A dietary cost of our appetite for gold

This Mothers Day, many moms will find their brood and mates proffering glittering booty: sparkling necklaces, earrings, bracelets, brooches, and rings fashioned in whole or in part of gold. There may also be gilded plates, glasses, and grandmas favorite–fragile, matched sets of hand-painted tea cups and saucers.

As women admire these tokens of their loved ones admiration, few will give passing thought to where the precious metal in their gifts came from. New research indicates that in some regions of the world, the mining of gold produces an unrecognized toxic fallout: fish dinners laced with methylmercury.

The problem traces to a common gold extraction process that relies on mercury. In Brazil, the mining of some 90 tons of gold per year releases roughly 130 tons of mercury into local rivers. Similar mercury releases are likely to occur in other developing countries where gold is mined.

One such country is French Guiana, where an official industry and a bootleg one have been mining gold since the mid-19th century. In a 1994 survey of mercury exposures, few people in French Guiana showed signs a problem–except in southern native Amerindian communities. Four Wayana villages along the upper Maroni River were most affected. Their 500 residents live in relatively isolated communities accessible only by helicopter or dugout canoe.

To identify the source of the Wayanas tainting, Alain Boudou of the aquatic ecosystems toxicology laboratory in Arcachon, France and his colleagues interviewed local village chiefs, health agents, and school teachers to learn more about the local communities largely subsistence culture and diet. Because aquatic environments are especially effective at transforming elemental mercury into toxic methylmercury, and because fish readily accumulate that toxic form of the metal, the scientists decided to probe the villagers fishy diet.

They selected 23 households, totaling about 165 people, throughout the area. For a week in the spring and again in the fall, the researchers logged all of the fish consumed in each household, noting the species, number, size, and weight. To calculate per capita consumption, the scientists also recorded who had been present each day to eat those fish.

The researchers then organized fishing expeditions to acquire the same species from the same portions of the river for mercury analysis and took samples of human hair, which stores residues of the methylmercury that individuals ingest.

In the May Environmental Health Perspectives, Boudous group reports finding that hair from 57 percent of the people sampled confirmed a tainting with mercury in concentrations that exceeded 10 micrograms per gram of hair, the safety limit set by the World Health Organization. Of the 270 fish caught in local rivers, representing 4 dozen species, roughly one in every seven exceeded the U.S. and Brazilian safety limit for mercury–500 nanograms/gram wet weight.

Fish that eat other fish, rather than plants, had the highest mercury contamination and accounted for most of the fish-borne mercury eaten by the Wayanas. In fact, the French researchers found that just four piscivorous species, constituting 28 percent of fish consumption, accounted for 72 percent of the local communitys mercury exposure from fish.

Boudous team calculated that adults in these communities were consuming an average of 40 to 60 micrograms per day of mercury–some 6 to 10 times the international average consumption per individual from all sources. Moreover, the scientists note, because birds and other game also bioaccumulate the toxic heavy metal, total dietary mercury intake among the Wayanas is undoubtedly higher than the new calculations would indicate.

Because methylmercury poisons the nerves, overly exposed children often exhibit behavioral impairments and delays in learning. Not surprisingly, Boudou and his coworkers found evidence of such problems in Wayana children.

Though there may be natural sources of mercury in the Wayana environment, much of the metals tainting of local aquatic life and its predators appears to have come from gold mining upstream, the researchers note.

Nor are people the only creatures at risk. In the April Conservation Biology, an international team of researchers reports evidence that fish downstream of gold-mining operations in Brazil’s Amazon basin “are already at high risk of mercury [poisoning], especially reproductive failure.”

Animals that feed on mercury-enriched fish also face poisoning, the scientists note. They point to the Amazon’s giant otter (Pteronura brasilensis), the most endangered otter in the world. The concentration of lead in some fish caught in the Brazilian study exceeded the “maximum tolerable [lead] intake for European otters,” the researchers found. Because it eats mainly fish, the Amazonian river dolphin, or boto (Inia geoffrensis), faces a similar risk of mercury poisoning. This dolphin is also at risk of extinction because of other pressures.

The Amazonian jaguar (Panthera onca), which dines on both fish and giant otters, also risks mercury poisoning. However, the scientists point out, at present, “there is no information on toxic levels of dietary mercury for this species.”

All this just goes to show that even the clean lines of a tiny gold earring post may have left a relatively toxic footprint somewhere upstream in the global environment.

Janet Raloff

Janet Raloff is the editor of Science News for Students, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer.

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