Mercury concentrations along the Amazon estuary and plume: Spatial trends and geochemical processes

Mercury (Hg) contamination in the Amazon is well-known globally and well-studied. However, there remains a notable research gap regarding Hg contamination in the coastal zone and continental shelf of the Amazon region. Therefore, the aim of this study was to evaluate the levels of Hg concentrations in water and sediments within the coastal region and continental shelf of the Amazon, identifying spatial trends and the main geochemical processes that influence the transport and fate of this metal in the area. The Hg concentrations in the unfiltered water varied among the analyzed regions (p < 0.0001), with the highest values recorded in the estuary of the Pará River, 13.8 pmol L^-1, and the lowest values, 1.5 pmol L^-1, in the Amazon River North Plume. The Hg concentrations in the study regions were found to be positively related to total organic carbon (TOC; R² = 0.537, p < 0.0001) and with suspended particulate matter (SPM; R² = 0.227, p < 0.0001) in water samples, with these matrices being identified as geochemical supports for the dynamics and transport of Hg in the continental shelf. The δ¹³C of organic matter presented a median of -28.0 ‰ in the dissolved fraction, -24.2 ‰ in the particulate fraction and -24.7 ‰ in the sediments. The median δ¹⁵N values were 2.2 ‰ in the dissolved fraction, 5.0 ‰ in the particulate fraction and 3.8 ‰ in the sediments. The isotopic and elemental composition of organic matter, along with the negative relation between Hg concentrations and salinity in the study areas (R² = 0.301, p < 0.0001), indicates that the Hg present on the continental shelf primarily originates from the Amazonian drainage basin.