Assessing the contribution of natural sources to regional atmospheric mercury budgets

Naturally mercury-enriched substrate is a long-lived source of mercury to the global atmospheric mercury cycle. Field flux chambers, laboratory gas exchange chambers and micrometeorological methods may be applied to estimate emissions from these sources. However, field chamber experimental design may affect the magnitude of the fluxes measured, and the laboratory chamber only provides a minimum estimate of flux. Many factors, such as mercury concentration and speciation in substrate, light, precipitation, and temperature, influence the emission of mercury from the substrate. Mercury concentration in the substrate is a dominant factor controlling emissions and may be used to predict emissions from regions of mercury enrichment. Mercury fluxes measured from three areas of natural enrichment and three areas with low levels of mercury enrichment are 1-5 orders of magnitude greater than the value applied to global belts of natural enrichment. Preliminary scaling of emissions from one of these areas and for western North America indicates that mercury enriched areas may be significant sources of mercury to the atmosphere, and that their contribution to regional and global atmospheric budgets needs to be reassessed.     

Organochlorine contaminants in endangered Steller sea lion pups (Eumetopias jubatus) from western Alaska and the Russian Far East

Investigations into the cause of the Steller sea lion population decline have focused on numerous factors, including exposure to toxic contaminants such as organochlorines (OCs). OCs, such as polychlorinated biphenyls (PCBs), 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane, or dichlorodiphenyltrichloroethane (DDT), have been associated with various biological effects in marine mammals. We measured these compounds in whole blood of free-ranging Steller sea lion pups in order to determine the extent and magnitude of contamination across their geographical range. Of 212 pups analyzed for OCs, 76 pups (36 females and 40 males) were from western Alaska and the other 136 (63 females and 73 males) were from the Russian Far East. Concentrations of summation SigmaPCBs in the whole blood of pups from western Alaska ranged from 0.21 to 13 ng/g wet weight with a mean of 2.1+/-0.27 ng/g wet weight. In the Russian animals, summation SigmaPCB concentrations in the whole blood of pups ranged from 0.33 to 36 ng/g wet weight with a mean of 4.3+/-0.44 ng/g wet weight. summation SigmaDDT concentrations in the whole blood of pups from western Alaska ranged from 0.18 to 11 ng/g wet weight with a mean of 1.6+/-0.23 ng/g wet weight. In Russia, summation SigmaDDT in the whole blood of pups ranged from undetectable to 26 ng/g wet weight with a mean of 3.3+/-0.36 ng/g wet weight. Average OC concentrations were significantly higher in the blood of Russian animals compared to western Alaska (for PCBs and DDTs, p<0.001) and in both areas females had higher concentrations than males. Male pups from western Alaska had significantly lower levels of summation SigmaPCBs and summation SigmaDDT when compared to male pups from Russia (for PCBs and DDTs p<0.001). Female pups from western Alaska were significantly lower in summation SigmaPCBs than Russian female pups (for PCBs p=0.009) as were female pups for summation SigmaDDT levels between areas (for DDTs p=0.026). OC contaminants data indicate that Steller sea lion pups have measurable concentrations of these synthetic chemicals. While any physiological effect and the specific role these chemicals may have in either the decline or the failure of the endangered Steller sea lion population to recover needs to be further investigated, this study indicates specific areas and animals that may be most at risk.     

Air-soil exchange of mercury from background soils in the United States

The air-surface exchange of mercury (Hg) was measured, using a dynamic polycarbonate flux chamber, for soils with low or "background" Hg concentrations (<0.1 mg/kg) at eleven locations across the contiguous United States. Sampling locations included agricultural, desert, grassland, mixed and pine forest ecosystems (n=1326 soil flux measurements at 46 individual sites). An overall soil Hg flux of 0.9+/-0.2 ng/m2/h for these background soils was obtained by averaging the means for the different locations. Soil Hg fluxes were significantly lower in dark conditions than in the light for all but the grassland sites. Mean inlet air Hg concentrations were 1.0+/-0.1 ng/m3 in the dark and 1.3+/-0.2 ng/m3 in the light. Soil temperature inside and outside of the chamber, air temperature, relative humidity, and irradiance were measured concurrently with soil Hg flux. Soil-air Hg exchange was weakly predicted by environmental variables (R2 from 0.07 to 0.52). For a single location, flux was better correlated with soil moisture than other measured environmental parameters, suggesting that soil moisture might be an important driver for Hg emissions from background soils. In addition, based on data collected we suggest some quality control measures for use of Tekran 2537A analyzers when measuring low mercury fluxes. Using basic scaling procedures, we roughly estimate that natural emissions from soils in the contiguous U.S. release approximately 100 Mg/yr of Hg to the atmosphere.     

