Abstract
Changes in hydrochemistry and Hg distribution in the Madeira River from Porto Velho to the confluence with the Amazon River were studied in two cruises in 1997 and 2002. Water conductivity was similar in both periods, but the pH was significantly higher in 2002, in particular along the middle reaches of the river. Total suspended matter concentrations also increased from 1997 to 2002 along the same river portion, which is a result of forest conversion to other land uses, in particular pastures and agriculture accelerated during the interval between the cruises. Dissolved Hg concentrations were similar along the river in both cruises, but particulate Hg concentrations increased significantly along the middle portion of the river, although the suspended matter from 2002 was relatively poorer in Hg compared to that from 1997. Since particulate Hg represents more than 90% of the total Hg present in the river water, there was a significant increase in the total Hg transport in the Madeira River. Although gold mining has nearly ceased to exist in the region, the remobilization of Hg from forest soils through conversion to other land uses is responsible for maintaining relatively high Hg content in the Madeira River environment.
Highlights
The production of gold in the Amazon relies on the Hg amalgamation to separate fine gold particles from riverbank deposits and bottom sediments
These results suggest that large quantities of Hg are presently being remobilized from forest soils when converted to other soil uses
The draining area between stations 18 thru 10 witnessed deforestation rates increasing from about 13,000 km2.yr-1 in 1997 to nearly 23,000 km2.yr-1 in 2002 (INPE 2009). Soy production in this portion of the state increased from 20,000 tons.yr-1 in 1999 to nearly 100,000 tons.yr-1 in 2002. These results suggest that, between the two periods, total suspended sediments (TSS) impoverished in Hg were exported downstream from the previous mining areas, and larger quantities of suspended matter, poorer in Hg, were available for river transport due to the forest conversion followed by an increasing soil erosion
Summary
The production of gold in the Amazon relies on the Hg amalgamation to separate fine gold particles from riverbank deposits and bottom sediments. This simple, cheap and reliable technique produced about 2,000 tons of gold from 1975 to 2002, leaving behind nearly 3,000 tons of Hg in the region’s environment (Lacerda 2003). The Madeira River Basin was the second most important gold.
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