HEAVY METALS DISTRIBUTION IN
BURNT SOILS OF A RAIN FOREST AREA IN RORAIMA, BRAZIL.
Marcelo T. Nascimento;
Cristina Ma M. Souza; Luiz E. O. C. de Aragão; Dora Ma
Villela; Carlos E. de Rezende & Marcelo, G. de Almeida (Laboratório de
Ciências Ambientais, Universidade Estadual do Norte Fluminense, Av. Alberto
Lamego, 2000, CEP: 28015-620, RJ, Brazil, cristal@cbb.uenf.br)
A study on heavy metals distribution in burnt soils was
carried out, in a forest area in Roraima State one month after the worst fire
in the Amazon region. Burn forest areas has been an increasing activity in the
Amazon area as consequence of deforestation processes for human occupation. The
results showed that total concentrations of Al, Fe, Mn, Cr, Zn, Cu and Ni, were
in natural levels, but Pb, Cd and Hg values were higher. The sequential
extraction did not show any differences between the metals distribution in
burnt and unburnt areas.
Land use activities have been enhanced an intensity human
occupation in rural areas. In Brazil, some regions still occupied with
indigenous inhabitants have been devastated and intensely burnt. Fire has
became more usual in Amazon, especially
by small farmers to clean the soil for selective wood exploiting,
putting in danger adjacent areas. Nowadays, this practice is extending over the
whole in Brazilian Amazonia, from Rondonia to Maranhão.
Just in the last two decades several studies on natural
and anthropogenic heavy metal emission have been done. There has been a growing
recognition of the need to quantify natural sources and cycling regional
processes of metals (Lacerda & Salomons, 1991). This study aimed to compare
the heavy metal contents and their availability in burnt and unburnt soils on
Amazon area (Roraima state).
The Roraima state is characterized by a tropical wet and
sub-equatorial climate, with temperature between 200C and 380C
and annual average precipitation between 1000mm and 2250 mm. “El Niño”
phenomenon in 1997 changed the rainfall regime causing a very strong dry
season.
The study area was located in the “Trairão” Village in Roraima State, north of Brazil. It’s economy is mostly related with agriculture and cattle breeding, with a increasing perspectives for immigrants. The severity of the fire in this region was due to “El Niño” event that prolonged the dry season associated with low relative air humidity and absence care to prevent the fire (Nascimento et al., 2000). Four transects of 1Km long each were set up (November, 1999) in the area. Transects varied from undamage to severely damage (understory totally burnt) rainforest. Four plots (10m x 50m) in each transect was established with intervals of 250m.
Bulked soil samples were collected in each area, in the
first 10 cm with plastic trowel and packed in plastic bag until the laboratory.
The samples were homogenized, considering grain size smaller than 2mm, oven
dried (400 C) and weighted for chemical analysis. To determine the
granulometry distribution inside the plots, an aliquot (100g) of the total soil
was used (ABNT/NBR 7181, 1984).
The extraction method for total metals was performed
using 1g ±
0.001g of dried samples placed in teflon bomb with acid mixture HF/HNO3
in an oven (1100C) during 12h (Krause et al., 1995). For mercury,
the samples were wet digested using an oxidant mixture HCl/HNO3,
(3:1) and KMnO4 5% (Bastos et al., 1998). To certify the quality
control an internal soil sample supplied by Biophysics Institute in Rio de
Janeiro (Radioisotope Laboratory) was performed. Total carbon was determined
using CHNS analyser (Perkin Elmer).
Sequential extraction based on Ure et al., (1993) was
performed, allowing the metals speciation in four geochemical phases:
exchangeable, reducible, oxidazable and residual. For all studied metals more
than 80% was recovery, with variation coefficients for analytical replicates
bellow than 10%. All metal determinations were performed by ICP - AES (Varian
Liberty Series II), using a VGA accessory for Hg.
The average granulometry distribution of the soil samples
showed that the soils area are sand (table 1) with low variation coefficient.
This fraction is mostly represented by fine sand fraction. Coarse material was
not found in the analysed samples.
