mercury, copper and zinc contamination in soils and fluvial sediments from an abandoned gold mining...

ORIGINAL ARTICLE

Mercury, copper and zinc contamination in soils and fluvialsediments from an abandoned gold mining area in southern MinasGerais State, Brazil

Ricardo Cesar • Silvia Egler • Helena Polivanov •

Zuleica Castilhos • Ana Paula Rodrigues

Received: 4 June 2009 / Accepted: 30 October 2010 / Published online: 17 November 2010

� Springer-Verlag 2010

Abstract Mercury, zinc and copper contamination was

evaluated in soils and fluvial sediments from an abandoned

gold mining site at Descoberto Municipality (southern

Minas Gerais State, Brazil). Metals bioavailability and

potential mobility were studied through physical, chemi-

cal and mineralogical characterization, geoaccumulation

indexes calculations, mercury speciation and determination

of potentially bioavailable contents of zinc and copper.

Values of pH were in the neutral range, while organic

matter concentrations were very low. Mineralogical char-

acterization, in the total samples, indicated the presence of

quartz, kaolinite and gibbsite for all samples. Total mer-

cury, zinc and cooper concentrations were higher than the

limits recommended by Brazilian documents. Geoaccu-

mulations indexes revealed that most of the sediment

samples were low to moderately polluted by zinc and

copper, while just one sample was very strongly polluted

by mercury. Mercury speciation revealed the predomi-

nance of the elementary form for all samples, and low

concentrations for exchangeable, strongly bound and

residual fractions. Zinc and copper behavior was strongly

controlled by iron and aluminum concentrations, while

their bioavailable contents were very low in comparison

with the total concentrations.

Keywords Heavy metals � Bioavailability � Soils �Fluvial sediments � Gold mining

Introduction

Over the last decades, heavy metals contamination has

become an extremely important subject for the scientific

community, due to its diverse damage functions on the

environment. Anthropogenic contamination sources are

usually related to inadequate management of domestic

wastes, chemical, petrochemical, metallurgical and mining

industries, as well as agrochemicals application (Solomons

and Forstner 1984). Important lithogeochemical sources

are associated with hydrothermal deposits and weathering

of sulfide-bearing rocks (Reimann and Garret 2005).

Another potentially strong geogenic source is the volcanic

eruptions, which may liberate high concentrations of heavy

metals to the atmosphere (Matschullat 2000).

The biogeochemical behavior of heavy metals in the soil

and fluvial sediments is related to physical, chemical and

mineralogical properties of those compartments, such as

pH, organic matter content, oxidation potential, electric

conductivity, clay minerals, Fe and Al oxi-hydroxides and

texture (Yin et al. 1996; Hylander et al. 2000; Abollino

et al. 2007). Distinct sources of pollution associated with

specific chemical forms can also affect those processes

(Katz and Salem 1993; Peakall and Burger 2003; Peijn-

enburg and Jager 2003). The understanding of such

R. Cesar (&) � S. Egler � Z. Castilhos � A. P. Rodrigues

Laboratorio de Ecotoxicologia Aplicado a Industria Mınero-

Metalurgica, Servico de Desenvolvimento Sustentavel,

Centro de Tecnologia Mineral, CETEM/MCT, Av. Pedro

Calmon, 900. Cidade Universitaria, Rio de Janeiro, RJ, Brazil

e-mail: [email protected]

R. Cesar � H. Polivanov

Departamento de Geologia, Setor de Geologia de Engenharia

e Ambiental, CCMN-Instituto de Geociencias, Universidade

Federal do Rio de Janeiro, UFRJ, Av. Athos da Silveira Ramos,

274-Cidade Universitaria, Rio de Janeiro, RJ, Brazil

A. P. Rodrigues

Departamento de Geoquımica Ambiental, Instituto de Quımica,

Universidade Federal Fluminense, UFF, Outeiro Sao Joao

Baptista, s/n. Centro, Niteroi, Rio de Janeiro, RJ, Brazil

123

Environ Earth Sci (2011) 64:211–222

DOI 10.1007/s12665-010-0840-8

mechanisms is extremely important for determining

mobility, availability and toxicity of those elements in the

environment (Caussy et al. 2003), as well as to assess their

risks on human health and biota.

