MERCURY AND CARBON DISTRIBUTION IN SURFACE SOILS FROM TWO DISTINCT ALTITUDES IN A TROPICAL RAIN FOREST IN THE NORTH OF RIO DE JANEIRO STATE, S.E., BRAZIL.

 

Primo, W.S.; Carvalho, C.E.V.; Vilella, D.M.& Mazurec, A.P. (Laboratório de Ciências Ambientais, CBB, UENF. Av. Alberto Lamego 2000, Horto, Campos dos Goytacazes, R.J. CEP: 28.015-620. Brazil. Tel: (24) 726-3709, Fax: (24) 726-3720,  carvalho@cbb.uenf.br)

 

Abstract

The present work studied the mercury distribution in soils from two different altitude (250 m and 50 m) in order to evaluate the mercury contamination in the an  Atlantic Forest located at the Desengano State Park. Soils were collected from five parcels (30 X 40m) in each altitude (250 and 50m), with four sample per parcel. The highest Hg values were observed in samples collected in the parcels locate at 250 m (292 µg.kg-1), where the highest carbon values (9,0%) were also observed. The 50 m areas, less wooded, presented the lowest Hg (199 µg.kg-1 ) and carbon concentrations (3,4 %). The average Hg concentration for all the two altitude areas was considered very high when compared to the regional background soils levels (40 µg.kg-1). The results strongly suggests that the mercury that was used as fungicide in the past has already been transported to the adjacent ecosystems (Imbé forest). The main pathway seems to be the atmospheric deposition of particles originated from the burning of the sugar-cane plantations before harvesting.

 

Introduction

 

            The metals and their compounds are indispensable to economy of many countries and this have had key factory to this release by modern civilizations. Few of know metals don't have any application by the industry, and the commercial use has grow with the science and technology development (Nriagu, 1988).

The mercury contamination in North of Rio de Janeiro Estate can be divided in two main sources: a) gold mining activity that in a short time of operation, caused a significant impact in the environment (Lima 1990; Lacerda et al.,1993 and Souza 1994) and b) the organo-mercurial fungicide widely used in sugar cane plantations (Camara, 1990), the main economic activity in this region.

 Ancient agricultural methods, like burning the sugar cane plantation before the harvest, are probably a very important pathway for Hg transport to adjacent ecosystems. Although the Hg had a short permanence time in the atmosphere,   around 40% of the total  give out rapidly attain near the source in some kilometers (Lacerda & Salomons, 1991). As mercury could remain in soil for more then a hundred years (Semu, 1986, Semu & Singh, 1987), it is very probable that this element had already been spread all over the region.

           

Sampling Site

The Imbé mountains are located in North of Rio de Janeiro State (21o48`427``S, 41o40`775``W) (Figure 1). The local climate is characterized by dry winter and wet summer (Cfa). The annual rainfall is 1400 mm and the annual temperature is 19o C (SOS Mata Atlântica, 1991; UFRRJ/Pró-Natura; 1994). The local soils were fine and irregular, with high quantities of original rocks fragments, the soil type was classified by RADAMBRASIL (1983) as “Cambissolo Álico” with change form clay to medium clay. The vegetation is humid tropical rainforest with the canopy of 25m high and very well preserved at the study sites (RADAMBRASIL, 1983). Two sites in different altitudes were sampled: (1) at 50 m a.s.l.  (above sea level) and (2) 250 m a.s.l. In site 1 (50 m) the  forest presented an accentuate declivity when compared to the site 2 (250 m), although the last one presented a more diversified micro-relief. The 50 m forest soils are less acids (pH = 5.0) then 250 m forest (pH = 3.6).

 

 


 


Figure 1. Sampling areas are marked by a circle (Desengano State Park Map modified from, IEF, 1996).

 

Material and Methods

 

Soil samples were collected from five parcels (30 X 40m) to each altitude (250m and 50m), four samples were collected per parcel, with a plastic trowel, and packed in plastic bags during transport to the laboratory. The soils were oven-dried (40ºC) and wet digestion in triplicate, using the procedure modified from Bastos (1998). Mercury was determinate by an ICP/AES with vapor generating accessory (Varian, Liberty II). The detection limit of the method used was 0.5 µg.kg-1. Analysis performed with standard soil sample, provided by the IBCCF/UFRJ, showed a recuperation average of 99%.

Results and Discussion

 

            Table 1 present the average Hg concentrations and  standard deviation found in both study areas. The soil from sampling site 2 (250 m) presented the highest concentrations for Hg and carbon when compared to sampling site two although both sites were located in the same forest.

 

Table 1. Average Hg and carbon concentrations for both studied areas.

Sampling sites

250 m

50 m

Hg (µg.kg-1)

220 (± 38)

147 (± 47)

C (%)

6.0 (± 1.5)

3.5 (± 1.0)

 

Comparing the concentration observed for each altitude studied it is possible to speculate that differences are probably due to two main factors: a) the higher organic matter content present in these soils and  b) physical factors like the lower weathering of soils with lower forest cover. According to Lodenius (1987), in soils, mercury is usually strongly bound to organic matter and particles. This binding is almost unaffected by fluctuations in pH and chloride concentration. Lacerda et al. (1995) studying tropical Amazonian soils showed that the relation between mercury concentration and the organic matter, is so not important, at least in the Amazon region. According to these authors, in this case physical factors were more important that the chemical factors in explaining mercury dispersion and retention. Our results suggests that chemical factors seems to be very important, if we consider that the site 2 (250m) presented the highest carbon and Hg concentrations.

Higher Hg concentrations were observed in the Imbé Forest soils when compared to the background levels for the area (Table 2). This trend strongly suggests that the atmospheric transport of this element is an important step in the biogeochemical cycle of this element in the studied environment. The atmosphere is recognized to play an important role in global transport of this volatile pollutant (Hachiya, 1998). The high volatility of elemental mercury is responsible to a efficient dispersion though the atmosphere. As a result of its volatility, a significant reemission from receiving surface occur (Lodenius  & Malm, 1998). In the atmosphere Hg0 is transformed under the influence of sunlight and ozone into the soluble form (Hg+2) that is transferred by wet and dry deposition into the soil (Gosar et al., 1997).

             

Table 2. Comparative table showing others studies and the present .

Local

Hg (µg.kg-1)

Reference

Forest soils near to Madeira river (Amazon)

30 – 340

Malm et al. (1991)

Forest soils near to gold mining area (Amazon)

420 – 9900

Malm et al. (1991)

Paconé soils (Mato Grosso, Brazil)

50 – 180

Lacerda (1991)

Canada soil average

70

Mitra (1986)

Europe soil average

300

Mitra (1986)

Tropical Forest soil (Rondônia, Brazil)

100 - 950

Pfeiffer et al. (1991)

Average forest soils of Imbé

118

Present work

Average of soils in north of Rio de Janeiro Estate 

66

Primo (2000)

 

Conclusion

 

            The Hg concentrations found in soils of both studied sites at the Imbé Forest could be considered high when compared to local background levels. This contamination is probably related to the late use of mercury fungicides in the sugarcane plantations located in the vicinities of the studied areas. The ancient harvesting methodologies (burning the plantations before harvesting) probably enlarged the Hg input to the atmosphere and consequently the transport to adjacent areas. The preference winds blows in the implies in a large atmospheric mercury dispersion.               

 

References

 

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