CHEN Shi-bao, WANG Meng, Ll Shan-shan, ZHAO Zhong-qiu, E Wen-di

1 Key Laboratory of Plant Nutrition and Fertilizer, Ministry of Agriculture/Institute of Agricultural Resources and Regional Planning,Chinese Academy of Agricultural Sciences, Beijing 100081, P.R.China

2 School of Land Science and Technology, China University of Geosciences, Beijing 100083, P.R.China

3 National Agricultural Technology Extension and Service Center, Ministry of Agriculture, Beijing 100125, P.R.China

1. lntroduction

Soil depletion and degradation have been increasingly recognized as important environmental issues in China(Wei and Yang 2010). Heavy metal (HM) pollution in soils has become serious with rapid industrialization and urbanization over the last two decades (Tenget al.2014;Liet al.2015). The Ministry of Land Resources (MLR 2007) in China reported that more than 10% of cultivated land was heavily polluted by HMs from various emission sources, such as oil extraction and refining, mining and metallic smelting, waste rock stockpiles, industrial wastes or sewage irrigation (Liet al. 2015). Similarly, Songet al.(2013) collected data from 138 regions based on published documents and reviewed soil HM pollution of cultivated land in China and found that the probability of HM pollution in soils was about 16.67%, implying that 1/6 of cultivated land in China may suffer from HM pollution. HM-polluted soils occupy a comparatively large area of China, and the risks they created for humans, animals, plants, and groundwaters have become increasingly serious. Therefore, it is important to develop criteria and standards that can be used to assess the degree of risk from contaminated soils or to establish guidelines for their remediation.

Soil Environmental Quality Standards (SEQSs) are an important instrument for implementing soil protection policies. They can be applied as a decision-support tool in risk assessment of polluted soils and their impacts on human health, water resources, and other environmental areas (Atanassov 2008; ME 2011; Jarva 2016). In 1995,China enacted SEQSs (GB 15618-1995) specifying the maximum allowable concentrations of ten pollutants(eight heavy metals) based on protection targets and soil properties, as well as the corresponding monitoring methods. This standard has been considered as the most important legal basis and criteria for soil quality protection and pollution prevention in China since it has been issued(Tenget al.2014). However, with rapid industrialization and urbanization, Chinese SEQSs (GB 15618-1995) are outdated: they do not reflect the serious level of soil pollution and serve little useful purpose in assessing the performance of contemporary soil restoration processes. In recent years,review and research articles have provided assessment of Chinese SEQSs (1995) (Heet al.2004; Zhou and Qin 2005;Zhanget al.2014). Such studies help to further raise public awareness of soil HM contamination and to facilitate policies for revision of SEQSs. However, in China, there remains a lack of systematic development and scientific discussion about SEQSs, meaning that rules, regulations, guidelines,and implementation protocols are often fragmented and chaotic.

This paper provides an overview of China’s national SEQSs of HMs and local specific regulations or guidelines,and also covers the development of scientific and legislative frameworks employed in China to assess HM pollution.First, the progress of international SEQSs for HMs is reviewed. Then the scientific and legislative development of SEQSs for HMs in China over time is characterized. We follow this with an examination of the existing SEQSs to demonstrate potential regulatory deficiencies. The analysis in this paper indicates that a closer relationship between legislative policies and scientific information is needed for efficient control of the risks and hazards of HMs in soil.

2. lnternational SEQSs of HMs

It is commonly acknowledged that reliable information on geochemical background concentrations, specifically SEQSs from different countries is essential in soil contamination or remediation studies (EA 2004; Carlon 2007). SEQSs are thus necessary for implementing policies related to soil protection. SEQSs can be applied as a decision-support tool in risk assessment of polluted soils and their impact on human health, water resources and other environmental aspects (Atanassov 2008; ME 2011).SEQSs have been broadly adopted in many countries to regulate the management of contaminated land, and they usually come in the form of concentration thresholds (mg kg–1soil dry weight) of contaminants in soils, above which certain actions are recommended or enforced.

