Mercury Behavior in a TropicalEnvironment: The Case ofSmall-Scale Gold Mining inPocone, Brazil

Lazaro J. Oliveira, Lars D. Hylander,

Edinaldo de Castro e Silva

An estimated 50 tons of mercury (Hg) have been emitted by gold

miners in the Bento Gomes river basin, in the municipality of

Pocone, Brazil, since the 1980s. Since the mid-1990s, the state

agency for environmental protection, FEMA (Fundacao Estadual

do Meio Ambiente de Mato Grosso), has enforced regulations to

reduce Hg emissions to air and water and has also implemented

an environmental assessment program. The objectives of this

study were to evaluate efforts to reduce emissions of Hg to air

and water from nine improved amalgamation centers, and to

assess the pollution level in sediment at 25 sites around Pocone.

In spite of the fact that retorts were used, results showed large

emissions of Hg when burning amalgam, resulting in Hg air

concentrations above the limit for occupational air (50 mg/m3)

at all centers except one. Keeping washing water in closed

systems and dumping residues in specially prepared sites

reduced Hg emissions to watercourses. The average Hg

concentration of fine sediments (<74 lm) in the Bento Gomes

river basin was 104 ng Hg/g dry weight, three to four times

higher than the background level; large amounts of Hg-

contaminated sediments are re-suspended during the rainy

season. In conclusion, present emissions to local watercourses

have been efficiently reduced, but the use of retorts in improved

amalgamation centers has not adequately reduced Hg emissions

to air, which is why the use of Hg remains an occupational and

environmental problem.

Environmental Practise 6:13–26 (2004)

The activities of small-scale gold miners (garimpeiros de

ouro), who have been active at several locations in the

Amazon region since the 1980s, cause several types of

environmental degradation. Extensive excavation and use

of water cause erosion, and this silts up rivers and lakes.

The turbid water hampers biological production; this is

reflected in reduced fish captures. The fish are often

contaminated due to large emissions of mercury (Hg)

from the mining sites (Hacon, 1993; Lacerda et al., 1995;

Veiga et al., 1991; Vieira, 1990). Metallic Hg used in the

amalgamation technique was in previous instances care-

lessly emitted into the environment without any consid-

eration as to its toxic effects.

In Pocone, one of the major small-scale mining locales in

the state of Mato Grosso, Brazil, gold mining and related

Hg contamination has been studied since 1989, including

the social, economic, environmental, and health aspects

(Callil and Junk, 2001; Camara et al., 1997; Castro e Silva

and Oliveira, 1996; Guimaraes et al., 1998; Hylander et al.,

1994, 2000; Lacerda et al., 1991; Lacerda, Salomons, and

Pfeiffer, 1991; Malm and Guimaraes, 1996; Marins et al.,

1991; Nogueira, Silva, and Junk, 1997; Oliveira, Silva, and

Ozaki, 1990; Pasca, 1994; Pereira Filho, 1995; Portela, 1991;

Silva et al., 1996; Silva, 2000; Speller, 1992; Tavares, 1997;

Veiga et al., 1991; von Tumpling, Wilken, and Einax,

1995; von Tumpling et al., 1995). Yet, many topics—such

as emissions and miners’ exposure to Hg in improved

amalgamation centers, and environmental assessment of

the entire area—have not been addressed.

This study had two objectives. First, we wanted to evaluate

practices for reducing Hg pollution and to study emissions

from improved amalgamation centers; this was in order

to assess environmental impacts and miners’ potential

exposure to Hg after legislation to reduce Hg emissions

to air and water was enforced in Brazil in the mid-1990s

(FEMA, 1997b). Second, we wanted to assess present levels

of Hg and other metals by surveying the Bento Gomes

river basin around Pocone. This would serve as baseline

data for future studies and also for studying the impact of

present and historic gold mining by comparing present

levels to estimated background levels. Officials involved in

Affiliation of authors: Lazaro J. Oliveira, Department of Chemistry,

Federal University of Mato Grosso, Cuiaba-MT, Brazil; Lars D. Hylander,

Department of Limnology, Evolutionary Biology Centre, Uppsala

University, Uppsala, Sweden; Edinaldo de Castro e Silva, Department of

Chemistry, Federal University of Mato Grosso, Cuiaba-MT, Brazil

Address correspondence to: Lazaro J. Oliveira, Department of Chem-

istry, Federal University of Mato Grosso, Av. Fernando C. Costa/sn,

78 090-900 Cuiaba-MT, Brazil; (fax) þ55-65-815-8766; (e-mail) lazaro@

univag.com.br.

� 2004 National Association of Environmental Professionals

environmental assessment and in options for reducing

emissions of Hg from small-scale gold mines (garimpos)

will benefit from the results presented in this article.

