Riverine transport and water-sediment exchange of polycyclic aromatic hydrocarbons (PAHs) along the middle-lower Yangtze River, China
Graphical Abstract
Introduction
Ever increasing industrial and agricultural production and urbanization entail the discharge of a considerable amount of chemicals into the environment, which are currently at a level that can affect both humans and wildlife species (Lindim et al., 2016). Rivers are important channels through which land-based natural materials and anthropogenic contaminants enter the oceans; about 85% of the materials from land are transported into the sea via riverine runoff (Wu et al., 2019). Among these materials, polycyclic aromatic hydrocarbons (PAHs) are a ubiquitous group of chemical compounds formed by two or more aromatic rings of carbon and hydrogen atoms fused in various arrangements (Neff et al., 2005). PAHs can be toxic, mutagenic, and carcinogenic to ecosystems and human beings, thus leading to environmental and public health concerns ([Wang et al, 2018], [Gonzalez-Gaya et al, 2019]). Unlike other persistent organic pollutants, most of which are globally prohibited for production and usage, the current release of PAHs from primary sources is typically associated with human activities, such as industrial and agricultural production processes, vehicle exhausts, coal and biomass combustion, and emission of petroleum-related products ([Jiang et al, 2018], [Soliman et al, 2019]). Therefore, the fate of PAHs in the environment is currently of special concern for the public. PAHs occur ubiquitously in various components of the aquatic environment, as they enter surface waters mainly via atmospheric deposition, riverine runoff, municipal and industrial effluents, and/or oil leakages through multiple transportation processes ([Wang et al, 2018], [Wu et al, 2019]). Due to their strong affinity for particulate matter and biochemical persistence (Haritash and Kaushik, 2009), PAHs are most likely to precipitate into sediments that act as a natural repository for these hydrophobic organic contaminants (Li et al., 2017). However, when sediment resuspension occurs due to changes in hydrodynamic processes such as waves, tides and currents, trawling, ship traffic, and operation of water conservation projects (Feng et al., 2007), PAHs in sediments are released to the overlying water, causing secondary contamination (Liu et al., 2018). This has been demonstrated in river systems because of the seasonal variations with different hydrological processes ([Rügner et al, 2014], [Liu et al, 2016]), resulting in spatial-temporal variations in the transportation of riverine materials.
Moreover, rivers suffer from variations in water discharge and sediment load, due to human activities, such as freshwater extraction, sand mining of riverbeds, land use changes in river catchments, and damming of rivers, thereby altering the ecological functioning of river systems ([Yang et al, 2015], [Yang et al, 2018], [Zhao et al, 2015], [Zhao et al, 2017]). More in detail, reservoir construction has the most significant anthropogenic impact on riverine fluxes due to the homogenization of river flow regimes (Poff et al., 2007). The construction and operation of more than 45,000 large (i.e., > 15 m high) dams worldwide during the 20th century has severely altered the global flux of water and sediments from continents to oceans through the world's river basins (Poff et al., 2007), by intercepting ~ 25–30% of the global fluvial sediment (Gao et al., 2018). In particular, dams have been reported to sequester reactive phosphorus, and hence, reduce the downstream transfer of nutrients to floodplains, lakes, wetlands, and coastal marine environments (Maavara et al., 2015). However, studies on dam retention of highly toxic PAHs and the following transfer of these pollutants along the river continuum are limited (Dong et al., 2015). Furthermore, dams are reported to decrease riverine particulate organic carbon (POC) by trapping sediments; in fact, dam impoundment explained 69.31% of the POC flux decrease in Chinese rivers during the period 1953–2016 (Liu et al., 2020a, [Liu et al, 2020b]). At the same time, the transport of POC-associated PAHs also tends to change ([Dong et al, 2015], [Li et al, 2017]), especially for large rivers interacting with dam construction and intensive human activities.
