The occurrence of organochlorine pesticides (OCPs) in riverine sediments of hilly region of southern China: Implications for sources and transport processes
Introduction
Due to the rapid development of industry and agriculture, most regions in China have been confronted with serious pollution in recent decades. Pollutants are discharged into rivers directly or via its tributaries, most of which are drinking water resources (Rusina et al., 2019; He et al., 2020). Because most sources of freshwater are under immense stress from industrial wastewater, the extensive use of pesticides and other anthropogenic activities, >300 million people in China rely on hazardous drinking water sources (Zhang et al., 2010). Furthermore, rivers play a vital role in transporting contaminants undergoing water runoff and sediment discharge processes with their final deposition in sediments (Liu et al., 2019; Marchetti et al., 2019). Ma et al. (2015) reported that about 1.8 × 105 t nitrogen and 1.0 × 104 t phosphorus was running into Poyang Lake from its nearby rivers every year. Similar studies revealed that a dissolved inorganic nitrogen load of 1.3 × 106 t was delivered to an estuary of the Yangtze river in 2007 (Xu et al., 2013). Additionally, the annual flux of polycyclic aromatic hydrocarbons from the Pearl River Delta to the coastal ocean was 33.9 metric tons (Wang et al., 2007). Therefore, it is urgent to monitor and control river pollution for a sustainable development of the river basin.
Organochlorine pesticides (OCPs) are considered to be ubiquitous environmental pollutants worldwide due to their extensive use in past decades (Hu et al., 2009; Qu et al., 2018). Although OCP application in China was prohibited in the 1980s, their historical residues still persist at considerable levels in the environment (Tang et al., 2014; Kang et al., 2016). OCP residues enter into freshwater ecosystems through atmospheric bulk deposition, soil erosion, river input, and in other ways (Zhou et al., 2001). Due to the low water solubility and high n-octanol/water partition coefficient (Kow) values, these compounds in waters are adsorbed by suspended particles and then adhere tightly to the sediments. So, sediments can be regarded as effective sinks and a portion of the organic substances are temporarily not freely available (Yang et al., 2005; Hu et al., 2010). However, once they are disturbed by flood currents and wind-driven waves, sediment resuspension can lead to a new release of contaminants into the water (Wang et al., 2017; Ullah et al., 2020). Because OCPs and its catabolites are persistent in the environment and bioaccumulate readily, the potential for direct transfer of OCPs from the aquatic to terrestrial ecosystem exists given the OCP transfer in food webs (Kurek et al., 2019). Moreover, OCP residues in the environment pose adverse effects to organisms and humans, including carcinogenesis and immunodeficiency as well as endocrine and estrogenic disruption (Qu et al., 2019b; Ullah et al., 2020). Therefore, historically residues of OCPs in sediments cannot be neglected in environmental risk assessments.
The hilly region of southern China in four provinces encompassing Zhejiang, Fujian, Jiangxi and Hunan, are located north of the Nanling Mountains and south of the Yangtze River, which is such an important agricultural production base that pesticides has been widespread used to get rid of the insects (Gong et al., 2018; Wang et al., 2018). During the time period of 1951–1983, the total usage of DDT in this region was 47,500 metric tons, accounting for 17.6% of the total China usage (Li et al., 1999). It is also estimated that 1.1 million metric tons (22.3% of the total China usage) of HCHs were applied in agriculture and industry from 1952 to 1983 (Li et al., 2001). In 1960, chlordane production began in China, with Zhejiang Province serving as the largest user, consuming a total of 980 metric tons until 2013 (Wang et al., 2013). Moreover, the hilly region of southern China is crisscrossed by river networks and dotted with lakes. Rivers and lakes in this area, including Poyang and Dongting Lakes (the two largest freshwater lakes), play important roles in drinking water resources (Wang et al., 2018). However, this same hilly region is characterized with severe soil erosion owing to abundant rainfall and intense human disturbance (Wei et al., 2016). Between 22.2 and 30.0% of this region has noteworthy soil erosion (MWRC, 2014b). Heavy and concentrated precipitation (mean: 1330 mm/year) especially in the summer season led to aggravated soil erosion with subsequent OCP migration from soil to river during rapid surface flow (MWRC, 2014a; Niu et al., 2020). Therefore, this study focuses on examining the seasonal and spatial distributions of OCPs in surface sediments of major rivers in the hilly region of southern China. The objectives of this study are: (1) to elucidate seasonal variations of OCPs with riverine runoff and sediments discharge, (2) to elaborate spatial variations of OCPs with its historical application, runoff transport, and the internal characteristics of sediments, (3) to ascertain different forms of OCP composition and possible OCP sources, and (4) to evaluate the OCP ecological risks and human health risks of this region.
Section snippets
Sample collection
A stainless-steel grab sampler was used to collect riverine sediment samples at 39 localities in the hilly region of southern China (Fig. 1). Two field surveys were conducted during the dry season of November 2012 and the wet season of July 2013, when the rivers were at low and high water levels, respectively. According to the water systems, the targeted 13 rivers were categorized into three basins, including the Zhemin Basin (ZMB), the Poyang Basin (PYB) and the Dongting Basin (DTB). Three
Seasonal variation of OCPs
The concentrations of OCPs in surface sediments are depicted in Fig. S1. The total OCP concentrations ranged between 13.9 and 218.7 ng/g dw with a mean value of 52.0 ± 46.5 ng/g dw in the wet season and between 6.9 and 139.3 ng/g dw (24.9 ± 26.5 ng/g dw) in the dry season. As for individual OCP compounds, DDTs and HCHs were the predominant pesticides in the wet season with an average concentration of 19.4 ± 34.0 ng/g dw and 14.0 ± 16.9 ng/g dw followed by aldrins (6.3 ± 7.1 ng/g dw), endos
Conclusions
This study provides the levels of OCPs in surface sediments from 13 rivers in the hilly region of southern China. A relatively high abundance of OCPs was observed in the wet season versus that in the dry season. HCHs and DDTs were the primary components and mostly originated from historical usage. Riverine runoff and sediment discharge were the major contributors to the effect input of OCPs from basins during different seasons. The spatial differences among rivers and basins were attributed to
CRediT authorship contribution statement
Xionghu Gong: Investigation, Formal analysis, Data curation, Writing - original draft. Qianyu Li: Formal analysis, Writing - review & editing. Lu Zhang: Conceptualization, Investigation, Resources. Zhonghua Zhao: Conceptualization, Methodology, Data curation, Writing - review & editing. Bin Xue: Project administration, Writing - review & editing. Shuchun Yao: Resources, Writing - review & editing. Xiaolong Wang: Investigation, Project administration, Funding acquisition. Yongjiu Cai:
Declaration of competing interest
No conflict of interest exits in the submission of this manuscript, and it is approved by all authors for publication. We declare that the work described is an original research article that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.
Acknowledgments
This study was supported by National Natural Science Foundation of China (No. 41771519, No. 41671477), the Chinese Academy of Sciences' Science and Technology Service (STS) Network Program, Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07204005), the Key Cultivation Project of the Institute's “13th Five-Year Plan”: “Ecological Effect and Prevention and Control of Complex Pollution of Lakes” (NIGLAS2018GH05), the National Key Research and Development
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