SEGH Articles

Environmental Geochemistry and Consultancy in Amazonia: from my archive

24 July 2014
The problem: mercury losses from informal gold mining and health risks to the miners (garimpeiros), gold traders and local riverine communities, by Past President Iain Thornton.

A surprise telephone call from Switzerland in 1994, instigated by the then UK Department of the Environment, and an invitation to a meeting in Brussels with the leader of a Brazilian NGO-GEDEBAM  was the start of some five years somewhat demanding and indeed exciting research along the River Tapajos, a major tributary of the Amazon. The problem: mercury losses from informal gold mining and health risks to the miners (garimpeiros), gold traders and local riverine communities. With funding from a private philanthropist and DG1 of the European Commission, my initial role together with a social anthropologist from Cambridge (and the author of the Rough Guide to Brazil) was to lead a small team of British and Brazilian workers to sample blood and urine from workers in selected mining camps and trading posts and from communities potentially exposed to mercury from the consumption of contaminated fish, fish being the staple diet in villages along the river.

Overcoming problems of importing clean sampling and storage equipment into Brazil and with the hire of a small aircraft and pilot, a base was set up at the local headquarters of the Brazilian Army at Itaituba, the major gold trading site in Amazonia. Travel from Santarem on the Amazon to Itaituba on the Tapajos took 17 hours by river boat, mainly overnight, sleeping on deck in a hammock! We were met by the President of the garimpeiros union and a band of some 10 rather agitated miners and had to convince them that we were there to for the good of their health and not to threaten their mining activities. All was well and we established a good relationship that was of great help in the future. Landing on a grass strip at our first mining camp after a 1 hour flight over dense forest was an incredible experience. In due course, blood and urine samples were collected by an army physician seconded to our team at two gold mining camps and a fish eating village on the River Tapajos. Frozen samples were transported back to the UK for mercury analysis at the Department of Clinical Biochemistry, Southampton General Hospital. At one camp, mercury in urine in 22 of the 106 individuals sampled exceeded an acceptable level in industrial workers, some greatly so. Mercury in blood in some of the fish eaters exceeded a threshold associated with neurological change (Cleary, Thornton et al.1994). Mercury levels in selected fish ranged up to 2.6mg/kg fresh weight and 21 0f a total of 51 fish samples exceeded  the EC Environmental Quality Standard of 0.30mg/kg for “a basket of fish”. Some samples of floordusts from gold trading posts exceeded 1%Hg.

A feasibility study was then undertaken at the request of the European Commission, leading to a multidisciplinary project funded for a 4-year period by the EU, with financial control and management by Imperial College Consultants, and scientific leadership by the author. A brief account of this work has been previously reported (Thornton, 2012).  The programme  involved a) setting up and equipping two speciality laboratories for Hg analysis on University campuses in the Amazon region under the direction of Dr.Olaf Malm of the Federal University of Rio de Janeiro;  b)  recruiting a medical team from the university of Odense in Denmark, to undertake detailed blood and urine sampling and make health checks on exposed populations including specific tests to identify possible neurological disorders; c)  a German team to investigate  methods of improving   the mining process to cut down losses of Hg into the river system; d) a Brazilian team to undertake dietary studies on fish eating riverine communities. The outcome was fourfold: a simple modification to the mining process was developed, tested and demonstrated to the local miners, reducing mercury losses into the river system by as much as 70%: gold traders, who burn off Hg from the amalgam to purify the gold, were persuaded to install safety equipment to reduce losses of Hg vapour into the atmosphere and thus reduce human exposure: villagers, living along the river and depending on fish as their main dietary source, were found to greatly exceed WHO recommended guidelines for Hg intake; this was reflected in raised tissue levels. Medical tests did not show any major health effects (i.e. similar to those found in exposed populations in Japan), though measureable minor neurological disorders were found. Advice was given on the consumption of different fish species, as Hg accumulates up the food chain and pisciverous fish contain the highest Hg levels. At the end of the programme, the two laboratories were donated by the EU to the two host universities, who were encouraged to set up consultancy arrangements to finance their future operation.   Finally, a regional symposium was organised by Imperial College Consultants in Santarem to disseminate the results of the work to institutions in neighbouring Amazonian countries with similar gold mining activities.

Subsequent postgraduate studies, funded by Rio Tinto, examined factors influencing mercury accumulation in different fish species and confirmed the role of biomagnification and largest concentrations in pisciverous species (Howard, 2002). Further research led to the proposal that soil erosion is a major factor leading to mercury release to rivers, supported by evidence from cores of bottom sediments in rivers and floodplain lakes suggesting that there has been a considerable input of mercury and particulate matter from soils over the last 50 years (Roulet et al, 2000). It was further suggested that infrastructure arising from mining activity had accelerated agricultural development in the Tapajos catchment with an impact on soil erosion and mercury release (Howard et al, 2002).

The current situation is difficult to assess. Several multinational groups and charities have focussed on the area with the broad aims of improving the wellbeing both of the mining and trading community and the health of the local villagers. In parallel, collaboration between the garimpeiros and international mining groups has proceeded with extensive exploration in the Tapajos region. Current activity focuses on significant surface mineralisation in the “Tapajos Gold Province” which has recently been termed “one of the largest under-explored gold regions in the world”.

A balsa on the River Tapajos—a raft with a large sluice for separating mercury/gold amalgam from river sediment.

Heating mercury/gold amalgam in an open crucible with a butane torch, releasing mercury into the atmosphere.


by Past President Ian Thornton.



Cleary,D., Thornton, I., Brown,N., Kazantzis,G., Delves,T., Worthington,S., 1994. Mercury in Brazil. Nature 369, 613-614.

Howard,B.M. 2001. Mercury Accumulation in Fishes of the Rio Tapajos, Brazilian Amazonia. Unpubl. PhD Thesis, Univ. London.

Howard,B.M., Malm,O., Payne,I., Thornton,I., 2002.Mercury in Amazonia: Assessing the Role of Artisanal Gold. Mining Environmental Management November 2002, 20-21.

Roulet,M., Lucotte,M., Canuel,R., Farella,N., Courcelles,M., Guimaraes,J.R.D., Mergler,D.,Amorim,M., 2000. Increase in Mercury Contamination Recorded in Lacustrine Sediments Following Deforestation in the Central Amazon. Chemical Geology 165, 243-266.

Thornton,I., 2012. Environmental Geochemistry: 40Years Research at Imperial College, London,UK. Applied Geochemistry 27, 939-953.