Pb AND Hg DISTRIBUTION IN DATED SEDIMENT CORES FROM REMOTE HEADWATER LAKE AT ITATIAIA MOUNTAINS, SE BRAZIL

 

LD Lacerda*, MG Ribeiro Jr & JJ Abrão

Dept. Geoquímica, Universidade Federal Fluminense, Niterói, 24020-007, RJ, Brazil. *ldrude@fortalnet.com.br

 

ABSTRACT

Pb and Hg concentrations and accumulation rates changed significantly over the last 80 years at Itatitaia Mountains, SE Brazil, the largest industrialized region of the country. Lowest concentrations were measured from 1910 to 1940 (40 mg.kg-1for Pb; 30 ng.g-1 for Hg). Hg and Pb concentrations correlate significantly, but not with organic matter content, suggesting an industrial source of both metals. Average early century Pb deposition was c.a. 8.0 mg.m-2.yr-1, and peaks at sub-surface layers (40 mg.m-2.yr-1). Average Hg deposition rate was 36±4 µg.m-2.yr-1, between 1910 and 1940, peaks in the 1960`s (120 µg.m-2.yr-1), and decrease after the late 1970’s to 20 µg.m-2.yr-1. Present day Pb deposition rates are somewhat higher than, whereas Hg deposition rates are similar to, the average deposition rates reported for remote sites in North America and Europe.

 

INTRODUCTION

Pb and Hg are global scale contaminants, due to the dominance of the atmospheric transport, and their long residence time in the atmosphere. Anthropogenic emissions of Pb and Hg in North America and Europe, resulted in increasing deposition rates during the past 150 years, with a peak of maximum deposition during the 1960`s and 1970’s. More recently a relative decrease in deposition rates is observed, due to emission control policies implemented in the industrialized nations (Pirrone et al., 1998). However, no consistent data are available for South America, notwithstanding its significant anthropogenic contribution of trace metals to the atmosphere, in particular from Brazil, the largest and most industrialized country of this sub-continent.

Emissions of Pb and Hg in Brazil are due to the accelerated industrialization after World War II, particularly in the southeast. Recently, large amounts of Hg are being emitted in the Amazon region due to gold mining, but atmospheric deposition occurs in the Amazon itself (Lacerda & Salomons, 1998). Monitoring from environmental agencies and some few academic studies, have detected a decrease, at least for Hg concentrations, in fish and estuarine sediments in localized polluted areas in the southeast, in response to the implementation of emission control polices. No evidence exists for Pb, however.

This study presents for the first time, estimates of Pb and Hg atmospheric deposition rates based on dated sediment cores, collected in high altitude lakes in the Itatiaia Mountains, located in the most industrialized area in Brazil, which receives most inputs from Rio de Janeiro, São Paulo and Minas Gerais industrial parks.

 

MATERIAL AND METHODS

            The Itatiaia National Park is located in the Serra da Mantiqueira along the Paraiba do Sul River valley, between São Paulo and Rio de Janeiro cities (Fig. 1). Most of the area is occupied by the Atlantic tropical rain forest, dominating altitudes up to 1,500 m. Altitudes from 1,500 to 2,100 m, are dominated by clouded evergreen forest, substituted by highland prairies up to 2,800m. Rainfall is over 2,300 mm per year. Alkaline granites covered by latossols, cambisols and litolic soils, dominate the geology of the region. This geology is particularly poor in heavy metals and no occurrence of Hg-bearing rocks is known for region. Thus any Hg reaching the area is mostly from atmospheric origin.

Figure 1. Location of the Itatiaia Mountains, SE Brazil

 

            Samples were collected in one of the most remote lake of the highland prairies, located at approximately 2,700 m of altitude, following Porcella (1996). Two cores were collected by hand, inserting acrylic tubes into the sediment to a depth of 50 cm. Cores were sliced in 1.0-cm layers to a depth of 23 cm and in 5.0-cm layers. Samples were stored in acid-clean plastic bags and frozen for transport. Sediment samples were oven-dried at 50 ºC to constant weight. Approximately 1.0 g of the dried sample was digested in a closed system (60-70 ºC, 1.0 h) with a 50% aqua regia (4 mL HCl + 6 mL HNO3 + 10 mL H2O) in duplicate. The extracts were centrifuged (15 min, 2,000 rpm), and Hg was analyzed in a Bacharat Model CVAAS, with a detection limit, based on 3-times the value of the reagent blank, of 6 ng.g-1. Simultaneous analysis of Hg in reference standards (NIST-USA, Buffalo River Sediments, 60 ng.g-1) gave 58±6 ng.g-1 (n=15).

            Additionally, the organic matter content was determined in 1.0 g dried samples, after combustion (450 ºC, 24 h). The ashes were digested with a strong acid mixture (2 mL HNO3 + 3 mL HCl + 1 mL HF) at 60-70 ºC for 2 hours, in duplicate, to determine Pb concentrations through conventional flame atomic absorption spectrophotometry (AAS). Pb concentrations were used as tracers of industrial inputs (Blais & Kalff, 1993).

            Sub-samples from the two cores were dated through the analysis of the excess 210Pb, at the Laboratory of Geochemistry from the University of Nice, France. Excess 210Pb distribution in the cores were fairly consistent and gave an estimated sedimentation rate of 0.45 cm.yr-1, constant for at least the past 60 to 80 years.

