Biogeochemical factors controlling arsenic distribution in a densely populated tropical estuary (Guanabara Bay, RJ, Brazil)

The preferential arsenic (As) accumulation in the central sector of the Guanabara Bay (a heavily impacted coastal system located in SE Brazil) is different from most metal contaminants, which present higher concentrations at harbour areas and the rivers mouths that drain large areas with highly deficient pollution control. The estuary is characterised by organic contents and minerals gradients, which induce different redox conditions and different biogeochemical process. Elemental and isotopic analyses on bulk organic matter, mineralogical and textural analyses of sediments were conjugated with a three-step sequential extraction procedure, based on the BCR method (Community Bureau of Reference). This approach was used to evaluate the potential of arsenic mobility. The distribution of the sum of arsenic in the three fractions presented a significant positive correlation with manganese mainly in the oxidised fraction (F2), which corresponded to the most representative fraction conditioned by physicochemical processes. In the exchangeable fraction (F1), which represents the most reactive arsenic phase, this element seems to be associated with the labile organic matter. The high productivity in the central sector might result in a greater deposition of arsenic in the sediment through co-precipitation with manganese deposition in an oxidising environment. Organic matter and sulphide in reducing environments were also found to be important factors controlling the distribution and speciation of arsenic, as indicated by the oxidizable fraction (F3) data.