Table 1: Granulometry Distribution (%) in Trairão Soils and Roraima
Sampling Point Coordinates. Average and variation coefficient (%).
|
Transects |
Granulometry Distribution |
Coordinates |
|||
|
|
Stone |
Sand |
Silt /Clay |
|
|
|
T1 |
----------- |
82 ± 5 |
18 ± 2 |
30 33’ 10” N 610 49’ 39” W |
Azimute 2900 |
|
T2 |
---------- |
79 ± 2 |
21 ± 3 |
30 33’ 12” N 610 54’48” W |
30 33’ 22” N 610 55’20” W |
|
T3 |
---------- |
77 ± 2 |
23 ± 3 |
30 38’ 04” N 610 51’19” W |
30 37’43” N 610 50’48” W |
|
T4 |
---------- |
84 ± 4 |
16 ± 3 |
30 37’ 26” N 610 50’20”W |
30 37’ 09” N 610 52’ 07” W |
The average concentrations for all metals in the samples
are presented in table 2. In general the heavy metal concentrations showed the
following distribution: Al >Fe > Mn > Cr > Zn > Pb > Cu >
Ni > Cd > Hg.
The magnitude of the concentrations obtained for mostly
metals are in agreement with other areas (Azcue, 1987; Houtmeyers, 1985) and
below that considered for world average, except Pb, Cd and Hg in transect 2,
enriched up to five times for Cd and two times for Hg and Pb. It is noteworth
that this transect was not burnt at all, this transect represents the reference
background for the area. All the concentrations measured along this transect
was higher than the others, including total carbon (%).
Although the fire varied in intensity along the transects
(Nascimento et al., 2000), it was not
verified differences among the burnt areas. However, the transect 3 showed the lowest concentrations for some
metals such as Al (4361 mg.g -1), Fe (3932
mg.g -1 ), Cr (6.0 mg.g -1 ), Cu (2.2 mg.g -1 ) and Cd (0.21 mg.g -1 ). In fact, this was the transect that had its
understory totally destroyed by the fire.
The high variation coefficient found in all transects (burnt and unburnt areas), showed the high environmental variability that occurs in the soils
Table 2: Average, Variation Coefficient (%) and Concentration Ranges of
Heavy Metals in Soils (mg.g -1 ).
|
Metals |
Transects |
world average* (mg.g -1 ) |
|
|||
|
|
T1 |
T2 |
T3 |
T4 |
|
|
|
Al |
4666 ± 40 (2798 - 6051) |
13452 ± 55 (4194 – 22197) |
4361 ± 91 1860 - 10305 |
7612 ± 28 4921 - 10207 |
--------- |
|
|
Fe |
5507 ± 35 (3498 - 5670) |
7252 ± 97 (1798 – 17187) |
3932 ± 50 (2032 - 6702) |
5101 ± 31 (3780 - 7229) |
38000 |
|
|
Mn |
88.9 ± 51 (42.2 - 135) |
707 ± 82 (273 - 1545) |
109 ± 90 (55 - 148) |
182 ± 68 (71 - 346) |
850 |
|
|
Cr |
11.1 ± 104 (3.2 - 28.4) |
26.0 ± 66 (4.7 – 46.9) |
6.0 ± 34 (3.7 - 8.5) |
10.6 ± 33 (5.8 - 14.0) |
100 |
|
|
Zn |
8.1 ± 33 (5.8 - 11.9) |
22.7 ± 56 (10.1 - 40.4) |
10.9 ± 43 (4.3 - 15.4) |
14.1 ± 21 (11.2 - 18.1) |
50 |
|
|
Cu |
2.8 ± 49 (1.5 - 4.7) |
8.7 ± 80 (2.9 – 18.8) |
2.2 ± 36 1.3 - 3.1 |
3.0 ± 38 (1.9 - 4.6) |
20 |
|
|
Pb |
3.3 ± 41 (1.4 - 4.5) |
19.0 ± 115 (5.0 – 51.7) |
4.5 ± 31 (2.5 - 5.5) |
6.3 ± 35 (4.2 - 9.1) |
10 |
|
|
Ni |
6.1 ± 101 (2.2 - 15.2) |
6.3 ± 75 (2.1 – 12.6) |
1.3 ± 60 (0.81 - 2.4) |
0.75 ± 20 (0.58 - 0.94) |
40 |
|
|
Cd |
0.33 ± 67 (0.08 - 0.62) |
0.33 ± 82 (0.08 - 0.69) |
<DL - 0.63) |
0.36 ± 76 (0.15 - 0.75) |
0.06 |
|
|
Hg |