(1) Generally, the legislative framework of each country considers the following questions when their SEQSs or guidelines are developed (Provoostet al.2006):

(2) When SEQSs are exceeded, what actions are taken(further investigation or remediation)?

(3) What is a generic step-wise approach for developing SEQSs?

(4) Do SEQSs differ based on the type of land use?

(5) Which receptors are considered: human health, the ecosystem (ecology), groundwater and/or surface water?

2.1. Types of SEQSs

There are different names for SEQSs around the world. In Denmark, China, and Sweden they are called “soil quality criteria/standards” (Carlon 2007; Zhouet al.2007); in the USA they are “soil screening levels” (US EPA 1997); in the Netherlands they are “target values” (ME 2011); in the UK they are “soil guideline values” (EA 2009); in Australia they are “investigation levels” (NEPC 2011); and in Germany they are “trigger values/levels” (Carlon 2007). Though different names are used among countries, the countries have similar meanings for various risk levels and corresponding applications, which are demonstrated and displayed in Fig. 1.

2.2. Derivation of SEQSs

The derivation of SEQSs differs among countries. Generally,ecotoxicological and geochemical methods are widely adopted (Wuet al.1991; Zhou 1996; Zhou and Zhu 1997).For example, certain methods based on the species sensitivity distribution (SSD) or the assessment factor (AF)have been applied in derivation of quality criteria since the 1980s (US EPA 1985), and are becoming useful tools to deal with toxicological indexes in derivation (DEPA 1997; Zhouet al.2017). The Netherlands is a leading country on SEQS development. In the Netherlands, soil protection values are derived by a threshold of protecting 95% of theoretical species in the ecosystem to undertake risk assessment at an ecosystem level; specifically, its ecotoxicological criteria for data-rich trace elements aim at protecting 50% of the species population and enzymatic processes (EA 2004).The protection level in Canada (CCME 1999) is 75% and the guiding principle of the Canadian Council of Ministers of the Environment (CCME) is to aim for a level of ecological protection that ensures that remediated land has the potential to support most activities likely to be associated with its land use. The Netherlands and Canada use a specific statistical technique if there are sufficient data, like species sensitivity distribution for trace elements, and apply an assessment factor for trace elements with limited data.Sweden and Norway use the assessment factor approach(Provoostet al.2006). The United States Environmental Protection Agency (US EPA) uses ecological soil screening levels (Eco-SSLs), which are derived from the geometric mean of selected chronic toxicity data, and aim to protect ecological receptors that commonly come into contact with soil or ingest biota that live in or on soil. Although in this process, all toxicity data about different receptors can be applied, different countries might have their own selection criteria like pH range, soil types and etc.

2.3. Application scopes of SEQSs

Land use (agricultural, residential, recreational or industrial use) is primarily considered by many countries when legislating their SEQSs. For example, in The Netherlands,the site-specific risk assessment is determined based on the land use, although one generic SEQS is available (Swartjes 1999). Similarly, the US EPA uses a pragmatic policy for risk assessment where all lands and the organisms in or above them are protected to the same level (US EPA 2004). In Denmark, the compliance requirement with the consideration of soil depth is integrated along with land use (Pagh 1996).Canadian SEQSs are also set by differentiating its land use and protection levels (CCME 1999). Due to the bad impact of industrial activities on environment, most countries have to take into account of industrial land use when setting up their SEQSs, but with some exceptions. In Austria, the ecological risks from industrial land are combined into those from groundwater (NEPC 2011). In addition to industrial use, agricultural use is often seriously considered due to its close relation with human health. Agricultural SEQSs are usually derived on the basis of different assumptions and modellings instead of those applied for industrial and residential contaminated sites. It also should be noted that all countries apply SEQSs for the protection of human health,while some countries - like the Netherlands, Sweden,Norway and Canada - include protection of the ecosystem(complementary to the human health based standards)(CCME 1996; SEPA 1996; NPCA 1999). Alternatively, some countries or regions emphasize soil properties, soil types,extractants, etc., instead of land use in their SEQSs, such as in China where various pHs and soil grades are considered;trigger values in Germany account for extractants, soil types,and soil depth; limit values in Slovakia and the maximum admissible and precautionary values in Czech Republic consider extractants and soil types (Váchaet al.2016).

