solomon and Takacs [38] proposed that we should apply some research results of the functions of the sensitive ecosystem of the ecosystem of different scales， such as from the laboratory microcosm scale to the field scale. Then， Jensen and Mesman [39] proposed the method of "three in one" based on the three aspects of the chemical parameters， the toxicological parameters and the ecological parameters. Over the same period， Li Zhibo et al [40] adopted the two types of the toxicity parameters of the nature of organisms and the ecological processes on the ecological risk assessment of the pollution of the heavy metals of the soil near a smelting plant in Fuyang of Zhejiang， and used the probabilistic method to characterize the risks. At the same time， we should consider the variety of the two aspects of the biological sensitivity and the concentration of the pollutants， and conducted the overall evaluation of the ecological risk of the contamination. In 2010， the Holland Institute of Standards published the three-in-one guidelines for the assessment methods of the soil risks. Any special assessment endpoints are not covered in this guideline， and what replaced them is a series of the endpoints of the overall assessment. Recently， Faber and Wesem [29] proposed that we can apply the concept of the service functions of the ecosystem in the ecological risk assessment of the soil. In this method， the choice of the appraisal endpoints is based on the specific ecological service function that the structures and processes of a soil ecological system can provide.

Corresponding to the overall evaluation endpoints， Dagnina et al [30] put forward the "three in one" data of the comprehensive environmental science， using the "weight of evidence" method for the ecological risk assessment， that is to say， based on the soil chemical parameters including the concentration of the pollutants， and the eco-toxicological parameters for the individual biology level. Compare the corresponding reference values according to each parameter， and obtain the corresponding risk index， namely， the chemical risk index （ChemRI）， the eco-toxicological risk index （EtoxRI） and the ecological risk index （EcoRI）， and finally win the overall risk index according to their weights. For the choice of some reference values， Xia Jiaqi and Luo Yongming [41] proposed that we can apply the soil environmental background value， the second-level standard value of the soil environmental quality and the critical value of the soil pollution as the evaluation index in the evaluation of the soil pollution of the areas and sites. In conclusion， the assessment of the ecological risk of the soil pollution so far has developed from the single evaluation method system for the whole evaluation system.

5. Research prospect

With the rapid development of the urbanization， the environmental problems of the cities are getting more and more attention， and the researches on the combined pollution of the soil of the city is in deepening and perfection， but there are still many problems unsolved， especially the researches on the combined pollution of the heavy metals and the pesticides in the organic and the inorganic compound pollution in the cities， which is fundamental basis for the formulation of the pollution prevention and control measures in view of the present situation of the environmental pollution. 1） Deepening the studies of the mechanism of the effects of the environment of the compound pollution. Further reveal the toxicity methods and the mechanisms of compound pollution， including the exploration and mining of some functional genes at the molecular level. In the field of the researches on the compound pollution， we should strengthen the studies of the toxic mechanism of the compound pollution of the heavy metals and the typical pesticides on the soil microbial and animals and plants at the molecular level， and make clear the dose and effect and mechanism of the compound pollution. 2） Focus on the assessment and prediction of the ecological risks of the combined pollution. The composite pollution of the heavy metals and pesticides in the soil of the city exists widely， and the overall assessment of the soil ecological risks is the development trend of the researches on the ecological risk assessment. At present we should focus on the assessment and forecast of the ecological risks of the combined pollution of the heavy metals and the typical pesticide in the soil of the city. 3） Construct the relationship between the research results of the combined ecological effects of the compound pollution of the simulated soil of the laboratory and the real soil in the fields. This is not only the key of the researches on the ecological risk assessment of the soil pollution， but also is the key to reveal the generation mechanism of the ecological risks of the actual contaminated soil in the fields.

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