Study Area

Pocone is situated at the northwestern border of the

savannah wetland Pantanal, a 137,000 km2 alluvial plain in

central-western Brazil facing the border of Bolivia and

Paraguay; it receives water from a surrounding upland

drainage basin that occupies 359,000 km2 (Hamilton,

Sippel, and Melack, 1996). The area is hydrologically

divided into the Alto (high), Medio, and Baixo (low)

Pantanal. Floodplains make up about 20% of the upstream

watershed (Alto Pantanal) and most of the area in the

Baixo Pantanal (Alho, Lacher, and Goncalves, 1988). The

rain falling from October to March results in an annual

flooding of usually more than half, and often more than

90%, of the alluvial plain; on the other hand, permanent

open-water areas are minimal (Hamilton, Sippel, and

Melack, 1996). The annual fluctuation of the inundation

area is most extreme in the Alto Pantanal, where the

inundation is maximal in March and April and minimal in

October and November (Hamilton, Sippel, and Melack,

1996). The Alto Pantanal is the northernmost part of the

Paraguay River basin, and includes the tributary Rio

Cuiaba (approximately 15.58–17.58S and 558–598W). The

rivers run southward, and are separated from the Amazon

basin to the north by the Serra dos Parecis and Serra Azul

mountain chains (Brasil, 1974). In the Alto Pantanal, Rio

Paraguai has a slope of 0.03 to 0.15 m/km, while the east-

west slope for the plain is 0.3 to 0.5 m/km (Zeilhofer,

1996), causing Rio Cuiaba to flow faster than Rio Paraguai

through the plain before they join. Rio Cuiaba originates

in the Chapada dos Guimaraes highlands of crystalline

bedrock (mainly sandstone and magnetite) at an altitude

of 600 to 700 m above sea level (Godoi Filho, 1986) and

enters the Pantanal at an altitude of 100 m above sea level

(Zeilhofer, 1996). The Pantanal basin was created more

than 100 million years ago and has subsequently been filled

up by deposits of quartz sand and aluminum silicate clay

sediments with depths of 100 m or more over large areas

(Godoi Filho, 1986).

Alto Pantanal houses gold mines because veins of gold-

containing, coarser material are found in the clay layers at

some places on the margin of the plain (Pereira Filho,

1995). This material is excavated in huge open-cast mines,

as well as small pits, and processed for gold by adding Hg.

Most gold mines in the area are located in and around

Pocone, where all rivers and brooks in the Bento Gomes

(a tributary to Rio Paraguay) river basin were sampled for

Hg (Figure 1).

The area includes grass savannah wetlands of Pantanal

and, in areas of higher elevation, steppes or ‘‘cerrado’’

surrounding the Pantanal (Silva, 2000). The original brush

and sparse trees are largely cleared when using the land for

agriculture and mining. The climate is semi-humid

tropical, with daily average temperatures often above

388C in the wet season and annual average rainfall around

1700 mm (Pereira Filho, 1995). Daily average temperatures

in the dry season (May to September) are just above 208C

(Pereira Filho, 1995). From October to April, there are two

dominating wind directions, from the west and northwest,

bringing air heated in the western Amazon rainforest. In

the winter months of May to September, polar air (south

and southeast winds) may cause drastic drops in the

temperatures (Silva, 2000).

Materials and Methods

In the study of Hg emissions from improved amalgam-

ation centers, nine amalgamation centers in operation as

of January 1998 and two closed down before 1995 were

included. In the improved amalgamation centers, amal-

gamation is performed in closed vessels (Figure 2) and the

amalgam is burned in retorts. All water has to be

recirculated in a closed system (Figure 3); the residues

from the amalgamation are deposited in concrete tanks or

pits sealed with plastic sheets to avoid leakage.

The sampling included air, precipitated solid residues after

amalgamation, and suspended particulate matter in water,

circulating in the plants in two separate and closed water

circuits—one being the rain-fed reservoir used for flushing

the gold-bearing ore into the mills and centrifuges, the

other being a circuit including a series of three concrete

tanks (Figure 3) where water washes away residues from

the amalgam (evaporated water is replenished with water

from the outdoor reservoir or a well). Surface water was

sampled beside the pump inlets. The values of Hg in water,

shown in Table 1, are calculated by dividing the Hg found

in suspended matter (see below) by the water volume from

which the suspended matter was precipitated. Precipitated

solid residues from the amalgamation were sampled as

composite samples from the surface layer of the concrete

tanks used for final storage. The Hg content of the air was

determined outdoors and at two places indoors at each site

using a portable mercury vapor analyzer with a gold film

sensor (Jerome 431-X).

14 Environmental Practise 6 (2) June 2004

Figure 1. Location of sampling sites for mercury emissions in the Bento Gomes river basin, Pocone, Brazil. (Source: Salatiel Alves deAraujo.)

Mercury and Gold Miners in Brazil 15

In the environmental assessment survey, superficial stream

sediment (0 to 10 cm), soil, and suspended matter in water

were collected at 25 sites in the Bento Gomes river basin

around Pocone (Figure 1) in conjunction with an

environmental assessment program executed by the state

agency for environmental protection, Fundacao Estadual

do Meio Ambiente de Mato Grosso (FEMA, 1997a).

Whenever possible, the sites were chosen to include one

point upstream and one to several points in depressions

containing possible deposits of material transported

downstream from existing or historic amalgamation sites.

Sites sampled in earlier surveys were also included in order

to determine any historical trends. Samples were collected

at the end of the dry season in July 1997, at the beginning

of the rainy season in September 1998, and a third time in

May 1999, when water levels were receding. Composite

samples were collected within a 2 m diameter circle at each

location and packed in polyethylene bags. Soil samples

were taken adjacent to the sediment sampling sites. The

organic surface layer (with possible direct Hg deposition)

was removed, and a 10-cm-thick layer was then sampled.

Water pH, dissolved oxygen, temperature, conductivity,

and turbidity were measured at all sampling locations,

using a multipurpose field instrument (Horiba U-10).