With a length of ~ 6300 km, the Yangtze River is the largest river in Asia and the fourth largest in the world, in terms of both water runoff (9200 Mt/year) and sediment discharge (480 Mt/year; [Gao et al, 2013], [Wang et al, 2013]). It supports a population of 0.6 billion (i.e., 43% of China) and its contribution to China's Gross Domestic Product (GDP) in 2018 was 6 trillion dollars, equivalent to 45% of the national GDP (Kang et al., 2020). The Yangtze River basin is also the earliest industrialized region in China (Liu et al., 2019a, [Liu et al, 2019b], [Liu et al, 2019c]), with more than 30 petrochemical plants built along the river in past decades (Wang et al., 2010). Thousands of organic chemicals have been produced along its shores ([Wang et al, 2010], [Yu et al, 2016]), leading to massive discharge and accumulation of PAHs into the river ([Müller et al, 2008], [Müller et al, 2012], [Cai et al, 2017]) and into the Yangtze River Estuary (YRE) due to the water discharge and sediment deposition, as well documented by several previous studies ([Cai et al, 2017], [Liu et al, 2018], [Yu et al, 2018]). As mentioned above, there is currently a surge in dam construction along the Yangtze River ([Dai and Liu, 2013], [Dai et al, 2016], [Jiang et al, 2017]), with a total of 61,446 reservoirs constructed in the river catchment by 2016 ([Gao et al, 2018], Liu et al., 2019a, [Liu et al, 2019b], [Liu et al, 2019c]). In particular, the Three Gorges Dam (TGD), one of the world's largest dams, began to impound water in 2003. It was reported that from 1956 to 1958 (pre-dam period) to 2013–2015 (post-dam and soil conservation period), the sediment load in the river sub-basins decreased by 91%, whereas in the main river, sediment flux decreased by 99% at Xiangjiaba (i.e., in the upper reaches of the river), by 97% at Yichang (i.e., at the transition between the upper and middle reaches), by 83% at Hankou (i.e., in the middle reach), and by 77% at Datong (i.e., at the tidal limit; Yang et al., 2018). Consequently, a recent dramatic decline in biogenic component burial in the river delta was observed due to the decrease in sediment discharge (Liu et al., 2019a, [Liu et al, 2019b], [Liu et al, 2019c]). It was estimated that by 2030 the retention of reactive phosphorus (RP) in the dam reservoirs along the Yangtze River would be 34.8% compared to 12.8% in 2000, due to the construction of reservoirs that modify nutrient stoichiometry along rivers, and thereby, affect nutrient limitation and food-web dynamics in river-fed aquatic ecosystems (Maavara et al., 2015). Moreover, the recent decrease in sedimentary metals could also be largely attributed to intensified erosion triggered by the construction of dams, especially the TGD, and by soil conservation projects (Sun et al., 2019). However, the retention and the transport of PAHs associated with water runoff and sediment discharge from the upper reaches to the estuary still remain unexplored.
As a new socio-economic development plan for the Yangtze River economic zone was elaborated and officially published by the National Development and Reform Commission of China (NDRC) in 2016, priority should be given to environmental protection, and innovative methods to ensure the ecological health of the Yangtze River should be outlined (Kang et al., 2020). Effective pollution abatement methods should be implemented by elucidating the fate of PAHs in the aquatic environment of the Yangtze River from a comprehensive perspective, rather than emphasizing only the regions between the mouth of the river and the ocean, or the particular reaches of the megacities (Qi et al., 2014). The objectives of our study are to investigate: (1) the spatial-temporal distribution of PAHs in both surface water and sediments from the mainstream along the middle-lower reaches of the Yangtze River; (2) the sources of the residues of PAHs related to the surrounding economic development; and (3) the exchange of PAHs between water and sediments, to understand the transport mechanisms of PAHs in river ecosystems associated with water runoff and sediment discharge. Thus, the present study can provide a reference to understand the potential impact of the TGD on the transport and deposition of PAHs along the Yangtze River due to changing human pressures, and to help ecosystem managers to comprehensively evaluate the environmental quality of the river.
Section snippets
Sampling
Following geological, climatic, and geomorphological criteria, the Yangtze River may be divided into the upper, middle, and lower reaches. The middle and lower reaches start from Yichang city with a length of ~ 1800 km and a low-gradient coverage area of ~ 785,000 km2 (Liu et al., 2020a, [Liu et al, 2020b]). The mainstream of the middle and lower reaches of the Yangtze River was chosen as the study area; the distribution of the 84 sampling locations is shown in Fig. 1. The water and sediment
PAHs in water
The concentrations of total PAHs detected in the waters of the Yangtze River during both the wet season and the normal season are illustrated in Fig. 2a and b, respectively. During the normal season, PAHs were in the range of 2.4–761.2 ng/L with a mean value of 73.7 ± 129.7 ng/L, with a large variation across different sampling sites. Pyr had higher concentrations than other congeners, in the range of nd-479.5 ng/L (23.2 ± 80.5 ng/L), although only 23.5% of the sampling sites were found to be
Seasonal variation of PAHs
In terms of temporal distribution, the one-way ANOVA results indicated a distinct seasonal variation, where the concentrations of total PAHs in water were significantly higher in the wet season than in the normal season (significance level of 0.05); this can be further demonstrated by the wet/normal season ratios, shown in the Supplementary Fig. 2s. Ratios were higher than 1 in 88.1% of sites, thereby indicating higher residues of PAHs in the wet season. In contrast, previous studies reported
Conclusions
As the government's strategic mission with respect to the Yangtze River basin include economic development and environmental protection, a comprehensive understanding of the pollution status is particularly necessary to identify major environment problems induced by human activities.
PAHs in water and sediments along the middle and lower reaches of the Yangtze River come mainly from human activities, with coal and coke combustion as the dominant sources, followed by vehicle exhausts. This
CRediT authorship contribution statement
Zhonghua Zhao: Conceptualization, Methodology, Investigation, Data curation, Writing - original draft and Revision. Xionghu Gong: Investigation, Methodology, Resources, Software, Visualization. Lu Zhang: Supervision, Writing - review & editing. Miao Jin: Methodology, Investigation. Yongjiu Cai: Investigation. Xiaolong Wang: Supervision, Investigation, Project administration.
Declaration of Competing Interest
The authors declared that no conflicts of interest to this work.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (51839011, 41671477, 41771519), the Chinese Academy of Sciences Technology Service Network Program (STS), the Comprehensive evaluation of geological resources and environment in the Yangtze River Economic Belt (DD20190260), the 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
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