Most of critical areas of mercury (Hg) pollution are

associated with its use in the gold extraction. Hg liberated

during such processes can escape to the atmosphere, and

thus deposit on soils and fluvial systems (Roulet and

Lucotte 1995; Wasserman et al. 2003). Hg presence in the

aquatic system has a particular importance due to its high

potential biotransformation to methylmercury, which is

neurotoxic, teratogenic and much more toxic than the

inorganic forms (WHO 1990; Castilhos et al. 2001; Ohriel

et al. 2008).

In gold mining areas, during gravitic concentration

processes, other chemical elements with high density, such

as zinc (Zn) and copper (Cu), can also be concentrated,

generating geochemical anomalies and contamination hot

spots (Rodrigues-Filho and Maddock 1997). Important

sources of Zn and Cu pollution are also related to fungi-

cide, pesticides and herbicides applications, which usually

contain high levels of Zn salts and Cu arsenates (Gimeno-

Garcıa et al. 1996; Sharma et al. 2007). On the other hand,

Zn and Cu play a crucial role in the metabolism of many

organisms (Sharma et al. 2007). Damages on biota are

usually associated with the exposure to high concentrations

(Lukkari et al. 2005).

The identification of abandoned mines and their passives

is a challenge and one of the most important priorities for

environmental authorities in the Minas Gerais State

(southeastern Brazil). In the 19th century, Minas Gerais

was extensively explored by gold mining activities and, as

consequence, there are still many non-identified abandoned

passives and contamination hot spots, especially in the Iron

Quadrangle region (Matschullat et al. 2000; Mello et al.

2006; Windmoller et al. 2007). In this respect, indiscrim-

inate uses of Hg are historically well documented in Minas

Gerais (Windmoller et al. 2007), especially in small-scale

gold mining sites for processing alluvial deposits. In the

Brazilian Amazon, e.g., residues from abandoned gold

mining sites can contain more than 4.4 mg/kg of Hg

(Yallouz et al. 2008). Leaching and erosion processes can

mobilize such materials, contaminating surrounding soils

and aquatic ecosystems.

Descoberto Municipality could be used as good general

model for subsiding the understanding of impacts related to

old gold mining activities and contamination hot-spots in

southeastern Brazil. Descoberto represents, at a small-scale,

an excellent example of an abandoned gold mining site in

Minas Gerais, since it presents many similarities with the

activities historically performed in the Iron Quadrangle

region. Contaminated lands by old mining activities are

nowadays occupied by rural populations, besides being

widely used in the agriculture and for providing drinking

water. In addition, important ecosystems (Mata Atlantica

forest) were highly degraded by mining activities, and its

conservation strongly depends on the characterization of

contamination hot spots on the surrounding areas.

This paper proposes the study of the bioavailability and

potential mobility of Hg, Zn and Cu contamination in soil

and fluvial sediments from an old gold mining site in

southern Minas Gerais State, Brazil.

Materials and methods

Study area

In the early 19th century, rural areas of Descoberto

Municipality (southern Minas Gerais State, Brazil—

21�2703600S, 42�5800400W, 620 m elevation) were submit-

ted to gold extraction, using rudimentary techniques of

mineral processing. In 2002, local population detected

metallic Hg presence on soils surface, and in 2005 a small

critical area of contamination (with 8,000 m2) was inter-

dicted by public institutions of research (Branco et al.

2005). A small creek, known locally as Corrego Rico,

drains this critical perimeter (Fig. 1) and it is a tributary of

Paraıba do Sul River Basin, one of the most important

rivers of southeastern Brazil. Until its interdiction, this

critical area of contamination used to submit to livestock

and agricultural activities. In this area, some authors

determined total Hg concentrations in soils, sediments and

plants (Branco et al. 2005; Carvalho-Filho et al. 2005;

Egler et al. 2008; Cesar et al. 2008). Until this moment,

Cesar et al. (2008) were the only authors who determined

methylmercury, zinc and copper contents in distinct gran-

ulometric intervals of a soil sample and in earthworms

(Eisenia andrei) exposed to such materials. Thus, those

previous works basically involved preliminary mappings of

Hg contamination and some initial studies of ecotoxicity.