Fig. 1 Derivation procedure of screening values based on various risk levels and their associated applications (revised from Carlon 2007).

2.4. Comparison of SEQSs

Table 1 provides an overview of SEQSs for HMs mainly in agricultural or residential land from various countries. These threshold values are often considered as the criteria to determine if a deep investigation or remediation is needed.It can be seen from Table 1 that Norway and Sweden set the lowest SEQS values among others, which might be explained by the fact that the exposure routes of not only soil but also groundwater, consumption of fish and crustaceans or other selected ecotoxicological endpoints are included(NPCA 1999). The SEQSs among the selected countries show differences due to political, regulatory, and sociocultural factors. Historical (incidental) reasons seem to also have played an important role. Conversely the highest SEQSs are found from US EPA, followed by Switzerland and Belgium. It is because in these countries, only less exposure routes are considered when setting up their SEQSs (SAEfl1998; US EPA 2004). Another reason for high SEQS values in Belgium is the exclusion of ecotoxicological criteria(Provoostet al. 2006). In addition, differences in selected environment risk assessment models, the ecotoxicological criteria, selected human toxicological and parameter values,are reasons for substantial variations in the SEQSs for HMs(Provoostet al.2006). Nevertheless, SEQSs could never be uniform between countries, because country-specific elements (geographical and ethnological) and political decisions must be included.

Table 1 Comparison of soil environmental quality standards for heavy metals of different countries

3. Development of Chinese soil quality standards for HMs in science and management policies

In Chinese SEQSs (GB15618-1995), soils can be classified into three grades (A, B and C). Grade A soils are those in natural conservation areas, tea gardens or soils in which the observed concentration of metals is closer to the reference background or baseline values. The soils can be considered as multifunctional, without suspicion of risks for public health or the environment.

Grade B soils consist of farmland, vegetable, tea, fruit,and grazing lands. Here the soils are considered to be contaminated, but the level of pollution does not create immediate risks to humans, environment or particular land use. The concentration of pollutants in this grade can be regarded as the upper acceptable limit.

Grade C soils are suitable for forestry or areas with greater absorption capacity. Concentration of HMs in these soils are the maximum allowable concentrations with the consideration of protection targets and soil properties.Exceeding these values implies a danger, and it has to be avoided through remedial action (intervention) or changes of actual land use.

Accordingly, grades A, B and C are defined as pristine,moderately enriched, and extremely impacted soils,respectively. This standard has become the most important legal basis and criteria for soil quality protection and pollution prevention in China since it has been issued. In 2006 and 2007, the State Environmental Protection Administration of China issued Environmental Quality Evaluation Standards for Farmland of Edible Agricultural Products (HJ/T 332-2006), Environmental Quality Evaluation Standards for Farmland of Greenhouse Vegetable Production (HJ 333-2006). Comparison of these SEQSs (GB15618-1995) is also displayed in Table 2. The values from HJ/T 332-2006 or HJ 333-2006 are much lower than the Grade B of SEQSs especially for Pb, which might be explained by their different applied scopes, as HJ/T 332-2006 and HJ 333-2006 are evidently suited for the soils on which edible agricultural products grow.

The development and management policies of Chinese SEQSs for HMs have temporally evolved in four stages,based on the number of research articles through searching the key words of “soil” and “heavy metal” and“standard” contained as the main topic in Chinese CNKI and ScienceDirect databases (Fig. 2). For comparison,almost five times more published articles were found in CNKI than that in SCI year by year, which might reflect the serious issues of our current SEQSs to some extent. The four stages are founded stage, acceptable stage, disputed stage, and revised stage as explained below:

?

