Marc Roulet

Effects of Recent Human Colonization on the Presence of Mercury in Amazonian Ecosystems

Three Hg sources were characterised and mass balance calculations were used to determine their relative contributions to the contamination of the Amazonian environment. About an order of magnitude more Hg is emitted to the atmosphere by goldmining activity than by the burning of forest biomass. However, anthropogenic atmospheric Hg cannot account for the high Hg burdens found in terrestrial ecosystems: deposition of Hg from goldmining sources is estimated to account for less than 3% of the Hg present in the surface horizons of soils. We propose that erosion of deforested soils following human colonization constitutes a major disturbance of the natural Hg cycle. Deforestation thus increases soil Hg mobilisation by runoff, which may explain the increase of Hg burdens in Amazonian aquatic ecosystems in newly colonized watersheds.

The geochemistry of mercury in central Amazonian soils developed on the Alter-do-Chão formation of the lower Tapajós River Valley, Pará state, Brazil.

In an oxisol-spodosol system developed on the terrestrial surface of the lower Tapajós Valley, the determination of total mercury (Hg), organic carbon (C), iron and aluminum oxy-hydroxide (Fe(cdb) and Al(cdb)) concentrations in the surface soil horizons are used to characterise the geochemical processes controlling the accumulation of Hg in soils under natural vegetation cover and in deforested and cultivated sites. Oxisols from the plateau have homogeneous and relatively high background Hg contents and burdens constituting an important natural reservoir of Hg for the region (90-210 ng/g dry wt. and 19-33 mg/m2 for the first 20 cm). The Fe(cdb) and Al(cdb) contents associated with the fine fraction (< 63 microns) of the soil suggest that oxy-hydroxides and, particularly Al-substituted Fe oxy-hydroxides, control the Hg concentrations observed in all of the soils of the study region. Consequently, the geochemistry of these minerals along the slopes governs the accumulation or the release of the Hg according to the natural evolution of the soil cover and/or following the degradation of soils by erosion after deforestation and cultivation. These observations have important implications for the interpretation of Hg contamination patterns observed in Amazonian aquatic systems that could be linked to different drainage sources of Hg from the terrestrial surface. The sandification and podzolisation that is characteristic of the evolution of numerous pedological systems in the equatorial Amazon could be responsible for exportation of the naturally accumulated Hg, as for other metals, by acidic complexation and migration to the black waters of the Amazon. In the central Amazon region, as a result of the fragility of the soil cover, deforestation and cultivation, affecting principally the superficial soil, promote the selective erosion of fine particles enriched in oxides and Hg. The erosion of soil could be responsible for an important release of Hg, transported in particulate form by drainage waters.

Mercury net methylation in five tropical flood plain regions of Brazil: high in the root zone of floating macrophyte mats but low in surface sediments and flooded soils.

Mercury net methylation in five tropical flood plain regions of Brazil: high in the root zone of floating macrophyte mats, but low in surface sediments and flooded soils

Sequential analysis of hair mercury levels in relation to fish diet of an Amazonian population, Brazil.

Several studies in the Amazonian Basin have shown that riverine populations are exposed to methylmercury through fish consumption. It has been suggested that seasonal variations in hair mercury observed through sequential analyses may be related to the changes in fish species ingested by the local communities. The aim of the present study was to investigate the relationship between fish-eating practices and seasonal variation in mercury exposure. A group of 36 women from a village located on the banks of the Tapajós River, a major tributary of the Amazon, comprised the present study population. An interview-administered questionnaire was used to gather information on socio-demographic characteristics, fish-eating practices and other relevant information. The women also provided hair samples of at least 24 cm in length for mercury analysis. Hair total and inorganic mercury concentration was measured using a cold vapor atomic absorption analytical method. Trigonometric regression analysis was done to assess the seasonal variation of total mercury levels. Variations in inorganic mercury were examined by repeated measures analysis of variance, and analysis of contrast variable with a polynomial transformation. The results showed that hair mercury levels varied with the season. Higher levels were observed in months corresponding to the dry season, with lower levels in the rainy season. Herbivorous fish predominated the diet for 47.2% of the women during the dry season, but this rose to 72.2% during the rainy season. Those who reported eating fish daily had higher mercury levels in hair compared to those who only ate fish a few times per week. Retrospective mercury analyses, evaluated by the quantity of mercury present in each centimeter of hair, indicate that mean mercury level of the population decreased over the 2 years prior to the study. The percentage of inorganic mercury over the total mercury in hair increased towards the extremities of the hair strand. Higher percentages of inorganic mercury were found for the group who ate more fish (on a daily consumption basis). These results support the assumption that there are seasonal variations in methylmercury exposure and also a relationship between type of fish species consumed and the resulting hair mercury levels.