In the laboratory, all sediment and soil samples were dried

(,508C) and dry sieved (,74 lm) with nylon mesh sieves.

Material suspended in the water was flocculated and

precipitated by the addition of aluminum sulfate,

Al2(SO

4)3(Silva et al., 1996). Twenty milliliters of a 10%

Al2(SO

4)3solution and about 20 ml of sodium hydroxide

(NaOH, 1 N) were added to 20 L of water to get a pH

greater than 7. The supernatant was discarded and the

precipitate dried (40–458C) and crushed in a mortar. A 2 g

sample of the precipitates or of the fine fraction of soil and

sediments was digested in 10 ml aqua regia (HCl:HNO3

3:1), in a glass tube with a glass pear, at 1208C for 2 hours in

a block digester (Goncalves and Paiva, 1995).

The total content of Hg in the twice-diluted extract was

determined by cold vapor atomic absorption spectro-

photometry (CVAAS; Varian model VGA 77 coupled to

a Varian AAS model 200). Other metals were determined

by AAS. Organic matter was determined by wet digestion

with dichromate (Walkley and Black, 1934).

Distilled and de-ionized water and laboratory-grade

chemicals were used in all experiments and for preparing

solutions. The accuracy of the Hg analyses was checked by

Figure 2. Closed amalgamation drums are compulsory in Pocone, Brazil’s improved amalgamation centers in order to reduce mercuryemissions. (Source: Gercino Domingos da Silva.)

16 Environmental Practise 6 (2) June 2004

comparison with certified reference materials (GBW 07309

stream sediment with 83 ng Hg total/g dry weight, NIST

Buffalo river sediment 2704 with 1,470 ng Hg total/g dry

weight). Obtained values were 72.5 (SD 2.7, n ¼ 6) for

GBW and 1,313.5 (SD 68.5, n ¼ 14) ng Hg/g dry weight for

NIST, indicating stable results just below the certified

values. All samples were digested in duplicate, and each

extract was analyzed twice. The precision of all metal

analyses was verified by internal standards and replicated

analyses. The detection limit (3 SD) for Hg was 0.5 lg/LHg in the extract, and 5 ng Hg/g dry weight in the

sediment when using 2 g samples and a final extractant

volume of 20 ml. The precision (1 SD) was about 10% in

typical samples containing 10–100 ng Hg/g dry weight.

Results and Discussion

Emissions from Improved Amalgamation Centers

TheHg concentration in,74 lmresidues from the sampled

amalgamation centers ranged from 48 to 459 lg/g, with an

average of 168 lg/g (Table 1). These variations depended

partly on amalgamation time, which helps determine the

concentration and granulometry of gold and other metals

left in the sample. Fine gold particles need a longer time to

amalgamate than coarser gold grains, which is why the

amalgamation times generally vary between 0.5 to 2 hours,

depending on the characteristics of the mineral concentrate

to amalgamate. The concentrate is obtained bymilling gold-

bearing minerals, from which particles with higher density

are separated by centrifuges. From the 100 to 600 tons of

minerals flushed to the mills by water, 0.07 tons of

concentrate is received. The amalgamation process, and

consequently the Hg content in the residues, depends also

on the quantity of Hg added, the condition of the

equipment, and the skill of the operator. Silva (1996) found

that after excessHghad been separated from the amalgamby

squeezing it through a fabric, the amalgam lost, on average,

40% of its weight during the burning due to evaporation of

Hg. Mercury still remains after burning, which is then fused

with borax at the gold dealer to get gold bullion. The gold

content in burned amalgam received by the gold dealers

varies between 80% and 95%, according to various gold

dealers in Cuiaba.

The large amount of gold left in the residues (MacDonald,

1983) indicates that amalgamation is not the best available

technique for gold extraction. As a consequence, some

amalgamation residues from the Pocone area have been

extracted a second time by leaching with cyanide,

a technique used in other gold fields (Hylander, 2001);

Hg left in the residues is dissolved by cyanide and becomes

an additional health and environmental hazard (Matlock

et al., 2002).

Table 1 indicates that even when retorts are used, the Hg

concentration in air is nearly always well above the limit

concentration recommended for occupational exposure,

50 mg Hg/m3 (World Health Organization, 1976). This

indicates potential health effects, which at the studied

amalgamation centers are partly counteracted by ventilated

structures (generally without walls) and by ventilation with

rustic fume hoods above the retorts. The results also

indicate that although 99% of the Hg may be recovered by

retorts (Farid et al., 1992; Farid, Machado, and Silva, 1991),

substantial quantities of Hg may still be lost to the

environment when using retorts. This warning should be

considered when planning to use this approach for small-

scale miners. These results also indicate that retort

designers face substantial challenges. Outdoors, Hg con-

centrations in the air were markedly lower, but at more

than half of the centers they were 5 mg Hg/m or more at

a distance of 5 to 15 m from the amalgamation centers in

the downwind direction (Table 1).

It is evident that the Hg content, mainly attached to

particulate suspended matter, will increase markedly in the

Figure 3. In Pocone, Brazil’s improved amalgamation centers,water for washing residues from the amalgam must berecirculated in a closed system, using a series of three concretetanks that precipitates solid particulates. (Source: WanderleiMagalhaes de Resende.)

