Assessing mercury contamiantion in the Amazon

3. Discussion

3.4.3. Soil erosion

Recent studies indicate that soil erosion may influence the natural biogeochemical cycle of mercury in the Amazon (Forsberg et al., 1999; Lechler et al., 2000; Roulet et al., 2000). The term soil erosion is used when anthropogenic activities such as deforestation, cultivation and mining result in soil erosion. Roulet et al. (2000) analyzed vertical profiles of sediments from the Tapajos River for mercury, textural indicators (water content and dry density), mineralogical indicators (iron and alu-minum associated with oxyhydroxides and aluminumsilicates) and organic indicators (carbon, nitrogen, C/N ratio). The results demonstrate that soil erosion is responsible for an overall enrichment of recent sediments by fine clay particles rich in mercury. Furthermore, the mercury levels in the sediments of the Tapajos River had the same relationship with aluminosilicates of soils.
Addition-ally, the activity of lead-210 suggested that surficial sediments originated from eroded soils. A preliminary dating indicated that the environmental changes recorded in the sediment began sometime between the 1950s and 1970s. This coincides with the colonization of the Brazilian Amazon.
Direct relationship between arsenic (As) and Hg were also found in sediments of the Madeira River, suggesting that As and Hg originated from a soils (Lechler et al., 2000).
Studies in the Tapajos and Madeira region have shown a lack of temporal and spatial Hg trends downstream of gold mining areas. (Lechler et al., 2000; Roulet et al., 1998b). This suggests that the mercury found in the various environmental compartments is related to a regional geological source rather than a local anthropogenic source. The geological source seems to be the naturally occurring mercury in soils.
In summery, anthropogenic activities in the Amazon, especially gold mining and soil erosion, seem to be important sources of Hg contamination to the aquatic environment. Soil erosion may have a more regional effect, while the effect of gold mining is more local. Natural soils may represent the largest reservoir of mercury in the Amazon region.

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