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Abstract
High mercury levels in the blood of fish-eating people in the Amazon have been attributed to gold mining activities conducted by informal miners. However, the high deforestation rate in the region has not been recognized as contributing to this environmental problem. About 90 tonnes of organic mercury from the biomass are estimated to be emitted annually to the atmosphere and precipitated in the aquatic systems for rapid transformation into methylated forms. This is a conservative assessment and may be more than 6 times this rate.
Mercury emissions from informal mining operations, or "garimpos" have been described as a potential epidemic in the Amazon region.These operations expanded considerably in Brazil in the 1980s as a consequence of poorly conducted agricultural policies, increased inflation and high unemployment. Mercury is used to amalgamate gold and a misunderstanding of adequate technical procedures and side-effects has caused considerable occupational hazards and contamination of other communities not directly involved with mining activities1.
Based on gold production, one estimate has been made that 170 tonnes of Hg enter "garimpos" annually2. Mercury losses of 40% are typical when amalgam is distilled in open pans, so about 70 tonnes of Hg are lost if retorts are not used at all by miners. About 80% of these losses are emitted directly to the atmosphere while the remainder is discharged with amalgamation tailings to watercourses3. Toxic vapour taken up in worker's lungs is mirrored by high Hg content in their urine. Samples as high as 840 ppb Hg have been recorded (normal levels are below 10 ppb)3.
High Hg levels have been observed in fish from localities well away from mining activity and Hg has been measured in blood samples taken from residents of Jacareacanga, a city 200 km south of mining activity on the Tapajós River and from inhabitants of a fishing village, Brasilia Legal, 250 km to the north. Fish is the main diet in these communities and about 90% of the mercury in fish is methylated4.
Mercury has an affinity for organics in soil; raw humus ranges from 0.2 to 1.9 ppm of Hg5. Our analysis of 0.8 ppm Hg in fulvic acid isolated from an non-mineralized and uncontaminated soil from British Columbia, confirms the capacity of these substances to take up mercury from any source. These complexes will methylate by biotic or abiotic processes to be directly channelled into the food chain6.
In 1972, fish from reservoirs in northern Manitoba showed high Hg levels. No man-made source could be identified precisely, but the high Hg background of organic soils associated with a new impoundment suggested that biomethylation had occured. Natural forest fires were also attributed as a source but the amount released annually was calculated at 20 g/ha ( less than 0.02% of provincial annual natural emissions)7, creating a high short-term emission pulse. The evaluation assumed 0.4 ppm as the Hg level in timber, but only 0.08 ppm was thought to be lost during fires.
Worldwide, wild forest fires are estimated to release 20 tonnes of Hg to the atmosphere, which is less than 1% of natural emissions8. Intentional wood combustion represents 60 to 300 tonnes of Hg (about 5% of man-made emissions in 1983)9, an estimate based on a range of 0.1 to 0.5 ppm Hg in wood. Today, given high rate of deforestation by fires in the Amazon, Hg emissions derived from wood combustion must be a more important source. Deforestation in the Amazon is between 35,000 (1987) and 50,000 (1988) square kilometres per year producing mostly cattle pasture10.
Natural mercury levels in plants range from 0.001 to 0.1 ppm (dry weight). In forest ecosystems, this increases to 0.01 to 0.3 ppm11, while crops grown in soils containing less than 0.04 ppm Hg vary from 0.004 to 0.09 ppm12. Little is known about Hg distribution in the Amazon flora but aquatic macrophytes show levels between 0.1 to 1 ppm.
The temperature range encountered in vegetation fires is between 650 and 1100°C13. At 200 - 300°C, destructive distillation of about 85% of organics occurs. Mercury compounds are volatile between 25 - 450°C, and organic mercurials usually have lower boiling points than inorganic ones. When fossil fuel is burnt, more than 90 % Hg is lost14. Because most mercury in wood is present in organic form, it can be assumed that about 90% of Hg is lost from above-ground biomass. It is also reasonable to infer that as with other metals, the remainder becomes weakly bound to the ash to be leached by runoff water.