Zn and Cu distribution and speciation data, e. g., were not

the most important interests of such researches.

Descoberto Municipality is located at a transition area

between two well-defined geological unities: Complexo

Juiz de Fora and Megassequencia Andrelandia. The critical

contamination perimeter is located on the gneissic rocks of

Megassequencia Andrelandia, whose essential mineralogy

is composed by plagioclase, potassic feldspar, grenada,

quartz, biotite, sillimanite, and orthopyroxene. These rocks

consist in the parent material for the soils, which are fre-

quently transported through the slopes.

Descoberto also has expressive bauxite deposits, now-

adays explored by Companhia Brasileira de Alumınio

(CBA). Branco et al. (2005) report the existence of gold

from primary origin associated with thermal events, while

212 Environ Earth Sci (2011) 64:211–222

123

in the 19th century gold alluvial deposits were widely

explored.

Samples

Sampling campaign was performed in July/2007. Sampling

point’s selection was based on a preliminary Hg contami-

nation map, emitted by Branco et al. (2005), and the

sampling collection was preferably performed in critical

areas of contamination (Fig. 1). Soil and fluvial sediments

were collected superficially (20 cm), since Branco et al.

(2005) indicated that Hg contamination was restricted to A

horizon (the top layer of the soil profile). 22 samples (11

soils and 11 sediments) were collected. In the field, sam-

ples were maintained under a temperature of about 5�C

immediately after the sampling, and at the laboratory they

were air dried and disaggregated. Afterwards, samples

were fractioned using nylon sieves of 1,700 lm (in order to

remove stones, roots and other larger particles), homoge-

nized and sent to chemical and mineralogical analysis.

In order to study the influence of distinct grain sizes in

the Hg distribution, samples were again fractioned using

sieves of 75, 150 and 180 lm. Thus, the following gran-

ulometric intervals were generated: 1,700–180, 180–150,

150–75 lm and \75 lm.

Determination of pH and organic matter

pH was measured in water (1:2.5—soil:water), according

to EMBRAPA (1997) procedures. Organic carbon contents

were quantified by elementary analysis, using LECO SNS-

2000 equipment. Organic matter concentrations were

obtained through the multiplication between organic car-

bon content and a factor of 1,724 (EMBRAPA 1997).

Mineralogical characterization

The identification of mineral groups was performed by X-ray

diffraction. The X-ray Diffractograms (XRD), obtained by

powder method, was collected on a Bruker-D4endeavor

Soil sampling points

Sediment sampling points Marked area: interdicted perimeter

Fig. 1 Geographical localization of Descoberto Municipality (MG, Brazil), including soils and fluvial sediments sampling points

Environ Earth Sci (2011) 64:211–222 213

123

equipment with radiation Co Ka (35 mA kV/40). The

qualitative interpretations of the spectrum were made by

comparison with the standards contained in the database

PDF02 (ICDD), Bruker software DiffracPlus.

Mercury determination

Total mercury (HgT) determination was performed using

LUMEX equipment (R A 915 ?), an atomic absorption

coupled to the thermodesorption technique. To guarantee

the quality of results, samples were simultaneously analyzed

with certified materials (NIST 2709 San Joaquin Soil).

Hg speciation followed the method proposed by Lechler

et al. (1997), which was only applied to the \75 lm

fraction. This method firstly consists in the heating of the

sample under a temperature of 180�C. The difference

between HgT and Hg determined in this sub-sample cor-

responds to metallic Hg concentration. Such sub-sample is

submitted to an extraction using MgCl2 (0.05 M), for

determining exchangeable Hg fraction. After that, the res-

idue of previous step is extracted using an acid solution

composed by HCl (0.5 M), and Hg concentration deter-

mined in solution corresponds to strongly bound fraction.

