TIDAL REMOBILIZATION OF MERCURY FROM MUD FLAT ACIDIC PORE WATERS

R.V. Marins¹*, L.D. Lacerda (Dept. Geoquímica, Universidade Federal Fluminense, Niteroi, 24020-007, RJ, Brazil) & Stephane Mounier (RCMO, Universite de Toulon, France)

1 - Present address: Univerisdade Federal do Ceará, Departamento de Geologia, Campus Universitário do PICI, Bl. 912/913, CEP: 60455-970, Fortaleza, CE, Brazil.

*ldrude@fortalnet.com.br

ABSTRACT

This paper presents the distribution of Hg species in porewaters from mudflats of a sub-tropical coastal lagoon, Sepetiba Bay, in SE Brazil. The results showed that reactive inorganic Hg species reach very high concentrations at deeper layers of the sediment cores, associated with very acidic pH and oxidation by incomming tides. Organic Hg on the other hand, dominates the surface layers of the cores, which are highly enriched in DOC. DOC profiles suggest that inorganic reactive Hg is complexed with dissolved organic matter along the core, allowing its migration to the surface of the sediment. The reacitons within these mud flat sediments may play a significant role in increasing Hg bioavailability to adjacent Sepetiba bay food chains.

INTRODUCTION

Exposure to monomethyl-Hg through fish consumption is the main public health concern of local environmental authorities, although the natural reactions leading to the formation of this substance is still far from comprehesion, but the role played by microbial metabolism in sub-oxic environments is fundamental to this process (Rudd, 1995). Sepetiba Bay is a key ecological area of the Rio de Janeiro coast where Hg is an important environmental issue, particularly due to its potential of export and contamination of adjacent tourist areas. The Bay is surrounded by about 40 km² of tidal flats, mostly occupied by mangroves and salt marshes. These intertidal areas play a significant role in mantaining abundant populations of economically valuable estuarine species, largely used by the local population and exported to nearby Rio de Janeiro city. Intertidal sediments are considered as sinks for heavy metals entering Sepetiba Bay, in particular of Cd, Zn and Pb (Lacerda, 1998). However, only about 15% of total Hg load to Sepetiba Bay are permanently buried in these sediments (Marins et al., 1999), suggesting that post-depositional mobilization may results in significant Hg diffusion from mudflats pore waters to surface waters of Sepetiba Bay (Lacerda et al., 1999). Mercury mobilization from these sediments for example, was observed due to changes in physical and chemical conditions of the sediment induced by the root metabolism of the sea grass Spartina alterniflora (Marins et al., 1997). Therefore, changes in the pore water conditions due to stronger forces, such as tidal forcing, may result in high Hg released from these sediments, which may be kept in solution associated with the typically abundant dissolved organic matter.

In this study, pore waters samples from a mangrove-salt marsh mudflat were collected and analyzed for the concentrations of Hg species and of different physical and chemical parameters, in order to investigate the processes involved in the post-depositional mobilization of Hg.

MATERIAL AND METHODS

Pore waters were collected from two mud flat areas, in Sepetiba Bay, during tidal flooding (Figure 1) IEF a bare mud flat adjacent to mangroves and EG a mud flat covered by the sea grass Spartina alterniflora. Vacuum operated collectors were introduced into the sediments at low tide for a period of about 2 hours and taken out when tidal waters were reaching the collectors sites. During a second campaign, pore waters were collected only during the peak of the low tide period. Samples were collected using a syringe and Tygon tubes, at 5cm intervals to 50cm of depth, directly into pre-washed 250 ml Teflon bottles. Samples were double bagged and transported in an ice box to the laboratory. Samples were analyzed between 4 to 6 hours after collection. (For details of sampling apparatus and strategies see Lacerda et al. (1999).

Dissolved gaseous Hg (DGM), Reactive Hg (R-Hg), total dissolved Hg (T-Hg) and organic Hg (Org-Hg) were determined following accepted protocols through AFS in a Tekran 2500 equipment. 50 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. 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. Dissolvde organic carbon (DOC) was determined by high temeprature catalytic oxidation with a TOC 5000 Shimadzu analyser.

RESULTS AND DISCUSSION

Table 1 presents major chemical and physical parameters of pore waters from Sepetiba Bay mud flats. The stickling feature of these pore waters are the extreme acidity and high oxidation state. Variation of Eh and pH suggest a strong oxidation of these sediments by the incoming tidal waters, in a manner similar to acidic mine drainage. Oxidation of these sulfidic (Table 1) sediments resulted in acidic pH (1.9 to 2.8 at both sites) in pore waters below 20cm of depth at the IEF site and along the entire profile at the EG site, and high Eh (+70 to +470).