Deforestation modifying terrestrial organic transport in the Rio Tapajos, Brazilian Amazon

The concentration and biomarker compositions of sedimentary organic matter (OM) as well as fine and coarse sus- pended particles were analysed to identify the impact of deforestation on the transport of terrigenous organic matter (OM) in the Rio Tapajos, a major tributary to the Amazon. Substantial shifts in the concentration and composition of recently deposited sedimentary OM suggest that intensive deforestation over the last few decades has considerably modified the natural inputs of sedimentary materials to the aquatic ecosystems by disrupting the terrigenous fluxes of humus and soil materials from the drainage basin. The observed compositional changes of bulk OM and land derived biomarkers (e.g. lignin) in recent sediments illustrate a sedimentary enrichment in OM from soil horizons that, under normal forest cover, tend to be retained in the drainage basin. On average, the recently accumulated OM is nitrogen- rich ((C/N)a=12-15) and more highly degraded ((Ac/Al)v=0.4-0.6 and DHBA/V=0.15-0.20) than deep materials ((C/N)a=20-30, (Ac/Al)v=0.25-0.4, and DHBA/V=0.05-0.10), showing that this recently accumulated material is more humified than original inputs to the aquatic system, and consistent with increased exportation of fine eroded mineral and organic particles from surface soils along river banks. The present study illustrates the relevance of using OM oxidation products in sediment profiles to evaluate deforestation impacts on aquatic ecosystems and to characterize the nature of eroded soil materials, complementing studies on mineral/metal cycling. # 2001 Elsevier Science Ltd. All rights reserved.

Methylmercury in water, seston, and epiphyton of an Amazonian river and its floodplain, Tapajós River, Brazil.

Levels and accumulation of MeHg were characterized in filtered water, suspended organo-mineral matter, phytoplankton, zooplankton and epiphyton during the dry season and the wet season. In open water of the lentic and lotic ecosystems, the MeHg in filtered water (< 0.2 microm) was near or below the detection limit (< 0.02-0.03 ng/l). These concentrations represent < 5% of the Hgtot. content in filtered water. Inundated forests (Igapó) and macrophyte floating mats were the only sites where MeHg was significantly detected (0.07-0.24 ng/l), representing 3-22% of the Hgtot. in filtered water. MeHg concentrations in organo-mineral suspended matter (2-26 ng/g dry wt. representing 0.6-7.3% of Hgtot.) were correlated with the N content but not with the C content. Data suggest that MeHg enrichment of suspended matter is strongly influenced by the presence of degraded planktonic remains relatively rich in N and MeHg. In zooplankton, MeHg concentrations (20-140 ng/g dry wt.) increased from the dry season to the end of the wet season. This increase was followed by higher proportions of MeHg during the wet season in comparison to the dry season (15-40 to 50-70% of the Hgtot.). The epiphytic material collected from the roots of macrophyte floating mats contained 2-8.5 ng/g dry wt. of MeHg. The proportion of MeHg to Hgtot. in epiphyton (1.5-8.3%) correlated with its C and N contents. The data suggest a greater bioavailability of MeHg in the Tapaj6s River ecosystems due to the seasonal increase in water level and the consequent inundation of the floodplain. Inundation favours the development of large macrophyte floating mats which increases the bioavailability of epiphyton to herbivorous/detritivorous fish. The root zone of floating macrophytes and the flooded organic horizon of the Igapó forest are the only sites along the Tapajós River where significant MeHg can be detected in the water column and sediment. This new study supports the hypothesis that MeHg production and transfer to the first link of the food chain in Amazonian river systems is closely related to organic matter biogeochemistry in the floodplain environment.

METHYLMERCURY PRODUCTION AND ACCUMULATION IN SEDIMENTS AND SOILS OF AN AMAZONIAN FLOODPLAIN - EFFECT OF SEASONAL INUNDATION ?

