Dredging impact on trace metal behavior in a polluted estuary: a discussion about sampling design

Abstract

Dredging impact on trace metal behavior in a polluted estuary: a discussion about sampling design Studies involving coastal sediment resuspension have shown that trace metals could be released to the water column due to changes in physical-chemical conditions. Therefore, if environmental agencies adopt screening protocols with insufficient evaluations of contaminant mobilization risks for a given area, the prediction of environmental impacts caused by dredging may be limited. This work evaluates the influence of spatial variation on the geochemical mobility of trace metals (Cd, Cu, Ni, Pb, Zn) after artificial sediment resuspension, using as study case the Iguaçu River estuary (Guanabara Bay, Brazil). The Iguaçu River drains the industrial complex of the metropolitan area of Rio de Janeiro State, besides the presence of agriculture and the input of untreated domestic wastes. Surface estuarine sediments were submitted to resuspension experiments in an open system, during 1h and 24h of agitation on local water. A clear tendency of metals' solubilization was observed after resuspension, especially considering the dissolved concentrations of Cu (average: 8.0 µg L-1) and Zn (average: 0.9 mg L-1), especially for the samples from the transects 2 and 3. However, evaluations of water quality changes due to sediment resuspension are not requested by the legislation regulating the dredging activities. In the sediments, the results suggested a higher geochemical mobility of Cu, indicated by the massive increase on the bioavailable fraction after resuspension, mainly on the transect number 3, on the river's mouth. The effects of resuspension were distinct between samples, suggesting that even in a small scale, important differences on metals' mobility are found. Thus, the combined assessment of changes in metal concentrations in water and in the metal partitioning linked to the solid-phase was demonstrated to be a promising tool for predicting the environmental risks of dredging due to changes in the bioavailability of metals.

Bioavailability Assessment of Metals from the Coastal Sediments of Tropical Estuaries Based on Acid-Volatile Sulfide and Simultaneously Extracted Metals

“…In the aquatic environment, Zn occurs mainly in the form of Zn +2 , the ion shows affinity with organic matter, Fe and Mn oxides and hydroxides, and the fine fraction (Fonseca et al, 2013). In this study, Zn was positively correlated with TOC, Fe, Mn, and clay in both bays, as reported in previous studies in Guanabara Bay and Sepetiba Bay, which found similar correlations [18,32,40,42].…”

“…Guanabara Bay is a eutrophicated environment [8,20,26,32], and in this type of environment, one of the main complexants of metals is organic matter [20,26,27]. The organic carbon reduces the solubility and toxicity of metals [39].…”

“…The organic carbon reduces the solubility and toxicity of metals [39]. Monte et al (2019), in a study in the Iguaçu River estuary, attributed organic matter as the main complexant in the region, followed by sulfides and oxides and hydroxides of Fe and Mn.…”

“…The effect range low (ERL) and effect range medium (ERM) definitions were based on strongly bound extraction [31]. However, the current study also compared the potentially bioavailable fraction with the safety reference levels since this reactive phase represents the risks associated with biota exposure to bioavailable metals [32].…”

Myco-Barriers as Sustainable Tool for Port Seawater Decontamination from Metals

“…Moreover, in the port basins, sediments can also represent a collector for PTEs [3,4] and a secondary source of PTEs due to their handling during dredging activities or resuspension during vessel manoeuvres. In fact, in these cases, sediment movements can promote elements dissolution and transfer in the water column [5]. Despite the numerous PTEs inputs, the rate of port seawater contamination by PTEs is difficult to evaluate because ports are open systems, and toxic elements tend to be very diluted in the water column.…”