3.1. Founded stage (1980–1994)

Considering the relatively low levels of pollution together with weak environmental management in the past, the awareness of SEQSs is late to emerge in China, but fundamental work was conducted relatively early on (Tenget al.2014). For example, between 1980 and 1990, the National Science and Technology Research Project on Soil Environmental Capacity in China was established (CGSC 1986). The influencing factors, mathematical models, and determination methods of soil-environmental capacity were also established(Xia 1994). Some books about soil-environmental capacity such asResearch on Soil Environmental Capacity(Xia 1986),Soil Environmental Capacity and Its Application(Xia 1988), andChina Soil Environmental Capacity(Xia 1992)were published. These investigations are very important because they provide basic data for regulating Chinese SEQSs released in 1995.

3.2. Acceptable stage (1995–2005)

During this period, keen awareness was paid to the seriousness of harms from soil HMs on humans. More attempts have been made to prevent or eliminate soil HM pollution through enacting various policies (Liet al.2015). The Chinese government implemented a nationwide systematic program, the “Multi-purpose Regional Geochemical Survey”, which was composed of soil survey,ecological assessment, land evaluation, and monitoring each year since 1999 (Chenget al.2004, 2005; Yang Zet al.2004, 2005; Xi 2005). Researchers also developed more remediation techniques to decrease HM concentrations in soils lower than the required level in GB 15618-1995(Caoet al.2003; Yang Yet al.2004; Lin and Lin 2005).Therefore, in this decade, not many articles about SEQSs were published as shown in Fig. 2. There were, however,some debates about the initially proposed SEQSs.

3.3. Disputed stage (2006–2010)

New challenges continued to emerge following the acceptable stage. In this period, serious HM pollution in cropland was found in some regions of the country, severe food safety concern was aroused across the country.Unfortunately, more than 10-year application of soil quality standards seemed to be outdated, due to not effectively reflecting the real situation of HM pollution (Caiet al. 2006;Wang 2007). Therefore, more and more bottlenecks,deficiencies or gaps were raised.Overemphasized uniformity China is a large country with various regions that have different soil properties,land use types, complex climate types, and varied geological backgrounds. Because of this, the migration and transformation of HMs in different soils are highly unlikely to be identical, making it very difficult to use a standard threshold value of HMs for all areas. In other words, the revision of SEQSs is suggested to distinguish land use types, like residential, recreational, or industrial use, as well as their associated functions and quality requirements.It is also important to note that the accumulation ability of agricultural crops for HMs greatly differs. As can be seen in Table 3, the toxicity threshold of Cd in soil highly depends on the parameters of soil types, agricultural crop species,and soil pH. For example, the standard value of Cd as 0.3(pH＜7.5) or 0.6 (pH>7.5) in SEQSs seems to be over/under rated: Cd content at paddy rice grain would not exceed its maximum limit, until up to 2.8 mg kg–1of Cd concentration at meadow cinnamon soil from Beijing, China. Conversely,only 0.26 mg kg–1of Cd in paddy soil from Daye City, Hunan Province, China would induce Cd in rice grain to exceed the limit. Therefore, SEQSs for soil contamination assessment should be land use, land type, and plant growth specific.

Difficulty in application of Grade BIn GB 15618-1995,Grade B values were set up according to the least soil environmental threshold capacity from various types of soil across the country, in other words, values lower than these threshold values indicated soils were not polluted.Soil environmental capacity determined using grade B values would definitely be lower for most sites, and grade B thresholds also could not reflect self-cleaning capacity of soils. Sometimes the soils seem to be polluted by HMs because their concentrations are higher than the settings in Grade B, while crops growing on them might not exceed their corresponding food safety standards, andvice versa. To date, there are different statistical criteria to derive heavy metal reference values in soils. The broadly recommended approach is the European Commission Technical Guidance Document (ECTGD) methodology for deriving predicted no-effect concentrations (PNECs) for different soil compartments. PNECs are concentrations below which unacceptable effects on organisms would most likely not occur, while in the case of distributionbased extrapolations, the level of protection is at the 95th percentile (EA 2004). Comparison of Zn limit values from our current SEQSs with PNEC values derived from HC5(the fifth percentile level of this distribution, representing to protect 95% of ecologically representative species)is displayed in Fig. 3. There are clear inconsistencies between PNEC and SEQSs values, particularly for pH>7.5,in which SEQS values are much lower than PNECs,indicating that SEQSs are too strict, and resulting in misunderstanding that Zn contents in various soils have exceeded the standard values, and further unnecessary remediation action might be conducted.