The difference between HgT and the sum of the other

geochemical phases corresponds to residual fraction.

Zinc, copper, iron and aluminum determination

Total zinc, copper, aluminum and iron analysis (ZnT, CuT,

AlT and FeT) was performed using 1 g of sample, followed

by an acid digestion with a mixture composed by

HF:HCl:HClO4 (2:1:1), and subsequent determination by

atomic absorption (Varian, Spectra 55b). To guarantee the

quality of results, chemical analyses were made in tripli-

cate and compared with certified samples (NIST 2709 San

Joaquin Soil).

Potentially bioavailable concentrations of zinc and copper

were quantified using one gram of sample (\75 lm fraction),

and an acid extraction with 25 mL of HCl (0.1 mol/L),

according to the methodology described in DePaula and

Mozeto (2001). After adding such solution, samples were

shaken during 2 h, centrifuged and metals in solution were

determined by ICP-MS (Jobin–Yvon, Ultima 2).

Evaluation of contamination intensity

Metal contamination levels were evaluated through com-

parison with Brazilian documents proposed by the Envi-

ronmental Company of the Sao Paulo State (CETESB

2005) and the National Council for the Environment

(CONAMA 2004), for soils and fluvial sediments quality,

respectively (Table 1). The quantitative assessment of

metal contamination in sediments was performed through

Geoaccumulation Indexes (IGEO) calculations, using

background values determined in the standard shale

(Muller 1979) (Eq. 1) and total metal contents in the

\75 lm fraction. The IGEO has been traditionally used for

many authors (Rodrigues-Filho and Maddock 1997; Martin

2004; Ruilian et al. 2008), and the results consist of good

parameters for future comparisons. This index varies

between 0 and 6 units and allows classifying the samples in

different levels of contamination (Table 2).

Rodrigues-Filho et al. (2002), when describing a Holo-

cene climatic transition in southeastern Brazil, quantified

0.350 mg/kg of HgT in allogenic sediments, dating

8,000 ± 50 14C yr B.P, from cores collected in the Silvana

Lake (Minas Gerais State). Since there were no mining

activities in such time, Hg geoaccumulation indexes were

calculated according to this regional geochemical back-

ground for the Minas Gerais State. It is also important to

note that Branco et al. (2005) and Rodrigues-Filho et al.

(2002) quantified total mercury contents in groundwater

samples in the Descoberto and Lake Silvana regions, and

found concentrations lower than the detection limits. Thus,

those sediments are free from groundwater contamination

IGEO ¼ Log2Me=NBNMe ð1Þ

Table 1 Mercury, zinc and copper concentrations values recommended by CETESB (2005) and CONAMA (2004) for soil and aquatic

sediments quality

Toxic metals Values recommended for soils quality (CETESB 2005) Values recommended for aquatic

sediments quality (CONAMA 2004)

Reference limit

(mg/kg)

Prevention limit

(mg/kg)

Agricultural intervention

limit (mg/kg)

Level one

(mg/kg)aLevel two

(mg/kg)b

Mercury 0.05 0.5 12 0.17 0.486

Zinc 60 300 450 123 315

Copper 35 60 200 35.7 197

a Limit of low probability of adverse effects on biotab Limit of high probability of adverse effects on biota


123

where, Me is the metal concentration in the sediment and

NBNMe is the metal geochemical background in the standard

scale (Hg = 40 mg/kg; Zn = 95 mg/kg; Cu = 39 mg/kg).

Statistical analysis

In order to identify possible affinities between heavy

metals and geochemical supports (iron, aluminum and

organic matter), Pearson’s product-moment correlation

coefficient and linear regression analysis were performed

using the program Statistica for Windows.

In order to evaluate significant differences among mer-

cury concentrations in distinct granulometric fractions,

Wilcoxon matched pairs test was performed.

