Seasonal dynamics of polycyclic aromatic hydrocarbons between water and sediment in a tide-dominated estuary and ecological risks for estuary management

https://doi.org/10.1016/j.marpolbul.2020.111831Get rights and content

Highlights

  • Understanding PAH dynamics between water and sediment is particularly important.

  • The Humen estuary was polluted at a moderate-to-high level.

  • TOC significantly influenced the behaviors of PAHs.

  • PAH sources were apportioned via diagnostic ratios.

Abstract

Understanding the transportation of polycyclic aromatic hydrocarbons (PAHs) across the water-sediment interface can help researchers to partition their sources while being particularly important for managing PAH input. This study fully explored the PAH dynamics between water and sediment in a tide-dominated estuary. The monthly concentration of ΣPAHs in sediments ranged from 325.47 to 1098.49 ng/g (dry weight), while that in water varied from 154.00 to 725.80 ng/L. The PAH levels found in the present study were relatively high in comparison with other estuarine systems worldwide. The high-molecular-weight PAHs were more readily redissolved from sediment to water, while the low-molecular-weight PAHs were mostly in an unsaturated state with diffusion occurring from water to sediment. The seasonal differences of ΣPAHs were significant and were largely controlled by the changes in sediment properties, marine currents, and water temperature. The diagnostic ratios revealed that predominant sources of PAHs were pyrogenic processes and petrogenic inputs.

Introduction

More than 100 types of polycyclic aromatic hydrocarbons (PAHs) have been found in aquatic environments via several paths including riverine input, atmospheric deposition, or stream inflows (Lang et al., 2008; Gu et al., 2016). These PAHs tend to be adsorbed onto the particles because of their hydrophobicity. They tend to accumulate into sediments, and they may enter marine organisms including fishes (Wang et al., 2010). The PAHs in an aquatic environment (sediment, pore-water, and water) have attracted attention worldwide because of their characteristics of carcinogenicity, persistence, toxicity, and mutagenicity (Harrison et al., 1996; Timoney and Lee, 2011; Deng et al., 2013; Niu et al., 2020). Some of them are on the United States Environmental Protection Agency list of priority pollutants, and have significantly affected the quality of the environment and created threats to human health (Qiu et al., 2009; Moeckel et al., 2014). The PAHs are generally derived from anthropogenic activities, with pyrogenic processes and petrogenic sources acting as the two major sources (Yunker et al., 2002; Chen et al., 2004; Zhang et al., 2016). Understanding the dynamic mechanisms of PAHs is therefore important for predicting their environmental fate and managing the PAH input into marine estuaries.

A water-sediment system forms the boundary interface between water and sediment (Santschi et al., 1990). The diffusion of PAHs between water and sediment is associated with the influence of physical and biological processes. These processes include particle re-suspension into the water body caused by strong hydrodynamic forcing, and particle mixing within the sediment caused by marine currents. The importance of water-sediment exchange processes for determining the spatiotemporal variations of the characteristics of PAHs has rarely been documented (Kirso et al., 1990; Xia and Wang, 2008).

A tide-dominated estuary is characterized by strong interactions between river discharges and tidal currents. Sediment is an important pool that stores organic pollutants and is a potential secondary pollution source when the river discharge and tide strongly interact (Liu et al., 2012; Zhang et al., 2012). Previous studies have reported on the distribution of PAHs in water and sediment in tide-dominated estuaries (Maskaoui et al., 2002; Liu et al., 2017, Liu et al., 2018), but knowledge on the dynamic mechanisms of PAHs across the water-sediment interface still remains poor. The mixing at the interface in an estuary is one of the most important dynamic processes occurring with estuaries (Liu et al., 2018). The estuarine processes not only influence the gradients of physicochemical factors that affect the properties of PAHs in water and sediment, but they also affect the transportation of PAHs between water and sediment in an estuary.

