Geostatistical mapping and quantitative source apportionment of potentially toxic elements in top- and sub-soils: A case of suburban area in Beijing, China

Highlights

• Most V, Cr, Ni and As were endogenous, while Cd, Zn, Pb and Hg were anthropogenic and exogenous.

• Urban emission was identified as a major source for 8 PTEs in suburban soils.

• Relative contributions of three sources to 8 PTEs were estimated by PMF.

Abstract

The risk assessment and source identification for potentially toxic elements (PTEs) in soils, particularly agricultural soils from megacities, are significant for environmental protection and pollution control. In this study, an intensive sampling (4127 topsoil samples and 994 subsoil samples) was conducted in the Shunyi District, Beijing, which is a suburban area with extensive cropland cover and has been impacted by the megacity over several decades. Concentrations and distributions of 8 PTEs, including V, Cr, Ni, As, Cd, Zn, Pb and Hg, were determined, and their possible sources were quantitatively assessed by principal component analysis (PCA), redundancy analysis (RDA), positive matrix factorization (PMF) analysis, and anthropogenic contribution ratio method. Among 8 PTEs, Zn, V and Cr exhibited significantly high concentrations in soils, with means of 68.29, 68.19 and 52.13 mg/kg, respectively, followed by Pb (23.84 mg/kg), Ni (22.91 mg/kg), As (8.30 mg/kg), Cd (0.15 mg/kg) and Hg (0.05 mg/kg). RDA and PCA demonstrated that the rock weathering was a significant source of V, Cr, Ni and As, and the local emissions and atmospheric deposition respectively contributed most of Cd, Zn and Pb, and of Hg in soils. This source category was confirmed the spatial variations of anthropogenic contribution ratios to individual PTEs. PMF results showed that the local emissions contributed 96.3% of Cd, 44.4% of Zn and 32.0% of Pb in soils, and the atmospheric source carrying urban pollutants amounted to 78.7–80.2% of Hg. In this case, several effective analysis methods have been successfully applied to quantify the impact of a megacity to PTEs in suburban soils. These results improve understanding of the contamination status of PTEs in suburban soils from Beijing megacity, and provide basis for policymaker regarding environmental protection and pollution control.

Introduction

Soil, as an important component of terrestrial ecosystems, plays a crucial role in biochemical transformation, the cycling of elements, and the filtration of water (Luo et al., 2012). It also serves as a reservoir for potentially toxic elements (PTEs), which could be transported from atmosphere, biomass and hydrosphere (Lv et al., 2014). Because of the environmental persistence, high toxicity and great bioaccumulation, the presence of PTEs in soils may threaten ecological safety and endanger the health of organisms through food chain (Akinwunmi et al., 2017, Khan et al., 2008, Lin et al., 2017, Tóth et al., 2016). In addition, PTE exposure could directly reduce immune function through suppressing immune system, as well as through indirect effects to the microbiota, both of which would increase risks from potential diseases (Keesing et al., 2010). Therefore, global concern has been raised over PTE contamination in soils (Chen et al., 2016a, Cortada et al., 2018, Gbadamosi et al., 2018, Ravankhah et al., 2017). Studies focusing on the occurrence and pattern of PTEs in soils would improve understandings for their environmental loads and prediction of human exposure risks.

Both natural and anthropogenic sources could contribute PTEs to soil environment (Yuan et al., 2013). Natural source of PTEs is generally a result of parent rock weathering or pedogenesis (Lin et al., 2017, Wang et al., 2019a), and anthropogenic pollutants are directly emitted from human activities (Wang et al., 2019b, Xiao et al., 2019, Zhao et al., 2014). In developing countries, the rapid economic development sharply increased anthropogenic emissions of PTEs, which would be transported atmospherically and deposited into nearby soils (Jiang et al., 2017). This may result in soil contamination at regional and even global scales. In China, a series of reports found that soils were obviously contaminated by PTEs, which increased from 185 mg/kg in 1990 to 400 mg/kg in 2015 (Chen et al., 2015, Pan et al., 2018, Zhang et al., 2018b). Over the past decades, the fast urban expansion in China has dramatically increased the industrial and municipal wastewater discharges, from 17.6 to 32.1 billion tons and 35.1 to 65.9 billion tons, respectively (Oyang and Wang, 2000, Pan et al., 2018). These sources, together with the overwhelming traffic emissions, resulted in a substantial input of PTEs into urban soils, and therefore, much attention has been paid to the urban soil contamination in some Chinese megacities (Jin et al., 2019, Liu et al., 2019, Lv and Liu, 2019). Beijing, the capital of China with a population over 20 million, is experiencing high rate of urbanization and associated soil contamination in urban areas (Chen et al., 2010, Liu et al., 2016, Wang et al., 2012), in which the PTE concentrations were detected as high at ~300 mg/kg (Chen et al., 2015). A recent study in Beijing by Yu et al. (2019) noted that a number of PTEs, including Hg, Cd, Pb, As, Zn, Cr, Ni, Co, and V would be dispersed outside with soil particles, and even be transported from urban areas to remote areas along a certain pathway. This finding may indicate that the particle-borne PTEs would be deposited into suburban soils with various distances from cities. However, few attention has been paid to PTEs in suburban soils, which may serve as a sink area for the urban emissions (Lin et al., 2017). This presents a knowledge gap regarding the impact of urban emissions on PTEs in suburban soils.

Because the human activities in suburban areas would also release PTEs, the levels and patterns of PTEs in suburban soils may not correspond to the contribution from urban transport. In Beijing, the Shunyi District is a typical suburban area. Although soils from the Shunyi District were reportedly contaminated by Beijing urban emissions (Wang et al., 2018), applications of chemical fertilizer, sewage sludge, and livestock manure for agricultural purpose would increase the soil PTE levels in this area (Luo et al., 2009, Xu and Zhang, 2017, Yang et al., 2013). In addition to the anthropogenic activities, the soil parent materials would significantly affect PTE concentrations in suburban soils as well (Guagliardi et al., 2018). However, the relative importance of rock weathering or pedogenesis to PTEs was unclear or previously neglected (Lin et al., 2017, Wang et al., 2019a).

To evaluate the various contributions to PTEs in soils, the geostatistical mapping has been used to identify the spatial features and possible sources of PTEs in soils (Li et al., 2014, Li and Feng, 2012, Men et al., 2018, Xie et al., 2011). Recently, a newly-developed method combining geostatistical analyses, multivariate analyses, redundancy analysis and robust geostatistics, has been successfully applied in discriminating the soil PTEs inherited from rock weathering from those derived by anthropogenic sources (Lin et al., 2017, Wang et al., 2019a). This method may provide knowledge of source apportionment of PTEs in soils from the Shunyi District, and further qualify the relative contributions from different sources. It was also interesting to compare this method with a traditional source appointment method, such as positive matrix factorization (PMF) model (Lv, 2019, Lv and Liu, 2019, Zhang et al., 2018c). In this study, the goals are: (1) to investigate concentrations and spatial distributions of PTEs in topsoils and subsoils from the Shunyi district, Beijing; (2) to discriminate anthropogenic contributions from natural source of rock weathering; and (3) to apportion quantitatively the urban emission for PTEs in suburban soils.