Results

Physical, chemical and mineralogical characterization

Granulometric analysis indicated that soils and sediments

were extremely coarse, since the 1,700–180 lm fraction

corresponded, in average, to 76 and 70% of the granulo-

metric distribution, respectively (Table 3). Determination

of pH in soils and fluvial sediments revealed values

between 5.5 and 7.9 units. Organic matter contents were

between 0.3 and 3.0% (Table 3).

The mineralogical characterization, in the total samples,

indicated the presence of quartz, kaolinite and gibbsite for

all samples (Tables 3 and 4). Sillimanite and microcline

were only detected for two samples (SD-08 and SL-03)

(Figs. 2, 3).


CuT contents in soils indicated that about 18% of samples

were below the quality reference limit (CETESB 2005)

(Table 3). ZnT concentrations were higher than reference

limit for all the samples, and lower than prevention limit.

In comparison with Cu and Zn, Hg contamination levels

were more intense, since 64 and 27% of samples were

higher than the prevention and reference limits proposed by

CETESB (2005), respectively.

Determination of CuT and HgT in sediments denoted

that about 82 and 27% of samples, respectively, were

higher than the ‘‘Level One’’ established by the Brazilian

legislation (CONAMA 2004), while only one sample was

higher than such level for ZnT (Table 3). Geoaccumulation

indexes (IGEO) of CuT and ZnT indicated that about 91

and 82% of the sediments were in the ‘‘class 1’’, respec-

tively. The other samples were in the ‘‘class 0’’ (Table 3).

The highest IGEO class (6) was obtained for HgT in only

one sample, while two samples were in the ‘‘class 1’’.

Biogeochemical behavior of mercury

Hg determination in distinct grain sizes revealed the

existence of an affinity between particle size and Hg

retention, since the finest fraction indicated much higher

HgT contents in comparison with coarse ones (Table 4,

p = 0.003). In the soils, e.g., although the finest fraction

(\75 lm) only represents about to 6.3% of the granulo-

metric distribution, it fixes around 35% of the HgT

(Table 4). No significant differences were detected among

the coarse fractions.

Hg speciation denoted the predominance of the metallic

form in all samples (Table 4—in average, 95 and 78% of

the HgT, in soils and sediments, respectively). Strongly

bound, exchangeable and residual Hg concentrations were

extremely low (Table 4). In comparison with the soils, Hg

contamination in the sediments seems to be lower (soil Hg

average = 3.29 ± 3.98 mg/kg; sediment Hg average =

0.14 ± 0.06 mg/kg). No significant relationships between

HgT and AlT/FeT/organic matter were detected.

Biogeochemical behavior of zinc and copper

In the soils, significant positive relationships were detected

between zinc/copper and some geochemical supports: (1)

total Cu and Fe (y = 6.771 ? 6.574x, p = 0.0004,

R2 = 0.77); and (2) total Zn and Fe (y = 7.028 ? 13.686x,

p = 0.002, R2 = 0.69). In the sediments, positive rela-

tionships were observed between: (1) total Cu and Fe

(y = 20.447 ? 3.109x, p = 0.006, R2 = 0.58); and (2)

total Cu and Al (y = 23.277 ? 2.541x, p = 0.005;

R2 = 0.74).

The determination of bioavailable concentrations of Zn

and Cu indicated values about 1% of the total contents

(Table 4). Positive correlations between total concentra-

tions of Zn and Cu were detected in soils (r = 0.88) and

sediments (r = 0.77). No significant relationships were

Table 2 Geoaccumulation Indexes (IGEO) of heavy metals in sedi-

ments of Reno River (Germany)

Intensity of pollution Accumulation in the

sediment (IGEO)

IGEO’s

classes

Very strongly polluted [5 6

Strongly to very strongly polluted [4–5 5

Strongly polluted [3–4 4

Moderate to strongly polluted [2–3 3

Strongly polluted [1–2 2

Low to moderately polluted [0–1 1

Practically non-polluted \0 0

Adapted from Muller (1979)


123

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123

found between ZnT and CuT contents and organic matter

(O.M.). Because bioavailable contents were only deter-

mined in the \75 lm fraction, it was not possible to cor-

relate them with O.M., since the last one was only

quantified in the \1,700 lm fraction.