Spatial and temporal changes of chlorinated pesticides, PCBs, dioxins (PCDDs/PCDFs) and brominated flame retardants in human breast milk from Northern Russia

This study presents for the first time temporal changes of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) in Russian human breast milk samples. Concentrations of OCPs and PCBs in samples from three locations in the North West of Russia in 2000-2002 (n=42), were compared to corresponding levels measured in 1993-1996 (n=58). In addition brominated flame retardants (BFRs), consisting of polybrominated diphenylethers (PBDEs) (including BDE-209) and hexabromocyclododecane (HBCD) were analysed in samples from 2000-2002 (n=37). The present levels of SigmaDDTs and SigmaHCHs were 5 and 10 times higher than corresponding levels in the neighbouring country Norway. Median concentrations of SigmaHCHs (196 microg/kg lw), SigmaCHBs (19.7 microg/kg lw) and SigmaPCBs(16) (316 microg/kg lw) were highest in Murmansk. The percentage of p,p'-DDT to SigmaDDTs and ratio DDE/DDT suggest possible ongoing use of DDT in Russia. Levels of PBDE were low and dominated by the congeners BDE-47 and BDE-153. The deca brominated BDE-209 was detected in all analysed samples (median concentration 0.19 microg/kg lipid). Levels of SigmaOCPs and SigmaPCBs decreased 56 and 30% in Murmansk and 36 and 43% in Arkhangelsk during the study period. The decline of SigmaOCPs was significant at both locations (p<0.05-p<0.0001). For SigmaPCBs, the decreasing trend was only significant in Arkhangelsk (p<0.0001). In addition, a decline of Sigmatotal TEQs (SigmaTEQs of PCDDs/PCDFs, non-ortho- and mono-ortho PCBs) was observed in Murmansk and Arkhangelsk during the study period. The exposure of infants by PCDDs/PCDFs and dioxin-like PCBs is still exceeding the daily tolerable intake (TDI) in North West Russia. However, the concentrations of PCDDs/PCDFs and dioxin-like PCBs seem to decline very rapidly.     

Foliar exchange of mercury as a function of soil and air mercury concentrations

Previous research has indicated that foliar mercury (Hg) flux is bi-directional, with influence from both atmospheric and soil Hg. This work investigated the role of soil and air Hg concentrations on foliar Hg exchange using a single-plant gas-exchange system. The exchange of Hg vapor with aspen seedlings grown in soil Hg concentrations of 0.03+/-0.01, 5.8+/-0.5, and 12.3+/-1.3 microg g(-1) and exposed to atmospheric Hg concentrations of 2.4+/-0.5, 11.0+/-0.9, and 30.4+/-2.2 ng m(-3) was measured. At background atmospheric Hg concentrations of 2.4 ng m(-3), foliage released Hg at all three soil Hg concentrations and fluxes ranged from 1.6 to 5.5 ng/m(2)/h. At higher atmospheric Hg concentrations (>11 ng m(-3)), net deposition to foliage ranged from -9 to -47 ng/m(2)/h, exhibiting increase uptake with higher air Hg concentrations. Fluxes associated with aspen showed an immediate response to changes in atmospheric Hg concentrations. Compensation points, the air concentration where no net flux of Hg vapor occurred, were 3-4 ng m(-3) in the light and 2-3 ng m(-3) in the dark for trees grown in soils of 0.03 and 6 microg g(-1) Hg content, and 5-6 ng m(-3) in the light and 2.5-3.5 ng m(-3) in the dark for trees grown in 12 microg g(-1) Hg soils.     

Fulvic acid in microlayer waters of the Gerlache Inlet Sea (Antarctica): their distribution in dissolved and particulate phases

In this work we studied the distribution of humic compounds between microlayer and subsurface waters. Both dissolved (DFA) and particulate (PFA) matter were considered. Humic acids were not found. The amount of total particulate matter is the same both in microlayer and subsurface waters. Conversely, particulate organic matter and fulvic acid concentrations were greater in microlayer waters. Enrichment factors (microlayer PFA/subsurface PFA and microlayer DFA/subsurface DFA) show a greater presence of fulvic acids in the microlayer. Moreover, the enrichment process is greater for the particulate phase. C/N values both in microlayer and subsurface waters are greater for DFA with respect to PFA and C/N values for DFA of microlayer are greater than that of subsurface waters. So DFA could be older than PFA, and DFA of subsurface waters younger than that of microlayer waters. For PFA S/C ratios in microlayer waters are greater than that of subsurface waters, underlining that the enrichment process is selective as regards sulphur-containing structures.     