High critical values of PbThe high values of Pb in SEQSs are briefly considered not to reflect the actual issues, due to the following possible reasons (Yuan and Wang 2000; Heet al.2004): 1) Research found that the toxicity pathway of soil Pb on children was through “soil-hand-mouth”, rather than “soil-food-human body”, and when the concentration of soil Pb was greater than 100 mg kg–1, blood Pb level could exceed the maximum permission concentration of 15 μg mL–1; and 2) soil pollution investigation suggested that Pb concentration in grain crops exceeded the standard values but the soils they grew on might not exceed Pb values of Grade B in SEQSs.

Fig. 2 Number of articles about Soil Environmental Quality Standards of heavy metals published in Chinese journals from CNKI database and SCI journals collected in ScienceDirect from 1996–2016.

lmpracticability in employing total amount of HMsCurrent SEQSs adopt a total amount to restrict HM contents in soils.However, due to the fact that the main source of soil HMs is parent material, and the available HM contents to generate phytotoxicity might be lower, the soil HM contents have already exceeded SEQSs, but the associated human health risk or ground water pollution might not be induced (Caiet al.2006; Zhanget al.2014). Therefore, the establishment of reference values/available contents for HMs is necessary for soil quality management. For example, Cr(VI) from mining could pollute soils and underground water based on identification standards for hazardous waste-identification for extraction toxicity (GB5085.3-2007), and the standards require the maximum leaching value of Cr(VI) to be less than 5 mg L–1, and less than 15 mg L–1for total Cr. Unfortunately,there are no corresponding requirements/limitations of Cr(VI) in current SEQSs. In addition to Cr(VI), methyl mercury (MeHg) produces severe pollution on mining or surrounding areas, and up to 96.4% of MeHg in total Hg was found to accumulate in rice grains when grown in Hgcontaminated regions (Qiuet al.2006).

Admittedly, more and more questions about SEQSs have been raised by scientific research and political strategy,that is why the number of publications concerning SEQSs increased steadily year by year in this period (Fig. 2). In 2008, the Ministry of Environmental Protection, China,tried to update Chinese SEQSs by increasing the land use categories and setting three-levels of threshold values.Unfortunately, it has not yet been released due to some reasons.

Table 3 Threshold values of soil Cd toxicity to agricultural products1)

3.4. Revised stage (2011–now)

Fig. 3 Comparison of Zn values from Chinese Soil Environmental Quality Standards (GB15618-1995) with predicted no-effect concentrations (PNEC) values derived from the fifth percentile level of this distribution (the data come from the supported project). Cb, background concentration of Zn.