Discussion

Physical and chemical characterization

Soils and sediments are extremely sandy and revealed

similar textures, suggesting that the first ones are frequently

transported to Rico creek by runoff. This process may favor

the mobilization of contaminated material to the drainage

and, consequently, increase the risk of heavy metal dis-

semination to the neighborhood areas.

The values of pH are in the neutral range and have a low

potential of heavy metals mobilization in the environment.

The concentrations of organic matter were low and can be

attributed to the very sandy texture of these soils and fluvial

sediments, since coarse particles do not fix organic matter.

In addition, the organic matter values are in agreement with

the contents quantified by Palmieri et al. (2005) (between

0.5 and 21.7%) in tropical soils of South America.

Mineralogical characterization

Kaolinite and gibbsite abundance indicates that the envi-

ronment suffered intense weathering process. Gibbsite

presence was expected due to bauxite deposits in the

regional geology, in agreement with high AlT levels

detected in those materials. In fact, Branco et al. (2005)

also determined high amounts of gibbsite in those soils.

The presence of sillimanite and microcline are related to

the mineralogy and weathering of rocks of the Mega-

ssequencia Andrelandia.

In terms of heavy metals bioavailability, the abundance

of gibbsite is a very important point. It is well known that

Fe and Al oxy-hydroxides (including gibbsite) are able to

form stable complexes with heavy metals, decreasing its

mobility and biotic assimilation. Rodrigues-Filho and

Maddock (1997) and Rodrigues-Filho et al. (2002), e.g.,

report the importance of gibbsite in Hg adsorption

Fig. 2 Representative X-ray

diffractogram of a sediment

sample collected in the Rico

Creek, Descoberto Municipalty,

Minas Gerais State (Brazil)

Fig. 3 Representative X-ray

diffractogram of a soil sample

collected into in the rural area of

the Descoberto Municipalty,

Minas Gerais State (Brazil)


123

processes in fluvial and lake systems, decreasing Hg bio-

availability. Weerasooriya et al. (2003) also describe some

arsenic adsorption mechanisms on gibbsite, under different

physical–chemical conditions, suggesting that such mineral

may play a vital role in the mobility processes, especially

under basic pH values.


The reference values proposed by CETESB (2005) corre-

spond to the pedogeochemical background determined in

the Sao Paulo, while the prevention values indicate con-

centrations which may provoke damages to terrestrial

biota. Thus, 18% of the samples were higher than the

background limits for CuT. Although all the samples

present concentrations higher than the background levels

for ZnT, the contents do not represent high potential risks

on biota (lower than prevention limit). In fact, Cesar et al.

(2008) and Egler et al. (2008), when testing the ecotoxicity

of those soils using earthworms, indicated the occurrence

of insignificant lethal effects on those organisms. On the

other hand, such authors do not refute the hypothesis of

physiological effects on the exposed worms, since they

were able to bioaccumulate high amounts of metals,

especially Zn.

In respect to Hg, 27% of the samples were higher than

the Sao Paulo background levels (CETESB 2005), while

67% of the soils presented contents able to cause adverse

effects on biota (prevention limit). It is important to

emphasize that Hg is a non-essential metal, and such high

concentrations substantially increase the risks on human

health and biota.

CuT and HgT concentrations in sediments suggest the

existence of high potential ecological risks on fluvial

benthonic biota (higher than level one—CONAMA 2004).

However, those risks must be better investigated by per-

forming ecotoxicological tests, since the standard values

adopted by Brazilian legislation (CONAMA 2004) are

based on the Canadian legislation. Therefore, such values

do not necessarily reflect the dynamism of tropical envi-

ronments, once they were estimated using materials and

bioassays with organisms of temperate climates. Geoac-

cumulation indexes (IGEO) of CuT and ZnT indicated that

most of the sediment samples could be considered ‘‘low to

moderately polluted’’, suggesting a reasonable level of

contamination. In the case of Hg, one sample was classified

as ‘‘very strongly polluted’’, and two samples as ‘‘low to

moderately polluted’’.