During the sampling at the peak of low tide, pH remained between 6 and 8. Eh was not available, but previously sampling at the same site showed that Eh were always lower than -100 mV (Lacerda et al., 1999; Marins et al., 1997). DOC concentrations were very high (123 mg/l) along the top 10cm at the IEF site and 37 (mg/l) at the EG site, decreasing to 5 to 15 mg/l below 20cm.

Table 1. Major physical and chemical parameters measured in pore waters from mud flats of Sepetiba Bay, SE Brazil.

IEF Site: Flooding	Salinity	S^2- (µM)	Eh (mV)	pH	DOC (mg L^-1)
5 - 10 cm	32	1.4	+200	7.7	123
20 cm	32	0.8	-	-	16
30 cm	34	1.1	+330	2.1	5
40 cm	34	2.4	+280	1.9	6
50 cm	34	7.8	+470	1.7	15

EG Site: Flooding	Salinity	S^2- (µM)	Eh (mV)	pH	DOC (mg L^-1)
5-10 cm	25	1	+430	1.9	24
20 cm	25	102	+280	2.8	5
40 cm	22	277	+120	2.5	4
50 cm	21	186	+70	2.6	7

IEF Site: Low tide	Salinity	S^2- (µM)	Eh (mV)	pH	DOC (mg L^-1)
10 cm	24	0.1	-	6.3	-
30 cm	25	0.5	-	7.4	-
50 cm	25	6.2	-	6.8	-

The concentration distribution patterns of Hg species were similar in both sites. DGM was very low in all profiles, being detectable only at the top layer of the cores collected during the flooding campaign (1.0 and 2.6 pM, at EG and IEF sites respectively), probably as a result of more oxygenated waters at the surface of the core, favoring demethylation (Mason & Sullivan, 1998).

Inorganic reactive Hg (mostly Hg ²⁺), was lower at surface during the flooding campaign, ranging from (0.2 to 17 ng/l) and higher below 30 cm, attaining maximum concentrations at 50 cm of depth of up to 21.0 ng/l and 32.2 ng/l at the EG and IEF site respectively. During the low tide sampling reactive inorganic Hg was very low (< 1 ng/l). Under the physical and chemical conditions of the pore waters below 30 cm presented in table 1, Hg²⁺ can be kept in solution, resulting in high inorganic-reactive Hg fraction. (Table 2).

Organic-Hg was the dominant Hg species at more surface samples (above 30 cm), in particular at the EG site, with highest values at the surface (91-246 ng/l), but decreasing to lower concentrations at deep layers (< 1 ng/l) to 14 ng/l).

The results suggest that Hg²⁺ generated by acidity under strong oxidation conditions promoted by root metabolism (at the EG site) and tidal flux, may be kept in solution associated with DOC, and thus easily diffuse upward when pH of surface layers increase or the sediment becomes reducing. The availability of high concentrations of DOC may facilitate the migration of Hg through pore waters.

Table 2. Concentrations of mercury species in pore waters of mud flats from Sepetiba bay, SE Brazil (ng L^-1)

IEF Site: Flooding	DGM	Reactive inorganic	Organic-Hg
5 - 10 cm	1.5	15.6	91
20 cm	2.6	nd	59
30 cm	nd	11.2	45
40 cm	nd	24	0
50 cm	0.7	32.2	14

EG Site: Flooding	DGM	Reactive inorganic	Organic-Hg
5-10 cm	1.0	16.9	246
20 cm	nd	5.6	20
40 cm	nd	7.3	0
50 cm	nd	21	0

IEF Site: Low tide	DGM	Reactive inorganic	Organic-Hg
10 cm	nd	0.5	15
30 cm	nd	0.4	10
50 cm	nd	0.2	20

ACKNOWLEDGEMENTS

Thanks are due to Fundação de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) for providing support for this work. The Conselho Nacional de Desenvolvimento Científico e Tecnológico provide grants for RVM and LDL.

REFERENCES

Lacerda LD. (1998) Diffuse Pollution and Biogeochemistry of Trace Metals in Mangrove Ecosystems. ISME, Okinawa, 68p.

Lacerda LD, Ribeiro MG, Gueiros BS. (1999) Mangroves & Salt Marshes 3:105-111.

Marins RV, Lacerda LD, Gonçalves GO, Paiva EC. (1997) Bull. Environm. Cont. Toxicol. 58: 733-738.

Marins RV, Lacerda LD, VillasBoas RC. (1999) In: Mercury Contaminated Sites. (Ebinghaus R, RR Turner, LD Lacerda, W Salomons, O Vasiliev, Editors), Heildelberg, Springer, pp:207-220.