Distribution and Geochemistry of Arsenic in Sediments of the World’s Largest Choked Estuary: the Patos Lagoon, Brazil

Abstract

The arsenic (As) contents of sediment cores from the Patos Lagoon in southern Brazil were measured to better understand the biogeochemical cycling of As in the estuary. Sediment cores (ca. 60 cm) were obtained from three locations within the estuary to capture possible changes in As content across the salinity gradient (i.e., where saline, brackish, and freshwater dominated). Two sediment cores were collected at each location, one beneath open water and the other from the fringing salt marsh. Along with As, we quantified the particle size; redox potential (Eh); manganese (Mn), iron (Fe), total organic carbon (TOC), and free (dissolved) sulfide concentrations; and acid volatile sulfide (AVS) and chromium reducible sulfide (CRS) contents. Bioturbation supports oxygen penetration to depths between 20 and 30 cm below the salt marsh surface, where an As- and Fe-rich zone was identified (3-fold higher As than mean As contents of the sediments). A similar subsurface peak of As, Fe, and Mn occurs in the open-water cores, albeit at greater depths between 40 and 50 cm below the surface. The subsurface peak has As concentrations that are 2-fold higher than the average for each core. Vertical profiles of Eh, free sulfides, and CRS for the shallow open-water cores showed similar distribution at depths of 50 cm, suggesting that pyrite formation is an important sink for As in the open-water cores. The data demonstrate clear differences in the geochemical conditions for salt marshes and shallow open waters that can have important implications for As distribution in estuarine sediments.