In 2012, the Ministry of Agriculture began to investigate baseline heavy metal pollution in agricultural areas. The objective of this investigation was to determine the historical and current pollution status and to ascertain pollution characteristics and distribution of As, Cd, Cr, Hg, and Pb in agricultural areas, and also to provide background information for revision of SEQSs (BSEMA 2012). At present, this investigation is still being carried out at the national scale, but a preliminary finding is that the probability of heavy metal pollution in soils cultivated in China is about 16.67%, implying that 1/6 of cultivated land in China may suffer from heavy metal pollution. Regional research and investigations about HM pollution status are still ongoing, (Lianget al.2017; Luet al.2018). The 12th Five-Year Plan for science and technology and for environmental protection pointed out that efforts should be made to focus on major industries and critical regions to more effectively prevent soil pollution. Subsequently,technical standards, such as Technical Policy for Prevention and Control of Arsenic Pollution and Technical Policy for Prevention and Control of Mercury Pollution were issued in 2013, requiring life-cycle tracking and control of heavymetal emissions, especially in mineral extractions, metal smelting, and waste disposal activities. Meanwhile, the Council General Office of the Central People’s Government issued “The notice of soil environmental protection and comprehensive management work arrangement in the near future” in January of 2013. The objective of this notice was to: determine the soil quality status of soils across China; establish a soil environmental quality monitoring network; and gradually set up soil environmental protection policies, regulations, and standard systems. Several guidelines have already been issued including Technical Guidelines for Site Risk Assessment (HJ 25.3-2014),Technical Guidelines for Environmental Site Monitoring(HJ 25.2-2014), Technical Guidelines for Risk Assessment of Contaminated Sites (HJ 25.3-2014), and Technical Guidelines for Site Soil Remediation (HJ 25.4-2014), in which the reference values of HMs were provided. In June 2014, the Ministry of Environmental Protection held a symposium to revise SEQSs and suggested that environmental quality of agricultural land be the main determinant for guidelines; that the land use categories needed to be expanded by including construction sites; and unique background values across the country should not be used to create guidelines. In addition, the quantity of publications on SEQSs has increased steadily during this period, and the peak stage occurred around 2014–2016 with a steadily increasing trend. In general, indicating the deficiencies or gaps of SEQSs has aroused public awareness, and revision of SEQSs might be enforced in the near future.

4. Conclusion

SEQSs in China (GB 15618-1995) could be an important tool for more realistic assessment of the degree of contamination of soils polluted by HMs since they were issued. However,with current rapid industrial development and urbanization,soil quality in China is subject to new problems. It is urgent to carry out studies to promote development or revision of current SEQSs, and to efficiently control the risks and hazards of soil HMs. This paper discusses international SEQSs of HMs as well their development in China over time,then examines current Chinese SEQSs to demonstrate their potential regulatory deficiencies by referring to international SEQSs.

Herein, the main recommendations in science and policy with regard to SEQSs revision are summarized as follows:

1) The toxicological databases for HMs are still insufficient.Further research must focus on long-term transformation or migration of HMs in soils, and their potential risks on ecosystem and human health. In particular, the risks of HMs on mortality, reproduction, and growth in microbes,terrestrial plants, and invertebrates should also be specified.

2) A sharp focus is needed in prioritising research and data collection efforts for identifying HM contents present at natural background levels, and characterizing their species,mobility, and availability in soils. In addition, the types,physical, or chemical characteristics of soils, dry or paddy lands need to be characterized.

3) Currently available statistical criteria need to be used to derive HM standard values in soil such as the ecotoxicological method and geochemical method or species sensitivity distribution and assessment factor methods. The use of multiple methods is suggested depending on the pedological characteristics of a given geographical area and the data type of distribution of HM concentrations.

4) There is a need to develop studies in China to establish specific reference values of HMs and to use them as prevention, protection, and recuperation tools for soils,especially in agricultural soils.

5) Establishing SEQSs according to land use including agricultural, commercial, industrial urban park, and residential use is of great practical significance.

6) Suggestions are put forth that forms of highly toxic HMs like Cr(VI) and MeHg should be included in SEQSs,and the employment of extractable speciation instead of total amount of HMs should be applied as an evaluation index,when SEQSs are revised.

China’s experience has shown that policy and science should be linked to work in tandem to manage soil quality issues and develop SEQSs. Scientists need to focus on the applicability as well as the robustness and strictness of their science, and policy analysts should strongly consider scientific results and expertise.

Acknowledgements

This research was financially supported by the National Key Research and Development Program of China(2016YFD0800707), the Key Technologies R&D Program of China during the 12th Five-Year Plan period (2015BAD05B03)and the National Natural Science Foundation of China(41271490).

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