We have calculated the amount of Hg emitted by deforestation from estimates of biomass distribution in the Amazon15. We assume most Hg compounds are released from above-ground biomass even without complete combustion, whereas only a minor amount volatilizes from surface soil. Our evaluation of the efficiency of Hg release is shown in the table. Using a conservative estimate of Hg levels in plants and organic matter of 0.05 and 0.3 ppm respectively, the unit Hg emission is 17.6 g/ha (1.76 kg/km²). Considering that 50,000 km² was burnt in 1988, 88 tonnes of Hg likely were emitted to the atmosphere that year, (26 % above the estimate for mining emissions). The total area consumed up to 1991 is estimated at 404,000 square kilometres16. So, over 710 tonnes of Hg have been released from this source. If the wood levels claimed by Nriagu9 are used (0.1 to 0.5 ppm), Hg emissions from above-ground wood alone would be 117 to 585 tonnes/year.
Release of water-soluble Hg[II] from burning fossil fuel depends on the presence of chloride and/or active particulate matter but can be as high as 50 % of total mercury14,17. Combustion gases from forest fires would follow the same pattern representing imminent danger as methylation is enhanced in aquatic environments. In contrast, the conditions during amalgam-burning do not favour oxidation of mercury vapour either thermodynamically or kinetically17. Deforestation has not been considered in monitoring programs in the Amazon, although fly-ash transport is now being investigated. It is clear that deforestation is a major source of mercury emission in a form more dangerous than that emitted by "garimpos". Mercury emissions from any source must be stopped but all significant villains should be recognized.
References
(1) Veiga, M.M. and Meech, J.A. Proc. Int. Symp. on AI in Material Process. 31st Annual Conf. of Metal. of CIM, Edmonton, AB, 107-118 (1992).
(2) Ferreira, R.C. and Appel, L.E., Estudos e Documentos Ser., 13. CETEM/CNPq, Rio de Janeiro, (1991) (in Portuguese).
(3) GEDEBAM - Report for the Commission of the European Communities. GEDEBAM and SOL 3 groups, Brazil-Switzerland-UK, (1992).
(4) Malm, O. thesis. Inst. Biophysics of the Univ. Federal Rio de Janeiro, (1991) (in Portuguese).
(5) Jonasson, I. and Boyle, R.W. In: Effects of Mercury in the Canadian Environment. p.28-49. National Research Council of Canada, Ottawa (1979).
(6) Meili, M. Water, Air and Soil Pollution, 56, 333-347 (1991).
(7) Williamson, D.A. In: Technical Appendices: Summary Report of the Canada-Manitoba Agreement on the Study and Monitoring of Mercury in the Churchill River Diversion. 1, 1-162 (1986).
(8) Nriagu, J.O. Nature, 338, 47-49.(1989).
(9) Nriagu, J.O. and Pacyna, J.M. Nature, 333, 134-139 (1988).
(10) Fearnside, P.M. The Ecologist 19, 214-218 (1989).
(11) Pendias, A.K. and Pendias, H., Trace Elements in Soils and Plants. CRC Press, (1992).
(12) Gracey, H.I. and Stewart, J.W.B. Can J. Soil Sci. 54, 105-108 (1974).
(13) Raison, R.J.; Khanna, P.K.; Woods, P.V. Can. J. For. Res. 15, 132-140 (1985).
(14) Pacyna, J.M. and Münch, J. Water, Air and Soil Pollution 56, 51-61 (1991).
(15) Jordan, C.F., An Amazonian Forest: the Structure and Function of a Nutrient Stressed Ecosystem and the Impact of Slash-and-burn Agriculture. UNESCO, Pathernon Pub. Group. Series Man and the Biosphere (1989).
(16) Anonymous. Almanaque Abril. Ed. Abril, São Paulo, Brazil, (1993) (in Portuguese).
(17) Hall, B.; Schager, P.; Lindqvist, O. Water, Air and Soil Pollution 56, 3-14 (1991).
Table 1.- Estimated Hg emissions from the Amazonian biomass during deforestation.
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