Research PaperSea-level-rise-induced flooding drives arsenic release from coastal sediments
Graphical Abstract
Introduction
In many of the world’s urban coastlines, sediments have turned into significant contaminant repositories and potential sources of pollution release to coastal water bodies (Ankley et al., 1996, Warnken et al., 2001). However, due to the multicomponent dynamic nature of coastal sediments, the fate of these coastal contaminants is unknown. Sea-level-rise(SLR)-induced flooding further complicates the prediction of coastal contaminant fate because it affects the biogeochemistry and physics of the sediments and overlying water (Borch et al., 2010). Given that nearly 40% of the global population inhabits within approximately 100 km of coastline (United Nations, 2007) and the fact that these populations are growing at twice the global average (Bijlsma et al., 1996), the impacts of SLR on the long-term fate of coastal pollutants are of environmental, economic, and ecological importance and expected to influence a large population. In recent years, climate change has increased the intensity and frequency of extreme weather events and raised the global mean sea level, resulting in increased flood risks (Hoozemans et al., 1993, Bijlsma et al., 1996). By 2100, this pattern is expected to continue, and extreme sea levels that were once historically rare will become common, leading to severe flooding in global coasts (IPCC, 2021). Such hydrologic changes will certainly influence existing contaminations along the coasts; yet, the mechanism and extent of the impact are not fully understood.
The physical disturbances caused by flooding can increase the diffusive flux of contaminants at the sediment-water interface (SWI) by more than an order of magnitude (Lorke et al., 2003, Wengrove et al., 2015), mobilize sediments into the water column (i.e., cause sediment resuspension), and facilitate the transport of resuspended contaminants away from their original site of deposition. Sediment resuspension further changes the chemical environment experienced by particles when they are eroded from anoxic bed sediments and enter the oxygenated overlying water (Saulnier and Mucci, 2000, Eggleton and Thomas, 2004). Such changes in the oxygen level threaten to release redox-sensitive toxic elements such as arsenic (As) into the water. The fate and bioavailability of metal(loid)s such as As are therefore regulated by the potential oxidation of anoxic sediments, metal sulfides, and reduced iron (Fe) and manganese (Mn) throughout flooding and resuspension (Klinkhammer et al., 1982, Shaw et al., 1990, Morse, 1994, Simpson et al., 1998, Simpson et al., 2000; Chen, 2004; Bushey, 2008).
To predict the fate of sediment-bound contaminants during flooding and resuspension, it is crucial to consider and understand both the chemical (i.e., sediment and water (geo)chemistry) and physical (i.e., local hydrodynamics and sediment erodibility) processes impacting the sediments. On the physical side, although much research has been conducted on quantifying the release of contaminants under quiescent conditions, the release and speciation of pollutants during turbulent flooding conditions in historically contaminated coastal sediments, is not well understood. In addition, while prior studies have focused on the chemical processes affecting pollution mobilization during flooding (e.g., Cantwell et al., 2002, Cantwell et al., 2008; Kalnejais et al., 2007, Kalnejais et al., 2010; Wengrove et al., 2015), the corresponding behavior of anionic and cationic pollutants has not been resolved. Overall, the current literature lacks an integrated investigation tailoring both chemical and physical aspects acting on pollution release under turbulent flooding conditions. To address this knowledge gap, the current study examines the chemical and physical impacts of flooding and sediment resuspension on the release of anionic (i.e., As and NO3) and cationic (i.e., NH4, Co, Ni, Mg, Mn, Pb, and Zn) pollutants from a contaminated site in coastal Wilmington, DE, which is a densely populated coastline expecting about 1 m of SLR by 2100 (Love et al., 2013). Particularly, in the current study, the behavior of As, as a highly concentrated and toxic element, is detailed during flooding.
Even though As is one of the most prevalent toxic elements in the environment, the concurrent variations in As release and speciation over the range of shear stresses that occurs in the natural environment have not yet been assessed. The labyrinthine world of As toxicity, mobility, and fate in the environment is defined by a complex series of controls dependent on the environmental conditions, chemical speciation, and biological processes. Here, to unravel the complex nature of As behavior during turbulent flooding conditions, we focused on the impacts of changes caused by flooding on the release and speciation of As. To that end, a laboratory-based sediment erosion chamber (EROMES) was employed to impose a series of various shear stresses ranging from quiescent to extreme storm conditions that are typically encountered in coastal environments. The concurrent physical and chemical impacts of moderate to extreme flooding on the release and speciation of As and selected elements are quantified in this novel combination of experimental conditions for the first time. Such information provides new insights into the processes controlling the long-term fate of pollutants in contaminated and tidally impacted coastal sediments affected by climate change and SLR. The data from this research plays a crucial role in developing accurate speciation and transport predictive models to understand how climate change affects the cycling of contaminants in polluted coastal sediments.
Section snippets
Sediment sampling and characterization
Sediment samples were collected from a highly contaminated coastal site in Wilmington, DE, projected to be inundated by 1 m of SLR by 2100 (see Fig. S1). The site is adjacent to the Christiana River, along the banks of a tidally-influenced ditch constructed as part of a remediation effort for a neighboring U.S. EPA superfund site, and it floods periodically from the Christina River and urban runoff. Three groups of surface sediments with different chemistries were collected from the lower (LDS)
Natural sediment characteristics
Despite being collected from adjacent areas, ditch (UDS and LDS) and riverbank (RBS) samples had distinct geochemical compositions with different contamination levels. The chemical properties and elemental composition of sediments are summarized in Table 1 and Table 2, respectively. Strikingly, more than 5% As was found in the UDS and LDS (see Table 2). Arsenic, Fe, Mg, and P were enriched in the UDS and LDS compared to RBS, while RBS was more enriched in Co, Cu, S, and Pb, where
Conclusions and environmental implications
The next several decades are likely to witness a considerable rise in sea levels accompanied by frequent and severe flooding events. An understanding of the fate and behavior of legacy coastal pollutants is, therefore, crucial in flood-prone contaminated coasts where geochemical conditions can alter due to climate change and SLR. In the current paper we outlined a new approach to quantify pollution release to coastal waters during SLR-induced flooding events. Monitoring three sets of
CRediT authorship contribution statement
Izaditame, Siebecker, Sparks: substantial contribution to conception and design. Izaditame, Siebecker: substantial contribution to acquisition of data. Izaditame, Siebecker: substantial contribution to analysis and interpretation of data. Izaditame: drafting the article. Izaditame, Siebecker, Sparks: critically revising the article for important intellectual content. Izaditame, Siebecker, Sparks: final approval of the version to be published.
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.
Acknowledgements
This research is a part of the Delaware EPSCoR’s (Established Program to Stimulate Competitive Research) Project WiCCED (Water in the Changing Coastal Environment of Delaware) supported by the National Science Foundation. The work was financially supported by the National Science Foundation EPSCoR Grant No. 1757353, the Multistate State Hatch Project, NC1187, and the State of Delaware. The authors thank Dr. Linda Kalnejais and Dr. Kai Ziervogel, University of New Hampshire, USA, for their
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