LAUSR

We investigate the distributions of the metalloid arsenic (As) and metals iron (Fe) and manganese (Mn) in the sediments of two pristine areas of a biological reserve in the Patos Lagoon Estuary. This area is occupied by Spartina alterniflora and by Neohelice granulata crab colonies and low concentrations of As are expected. The bioturbation/bioirrigation of sediments by crabs and the roots of plants lead to the penetration of oxygen below the oxic/suboxix division and the subsequent precipitation of Fe-Mn hydroxides. Ferruginous incrustations and nodules along roots and crab channels propagate to depths of over 35 cm and sediment contains up to 33 mg kg− 1 of As. The metalloid distribution in sediments is strongly correlated with that of Fe but not with Mn. This study revealed that areas with biologically disturbed sediments could demonstrate contamination in As, which is not anthropogenic in origin.