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Abstract
Dredging and tailings disposal can mobilize mercury (Hg) accumulated in bottom sediments, increasing the exposure of the aquatic biota. In the Mucuripe Harbor, NE, Brazil, dredging is performed regularly, but the impact on Hg mobilization is unknown. This paper presents results from the monitoring of a dredging operation to characterize and quantify an eventual Hg mobilization. The results showed that remobilization is significant and is associated with suspended solids. Further deposition of the Hg-enriched, remobilized, suspended solids increased Hg concentrations by a factor of 1.2 and 2.0 in harbor and shelf sediments, respectively. Maximum concentrations in harbor and shelf sediments reached 79.9 ng.g-1 and 32.4 ng.g-1 (20 and 9 higher than the regional background, respectively). The progressive increase in Hg concentrations may result in higher exposure to the local biota.Keywords: metals, mobilization, contamination, coastal sediments.
Highlights
Dredging to keep navigable harbor accesses and maneuvering areas changes the chemical and biological characteristics of bottom sediments and the overlying water column, both at the operation and at disposals sites (Caplat et al, 2005), generally increasing sediment toxicity (Baruaem et al, 2012; Moreira et al, 2017; Saes et al, 2019)
The mobilization of contaminants is of relevant environmental concern, in particular those elements quickly incorporated into food chains that may result in increasing exposure to pollutants
A critical review of Hg sediment quality values (SQV) for the protection of benthic invertebrates shows low predictive value of SQVs when Hg concentrations are above background levels (Conder et al, 2015), in Brazilian shelf sediments background Hg concentrations are on the order of 5 to 30 ng.g-1 (Marins et al, 2004)
Summary
Dredging to keep navigable harbor accesses and maneuvering areas changes the chemical and biological characteristics of bottom sediments and the overlying water column, both at the operation and at disposals sites (Caplat et al, 2005), generally increasing sediment toxicity (Baruaem et al, 2012; Moreira et al, 2017; Saes et al, 2019) Amongst these changes, the mobilization of contaminants is of relevant environmental concern, in particular those elements quickly incorporated into food chains that may result in increasing exposure to pollutants. Mercury is regulated under the Conama (2012) act and the proposed limit concentrations in sediments range from the Level 1 (300 ng.g-1), when no specific disposal is required, to Level 2 (1,000 ng.g-1), when pre-disposal treatment is required These concentrations, may be 10 to 100 times higher than the local background of Hg in Brazilian shelf sediments (Marins et al, 2004). Virtually any dredging of merely Hg-enriched sediments can pose environmental threats to aquatic organism
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