Phosphorus cycling through phosphine in paddy fields

Phosphine emission fluxes from paddy fields, phosphine ambient levels in air, and the vertical profile of matrix-bound phosphine in soil have been measured throughout the growing season of rice in Beijing, China. It was found that both the seasonal and diurnal emission fluxes and ambient levels fluctuate significantly. During the drainage period, phosphine released from the soil with the highest diurnal average flux on the first period of drainage (approx. 17.7 ng m(-2) h(-1)), whereas its highest ambient level (approx. 250 ng m(-3)) occurred at 06.00 h. During the flooded period, phosphine emission was low, and the peaks of phosphine emissions occurred at midnight. The average flux of PH3 emission for the whole season was found to be approximately 1.78 ng m(-2) h(-1). The mass fraction of matrix-bound phosphine is approximately 0.18 approximately 1.42 x 10(-7) (m/m) part of organic phosphorus or 3.4 approximately 9.2 x 10(-9) (m/m) part of total phosphorus in paddy soil. The amount of phosphine emitted to the atmosphere was only a small fraction of the phosphine that remained in the soil in the matrix-bound form. Soil serves both as the source and the sink of PH3.     

Modelling of mercury emissions from background soils

Emissions of volatile mercury species from natural soils are believed to be a significant contributor to the atmospheric burden of mercury, but only order-of-magnitude estimates of emissions from these sources are available. The scaling-up of mercury flux measurements to regional or global scales is confounded by a limited understanding of the physical, chemical and biochemical processes that occur in the soil, a complex environmental matrix. This study is a first step toward the development of an air-surface exchange model for mercury (known as the mercury emission model (MEM)). The objective of the study is to model the partitioning and movement of inorganic Hg(II) and Hg(0) in open field soils, and to use MEM to interpret published data on mercury emissions to the atmosphere. MEM is a multi-layered, dynamic finite-element soil and atmospheric surface-layer model that simulates the exchange of heat, moisture and mercury between soils and the atmosphere. The model includes a simple formulation of the reduction of inorganic Hg(II) to Hg(0). Good agreement was found between the meteorological dependence of observed mercury emission fluxes, and hourly modelled fluxes, and it is concluded that MEM is able to simulate well the soil and atmospheric processes influencing the emission of Hg(0) to the atmosphere. The heretofore unexplained close correlation between soil temperature and mercury emission flux is fully modelled by MEM and is attributed to the temperature dependence of the Hg(0) Henry's Law coefficient and the control of the volumetric soil-air fraction on the diffusion of Hg(0) near the surface. The observed correlation between solar radiation intensity and mercury flux, appears in part to be due to the surface-energy balance between radiation, and sensible and latent heat fluxes which determines the soil temperature. The modelled results imply that empirical correlations that are based only on flux chamber data, may not extend to the open atmosphere for all weather scenarios.     

Seasonality of diffusive exchange of polychlorinated biphenyls and hexachlorobenzene across the air-sea interface of Kaohsiung Harbor, Taiwan

Gaseous and dissolved concentrations of polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB) were measured in the ambient air and water of Kaohsiung Harbor lagoon, Taiwan, from December 2003 to January 2005. During the rainy season (April to September), gaseous PCB and HCB concentrations were low due to both scavenging by precipitation and dilution by prevailing southwesterly winds blown from the atmosphere of the South China Sea. In contrast, trace precipitation and prevailing northeasterly winds during the dry season (October to March) resulted in higher gaseous PCB and HCB concentrations. Instantaneous air-water exchange fluxes of PCB homologues and HCB were calculated from 22 pairs of air and water samples from Kaohsiung Harbor lagoon. All net fluxes of PCB homologues and HCB in this study are from water to air (net volatilization). The highest net volatile flux observed was + 172 ng m^− 2 day^− 1 (dichlorobiphenyls) in December, 2003 due to the high wind speed and high dissolved concentration. The PCB homologues and HCB fluxes were significantly governed by dissolved concentrations in Kaohsiung Harbor lagoon. For low molecular weight PCBs (LMW PCBs), their fluxes were also significantly correlated with wind speed. The net PCB and HCB fluxes suggest that the annual sums of 69 PCBs and HCB measured in this study were mainly volatile (57.4 × 10³ and 28.3 × 10³ ng m^− 2 yr^− 1, respectively) and estimated yearly, 1.5 kg and 0.76 kg of PCBs and HCB were emitted from the harbor lagoon surface waters to the ambient atmosphere. The average tPCB flux in this study was about one-tenth of tPCB fluxes seen in New York Harbor and in the Delaware River, which are reported to be greatly impacted by PCBs.     

