Relationship of mercury with aluminum, iron and manganese oxy-hydroxides in sediments from the Alto Pantanal, Brazil

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Abstract

Sediments from nine floodplain lakes in Pantanal were analyzed for a large-scale (300 km) survey of mercury (Hg) load in sediments and soils of the Alto Pantanal and to study the relationship between Hg and reactive aluminum, iron, and manganese oxy-hydroxides. The results were compared with the Hg content in river and stream sediments from the Poconé gold mining area, where Hg has been extensively used and still is in use. The results indicate that the Hg concentrations were elevated in river sediment close to the mining area in Bento Gomes river basin (average in the <74-μm fraction 88.9 ng Hg g−1 dry wt.; interquartile range 50.3–119.5), but there was no clear indication that the local Hg emissions have contaminated the remote floodplain lakes, where concentrations were surprisingly low (average in the <74-μm fraction 33.2 ng Hg g−1 dry wt. sediment; interquartile range 18.4–46.8), in particular when considering geochemical characteristics of the sediment. The sediment from the floodplain lakes contained less Hg-tot and more reactive iron oxy-hydroxides than soils from the Tapajós area in the Amazon basin. This resulted in a mass ratio between Hg and amorphous oxy-hydroxides of only 5×10−6 for Hg-tot/Fe-oxa (interquartile range 3–7×10−6).

Introduction

Artisanal gold-mining has been suspected to be responsible for the release of much of the mercury encountered in tropical floodplains such as the Amazon and the Pantanal in Brazil (e.g. Lacerda et al., 1990). During the goldrush of the past decades, an estimated 2000 t of Hg has been released in the Amazon region (Veiga et al., 1995). Furthermore, the Spaniards brought another estimated 260 000 t of Hg into Latin America between 1550 and 1930, mostly used for silver amalgamation (Nriagu, 1993, Lacerda, 1997). On the other hand, most Hg in remote soils and sediments is supplied by atmospheric deposition (Fitzgerald et al., 1998), which can be of natural or anthropogenic origin. Large quantities of Hg can be accumulated in some soils and can eventually reach surface waters, in particular at improper management. Forsberg et al. (1999) estimated that the soils in the Rio Negro basin (a tributary to the Amazon river) contains considerably more Hg than could possibly be of anthropogenic origin, in spite of three times higher global Hg deposition nowadays compared to preindustrial deposition. This Hg is leached from the soil by organic acids and can cause Hg concentrations in fish to exceed the limit for safe consumption (Nakazono et al., 1999). Roulet et al., 1998a, Roulet et al., 1998b suggested that Hg entering the Amazon soil is initially firmly bound to soil constituents such as sesquioxides and crystalline Fe and Al oxy-hydroxides. It is first when the soil or sediment reaches saturation that Hg leaks into the water courses in potentially bioavailable forms. The bound Hg can also reach rivers if the soil constituents themselves are relocated to the water courses due to erosion or wash out of soil particles. Roulet et al., 1998a, Roulet et al., 1998b estimated that 97% of the Hg burden to the lower Tapajós river (a tributary to the Amazon river) originates from soil eroded Hg, in spite of direct pollution from upstream (>50 km) gold mining using Hg. They also demonstrated that the Hg burden is not related to organic matter content in the oxisols and spodosols under rainforest in the Tapajós valley, contrary to what is known from temperate areas with boreal forests (e.g. Meili, 1997). Instead they identified crystalline Al and Fe oxy-hydroxides to be the soil constituents determining the capacity to sorb Hg. Also amorphous Al and Fe oxy-hydroxides as well as Mn oxy-hydroxides have capacity to sorb heavy metals (McKenzie, 1980), but were not as abundant in the actual soils.

The aim with the present study was to survey the Hg levels in sediments and soils of the Alto Pantanal and to quantify relationships between the sediment Hg content and the abundance of amorphous and crystalline Al, Fe, and Mn oxy-hydroxides in Alto Pantanal. An additional aim was to estimate Hg amounts relocated by water courses from gold mining areas, where Hg has been extensively used and still is in use. The presented data are obtained in a project financed by Sida/SAREC (Swedish International Development Co-operation Agency/Department for Research Co-operation) with the aim to study the accumulation and transformation of mercury in tropical rivers and wetland in Brazilian Pantanal and in the environmental monitoring program run by FEMA (the governmental, environmental foundation of Mato Grosso state) in the Bento Gomes river basin, Brazil.

Section snippets

Study area

The savanna wetland Pantanal is a 137 000-km2 large alluvial plain in central-western Brazil facing the border of Bolivia and Paraguay, receiving water from a surrounding upland drainage basin that occupies 359 000 km2 (Hamilton et al., 1996). The area is hydrologically divided into the Alto (high), Médio and Baixo (low) Pantanal. Floodplains make up approximately 20% of the upstream watershed (Alto Pantanal) and most of the area in the Baixo Pantanal (Alho et al., 1988). The rain falling from

Material and methods

The samples were collected during three campaigns, two at the end of the dry seasons in August–September/October 1997 and 1998, and the third campaign at the end of the rainy season in March 1998. Water pH, dissolved oxygen, temperature, conductivity and turbidity were measured in the field with a multipurpose field instrument (Horiba U-10) at all sampling locations.

Superficial stream sediment was collected at 40 sites in the Bento Gomes river basin around Poconé (<30 km) at the end of the dry

Results

A wide variety of surface waters was found in the Alto Pantanal. Rio Cuiabá is a white water river with a high load of suspended material, which increases downstream from the city of Cuiabá (Table 1). The tributary Rio Mutum has dark water with signs of oxygen depletion, and with an insignificant load of suspended material so the dissolved organic matter will dominate its characteristics (Table 1). This type of water is known to increase the mobility and probably also the bioavailability of Hg (

Discussion

Low Hg-tot concentrations were found in sediments of the Alto Pantanal, except in the immediate vicinity of a major gold-mining site. Also relative to the abundance of sesquioxide forming metals, Hg-tot concentrations were lower than in other tropical floodplain areas. The Hg-tot concentration in the lake sediments ranged from below 10 ng Hg g−1 dry wt. in some deep sandy horizons to above 50 ng Hg g−1 dry wt. (Fig. 2). River bottom sediments sampled in 1992 from the same area as the present

Conclusions

The Hg-tot content in fine sediments (<74 μm) of the Alto Pantanal, apart from the immediate vicinity of present or former gold mining activities, is generally below 50 ng Hg g−1 dry wt., which is lower than in the Amazon region as well as in many temperate areas. Hg-tot/Al and Hg-tot/Fe ratios were lower than those presented by Roulet et al. (1998a) for soils and sediments in the Tapajós basin and other tropical areas of South America, even though only amorphous metal oxy-hydroxides were

Acknowledgements

Work supported by Sida/SAREC (Swedish International Development Co-operation Agency/Department for Research Co-operation), PRODEAGRO (the program for agro-environmental development in Mato Grosso state, Brazil), and the Federal University of Mato Grosso, Brazil. We thank Leila M.C. Singulane, Sandra M. Laet, Luis B. Barreto, and Salatiel A. Araujo at the mining division of FEMA (the governmental, environmental foundation of Mato Grosso state) for technical support. A special thanks to

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    Present address. Institute of Applied Environmental Research (ITM), Stockholm University, S-106 91 Stockholm, Sweden.

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