0.03 ±36 (0.01 - 0.04) |
0.06 ± 38 (0.03 - 0.08) |
0.04 ± 59 (0.01 - 0.06) |
0.03 ± 16 (0.03 - 0.04) |
0.03 |
|
|
C % |
0.90 - 1.3 |
1.1 - 5.9 |
1.9 - 3.1 |
1.2 - 2.3 |
----- |
|
|
C/N |
15 - 18 |
12 - 13 |
14 - 18 |
11 - 15 |
----- |
|
<DL=
lower than detection limit of the instrumental analysis * Fórstner & Wittmann, 1984
The
results obtained from sequential extraction are presented in table3. The
transects 2 (unburnt) and 3 (burnt) were choose. All metals in natural area (T2) presented a predominant distribution in
residual phase (higher than 70%), except Mn (47%). This association with
residual phases indicates natural sources for these elements (Förstner &
Wittmann, 1984).
Table 3: Percentage (%)
distribution of heavy metals in
geochemical phases of soil.
|
Phases |
Metals |
|||||||||
|
|
Al |
Fe |
Mn |
Cr |
Zn |
Cu |
Pb |
Ni |
Cd |
|
|
|
Exc |
0.5 |
-------- |
1 |
------- |
-------- |
24 |
-------- |
8 |
------- |
|
T3 |
Red |
1 |
3 |
39 |
------- |
2 |
18 |
13 |
6 |
------- |
|
|
Oxi |
0.5 |
-------- |
10 |
16 |
18 |
19 |
-------- |
17 |
------- |
|
|
Res |
98 |
97 |
50 |
84 |
80 |
39 |
87 |
69 |
100 |
|
|
Exc |
0.1 |
--------- |
15 |
-------- |
1 |
14 |
------- |
7 |
------- |
|
T2 |
Red |
0.5 |
1 |
35 |
-------- |
2 |
7 |
13 |
9 |
4 |
|
|
Oxi |
0.4 |
1 |
3 |
21 |
6 |
7 |
------- |
14 |
------- |
|
|
Res |
99 |
98 |
47 |
79 |
91 |
72 |
87 |
70 |
96 |
Exc =
exchangeable; Red = reducible; Oxi = oxidazable; Res = residual
Manganese
showed similar percentages between the
transects, with a good relation in reducible phase, representative by manganese
and iron oxides. The elements Cr (16 and 21%), Zn (18 and 6%), Cu (19 and 6 %)
and Ni (17 and 14%) had some association with oxidazable phase in both
transects (T3 and T2,
respectively), suggesting the chemical affinity to form organic matter
complexes.
The
results found from sequential extraction did not show a relationship between
the burnt areas and the metal phase associations, since the distribution
observed was similar between the two
transects. Although Mn, Cu and Pb showed the highest differences between
unburnt and burnt areas, it was difficult to confirm their lost, since the
environmental variability is high inside the transect.
The
higher concentrations for Cd, Hg and Pb compared with the world average values,
suggest the need of further studies for these metals in the area. The
dispersion mechanisms specially for Hg, are probably related to anthropogenic
sources such as gold mining activities, spread out in Amazon region in the
80´s. The Hg released to the atmosphere during the burn amalgam may be
transported to the study area, that is located
near to the “Santa Rosa” gold mining. Since Hg could remain in the soil
for more than a hundred years (Lacerda & Salomons, 1998), it is very
probably that this element have already been dispersed all over the region.
Acknowledgment: to Guttemberg de Oliveira head of Maracá ecological field station (IBAMA) for the loggistic support and IBAMA (DF) for finantial support to field experiments.
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