Mercury and Gold Miners in Brazil 17

water when residues are washed from the amalgam (Table

1). The coarser, suspended particles will precipitate in the

water tanks of the circuit; the precipitates are regularly

transferred to a final residue storage, generally large

concrete tanks from which amalgamation residues were

sampled. These residues have a 5 to 22 times lower Hg

content than the suspended matter found in water used for

washing the amalgam (Table 1), because there is a large

fraction of quartz grains with poor affinity for Hg in the

precipitated material, while clay colloids with Hg adsorbed

remain in suspension (Brady and Weil, 1996). Mercury

concentrations of residues are generally higher than those

of suspended matter in water used for flushing the gold-

bearing ore into the mills and centrifuges, garimpos 1 and 3

being notable exceptions, due to unexpectedly low Hg

content in these residues (Table 1).

The closed water circuits for water used to wash away

amalgam residues, along with appropriate storage of the

residues at the amalgamation centers, have reduced Hg

emissions to watercourses in the Bento Gomes basin.

Similar experiences in Venezuela lasted for only a limited

period. Several amalgamation centers have been con-

structed in Venezuela (Veiga, 1997), but none of them were

in operation in 2001, according to our observations,

which have been confirmed by other professionals in

the area. This indicates that the construction of amalgam-

ation centers in and of itself is not enough to reduce

Hg emissions; their operation must be supported and

inspected.

Table 2 shows some physical and chemical parameters of

the water at the 10 amalgamation centers. The data from

1996 were measured during the time when gold miners

consulted FEMA for permission to operate according to

the introduced laws for environmental protection, and

data from 1998 are from inspections carried out by FEMA

to ensure that operations followed given rules. Water used

to feed the mills with minerals (freshwater reservoirs fed by

a river or creek) is more turbid and has a lower level of

dissolved oxygen than water found in the watercourses

feeding the reservoirs. The turbidity originates with the

flushed minerals. Within the actual temperature range

(208 C to 348 C), between 9.11 and 7.09 mg oxygen/L water

may maximally be dissolved, which indicates that the stag-

nant water in the reservoirs is not sufficiently oxygenated;

this may stimulate methylation of Hg.

In nearly all centers, natural water in the reservoirs is

generally slightly acidic, but the water used to wash away

Table 1. Mercury concentrations obtained, 1998 (air, precipitated solid residues after amalgamation, and suspended particulate matterin water), at gold mining sites (garimpos) in the Bento Gomes basin, Pocone, Brazil

Gold mining site

Air (lg Hg/m3)

Residues from

amalgamation

(lg Hg/g dw)

Indoors

Outdoors

Suspended matter in water

(ng Hg/g dw) Water (lg Hg/L)c

Residuesa Reportb GW AM GW AM

Garimpo 1 98 954 10 48 112 923 0.3 6

Garimpo 2 23 196 5 73 42 1,579 ,0.1 15

Garimpo 3 29 15 ,1 51 81 333 0.2 4

Garimpo 4 128 .999 49 459 245 2,267 0.4 13

Garimpo 5 54 90 5 153 39 1,182 ,0.1 17

Garimpo 6 6 147 78 98 58 1,222 0.4 11

Garimpo 7 18 74 ,1 253 62 1,182 x 13

Garimpo 8 9 65 ,1 136 98 1,059 x 9

Garimpo 9 132 985 7 238 195 2,111 0.4 19

Garimpo 10d x x ,1 x 53 x ,0.1 x

Garimpo 11d x x ,1 x 68 x 0.2 x

Average 55 .392 14 168 96 1,318 0.2 12

SD 51 444 25 132 66 596 0.1 5

Water was sampled from two separate circuits: water used for flushing the gold-bearing ore into the mills and centrifuges (GW) and water used for washing awayresidues from the amalgam in a closed circuit (AM).a Measured 0.3 m above the heap of dried and stored residues contaminated with Hg from amalgamation.b Close to the retort in operation, but with the fan in the fume hood disconnected.c Calculated by dividing Hg determined in suspended matter with the water quantity the suspended matter was collected from.d Operation stopped before 1995 (water in the tank sampled).

18 Environmental Practise 6 (2) June 2004

residues from the amalgam is either neutral or basic as a

result of lime added to the sedimentation tank to accelerate

and increase precipitation. In addition, before water is

recirculated in the amalgamation process, the water pH

often increases because of the addition of soda or washing

powder to the amalgamation drums in order to dissolve fat

and reduce the surface tension for optimal amalgamation

(Farid, Machado, and Silva, 1991). These changes in water

pH and conductivity may increase biological production

and possibly increase the methylation rate, but the

increased pH counteracts dissolution of Hg and thereby

reduces its transport by water in soluble form to other

Table 2. Physical and chemical parameters of recirculating water used for washing away gold-bearing concentrate from other minerals(GW) and used for washing away amalgam residues (AM), measured in reservoirs and tanks in July 1996 and January 1998

Mining sitea Year Source pH Eh (mV)

Conductivity

(lS/cm)

Turbidity

(NTU)

Dissolved O2

(mg/L)

Salinity

(%)

