A comprehensive comparison and analysis of soil screening values derived and used in China and the UK

Highlights

• China and UK approaches to derive soil screening values (SSVs) are compared.

• Results for carcinogens show the biggest difference in SSVs between countries.

• For non-carcinogenic substances the difference in SSVs is small.

• China can improve risk assessment and management of brownfield sites.

Abstract

China and the UK use different risk-based approaches to derive soil screening or guideline values (SSVs; SGVs) for contaminants. Here we compare the approaches and the derived values for 6 illustrative contaminants. China’s SSVs are derived using an approach developed in the US as follows: for carcinogens, acceptable level of risk (ACR) is set at 10⁻⁶ and the SSVs calculated as 10⁻⁶ divided by the soil exposure and toxicity data; for non-carcinogens, the hazard quotient is 1 and the SSV is calculated as 1 divided by the soil exposure and toxicity data. The UK’s SGVs are calculated by the CLEA model, for which the Average Daily Exposure (ADE) from soil sources by a specific exposure route equals the health criteria values (HCVs) for that route, whether for carcinogens or a non-carcinogens. The UK’s CLEA model is also used here to derive SSVs with Chinese input parameters. China’s SSVs, the UK’s SGVs and values for Chinese conditions derived using the UK approach were as follows (mg/kg): As, <1, 35, 20; Cd, 20, 18, 11; Cr (VI), <1, 14, 29; benzene, 1, 1, 2; toluene, 1200, 3005, 3800; ethyl-benzene, 7, 930, 1200. By comparing the differences in toxicity assessment and risk characterization, exposure assessment and parameter types in the methodologies to obtain SSVs in China and the UK, and by combining the CLEA model with Chinese parameterisation, these comparisons highlight that the difference in toxicity assessment and risk characterization methods of carcinogens results in the biggest difference in SSVs between the 2 countries. However, for non-carcinogenic substances, the difference of SSVs calculation method and SSVs is small. The difference in SSVs for carcinogenic substances is also related to the route of exposure. For volatile organic compounds, the presence of indoor respiratory exposure pathways greatly reduces the differences caused by toxicity assessment and risk characterization methods. For non-volatile substances such as heavy metals, the effects of toxicity assessment and risk characterization methods are significant. The SSV of As obtained by the CLEA model with Chinese parameters is closer to the background value of soil in China. In the management of non-volatile contaminated sites such as heavy metals in China, the CLEA model can be used for risk assessment and calculation of site specific SSVs. In the future, China can use the UK method to strengthen its toxicity assessment and risk characterization methods for carcinogenic substances, to reduce the uncertainty in the risk assessment of contaminated sites and improve the scientific management of contaminated sites.

Introduction

Soil pollution has become a widespread and serious problem in many regions of the world (Cachada et al., 2018; Chen et al., 2014; Barsova et al., 2019; Ramón and Lull, 2019; Kumar et al., 2018). In the past thirty years, environmental risk assessment has been widely adopted in many countries to manage soil pollution in contaminated land, and some countries (e.g. United States, United Kingdom, Netherlands, Canada and Australia) have developed risk based approaches to derive contaminant-specific values, to help with management of contaminant scenarios. These are designed to protect human health and manage soil pollution in accordance with national regulations. Soil screening values (SSVs) are derived by risk based approaches, and provide an important support tool for contaminated land management (United States Environmental Protection Agency, 1996; Swartjes et al., 2012; Environmental Agency, 2009a; CCME, 2006; National Environment Protection Council, 2011). SSVs are used to categorise the risk of soil contamination. For a specific land use, if the concentration of a given contaminant is less than the SSV, it is defined as being of no risk to human health. If it exceeds the SSV, this may trigger further surveys, risk assessments, potential changes of land use or remediation measures, depending on the national processes (United States Environmental Protection Agency, 1996; Environmental Agency, 2009a; CCME, 2006; National Environment Protection Council, 2011; Ministry of Environmental Protection of the People’s Republic of China, 2018a). However, SSVs in different countries are different in terms of definition, numerical values and inference methods. For example, Zhou et al. (2016) found standard values for arsenic varied by country, land use and definition (Zhou et al., 2016). Generally, standard values of industrial land are higher than those of commercial, residential and agricultural land. Standard values of some countries and regions place more emphasis on soil properties, soil types and extractants, not land types. Carlon et al. (Carlon, 2007) conducted a comprehensive analysis of SSVs in different European countries and found it was subject to geographical, biological, socio-cultural, regulatory, political and scientific factors. The value of SSVs in different countries in Europe are different in value and usage, and the influence of different factors is different. Many factors combine to result in differences, namely: i. the approaches used to derive the SSVs (e.g. hazard identification, toxicity assessment, exposure assessment and risk characterization); ii. the descriptors/parameters selected (e.g. the population/soil/site characteristics and building structure; the environmental conditions and parameters values); iii. The proposed land use or level of ‘acceptable risk’ (Claudio et al., 2007; Song et al., 2011; Wang and Lin, 2016; Xu et al., 2013). Due to these reasons, SSVs derived and used in different countries can be different, resulting in different management options being selected for the same soil concentration in different places. Thus, using scientific derivation methods, matching the parameter values of regional characteristics, and calculating the soil screening value to meet the risk level of policy requirements is the basis for scientific management of contaminated soils in a country and region, which is necessary to derive methods, parameter values, etc. In this paper, a comparative analysis of SSVs is carried out, to provide scientific reference for method selection and parameter determination. Our focus is China, as explained below.

The UK’s approach is one of the most established. Over the last 20 years, the Environment Agency (EA) has systematically released a series of regulations, standards and science reports to introduce how to deal with soil contamination in the UK (Environmental Agency, 2009a; Environmental Agency, 2009b; Environmental Agency, 2009c; Environmental Agency. Sci, 2009). Soil Guideline Values (SGVs) have been derived and widely applied to the investigation and management of contaminated land (Environmental Agency, 2009a). SGVs are defined as a starting point for evaluating long-term and on-site exposure risks to human health from chemicals in soil, below which the long-term human health risks are tolerable or minimal, above which further investigation should be undertaken. It uses the CLEA (Contaminated Land Exposure Assessment) model to derive SGVs.

In recent years, the Chinese central government and local government has started to pay attention to soil pollution by taking a series of actions (Hou and Li, 2017; Li et al., 2017). For example, in 2016, the 10-Chapter Soil Pollution Action Plan was issued. It’s purpose is to manage, control and prevent soil pollution and improve soil quality in China (People’s Daily, 2016). An early priority is to conduct relevant surveys of soil pollution, to define baselines of soil environmental quality (Council, 2016). In August 2018, the Chinese government released national standards for contaminants in agricultural soils and contaminated land, Soil Screening Values (SSVs) (Ministry of Environmental Protection of the People’s Republic of China, 2018a, 2018b). China’s SSVs are also derived using a risk-based approach. Fig. 1 shows the procedures used to derive SSVs in China and the UK.

The purpose and objectives of the study were therefore to:

• compare the derivation method of SSVs in China and the UK in terms of toxicity assessment, risk characterization and exposure assessment;

• identify the key differences in SSVs between China and UK and their main factors;

• provide some suggestions for the improvement of China’s national and local SSVs standard setting.

To achieve these goals, six chemicals were selected as examples, 3 inorganic and 3 organic, for which SSVs have been published. These are As, Cd, hexavalent Cr, benzene, toluene and ethyl-benzene.