This study investigated the spatialand seasonal variations of MeHg concentrations andburdens of different sediments and soils of theTapajós river floodplain, one of the majorclear-water tributaries of the Amazon. The smallfloodplain of the Tapajós is typical of Amazonianfloodplain ecosystems. The studied lakes are borderedby inundated forest (igapó), while floatingmacrophyte mats (Paspalum sp.) develop at themargin of lakes during the flooded season. During theflood, we observed very low MeHg concentrations in theopen water lake sediments (<0.5 ng g-1 d.w or<0.5 μg m-2 cm-1 d.w.) as compared to thesemi-aquatic sediments of the macrophyte zone (0.2–1.4 ng g-1 d.w or 1–3 μg m-2cm-1 d.w.) and the igapó semi-terrestrial soils (0.2–3 ng g-1 d.w or2–12 μg m-2 cm-1 d.w.). The litter horizon fromthe igapó soils showed the highest value of MeHg(4–8 ng g-1 d.w.) representing 0.2–2 μg m-2cm-1 d.w. at the sediment/water interface during theaquatic phase. The inundation had no effect on theconcentrations and burdens of MeHg in the sediment ofthe central part of the lake. The inundation had aclear effect on the methylation of Hg at the surfaceof semi-aquatic shoreline sediments (macrophyte zone)and semi-terrestrial forest soils, where MeHgconcentrations and burdens appeared to be 3 timesgreater following inundation. In all cores, total Hgconcentrations follow those of Fe and Aloxy-hydroxides, whereas the MeHg concentrations arelinked to organic matter quality and quantity. It issuggested that organic matter and inundation controlMeHg production and accumulation at the benthicinterface. These results confirm previousobservations, in the same study area, of net203Hg methylation potentials. The fresh andlabile organic matter in the litter of the igapóforest appears as the most important factor leading tosignificant enrichment of MeHg in these particularterrestrial/aquatic sedimentary environments.

Sulfate-Reducing Bacteria in Floating Macrophyte Rhizospheres from an Amazonian Floodplain Lake in Bolivia and Their Association with Hg Methylation

ABSTRACT Five subgroups of sulfate-reducing bacteria (SRB) were detected by PCR in three macrophyte rhizospheres (Polygonum densiflorum, Hymenachne donacifolia, and Ludwigia helminthorriza) and three subgroups in Eichhornia crassipes from La Granja, a floodplain lake from the upper Madeira basin. The SRB community varied according to the macrophyte species but with different degrees of association with their roots. The rhizosphere of the C4 plant Polygonum densiflorum had higher frequencies of SRB subgroups as well as higher mercury methylation potentials (27.5 to 36.1%) and carbon (16.06 ± 5.40%), nitrogen (2.03 ± 0.64%), Hg (94.50 ± 6.86 ng Hg g−1), and methylmercury (8.25 ± 1.45 ng Hg g−1) contents than the rhizosphere of the C3 plant Eichhornia crassipes. Mercury methylation in Polygonum densiflorum and Eichhornia crassipes was reduced when SRB metabolism was inhibited by sodium molybdate.

Characterization of pesticide consumption in the county of Santarém, Pará, Brazil

Many potentially harmful pesticides for both human health and the environment are used in Brazilian Amazon. However, no scientific datum on pesticide usage is presently available for this region. Consequently, it is difficult to assess which substances arc used and in which quantities. As an important premise for future work on pesticide contamination in the county of Santarem (State of Para, Brazil), a survey was conducted in order to qualify and quantify the use of some pesticides in this region. This investigation was made between January and March 1997 and August and October 1998 and revealed use of several organophosphates, synthetic pyrethroids and carbamates insecticides. Furthermore, many herbicides and fungicides were listed. These pesticides are used for agriculture, domestic, and sanitary programs. This paper also provides a first estimation of quantities of some insecticides commonly used in agriculture (chlorpyrifos, malathion, metamidophos and methyl-parathion). The annual consumption for these four compounds is estimated at ca. 1 910 kg. Organophosphate insecticide consumption in the county of Santarem seems to be lower than the Brazilian average in terms of «per capita» and «per agricultural area» consumptions. Nevertheless, this county uses toxic substances on sensitive environments such as floodplains (varzeas), making relevant a thorough study on the potential contamination of this environment and its biota.