GEOCHEMICAL STUDY OF HEAVY METALS IN BOTTOM SEDIMENTS AND BORDER SOILS FROM A LACUSTRINE SYSTEM OF CIMA LAKE, RJ, BRAZIL.

Marcelo G. de Almeida (marcelo@cbb.uenf.br); Cristina M. M. de Souza* and Luciano Guedes (Laboratório de Ciências Ambientais, CBB, UENF, Av. Alberto Lamego 2000, Horto, Campos dos Goytacazes, RJ, CEP: 28015-620, Brazil).

Abstract

The distribution of Fe, Mn, Zn, Cr, Ni, Cu e Hg in bottom sediments from a fluvial lake system and its adjacent areas (soils), was studied. The characterization of the system was complemented by granulometry distribution, carbon and nitrogen content and determination of the physic-chemical parameters in the water column. The concentration of metals were compared with non contaminated areas, characterizing a situation of background for the region. The exception was the high Hg levels, enriched by the fungicide application, since these substances had been used in a sugar cane plantation for a long time in the past.

Introduction

The lacustrine systems have been considered like deposition environments that often works as filters, retaining primary materials carried by rivers, like suspended particulate and dissolved substances. These materials originated by their watersheds, will be reflect the quality level of the system, since heavy metal contents have been associated with these compartments. (Lacerda,1994; Berner & Berner, 1996).

In Brazil, the most studies with mercury contamination and cycling has concerned with the Hg input due to gold mining activity in Amazon Region (Lacerda et al., 1988; Malm et al., 1990; etc.) and in the Pantanal (CETEM, 1989, etc). The use of organomercurial fungicides in agriculture as mercury source, has been very little investigated. The north of the Rio de Janeiro State, has it’s economy strongly dependent of large sugar cane plantations that widely used organomercurial fungicides in the past, before the banning the utilization in 1980 (Câmara, 1990).

The Cima lake with 14,2km² of area, is a tropical lacustre environment without manufactured pollution sources in it’s watershed. The system is supplied by two rivers (Imbé e Urubu) and drained to another Lake (Feia Lake) by the Ururai channel (Figure 1). During the last 40 years the original forest which was present along the lake, has been destroyed and replaced by pastures and sugar cane crops.

The aim of this work is to study heavy metals distribution in the lacustrine environment and adjacent areas. In this way, physic-chemical parameters in water column; the quantitative distribution of heavy metals (Fe, Mn, Zn, Cr, Cu, Zn and Hg), total Carbon and nitrogen in superficial soils (fraction<63µm) and bottom sediments (fraction<63µm), in both fluvial and lacustrine area, were considered . Bottom sediments were collected during different periods (may/96, october/96, february/97 and June/97) in six stations: Imbé River (IR), Urubu River (UR), Entrance Lake (EL), Middle Lake (ML), Lake Exit (LE) and Ururai Channel. Soil samples (n=19) were collected of different land uses around the hydric system, in a dried period.

Figure 1: Cima Lake Hydric System and Sample Localization.

Methodology

The soil and bottom sediment were analyzed in a fine fraction (<63µm). The wet digestion procedure was performed in triplicate, based on Krause et al.(1994). Heavy metals content (Fe, Mn, Zn, Cr, Ni, Cu) were measured by atomic absorption spectrophotometry. A certified fluvial sediment (Standard Reference Material 2704 - NIST) was used for quality control of the analytical procedure. The recovery levels showed more than 80% for all elements and variation coefficient for triplicates, bellow than 10%. Mercury was determined in ICP – AES, after acid digestion, described by Bastos et al. (1998), with vapor generating accessory (VGA 77). The detection limit of the method is 0,5 ngHg.g^-1. Analysis performed with an internal standard soil provide by the IBCCF/UFRJ, showed a recuperation average around 99%.

Results And Discussion

The representatives averages of environmental parameters (Table 1) present the following intervals pH=6.13-7.86; percentage saturation of O₂=68.7-109.5; conductivity (µS.cm^-1)=30-45; Eh (mV)= 248-311; T (ºC)=23.8-26.3; SPM (mg.L^-1)=2.6-9.4. It can be observed one increase of pH and percentage saturation of O₂in interior of Lake, when compared with tributaries. The carbon and nitrogen concentration was bigger in lacustrine area than tributaries. The elevate residence time of the water masses in addition to the low energy inside the lake, promote the deposition of the tributary suspended material. At the same time that autoctone particulate matter was produced, mainly due to primary production.

The intervals of metal concentrations showed the following variation range to bottom sediments (fraction<63µm) and soils (fraction<63µm), respectively: Fe=2.9-11.7//0.82-7.6%; Mn=105-2577//30-610µg.g^-1; Zn=97-231//36-215µg.g^-1; Cr=23-52//19-77µg.g^-1; Ni=31-58//18-47µg.g^-1; Cu=14-32//8.6-42µg.g^-1; Hg=113-239//70-212ng.g^-1. The results obtained in bottom sediments showed an accumulation tendency of Fe, Mn, in Lacustrine area (LE), what can be attributed the metabolic activity of phytoplancton that promote changes physic-chemical parameters (O₂ dissolved and pH) in water column, affecting the partitioning of redox-sensible elements among compartments, favoring their deposition. The concentrations observed in soil suggest erosion and runoff mechanisms like significant sources of metals for some regions of the lake. The concentration of metals (Fe, Mn, Zn, Ni, Cr and Cu) was compared with non contaminated areas, characterizing one situation of background for the region.

