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From Trickle to Torrent: The Amazon Part 2

That tiny, clear, trickle of fresh water that begins making its way to the sea from high up on Nevado Mismi in Peru, becoming the mighty Amazon River along the way, is not clear or fresh when it enters the Atlantic in Brazil.  Along its more than 4,000 mile path, 500 tributaries spill their waters into it; waters highly contaminated with mercury, innumerable tons of eroded soil, raw sewage, and garbage.

We wrote in last month’s article that, contrary to long held beliefs that all the mercury contamination came from mining activities, Dr. Donna Mergler, from the Institute of Science and Environment at the University of Quebec in Montreal, has identified another and even more dangerous source.  It is fact that for every kilogram of gold mined along the Rio Tapajos, a kilogram of mercury is used to extract it leaving some 130 tons of mercury in just one of the 500 riverine systems that make up the Amazon’s ecosystem.

However, when Dr. Mergler and a team of Canadian and Brazilian scientists began the CARUSO Project in 1994, a theory began to take shape that perhaps mercury from mining was not the only source of mercury contamination in the Amazon.  What they discovered while testing water and sediment samples along a route on the Rio Tapajos, which took their boats hundreds of kilometers down river from the mining sites, is that mercury levels remained constant.  It was this constancy that caused the emergence of their new theory; if the mercury was coming solely from mining then its concentrations should have lessened the further from the mining sites they sampled.  Instead, it was constant—but why?

All along their Rio Tapajos river route they witnessed deforestation; a land rush was underway with its rampant slash and burn agriculture creating food plots and pastures and, as a result, eroded soil was washing into the river on a massive scale.  The constancy of the high levels of mercury contamination was due to the release of mercury trapped in the eroded soil, soil that was once held in place by the lush vegetation of the Amazon Rainforest…a rainforest that no longer existed along the Rio Tapajos.  Mercury in soil is considered to be relatively harmless because it is not generally absorbed by plants but, remove the plants, take away the roots that hold the soil in place and the mercury is released; mercury contaminated soil washes into waterways and blows away on the wind.

According to Dr. Marc Lucotte, one of the Dr. Mergler’s associates and a member of the CARUSO team, “Tropical soils contain very high levels of natural mercury but when you cut down the forest, this mercury is released into the river.  Then it’s transformed by bacteria in the water into what we call a “bioavailable” form, which gets into the food chain.” 

The bacteria-transformed mercury is known as methylmercury, the most toxic form of mercury, and it is taken up by plankton.  Plankton eating fish then become contaminated with mercury and up the food chain the mercury moves becoming ever more dangerous; carnivorous fish feeding upon plankton eating fish carry cumulatively higher levels of mercury and, eventually, that mercury makes its way into human hosts.

Remember that the Rio Tapajos is only one of some 500 tributaries flowing into the Amazon River Basin and that the Amazon basin serves as drainage for rivers with origins in Peru, Ecuador, Bolivia, Columbia, Suriname, Guyana, and Venezuela.  Deforestation and mining are taking place in all of those countries, too. 

And, when the Amazon empties its 200,000 to 300,000 cubic meters of water per second into the Atlantic Ocean, the mercury contamination and other pollutants also flow forth whereupon plankton becomes contaminated, plankton eating fish are fed upon by carnivorous fish…up the food chain it goes.

Now, think of the development taking place throughout Central America and the Caribbean; mountainous countries and islands with growing populations expanding into the countryside, foreigners building vacation and retirement homes on land with little or no environmental protections in place.  Although the Amazon Rainforest, producing 20% of the earth’s oxygen and losing forest at a rate of more than 3,000 square miles per year, is, perhaps, the most critical area of concern…it is not the only area being denuded of plant and animal life…it is not the only area where mercury long held within the land is washing into the seas.

After 30 years as a wild and domestic animal rescuer, rehabber, and educator in the states, Becky Bauer became a scuba instructor and award-winning journalist covering the marine environment in the Caribbean.  She is a contributing photographer to NOAA.

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