Garimpo 1 1996 GW 6.4 135 98 440 4.32 0.00

AM 8.2 129 228 180 3.98 0.00

1998 GW 6.8 165 24 330 4.20 0.00

AM 7.4 140 173 45 5.61 0.00

Garimpo 2 1996 GW 7.5 74 50 6 6.99 0.00

AM 10.5 50 554 62 5.54 0.02

1998 GW 6.6 130 60 3 5.76 0.00

AM 11.1 132 1250 130 3.83 0.05

Garimpo 3 1996 GW 6.4 135 65 86 5.21 0.00

AM 7.6 121 265 45 5.23 0.00

1998 GW 6.9 101 95 49 5.06 0.00

AM 7.9 145 153 67 4.58 0.00

Garimpo 4 1996 GW 7.2 45 38 161 4.35 0.00

AM 8.6 165 65 390 3.56 0.00

1998 GW 6.4 140 24 8 5.23 0.00

AM 7.1 162 42 220 4.20 0.00

Garimpo 6 1996 GW 6.1 91 78 5 3.58 0.00

AM 8.1 47 83 39 5.12 0.00

1998 GW 5.8 26 46 3 1.72 0.00

AM 7.2 13 83 22 4.85 0.00

Garimpo 7 1996 GW 6.0 95 28 11 5.61 0.00

AM 7.2 101 38 65 3.27 0.00

1998 GW 6.2 52 67 13 5.13 0.00

AM 7.2 47 57 80 2.87 0.00

Garimpo 8 1996 GW 7.0 156 58 79 5.19 0.00

AM 10.2 231 97 167 4.56 0.00

1998 GW 6.9 123 107 25 4.84 0.00

AM 9.1 92 62 527 3.71 0.00

Garimpo 9 1996 GW 6.8 106 73 51 4.87 0.00

AM 7.5 84 59 45 4.29 0.00

1998 GW 6.5 113 49 150 4.87 0.00

AM 7.6 46 73 70 4.67 0.00

Garimpo 10b 1996 GW 6.8 112 21 6 5.10 0.00

1998 GW 6.4 91 47 3 5.10 0.00

Garimpo 11b 1996 GW 6.8 92 37 8 5.98 0.00

1998 GW 6.5 156 30 5 5.50 0.00

a Garimpo 5 not measured.b Operation stopped before 1995 (water in the tank measured).

Mercury and Gold Miners in Brazil 19

locales (Stumm and Morgan, 1996). However, nearly all

waterborne Hg in Pantanal is transported attached to

suspended matter (Hylander et al., 1999).

Survey in the Bento Gomes River Basin

Water parameters at the 25 sites included in the survey are

presented in Table 3. Water pH ranged from 8.6 at TQP 3 in

1997 to 4.1 at TQP 1 in 1999. Average water pH was highest

in 1997, when it was 6.6, and lowest in 1999, at 5.9. The

lower pH values in 1999 reflect increased quantities of

organic acids entering the watercourses, originating from

earlier decomposition of submerged vegetation descending

from flooded areas in April and May.

The oxidation-reduction (redox) potential (Eh) varied less

between the years, with an average of between 118 and

125 mV. It varied considerably between different sites,

however, especially in 1998, when measurements were done

at the beginning of the rainy season. Some sites develop

pronounced reducing conditions when large areas with

organic material get flooded and consume oxygen. Sub-

sequently the concentrations of dissolved oxygen were also

low at these sites, and were, on average, lower in 1998 and

1999 than during the dry season in 1997. For example, one

of the lowest concentrations of dissolved oxygen was

encountered in all years in the river Bento Gomes at BNG

08, a flat with practically no inclination and substantial

vegetation. The predominant pH and redox conditions

indicate that metallic Hg entering the aquatic system will

not easily be oxidized from Hg0 by chemical processes

(Hem, 1970).

The turbidity was, at nearly all sites, highest at the

beginning of the rainy season in 1998 and during the dry

season in 1997 (Table 3). The turbidity during the dry

season is mainly caused by domestic and wild animals,

including caimans, looking for water and food in the few

waters that are not dried up. The maximum turbidity

encountered (36 NTU) is well below the limit (100 NTU)

Table 3. Physical and chemical water parameters at 25 freshwater sites in Rio Bento Gomes river basin surveyed in July 1997, September1998, and May 1999

pH Eh (mV) Turbidity (NTU) Conductivity (lS/cm) Dissolved O2 (mg/L) Temperature (8C)

