Mercury distribution and speciation in waters of the coastal lagoons of Rio de Janeiro, SE Brazil

Abstract

Abstract The distribution and speciation of Hg in surface waters from the coastal lagoons of Rio de Janeiro State, Brazil, sampled in 1997–1998, showed highest concentrations of all Hg species (reactive, total dissolved and particulate) in lagoons affected by metropolitan areas, where total suspended solids (TSS) were, in general, very high. Total dissolved Hg varied from 18.2 to 55.2 pM, one to two orders of magnitude higher than open ocean concentrations. Highest values were found in lagoons located at the metropolitan area of Rio de Janeiro city, and the lowest values in the hypersaline Araruama lagoon. Reactive Hg (Hg 0 +HgII) concentrations varied from 0.18 to 0.43 pM, comprising less than 2% of the total dissolved Hg, a fraction much lower than frequently reported for the open ocean or coastal plain estuaries. The fraction of reactive Hg was negatively correlated with total dissolved Hg. Particulate and total Hg concentrations (dissolved plus particulate) ranged from 18 to 230 pM and 37 to 285 pM, respectively, with lowest values in lagoons with the lowest TSS concentrations. Mercury concentrations in TSS and bottom sediments, on a weight basis, varied from 0.7 to 2.9 μg g −1 and 0.12 to 0.44 μg g −1 , respectively. However, Hg concentrations in the water column did not correlate significantly with the Hg concentrations in TSS or in bottom sediments. The results clearly show that most dissolved Hg in these lagoons is in strongly bound forms. Also, high TSS due to resuspension of bottom sediments maintains large amounts of both dissolved and particulate Hg in the water column. Relatively low water–particle distribution coefficients ( K D =0.6–5.5×10 5 ) suggest sorption/desorption processes as responsible for this. These results suggest that the behavior of Hg in the coastal lagoons is similar to that described previously for the turbidity maximum zone of coastal plain estuaries. Therefore, a survey of Hg concentrations in key biological components of these ecosystems is recommended.