Temporal trends in gaseous mercury evasion from the Mediterranean seawaters

Mercury evasion from seawaters is considered to be one of the main natural sources of mercury released to the atmosphere. The temporal evolution of this mechanism is related to biotic and abiotic processes that produce mercury in its elemental form and as DGM. The efficiency of these processes depends upon the intensity of the solar radiation, the ambient temperature of the air parcel above the seawater, and the water temperature. In the Mediterranean region, the magnitude of these mechanisms are particularly significant, due to favorable climate conditions and to the presence of large cinnabar deposits that cross the whole region; all these synergic factors yield significant evasional fluxes of mercury from the surface water during most of the annual period. In this work, mercury fluxes were measured by using a floating flux chamber connected to an atomic absorption analyzer. Photosynthetic active radiation (PAR) and UV components of the solar radiation were measured using the same system adopted in the EC 'ELDONet project'. The measurements of the mercury evasional fluxes were carried out at three sites of the northern Tyrrhenian Sea during 1998. Two sites were located at unpolluted and polluted coastal areas, and the third was an offshore site. The evasional flux showed a typical daily trend, highest at midday when the ambient temperature and solar radiation were at the maximum, and lowest, near to zero, during the night. Besides the day-night behavior, a seasonal trend was also observed, with minimum values during the winter period (0.7-2.0 ng/m2 h) and maximum values during the summer (10-13 ng/m2 h).     

Historical contamination of Yukon Lake sediments by PCBs and organochlorine pesticides: influence of local sources and watershed characteristics

PCBs and other persistent organochlorine (OC) pesticides were analyzed in sediment cores collected from six lakes in Yukon Territory and one in northern British Columbia, Canada, with the objective of establishing sources and historical trends of these contaminants. DDT was found to be the most prominent OC in the sediment profiles of most of the lakes. Maximum sigmaDDT levels (3.47-2680 ng g(-1) dw) were observed in sediment slices dated to the 1950s from lakes near populated areas. In contrast, in more remote lakes (Hanson, Kusawa and Lindeman), the maximum sigmaDDT concentrations were observed in the sediments dated to the 1970s. Highest sigmaPCB and sigmaDDT concentrations were measured in sediments from Watson Lake, near a suspected PCB waste disposal site and in a region where DDT was heavily applied in the 1950s and 1960s. Elevated sigmaPCB concentrations [16.1-93.6 ng g(-1) dry weight (dw)] were also observed in sediments from lakes situated near populated areas, relative to Kusawa and Lindeman (11.1 and 12.7 ng g(-1) dw, respectively). Recent sigmaPCB fluxes ranged from 621 ng m(-2) y(-1) in Kusawa Lake to 16400 ng m(-2) y(-1) in Little Atlin Lake. The extremely high sedimentation rate (2050 g m(-2) y(-1)) in glacial fed Lindeman Lake gave rise to elevated fluxes of sigmaPCB (2410 ng m(-2) y(-1)) and other OCs, despite much lower concentrations in the sediment. Levels of hexachlorocyclohexanes (sigmaHCH), chlordane-related compounds (sigmaCHL), and chlorobenzenes (sigmaCBz) were in the low ng g(-1) (dw) range in all lake sediments, similar to concentrations previously reported for Arctic lakes in Canada, indicating that their major source was long range atmospheric transport. Contamination of the lakes with PCBs and DDT near populated areas of the Yukon Territory appears to be a result of regional activities rather than long range transport and deposition. The results also point to glacial runoff as a significant source of OCs to small, high elevation lakes (Lindeman), but not to larger lakes within the Yukon River drainage basin that are also affected by glacial sources (Kusawa, Laberge).     

The ebullition of hydrogen, carbon monoxide, methane, carbon dioxide and total gaseous mercury from the Cornwall Area of Concern

This paper reports the first ebullitive fluxes of hydrogen (H2), carbon monoxide (CO), methane (CH4), carbon dioxide (CO2) and total gaseous mercury (TGM) from the Cornwall Area of Concern (CAC). Although sediments were contaminated with mercury, bubbling was a negligible source of mercury for the global atmosphere. Indeed, the average emission of TGM through ebullition was 0.04 pg m(-2) h(-1). Measurements of H2, CO, CH4 and CO2 trapped gas concentrations and fluxes were used as indicators of diagenesis processes. The CAC represented a significant regional source of CH4 since the estimated ebullitive fluxes (3.5 mg m(-2) h(-1)) were similar to the CH4 emissions measured above typical flooded freshwater wetlands. As molecular diffusion is known as the main pathway of CO2 transport from water to the atmosphere, CO2 ebullitive fluxes were weak (0.39 mg m(-2) h(-1)). Bubbling of CO (1.6 microg m(-2) h(-1)) was 10 folds less important than CO fluxes measured over flooded freshwater wetlands. Finally, H2 emissions (0.001 microg m(-2) h(-1)) were negligible since the level of this trace gas is tightly regulated by microorganisms in anaerobic environments.     