Site ’97 ’98 ’99 ’97 ’98 ’99 ’97 ’98 ’99 ’97 ’98 ’99 ’97 ’98 ’99 ’97 ’98 �’99

BNG 1 7.5 6.7 6.8 104 85 120 36 33 9 175 97 170 7.50 5.58 7.71 23.6 26.7 19.7

BNG 5 7.1 6.7 5.8 121 91 128 10 6 2 126 144 68 5.20 4.37 4.41 22.4 32.0 23.8

BNG 7 6.3 6.7 5.6 78 280 125 10 7 1 90 103 98 5.80 2.88 3.70 29.0 32.2 28.3

BNG 8 6.5 6.5 5.1 99 115 93 2 8 1 146 159 63 2.99 1.40 2.79 24.4 30.8 25.4

CAA 1 6.3 5.6 5.6 45 12 53 10 7 1 31 52 38 5.23 3.56 4.12 27.0 27.8 22.8

CCO 1 6.1 6.4 5.5 65 �64 120 2 2 1 50 150 45 7.40 1.44 3.13 27.5 27.5 22.5

CCO 2 6.3 5.7 6.2 154 54 97 10 3 1 54 40 49 5.91 5.80 5.02 29.5 27.5 26.6

CFU 1 6.8 6.5 6.3 225 73 105 13 6 4 60 97 62 7.90 7.92 7.57 24.0 25.7 22.3

CFU 3 6.8 6.6 5.9 215 167 158 10 10 7 56 88 56 7.60 7.80 7.25 25.7 29.3 24.0

PIM 1 5.6 5.1 5.7 128 212 145 2 1 1 96 112 78 5.24 2.70 6.09 26.9 26.8 21.1

PIM 2 6.7 6.5 6.3 147 122 100 3 6 1 65 93 69 5.58 4.30 4.76 22.7 29.7 24.7

PIM 3 6.8 6.5 6.4 141 111 104 4 29 1 60 86 67 6.50 4.19 5.35 24.0 29.0 25.3

PIM 4 6.8 6.6 5.4 107 154 171 3 25 2 80 95 23 4.30 5.22 4.47 22.2 34.0 22.6

PRT 1 6.7 6.6 6.6 112 142 125 2 6 3 70 110 78 7.40 4.09 5.01 22.0 24.7 23.8

PRT 2 6.6 6.4 6.0 123 131 151 2 2 3 70 103 68 7.40 7.82 5.40 26.0 28.3 24.9

PRT 3 6.5 6.4 6.32 130 87 65 2 2 1 70 86 67 6.90 6.75 5.84 27.0 29.0 23.8

PRT 4 6.5 7.0 6.7 145 225 169 10 28 3 70 74 65 7.90 6.98 7.20 23.8 26.7 22.6

PRT 5 6.6 6.4 5.5 87 79 144 10 5 10 67 76 65 6.40 4.02 3.58 26.6 30.8 30.8

PRT 6 6.7 7.3 5.6 97 182 113 4 3 1 71 70 68 5.37 7.56 3.65 30.1 30.4 30.4

TEB 1 6.7 5.7 5.6 30 67 148 18 16 1 111 62 35 7.39 5.10 0.98 25.5 21.7 21.7

TEB 2 6.6 6.2 6.1 150 351 24 2 6 3 37 171 125 5.90 5.98 4.70 27.2 31.2 21.4

TQP 1 6.5 5.4 4.1 107 97 162 2 4 1 47 11 10 6.41 3.85 4.23 21.8 28.0 24.9

TQP 2 5.2 6.2 5.2 179 151 116 0 1 1 9 53 51 6.68 5.47 5.50 24.6 33.2 28.9

TQP 3 8.6 6.1 6.2 74 115 198 6 1 4 50 51 51 5.27 7.30 5.78 33.0 33.0 26.4

VGC 1 6.8 6.4 6.4 90 87 104 10 2 2 51 86 51 7.80 6.75 6.46 23.6 29.0 24.8

Average 6.6 6.3 5.9 118 125 121 7.4 8.8 2.6 71 89 64 6.32 5.15 4.98 25.6 29.0 24.5

SD 0.6 0.5 0.5 46 84 39 7.5 9.6 2.5 36 38 31 1.25 1.94 1.60 2.8 2.9 2.8

20 Environmental Practise 6 (2) June 2004

for water permitted for use in Brazil’s drinking water

(Brasil, 1986).

Electrical conductivity varied, on average, from 64 in May

1999 to 89 at the beginning of the rains in September 1998

(Table 3). Generally the conductivity is lower in the rainy

season than in the dry season due to dilution by rain that is

comparably free of salts (Silva, 1996). At the onset of the

rains, however, salts in dust are washed into the water-

courses; this is expected to have caused the increased

conductivity at the beginning of the rainy season of 1998.

Organic acids from decomposition of newly submerged

organic material may also have contributed to this

scenario.

Water temperature was, on average, 3.48 C higher in

September 1998 than in July 1997, and 4.58 C higher in

September 1998 than in May 1999 (Table 3). Air tem-

perature varied less between the years, and was, on average,

34.28 C in 1997, 34.58 C in 1998, and 32.58 C in 1999 (data

not shown).

The average Hg concentration of material suspended in

water sampled at 25 sites in the Bento Gomes river basin

was 364 ng Hg/g dry weight in 1997, 3,355 in 1998, and 345

in 1999 (Table 4). The median was 224 ng Hg/g dry weight,

the first quartile was 152, and the third quartile was 328 in

1997, while corresponding values for 1998 were 1,634, 529,

and 4,495 ng Hg/g, and for 1999 were 327, 82, and 555 ng

Hg/g. The Hg concentration in the suspended matter

transported by the creeks and rivers at the beginning of the

rainy season (September 1998) is about five times higher

than at the end of the same season. This indicates that Hg-

bearing particles precipitate in the watercourses, and that

they are re-suspended and flushed away by the rainwater,

together with Hg-bearing particles from gold mining

residues along the watercourses. Rodrigues et al. (1994)

determined Hg concentrations in suspended matter in Rio

Table 4. Mercury concentrations (ng/g dw) of suspended matter in water and sediment collected July 1997, September 1998, and May1999 at 25 sites in the Bento Gomes river basin, Pocone, Brazil