In comparison with other Brazilian mining sites, CuT

concentrations in soils were relatively lower than that

obtained in the Formiga creek (between 10 and 136 mg/kg)

at Pocone Municipality, Mato Grosso State (Rodrigues-

Filho and Maddock 1997). On the other hand, ZnT contents

were higher than the concentrations determined in sedi-

ments from the Guarandi creek (between 13 and 84 mg/g)

(Mato Grosso State) (Rodrigues-Filho and Maddock 1997).

In Addition, Hg concentrations in soils were significantly

higher than the contents determined in different regions of

the Brazilian Amazon, but much lower than the contents

quantified in the North Sulawesi mining site, Indonesia

(Table 5). HgT contents in sediments were similar to the

concentrations obtained in the Brazilian Amazon, and

lower than the values determined in the Iron Quadrangle

region (Minas Gerais State) by Windmoller et al. (2007)

(Table 5).

Biogeochemical behavior of mercury

The affinity between Hg and silt–clay fractions is possibly

related to the increase of the contact surface between Hg

and the sample, as well as to a possible kaolinite concen-

tration in the finest fraction (\75 lm). The quantification

of high contents of elementary Hg in soils and sediments in

fact confirms old gold mining activities performed in this

area. Due to its high volatility, it is probable that Hg is

being transferred to the atmosphere, and deposited on the

surrounding areas. In agreement with other gold mining

areas of Minas Gerais State, Windmoller et al. (2007) also

Table 5 Comparisons between mercury concentrations determined at the Descoberto Municipality (Brazil) and contents quantified in other gold

mining sites

Locality Hg (mg/kg)-soils Hg (mg/kg)-sediments Authors

Alta Floresta (southern Amazon) 0.01–0.11 0.4–0.250 Wasserman et al. (2007)

Madeira river basin (Rondonia

State, Brazilian Amazon)

0.041–0.346 0.041–0.340 Lechler et al. (2000)

Iron Quadrangle region (Minas

Gerais State, Brazil)

1.8 (maximum value) 0.04–1.100 Windmoller et al. (2007)

Talawaan river basin (North

Sulawesi region, Indonesia)

59.0 ± 151.6

(Hg average value)

91.0 ± 95.0

(Hg average value)

Rodrigues-Filho et al. (2004)

Descoberto (southern Minas

Gerais State, Brazil)

0.274–13.45 0.078–0.237 This study


123

quantified very high concentrations of metallic Hg in the

Iron Quadrangle region, suggesting that the atmosphere

could be a very important via of Hg dissemination in that

environment.

The predominance of metallic Hg has particular impli-

cations for this area. The exposure of human populations to

this Hg chemical form can provoke damages to the respi-

ratory, excretory and neurological systems, as previously

indicated by different authors (Drasch et al. 2001; Veiga

et al. 2005; Shandro et al. 2009). In future works, HgT

contents should be determined in urine samples of local

populations, since it is a good indicator of metallic Hg

intoxication (Veiga et al. 2005). Egler et al. (2008) eval-

uated HgT contents in plants from this area, and found high

concentrations in the roots and lower contents in the aerial

parts, thus suggesting low toxicological risks associated

with the ingestion of contaminated leaves (by humans and

neat). On the other hand, it is important to emphasize that

atmospheric Hg deposition on leaves may stimulate its

accumulation through stomata (Rea et al. 2001; Egler et al.

2006), especially due to the high volatility and abundance

of metallic Hg in this area.

Low concentrations of Hg strongly bound and

exchangeable fractions suggest a low potential of Hg oxi-

dation in this environment, in agreement with pH values in

the neutral range, which do not favor such process. The low

contents obtained for the residual fraction indicate a low

contribution of the geology in the contamination.