Table 1: Some Chemical and physical characteristics of the water column. Average (n=4) and standard deviation.

Site

Cond.

(mS.cm^-1)

(º C)

(mV)

% Sat.

O₂

Prof.

(m)

SPM

(mg.l^-1)

29.6

(2.3)

23.8

(3.0)

311

(98.7)

6.13

(0.58)

88.1

(6.61)

0.92

(0.4)

9.41

(5.4)

45.0

(8.3)

25.4

(3.4)

301

(58.6)

6.17

(0.22)

68.7

(6.46)

1.53

(0.3)

2.62

(1.12)

29.9

(1.9)

25.3

(2.3)

238

(21.2)

6.39

(0.22)

71.4

(18.8)

0.82

(0.4)

31.3

(2.8)

26.1

(2.0)

260

(4.0)

7.76

(0.72)

100

(6.02)

2.99

(0.3)

7.25

(0.87)

31.6

(2.8)

26.3

(1.7)

259

(28.0)

7.79

(0.91)

102

(6.54)

1.35

(0.5)

31.8

(1.9)

26.1

(1.9)

248

(28.4)

7.86

(0.98)

110

(12.16)

1.31

(0.5)

6.62

(1.21)

Table 2: Concentrations of Heavy Metals (µg.g^-1), total C and N (Average and standard derivation).

SITE

Fe*

Hg**

f***

(<63µm)

Soil (n=19) SA

4.06

(2.10)

224.8

(167.7)

117.9

(55.6)

40.1

(15.8)

29.5

(8.98)

20.2

(8.28)

151

(57.1)

2,45

(2.39)

0.62

(0.39)

6.14

(6.04)

Sediment

(n=4) TA

Fe*

Hg**

f***

(<63µm)

8.25

(0.28)

1717

(423)

231

(46.0)

46.8

(8.5)

58.0

(4,6)

15.7
(2.7)

168

(45.7)

1.0

(0.8)

0.31

(0.04)

2.66

(0.44)

2.87

(0.52)

105

(28.0)

108

(28.2)

23.2

(3.3)

31.4

(3.4)

14.4

(3.1)

239

(34.9)

14.0

(11.0)

0.52

(0.09)

6.18

(1.09)

5.05

(1.63)

446

(202)

128

(45.1)

30.5

(8.7)

39.1

(3.1)

15.3

(1.1)

170

(17.0)

23.1

(34.0)

0.48

(0.27)

5.87

(4.44)

4.26

(1.27)

267

(116)

96,8

(16.0)

31.7

(11.7)

39.5

(3.1)

15.2

(0.75)

185

(33.0)

75.2

(26.3)

0.36

(0.09)

3.73

(0.11)

11.7

(5.81)

2577

(2078)

131.3

(62.7)

33.0

(14.0)

44.1

(11.7)

18.9

(7.9)

112.7

(38.9)

16.4

(27.1)

1.06

(0.64)

8.08

(3.85)

10.0

(3.09)

1804

(117)

124

(52.4)

52.4

(10.8)

54.1

(13.9)

32.3

(10.6)

(27.8)

3.5

(57.1)

1.08

(0.54)

7.90

(5.0)

* Concentrations in %. ** Concentrations in ng.g^-1. *** Silt-clay fraction in percentage. SA= Spatial average between sample points of; TA= temporal average between 4 sampling (may/96, october/96, february/97 and June/97).

The exception was the highest Hg values observed in sediment samples (up to 230ng.g^-1) and soil (up to 210ng.g^-1) when compared with background level to sediment of non-contaminated Amazon Rivers (20-100ng.g^-1) (Lacerda & Pfeiffer, 1992) and the world average level for soils (70ng.g^-1) (Bennet, 1981). The Urubu river drains a more fertile and plain area where there are many sugar cane crops. By the other side the watershed of Imbé River with mountains difficult this culture.

In conclusion, the watershed of Urubu river with more elevated Hg concentration (239 ng.g^-1) should have two sources: deposition atmospheric and deposits of organomercurial in sugar cane crop. By other side, the watershed of Imbé River with mountains is probably the biggest source of others metals (Fe, Mn, Zn, Cr, Ni and Cu) through geologic weathering. The results strongly suggest that the mercury was used as fungicide in the past has been transported to the adjacent ecosystems, mainly through atmospheric deposition. As mercury could remain in soil for more than a hundred years, its very probably that this element had already been spread all over the region. The main pathway seems to be the atmospheric deposition of particles originated from the burning of the sugar cane plantations before harvesting, although the require of more studies are latent in order to elucidate this pathway.

References

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Acknowledgments

We would like to thank CNPq (processo nº521466/95-4 to C.M.M. Souza) and FENORTE by Financial Support.