Suspended matter Sediment

Site 1997 1998 1999 1997 1998 1999

BNG 1 448 270 184 25 23 21

BNG 5 152 na 387 39 40 9

BNG 7 152 984 80 26 133 70

BNG 8 104 824 513 198 57 22

CAA 1 272 na 659 79 104 1,319

CCO 1 160 3,990 16 58 178 46

CCO 2 1,408 na 58 147 96 nd

CFU 1 328 5,306 369 18 40 113

CFU 3 176 na na nd 66 21

PIM 1 448 330 87 59 270 30

PIM 2 152 8,870 597 53 47 672

PIM 3 152 546 621 48 83 29

PIM 4 104 7,738 328 97 26 20

PRT 1 272 856 na 69 31 na

PRT 2 160 246 79 27 69 30

PRT 3 1,408 1,042 9 51 121 66

PRT 4 328 5,986 765 29 76 18

PRT 5 176 608 1,356 348 88 19

PRT 6 448 4,116 359 50 27 12

TEB 1 152 na 316 125 87 92

TEB 2 152 476 289 245 86 40

TQP 1 104 1,634 658 167 193 109

TQP 2 272 18,706 121 92 179 313

TQP 3 160 4,224 83 58 46 18

VGC 1 1,408 354 nd 55 50 35

Average 364 3,355 345 90.1 88.6 135.8

SD 408 4,504 326 80.1 61.4 295.3

na ¼ not analyzed; nd ¼ not detected.

Mercury and Gold Miners in Brazil 21

Tapajos (a tributary to Rio Amazonas) at Itaituba, and

obtained similar results, ranging up to 31,700 ng Hg/g

downstream from gold mining areas. It is interesting to

note that although the water in 1997 (collected in the dry

season) was fairly turbid (Table 3), the Hg concentration of

those particles was generally several times lower than in the

suspended particles sampled in 1998 (collected in the rainy

season), indicating that Hg deposited on land during the

dry season is washed into the watercourses by the rain. The

average Hg concentrations of fine sediments (,74 lm)

sampled in the Bento Gomes river basin were 90, 89, and

136 ng Hg/g dry weight for the years 1997, 1998, and 1999,

respectively (Table 4). The average Hg concentration in

uncontaminated soil (,74 lm) from the area was 24.2 ng

Hg/g dry weight (SD 24.2, n ¼ 10), and the average Hg

concentration in fine sediments (,74 lm) from remote

lakes of the Pantanal, studied earlier, was 33.2 ng Hg/g dry

weight (Hylander et al., 2000). These values indicate

elevated Hg concentrations in sediments from the portions

of the Bento Gomes river basin that include the gold

mining area.

Similar to Hg concentrations in suspended matter, Hg

concentrations in fine sediment vary considerably between

sites and also within sites between the different years

(Table 4). This variability is common in contaminated

areas; the Hg concentration may vary several orders of

magnitude within meters. The average concentration for

all sites, however, is consistent for 1997 and 1998, but

increases in 1999, due to two high concentrations (672 and

1,319 ng Hg/g) encountered downstream from abandoned

amalgamation sites PIM 2 and CAA 1. These values

indicate re-suspension of contaminated sediment or

residues from historical amalgamation by the streams

during the rainy season and subsequent precipitation at

the sampled sites, which serve as sinks for Hg. Except for

these two high values, the Hg values (sediment) for 1999

were generally lower than the values for 1997 and 1998. This

may be due to the fact that organic matter content, which

generally has a higher concentration of Hg, was markedly

lower in 1999 compared to earlier years. The average

concentrations of iron and manganese (Table 5), and

copper and zinc (not shown) were also lower in 1999.

These values indicate a larger fraction of coarse particles of

the ,74 lm sediment, because the finest particles (with

a higher cation exchange capacity and sorption capacity)

were washed away during the rainy season of 1999.

Based on an extensive survey in the Tapajos basin,

Amazonas, Roulet et al. (1998) suggested that Hg entering

the Amazon soil is initially firmly bound to soil con-

stituents such as sesquioxides and crystalline aluminum

and iron oxy-hydroxides. It is when the soil or sediment

first reaches saturation that Hg leaks into the watercourses

in potentially bioavailable forms. They also demonstrated

that the Hg burden is not related to organic matter con-

tent in the rainforest’s oxisols and spodosols in the Tapajos

valley, contrary to what is known of temperate areas

with boreal forests (e.g., Meili, 1997). Instead, they

identified crystalline aluminum and iron oxy-hydroxides

as the soil constituents that determine the capacity to sorb

Hg. Amorphous aluminum and iron oxy-hydroxides, as

well as manganese oxy-hydroxides, also have the capacity

to sorb heavy metals (McKenzie, 1980), but were not as

abundant in the actual soils. Although the data in Table 5

relate total concentrations of aluminum, iron, and

manganese and not the crystalline or amorphous forms,

interesting observations can still be based on them. The

three to four times lower average concentration of iron in

1999 than in 1997 and 1998 indicates fewer iron oxy-

hydroxides available in the sediment, and therefore a lower

potential sorption capacity for Hg (Table 5). Data also

indicate that Hg bound to manganese oxy-hydroxides is

of subordinate importance in the Bento Gomes basin,

because of the low concentrations of manganese compared

to aluminum and especially iron.

Table 5. Concentrations of aluminum, iron, manganese, and organic matter at 25 sites in Bento Gomes basin, Pocone, Brazil

Soil (not

contaminated)

Sediment

Element 1997 1998 1999

Al (lg/g dw3 103) na na na 10.06 7.8

Fe (lg/g dw3 103) 1166 92 1676 96 1296 54 396 40

Mn (lg/g dw) 4106 397 8856 966 6186 512 3756 364

Organic matter

(lg/g dw3 103) 196 12 176 12 196 8 86 7

Each value is the mean of 25 samples6 SD.na ¼ not analyzed.