 

RESULTS AND DISCUSSION

            Organic matter distribution in core Itatiaia II, are presented in Figure 2 a, b.  Organic matter content is constant along the core and ranging from 92 to 97%, except in the first top cm, where it drops to approximately 70%, probably due to higher oxidation at the sediment-water interface.  The extremely constant values of organic matter content suggest that water table fluctuations, which may result in the redistribution of Hg and Pb, is of minor, if any, importance to this lake.

            Distribution of Pb and Hg (Fig. 2) showed lower concentrations, of c.a. 40 µg.g-1, and 30 ng.g-1, respectively, from the bottom of the core to 23 cm. Pb increased steadily to 80 µg.g-1 in sub-surface layers. Hg concentrations increased to reach a maximum of 420 ng.g-1 at the 16-18 cm layer. Contrary to Pb, Hg concentrations began to decrease towards the top of the core starting around 8 cm of depth, corresponding to the beginning of the 1980's. Surface concentrations ranged from 20 to 40 ng.g-1.

            Decreasing Hg concentrations in the surface of lake sediment has been observed in lakes of the northern hemisphere, and in continental shelf sediments of the Rio de Janeiro coast. However, this decrease has not been observed in the Amazon gold mining areas, which show increasing Hg concentrations toward the top of the sediment profile. Surface Hg concentrations in the Itatiaia Mountains Lake are lower than in sediments under the influence of gold mining in the Amazon. In the Pantanal region, Central Brazil, Hg concentrations range from 60 to 120 ng.g-1. In the Carajás Mountains, SE Amazon, lakes similar to Itatiaia Mountains lake, present Hg concentrations in surface sediments ranging from 80 to 310 ng.g-1, while at the Alta Floresta, S Amazon, they ranged from 80 to 210 ng.g-1 (Lacerda & Salomons, 1998). These data confirm previous ones suggesting the larger importance of Amazon Hg emissions relative to industrial emission in Brazil.

Pb and Hg deposition rates are shown in Fig. 3.  As expected, Pb deposition rates, increased from nearly constant values of about 8.0±2.0 mg.m-2.yr-1, from 1910 to 1940, to about 20 mg.m-2.yr-1, between 1950 and 1980, and reached a peak at the sub-surface of about 40 mg.m-2.yr-1. Notwithstanding the typical industrial emission-influenced profile, Pb deposition rates are 10-30 times lower than in other industrialized regions, typically ranging from 400 to 1,000 mg.m-2.yr-1 (Blais & Kalff, 1993).

Hg deposition rates ranged from a relatively constant value from 1910 to 1940, of 36±4 µg.m-2.yr-1, increasing from 1940 onward, following the industrialization of the Paraíba do Sul River valley, and peaked in the 1960`s, of c.a. 120 µg.m-2.yr-1. Contrary to Pb however, Hg deposition decreased from the 1970`s to present, to 15 to 30 µg.m-2.yr-1, at the surface. This is probably due to emission control measurements implemented by that time, in particular the banning of Hg-containing agro-chemicals and changing chlor-alkali plants technology which has significantly reduced Hg industrial emissions in Brazil (Lacerda, 1997). The 5 to 6-times decrease in Hg deposition rates observed, compares well with the decrease in Hg industrial emissions from c.a. 150 t in 1979, to about 30 t in 1995 (Lacerda, 1997).

The temporal variation in Hg deposition rates observed is similar to those reported in different regions of the northern hemisphere, which also reported peak deposition occurring in the 1960`s or 1970`s (Porcella, 1996). Also, peak depositions in the Itatiaia lake compare well with those reported for remote lakes in Midwest USA (Engstrom & Swain, 1997) and the Great lakes by Pirrone et al. (1998) of 135 µg.m-2.yr-1).

Present day average Hg atmospheric deposition measured in the Itatiaia Mountains (15 - 30 µg.m-2.y-1), however, are similar to recent average deposition rates reported for North America (Pirrone et al., 1998) and from northern Europe (Iverfeldt et al., 1995), which ranges from 9 to 30 µg.m-2.yr-1. These deposition rates are much lower than those measured in areas receiving direct atmospheric effluents from industrial sources in this region of Brazil, which reach 76 µg.m-2.yr-1 (Marins et al., 1996).

            Concluding, Hg atmospheric deposition in the Itatiaia Mountains, provides a similar figure in South America as in North America and Europe, with values showing a significant decrease in response to emission control measurements of the last 15 years. Unfortunately, theses are only effective for Hg, since Pb atmospheric deposition, contrary to reports for the northern hemisphere, showed no similar decrease in present day values.

 

References

Blais JM, Kalf J. (1993), Biogeochemistry 23:1-22.

Engstrom DR, Swain EB (1997), Environ. Sci. Technol. 31:960-967.

Iverfeldt A, Munthe J, Brosset C, Pacyna JM. (1995), Water Air Soil Pollut. 80:227-233.

Lacerda LD. (1997), Water Air Soil Pollut. 97:247-255.

Lacerda LD, Salomons W. (1998), Mercury from Gold and Silver Mining: A Chemical time-bomb? Berlin, Springer-Verlag, Berlin.

Marins RV, Silva Filho EV, Lacerda LD. (1996) J. Braz. Chem. Soc. 9:77-81.

Pirrone N, Alegrini I, Keeler GJ, Nriagu JO, Rossmann R, Robins JA. (1998) Atmospheric Environm. 5: 929-940.

Porcella DB. (1996) Protocol for estimating historic atmospheric mercury deposition. Technical Report - TR - 106768, Palo Alto, EPRI.