Evaluation of potential ecological risk, possible sources and controlling factors of heavy metals in surface sediment of Caohai Wetland, China

Highlights

• Cd, Hg, Pb, Zn, Cr, and Ni contents exceed background values in birds habitat.

• The predominant heavy metal species (except Cd and Pb) were in the residual fraction.

• Cd and Hg pose a medium to high environmental risk, others are low or no risk.

• Anthropogenic loadings, soil pH and NOM are key factors in affecting metal contents.

Abstract

Caohai, a plateau wetland in Southwest China, is a national nature reserve providing protection for a variety of threatened and endangered species of migrant birds (e.g., the black-necked crane Grus nigricollis). It has been experiencing the increasing environmental problems with heavy metals due to anthropogenic activities. However, the contamination of heavy metals in different habitats is unclear. Surface sediment samples from these habitats were thus collected to analyze the distribution characteristics, potential risk and possible sources for heavy metals (Cd, Hg, Pb, Zn, Ni, Cr, Cu, Be, and V). The results showed that all of Cd, Hg, Pb, Zn, Cr, and Ni concentrations exceeded the background values, and these elements (except Cr and Ni) presented comparatively high levels in habitat adjacent to urban in comparison with the other habitats. Based on the regression analysis, we found that metals with higher EFs (Cd, Hg, Pb, and Zn) were mainly controlled by anthropogenic loadings, while metals with lower EFs (Cr, Ni, Be, and V) were mainly associated with sediment properties (pH or NOM). In addition, the results from geo-accumulation index, Hakanson potential ecological risk assessment, and risk assessment code showed that Cd and Hg posed a medium to high environmental risk to the ecosystem, and the other heavy metals posed no or low risk. Therefore, to protect this wetland ecosystem and to supply a well habitat for migratory birds, greater efforts aimed at reducing anthropogenic discharges and remediating sediment contaminated with heavy metals should be pursued.

Introduction

The aquatic environment in many developing countries is threatened owing to large uncontrolled pollutant inputs from anthropogenic sources as a result of rapid socio-economic development and urban expansion (Chen et al., 2016a; Siddiqui et al., 2019). In particular, the presence of heavy metals in wetland sediment has attracted worldwide attention because of the toxicity of these contaminants, their persistence in the environment, and bioaccumulation when they enter food chains (Peng et al., 2009). Heavy metals are deposited in sediment by adsorption, hydrolysis, and co-precipitation processes and can also be released from sediments back into the water when environmental conditions changed, posing a potential threat to aquatic biota and human health (Demirak et al., 2006). Recognizing contamination characteristics and the potential ecological risk of sediment metals, as well as identifying possible sources and controlling factors are of key importance for wetland pollution prevention and biodiversity conservation management (Chen et al., 2016b; Ke et al., 2017).

Caohai, a typical and complete plateau wetland located in Guizhou province, Southwest China, was designated as a national nature reserve in 1992. The main focus of the reserve is to provide habitats for a variety of threatened and endangered species of migrant birds (e.g., Grus nigricollis) and to protect the plateau wetland ecosystem (Xu et al., 2015). However, Caohai wetland has been experiencing increasing environmental problems, such as eutrophication and heavy metal pollution in recent decades (Hu et al., 2017). Previous studies showed that the sediment was contaminated with Cd, Cr, Cu, Pb, Zn, and Hg at different levels (Zhu et al., 2011; Zhang et al., 2013; Hu et al., 2017), and what's worse was that these metals were significantly accumulated in zoobenthos and aquatic plants in Caohai wetland. This demonstrated that heavy metal pollution in Caohai wetland might pose a potential threat to the higher trophic-level organisms (Vrhovnik et al., 2016; Liu et al., 2018). There were number of potential anthropogenic sources that could discharge massive heavy metals into the Caohai wetland, including historical zinc smelting, farming, the discharge of municipal sewage, and fossil fuel combustion (Bi et al., 2007; Hu et al., 2017). Owing to the ecological sensitivity of birds overwintering and breeding in the Caohai wetland, even a small increase in heavy metal concentrations could have a significant effect on the ecosystem. However, the possible sources and factors controlling the spatial distribution of heavy metals in the black-necked crane habitat and their potential risk of exposure to these contaminants are still lacking.

Assessment indexes, such as the geo-accumulation index (Igeo), sediment quality guidelines, potential ecological risk index (PER), and risk assessment code (RAC) can be used to quantify the metal pollution level and the potential ecological risks in sediments and soils based on the heavy metal concentration and their speciation (Zahra et al., 2014; Xu et al., 2016a; Chen et al., 2016a). However, previous research had shown that the results of different assessment method were not consistent sometimes (Xu et al., 2016a). Therefore, it is essential to assess sediment quality using multiple approaches to gain more comprehensive and accurate information.

Identifying the source of heavy metals is crucial but very difficult because differentiating metals originating from anthropogenic and natural sources can be a challenge. Furthermore, the quantitative measurement of metal loading fluxes is unfeasible for all sources of pollution (Hu and Cheng, 2013; Li et al., 2015). Natural sources include the weathering of rocks and forest fires. Major anthropogenic sources of metal pollution include mining and smelting, electronics, agriculture, sewage sludge, and fossil fuel combustion (Hu and Cheng, 2013; Chen et al., 2016a). Several receptor models have been proposed to identify major sources of heavy metals over the past decades, such as positive matrix factorization, principal component analysis, cluster analysis (CA), multiple line regressions and factor analysis (Loska and Wiechuła, 2003; Chen et al., 2016b). These methods classify heavy metals into different groups based on their spatial variations in concentration (Ma et al., 2016). Metals in the same group are considered to originate from similar sources (Tang et al., 2010; Chandrasekaran et al., 2015; Li et al., 2015). Although these methods have been widely applied in various different environments, including lakes, rivers, and soils, they did not take the impact of environmental factors into account (Li et al., 2015). Many researches showed that the distribution and morphological composition of heavy metals in sediments are related to the physical and chemical properties of sediments, such as pH and the organic matter and iron‑manganese oxide content (Walker et al., 2004; Wang et al., 2014; Li et al., 2015; He et al., 2017). These environmental factors may influence the distribution of heavy metals in sediments by affecting their mobility (Wang et al., 2014). Hence, source identification methods should both consider the source loading and the environmental factors (Li et al., 2015). However, to date, few investigations of heavy metal levels in sediments have quantified or estimated the contribution of anthropogenic loads and environmental factors (Li et al., 2015).

From the knowledge gap outlined above, thus, the aims of this study were to (1) determine the spatial distribution of heavy metals in the surface sediment of black-necked crane habitats in the Caohai wetland, (2) identify the major sources and factors controlling the distribution of these metals in the sediment, and (3) assess the level of ecological risk posed by each of the heavy metals in the sediment.