22 Environmental Practise 6 (2) June 2004

The Fate of Mercury in the Bento Gomes River Basin

Concentrations of Hg in samples collected earlier from

some of the sites included in this study are presented in

Table 6. Values obtained in eight earlier studies agree well

with the present study and indicate a substantial contam-

ination of Hg in the area. The results indicate that the

major part of the Hg contamination in the Bento Gomes

basin occurred between the 1980s and mid-1990s, before

FEMA started to supervise the gold miners. An estimated

50 tons of Hg has been used in the area since the 1980s

(authors’ estimate, based on many years of communica-

tions with the miners). Secretaria Estadual do Meio

Ambiente (SEMA, 1988) estimated the Hg consumption

at 1.1 tons per year, based on an inquiry of miners. This is

likely to be an underestimate, because the miners give

lower values than the actual consumption in order to

avoid taxes on gold production related to Hg consump-

tion. In addition, some miners deny any use of Hg, in

order to avoid public condemnation.

Silva (1996) and Davis, Bloom, and Que Hee (1997)

suggested that the Hg used remains largely in metallic

form, and several studies suggest that part of it is deposited

in the sediment and soil in the vicinity of the place where it

is used (Fadini and Jardim, 2001; Lindberg, Kim, and

Munthe, 1995). Nelson et al. (1977) claim that 90% to 99%

of the Hg transported by the Kuskokwim River (Alaska)

was precipitated in the sediment, and Ramamoorthy and

Rust (1976) suggest that inorganic Hg will precipitate to

the bottom within five minutes. Earlier studies around

Pocone indicated yearly sedimentation of 10 to 15 mg Hg/

m2 in Corrego Fundo (von Tumpling et al., 1995). The two

high Hg values (sediment) in 1999 (Table 4) indicate that

when once emitted, Hg will be resuspended and reallo-

cated by torrential streams during the rainy season.

A logical question is: Where has the Hg gone? Silva (2000)

and von Tumpling, Wilken, and Einax (1995) concluded

that less than 10% of the area’s estimated 50 tons used are

in the piles of residue. Some Hg remains in soil and

sediment, but probably only limited quantities have been

transported away by the watercourses. Therefore evapora-

tion in metallic, inorganic, or organic form appears to be

the main pathway of escaped Hg. High temperatures

nearly all year round, shallow and clear waters in the

region, and soil and sediment with few reactive sites

facilitate evaporation, while high biological activity due to

annual flooding, high temperatures, and abundant organic

matter, phosphorus, and other nutrient sources possibly

result in biological transformation to volatile forms

(Guimaraes et al., 2000; Zeilhofer, 1996).

Conclusion

Mercury emissions from gold mining in the Pocone region

were substantially reduced in the 1990s because of actions

taken by FEMA to supervise and enforce the mandate that

gold miners reduce Hg emissions. Reduced mining

activities due to declining gold prices have also contrib-

uted. Direct emissions to watercourses have been reduced

substantially, although high Hg concentrations in sedi-

ments were encountered in a couple of places, probably

due to re-suspension during the rainy season of historically

emitted and settled Hg. Substantial quantities of Hg are

Table 6. Concentrations of Hg (ng/g dw) in sediment from different watercourses in Bento Gomes river basin, Pocone, Brazil,according to different studies since 1990

Author Piranema

Corrego

Fundo Piraputanga

Bento

Gomes

Tanque dos

Padres

Vieira (1990) nd–117 nd–255 nd–171 nd–242

Lacerda et al. (1991) ,20–180

Veiga et al. (1991) 20–220

Hylander et al. (1994) 10–40a

Pereira Filho (1995) ,40–600 ,40–60 100–1,850

Nogueira, Silva, and Junk (1997) 36–198

von Tumpling, Wilken, and Einax (1995) nd–150

Silva (2000) 9–202

This study (dry season, July 1997) 48–97 18 27–348 25–198 58–167

This study (rain starts, September 1998) 26–270 40–66 27–121 23–133 18–193

This study (rain ends, May 1999) 20–672 21–113 18–66 9–70 18–109

a Soil.nd ¼ not detected.

Mercury and Gold Miners in Brazil 23

still being emitted to the air, which is a potential health

hazard for the operators and also contributes to the global

Hg cycle. The results of this study show that the use of retorts

cannot reduce the emissions of Hg to an acceptable level. A

hot climate, a less complex grassland vegetation than in the

rainforest, and soil and water characteristics of the Pocone

area have resulted in a large part of the historically used Hg

being emitted into the air, soil, and water, leaving the area

via the atmosphere as a diffuse source, and now participat-

ing in the global Hg cycle (Lindberg et al., 2000). The long-

term effects of Hg still present in the Bento Gomes river

basin need further research, however, and measures should

be taken to further reduce emissions of Hg, which are still

unacceptably large.

Acknowledgments

We want to thank all institutions and individuals who contributed to this

study, in particular geolog Salatiel Alves Lopes de Araujo for develop-

ing the map; Wanderlei Magalhaes de Resende, Gercino Domingos da

Silva, and Antonio Joao Paes de Barros for photos and comments; and

all the garimpeiros who permitted sampling at their mining sites. This

work was supported by Fundacao Estadual do Meio Ambiente (FEMA-

MT), and also by the Swedish International Development Cooperation

Agency/Department for Research Cooperation (Sida/SAREC), and the

Federal University of Mato Grosso, Brazil.

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Submitted January 2, 2003; revised January 22, 2004; accepted February10, 2004.

26 Environmental Practise 6 (2) June 2004