A marked contrast of contamination intensity between

soils and sediments (much higher pollution levels in the soils)

suggest that geochemical supports may play an important

role in the Hg behavior in soils, decreasing its mobility in

direction to the drainage. However, the results do not indicate

any affinity between Hg and AlT/FeT/organic matter. In

future works, other pedogeochemical parameters will be

investigated in order to evaluate their influences in the

mobility of Hg. On the other hand, it is important to note that

Hg is mainly present in the metallic form, an electronically

stable chemical form which does not necessarily need to be

bound with other geochemical supports to get the stability.

Biogeochemical behavior of zinc and copper

The behavior of Cu and Zn in soils and sediments seems to

be strongly controlled by Fe and Al concentrations, indi-

cating the existence of possible affinities between Fe and

Al oxy-hydroxides and such heavy metals. These affinities

may be associated with the adsorption of Cu and Zn on

geochemical supports (in this case, Fe and Al oxy-

hydroxides), decreasing the potential mobility of those

heavy metals. In this respect, it is important to note the

abundance of gibbsite in those materials, as previously

indicated by the mineralogical characterization.

The low concentrations of bioavailable Cu and Zn in

soils and sediments may be associated with gravitic con-

centration processes, which are able to generate geo-

chemical anomalies of elements with high density (such as

Zn and Cu). It is possible that such positive correlations

between Zn and Cu do not necessarily indicate a relation-

ship of cause and effect, but they suggest a possible com-

mon genesis of Zn and Cu associated with anthropogenic

processes, since sulfide-bearing bedrocks containing Zn

and Cu are not present in the regional geochemistry. Ro-

drigues-Filho and Maddock (1997), when studying the

behavior of Zn and Cu in sediments from a gold mining site

in southern Amazon, found high concentrations of Zn and

Cu, positive correlations between such metals and low

levels of bioavailability. In that work, the authors sug-

gested mechanic mechanisms of concentration (gravitic

concentration processes) for explaining the origin of

anomalous contents of Zn and Cu.

The observation of very low bioavailable contents of Zn

and Cu (in comparison with the total contents) could justify

the absence of positive relationships between organic

matter and total concentrations of Zn and Cu. In fact, Yin

et al. (1996) report that humic substances are able to form

stable chemical complexes with ionic forms of metals

(more bioavailable), which play an important role in the

bioavailability processes. Consequently, the presence of

organic matter may decrease substantially the metal

exchangeable fractions in the soil, even in environments

with low organic matter contents, like the soils of Desco-

berto. In addition, Peijnenburg and Jager (2003) suggest

that Cu is usually more bound to organic carbon than Zn,

and that such process could directly affect the bioavail-

ability of those metals for soil macro-fauna (earthworms),

especially by ingesting contaminated grains.

Conclusions

Hg is predominantly present in the silt–clay fractions and in

its metallic form, which is extremely volatile and may be

easily transferred to the atmosphere. This aspect of Hg con-

tamination suggests a possible existence of other contami-

nation hot spots in the surrounding areas, which must be

carefully investigated in further screening studies. Besides

that, Hg is a highly toxic metal and its abundance substantially

increases the toxicological risks on human health and biota.

The bioavailable contents of Zn and Cu were very low

in comparison their total concentrations, suggesting the

occurrence of low adverse effects on the ecosystem. ZnT

and CuT were strongly controlled by Fe and Al concen-

trations, decreasing the mobility of such metals. In terms of

ecotoxicity, Zn and Cu contamination apparently have a

secondary importance in comparison with Hg levels. On


123

the hand, Zn and Cu were mainly quantified into the

interdicted perimeter (delimited by Hg levels), and further

investigations in the surrounding areas (other micro-basins)

must also be performed.

Acknowledgments The authors would like to thank the Environ-

mental Foundation of Minas Gerais State (FEAM) and the Centre for

Nuclear Technology Development (CDTN), especially Dr. Peter

Fleming and Dr. Otavio Branco, for their help during the sampling

campaigns. We also thank the geographers Ricardo Sierpe Silva and

Nilo Teixeira for confectioning the sampling points map. Ricardo

Cesar was supported by a scholarship program for graduate students

from the Brazilian National Council for Scientific and Technological

Development (CNPq). Without such financial help, this work could

not be executed.

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