DISSOLVED REACTIVE MERCURY SPECIATION IN WATER PROFILES FROM SEPETIBA BAY, SE BRAZIL

RV Marins, LD Lacerda*& HHM Paraquetti

Dept. Geoquímica, Universidade Federal Fluminense, Niteroi, 24020-007, RJ, Brazil

ABSTRACT

This paper presents the distribution of Hg species in waters of a sub-tropical coastal lagoon, Sepetiba Bay, in SE Brazil. The results showed that reactive Hg species: dissolved gaseous Hg (DGM) and other inorganic Hg species dominate the total dissolved concentrations. Analysis of water column profiles showed increasing concentrations of DGH in surface waters relative to bottom waters, probably as a result of higher reduction and demethylation rates due to intense primary productivity and microbial activity in the mesotrophic coastal water body.

INTRODUCTION

Sepetiba Bay is a key area of heavy metal contamination in southeastern Brazil. Mercury, among other metals, is an environmental issue of this Bay, particularly due to its potential of export and contamination of important tourist areas adjacent to Sepetiba Bay (Marins et al., 1999). Exposure to monomethyl-Hg through fish consumption is the main public heath concern of local environmental authorities. However, fish Hg content is not directly linked to total Hg content of waters, but on the concentrations of selected Hg species, in particular those included in the operational definition of reactive Hg species, which includes dissolved gaseous Hg (Hg⁰) and inorganic Hg(Hg²⁺) (Rolfhus & Fitzgerald, 1995; Lacerda, 1997).

Since reactive Hg species are the most important in terms of bio-accumulation from water and dominates the Hg dissolved concentrations in the water column, we investigated the vertical and horizontal distribution of the total reactive mercury, dissolved gaseous mercury (DGM) and of inorganic Hg (Inorg-Hg) concentrations in Sepetiba Bay, in order to understand the Hg transport to adjacent areas and the processes influencing it.

MATERIAL & METHODS

Surface water samples were collected in 6 stations at the inner section of Sepetiba Bay. In two other stations samples were also collected in water profiles to 50 cm to the bottom sediments (Figure 1). Surface samples were collected by hand by a diver using Teflon gloves and 500ml Teflon bottles. Water column profiles were sampled using Teflon van Dorn bottles. Samples were double bagged and transported in an ice box to the laboratory. Samples were analyzed between 4 to 6 hours after collection. 200 ml sub samples were purged with Hg-free Argon to evade and measure dissolved gaseous Hg (DGM). Sub-samples were treated with SnCl for reduction of the total reactive Hg fraction. Total reactive minus DGM was considered the dissolved inorganic fraction. Sequentially, the sub-samples were oxidized with BrCl followed by reduction with SnCl to determine total dissolved Hg. Dissolved organic Hg was considered as the difference between total dissolved and total reactive Hg. Mercury species were determined by atomic fluorescence spectrophotometry in a Tekran 2500 spectrophotometer. Detection limit of this analytical procedure was 0.1 pM.

Simultaneously to the Hg sampling, temperature, salinity and dissolved oxygen concentrations were measure in situ using portable equipment.

RESULTS & DISCUSSION

Table 1 resumes the Hg concentrations found in surface waters of Sepetiba Bay. Total dissolved Hg concentrations in surface waters of inner Sepetiba Bay ranged from 0.4 to 3.3 pM. Total reactive Hg ranged from 0.56 to 1.8 pM. These concentrations are in the range found for oceanic waters and pristine coastal waters and reflect the lack of significant contamination of Sepetiba Bay by Hg.

Table 1. Summary of Hg species concentrations (pM) and average percentage distribution in surface waters of inner Sepetiba Bay, SE Brazil.

Dissolved Gaseous Mercury	Dissolved Inorganic Mercury	Total Reactive Mercury	Dissolved Organic Mercury	Total Dissolved Mercury
0.16 - 0.41	0.4 – 1.39	0.56 – 1.80	<0.1 – 2.77	0.6 –3.33

Total reactive Hg in surface waters corresponded to 55 to 90% of the total dissolved Hg concentrations, whereas dissolved organic Hg contributed to 10 to 83%. Preliminary results of particulate Hg, showed much higher concentrations of up to 7.5pM. Thus corresponding to 70% of the total Hg content of inner Sepetiba Bay waters. However, Hg present in suspended particles could only be dissolved using strong acidic digestion, suggesting that particulate-Hg is of minor environmental importance to the region (Marins et al., 1999).

In general, highest reactive Hg concentrations occurred seaward, where particulate Hg is insignificant, but a positive relationship with DOC was reported (Lacerda et al., 2000). These confirm the importance of reactive Hg relative to more refractory species to the Hg cycling and bioaccumulation, and agree with previous speciation of marine surface waters, which consistently shows dominance of reactive species (Mason et al., 1998; Mason & Sullivan, 1999).

Dissolved gaseous Hg (DGM) ranged from 0.16 to 0.41 pM, about 30% of the total reactive concentrations, the major part (70%) of it being dissolved inorganic Hg species. The measured concentrations are in the range found for ocean waters. However, while in the open ocean DGM dominates the reactive fraction, in Sepetiba Bay, as a coastal water body, inorganic species dominates the reactive fraction (Mason & Sullivan, 1999). Higher inorganic Hg relative to DGM is probably related to the inputs from rivers and the high atmospheric Hg deposition reported for Sepetiba Bay (Marins et al., 1996).

Water column profiles of reactive Hg species also showed that Inorg-Hg dominates the reactive species with concentrations ranging from 0.56 to 1.80 pM, whereas DGM concentrations ranged from 0.12 to 0.41 pM (Figure 3,4). In general, inorganic Hg presented highest concentrations at the sub-surface samples (about 1.0 m of depth). DGM was only slightly higher at surface samples. However, when concentrations of reactive species are plotted as percentage of the total reactive concentrations (Figure 4,5), a clear increase in the importance of DGM, relative to other inorganic species, appears. DGM increases from a contribution of less than 5% in waters collected close to the bottom sediments, to 16% and 18% at the surface. Highest DGM concentrations in surface waters relative to deeper samples probably results from reduction of Hg species by plankton activity. This has been suggested to explain high DGM concentrations in the open ocean surface waters (Mason et al., 1998). Since Sepetiba Bay is a highly productive environment, this would easily result in higher reduction rates of Hg in surface waters. High demethylation rates may also contribute to the elevated DGM content of surface waters relative to deeper samples.

Concluding, these preliminary resultas highlights the importance of reactive Hg as the major chemical speciesavailable for biological uptake in Sepetiba bay. However, the high productivity of the bay may enhance Hg reduction and demethylation, increasing Hg transfer back to the atmsophere.

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