Assessing mercury contamiantion in the Amazon

3. Discussion

3.3. Background concentrations of mercury in the Amazon region

According to Lucas et al. (1996), sandification and podozolization processes control the evolution on slopes. In the Amazon this is seen as a clay gradient from high clay concentrations on plateau remnants to low clay concentrations at the valley bottom (Fig. 1). This is interesting because mercury is not homogeneously distributed over the various grain size fractions in soils and sediments, but is generally found in the fine-grained fractions consisting mainly of clay minerals.
In the very clayey ferrasols, typical of plateau remnants, total mercury concentrations varied between 212 to 439 ng Hg / g d.w. (Forsberg et al., 1999; Lechler et al., 2000; Roulet et al., 1998b). In mid slope ferrasols total mercury concentrations varied between 136 to 210 ng Hg / g d.w. (Roulet et al., 1998b; Malm et al., 1995). Finally, total mercury concentrations in podzolized ferrasols were circa 43 to 83 ng Hg / g d.w (Roulet al., 1998b).
Background mercury concentrations in surface sediment varied from 100 to 250 Hg / g d.w. in the Tapajos River (Roulet et al., 1998). In the Madeira River, from Porto Velho and 400 km downstream, mercury concentrations in sediments ranged from 245 to 439 ng Hg / g d.w. (Lechler et al., 2000). Although sample sites were downstream from gold mining locations no systematic downstream Hg-trends were observed in sediments or filtered water from the Madeira and Tapajos River (Lechler et al., 2000; Roulet et al., 1998, respectively).
Dissolved mercury concentrations in the Madeira, Tapajos and Amazon River were 0.5-3.7, 0.4-2.8 and 1.31 ng Hg / l, respectively (Lechler et al., 2000; Roulet et al. 1998a; Roulet et al. 1998a, respectively). Furthermore, mercury concentration in the fine particulate matter of the Tapajos River were 0.28-13.3 ng / l, which is one order of magnitude lower than previously reported Hg-concentrations in Amazonian waters (Roulet et al. 1998a).
Average total mercury concentration was 3.05 ng m3 in the air over the Amazon region (Artaxo et al., 2000). However, over pristine areas mercury concentrations ranged between 0,5-2 ng3 and over gold mining areas mercury concentrations were as high as 14.8 ng 3.
In summery, mercury concentrations in Amazonian soils, sediments and air are high compared to global averages. Mercury bounded to fine particulate matter in rivers is also higher than global values. Dissolved mercury concentrations in the Madeira and Tapajos River are similar to global averages.

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