Soil contamination in China:studies on the status, priorities, policies, management and risk assessment

As China is trying to balance economic development, environmental safety and human health, the Government has released strategic plans and legislation for soil contamination management. Aspects of the quality of China’s soils and management of soil contamination in China are addressed in this thesis. Soil environmental quality standards and science-based risk assessment of contaminants in soils are evaluated. China and the UK use different risk-based approaches to derive soil screening or guideline values (SSVs; SGVs) for contaminants. The approaches are compared and values derived 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. The difference in toxicity assessment and risk characterization for 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. In the future, China can use the UK method to strengthen its toxicity assessment and risk characterization for carcinogenic substances. Data was made available for this thesis from an extensive field and analytical campaign of human exposure to heavy metals in China. This was used to calculate the relative contributions of exposure to As, Cd, Cr, Hg and Pb from environmental media (air, water, soils) via the inhalation, drinking and the diet for different regions of China. Dietary exposure dominated, contributing ~90-97% of the total exposure for these elements. Exposure differences were observed with gender, age and region. This survey information can be used to derive exposures from soil-borne sources. Soil organic matter (SOM) and pH are critical soil properties strongly linked to carbon storage, nutrient cycling and crop productivity, but there is a lack of information on changes in these soil properties over time for China. This study used data from Chinese soil surveys to examine changes in soil pH and SOM across different land uses (dry farmland, paddy fields, grassland, woodland, unused land), with surface soil (0-20 cm) collected in the periods 1985-90 (Survey 1; 890 samples) and 2006-10 (Survey 2; 5005 samples) from two contrasting areas. In the southern part of China, the mean pH of paddy soils fell over the two decades between surveys - from pH 5.81 to 5.19 (p<0.001), while dry farmlands in the northern sampling area fell slightly (from pH 8.15 to 7.82; p<0.001). The mean SOM content of dry farmland soil rose in both areas and the mean SOM of paddy fields in the southern area also rose (all p<0.001). Woodland soil pH in the south increased from 4.71 to 5.29 (p<0.001) but no significant difference was measured in the woodlands of the northern area, although the trend increased. The SOM content of woodland top soils rose in the northern (p=0.003) and southern (p