In the present study, the tide-dominated region of the Humen river mouth in the Pearl River estuary (PRE; Fig. 1), as the main river mouth of the cities of Gugangzhou and Dongguan (an economically important region in China), was selected to examine the behavior of PAHs between water and sediment. The PRE system consists of complicated tributaries and river networks having approximately 322 branches and constitutes a major pathway of PAHs being transported to the South China Sea. The variations in hydrological conditions between the wet (April–September) and dry (January–March and October–December) seasons in the PRE are substantial (Lu et al., 2009; Liu et al., 2017). The Pearl River discharges pollutants through eight river outlets at Humen, Jiaomen, Hongqili, Hengmen, Hutiaomen, Jitimen, Yamen, and Modaomen. The Humen river mouth is a typical tide-dominated estuary with seasonal changes in estuarine dynamics; it is the main passage that transports organic pollutants from the large urban areas of Guangzhou and Dongguan. Previous studies have reported on the characteristics of PAHs in the PRE (Mai et al., 2003; Wang et al., 2010; Yuan et al., 2015), and our previous studies have investigated the estuarine behaviors of PAHs in the water column (Liu et al., 2017, Liu et al., 2018; Niu et al., 2018) and surface sediments (Niu et al., 2020) at the Humen estuary. However, documentation of the dynamic mechanisms of PAHs in the water-sediment system of the Humen estuary and the related ecological risks has not completed. Despite the above situation, the objectives of this investigation are to: (1) explore the seasonal changes in the concentration and composition of PAHs and the factors influencing PAH content in water and sediment, (2) document the diffusion of PAHs between water and sediment, (3) assess the related ecological risks, and (4) apportion the anthropogenic contamination sources of PAHs in this tide-dominated Humen river mouth of the PRE. Furthermore, the influences of seasons on river-marine dynamics and on the geochemical behaviors of PAHs between water and sediment have ecologically important implications for managing the input of PAHs in this estuary.

Section snippets

Sample collection

The eight river mouths of the Pearl River estuary system can be divided into two functional groups regarding to their estuarine processes: tide- and river-dominated. Among the four eastern river mouths, the sampling site of the Humen estuary analyzed here has only one gate functioning as seaward. In 2011, bottom water samples (0.5 m over the riverbed) and surface sediment samples (<5 cm) were continuously collected each month in the Humen estuary, using the traditional active sampling method

PAH concentrations

The seasonal dynamics of PAHs were detected in the Humen estuary. For the surface sediment samples (Fig. 2a), the dry weight of ∑2-, ∑3-, ∑4-, ∑5-, and ∑6-ring PAHs varied from 42.00 to 447.10 ng/g (mean 184.50 ng/g), 46.39 to 195.60 ng/g (91.18 ng/g), 108.70 to 299.20 ng/g (178.70 ng/g), 58.78 to 182.10 ng/g (98.79 ng/g), and 21.00 to 76.00 ng/g (35.40 ng/g), respectively. The ∑PAHs varied from 325.47 to 1098.50 ng/g (mean 588.64 ng/g). Heavily contaminated sediments (>500 ng/g, Maioli et al.,

Conclusions

The present study explored the seasonal dynamics and diffusion of PAHs between water and sediment of this tide-dominated Humen estuary, thereby helping to fill the knowledge gap of this complex region. The estuarine behaviors of PAHs varied widely throughout the sampling year and were greatly determined by the co-influencing factors of organic carbon content, sediment properties, and riverine input. Highly contaminated sediments (>1000 ng/g, dry weight) were mainly detected in the dry season

CRediT authorship contribution statement

Lixia Niu: Writing - original draft, Conceptualization, Formal analysis. Xiangxin Luo: Data curation, Software. Huayang Cai: Writing - review & editing. Feng Liu: Data curation. Tao Zhang: Software. Qingshu Yang: Supervision, Project administration, Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was financially supported by the National Key R&D Program of China (No. 2016YFC0402601) and National Natural Science Foundation of China (No. 21077139, 51709289, 51909290, and 41476073). The authors would like to thank the editor and anonymous reviewers for their time on this manuscript. The authors also thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

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