Chemical composition of suspended sediments in World Rivers: New insights from a new database

The aim of this paper is to present a new database on the chemical composition of suspended matter in World Rivers, together with the associated elemental fluxes. There is a lack of any recent attempt in the literature to update the pioneering work of Martin and Meybeck [Martin, J.-M., Meybeck, M., 1979. Elemental mass balance of material carried by major world rivers. Mar. Chem. 7, 173-206.] and Martin and Whitfield [Martin, J.-M., Whitfield, M., 1983. The significance of the river input of chemical elements to the ocean. Trace metals in sea water Wong, Boyle, Bruland, Burton, Goldberg (Eds) Plenum Publishing Corporation.] regarding the worldwide average major and trace element chemistry of riverine particulate matter.Apart from compiling a new database on particulate matter, this paper also aims to give a “snap-shot” of elemental fluxes for each continent. This approach should allow us to obtain new insights on weathering conditions in different environments and assess the influence of human activities on natural geochemical cycles. Finally, this study demonstrates the large uncertainties currently associated with estimating the flux of sediments transported by rivers.By comparing the riverine suspended sediment fluxes of some metals (Cd, Zn, Ni, Cu, Cr and Pb) given in this study with estimates of the anthropogenic fluxes of these metals to the atmosphere, soils and waters (natural ecosystems) [Nriagu, J.O., 1988. A silent epidemic of environmental poisoning. Environ. Pollut. 50, 139-161.], we can see that riverine fluxes are similar to anthropogenic fluxes. This casts light on the effect of human activities on the cycles of trace elements at the Earth's surface.     

Downward fluxes of particulate carbon, nitrogen and phosphorus in the north-western Adriatic sea

Downward fluxes of particles, organic carbon, total nitrogen and total phosphorus and the composition of the settled particulate matter were determined in the north-western Adriatic Sea at two coastal sites influenced by the outflows of the Po and Adige rivers and one offshore site. Vertical fluxes were strongly influenced by resuspension processes in addition to the primary flux and advection. The resuspended material contributed on average 34-43% of the total matter sedimented in the near bottom traps in coastal waters. Net annual vertical fluxes (due to primary flux and advection) of organic carbon, total nitrogen and phosphorus in the coastal stations were: 71-97 g C m(-2) year(-1), 8-14 g N m(-2) year(-1) and 2.1-2.3 g P m(-2) year(-1), with the highest values recorded at the station off the Po river delta. The offshore site was characterised by net annual fluxes of particulates, C, N and P approximately one order of magnitude lower than the above. The carbon export to the bottom was limited in the warm seasons when it constituted only 2-9% of primary production, due to high recycling and utilisation in the upper layer of the water column, increasing up to 8-18% in winter because of the instability of the water column and low biological utilisation.     

Thermokarst lakes of Western Siberia: a complex biogeochemical multidisciplinary approach

Western Siberia’s thermokarst lakes are highly dynamic hydrochemical systems that receive chemical elements from the surrounding peat soil and exchange greenhouse gases with the atmosphere, delivering dissolved carbon and metals to adjacent hydrological systems. Climate warming is likely to intensify the magnitude of these processes, thus seriously affecting the biogeochemical fluxes both on land and in the coastal zone of the Arctic Ocean. In this work, we review biogeochemical and morphological features of thermokarst water bodies comprising frozen palsa depressions up to large, kilometre-size lakes and drained lakes. Based on a compilation of more than a hundred analyses of these water bodies, we discuss the average concentration of organic carbon, as well as the major and trace elements, and predict the development of their chemical composition, CO₂ and CH₄ exchange with the atmosphere and effect on the riverine fluxes from the land to the ocean under the climate-warming scenario. The accelerating permafrost thaw and rising water temperatures in this region will probably shorten the life cycle of the thermokarst thaw water bodies, increase the fluxes of both CO₂ and CH₄ into the atmosphere, increase the concentration and delivery of dissolved organic carbon and related trace metals to the hydrological network and increase the potential bioavailability of micronutrients. Thus, the impact of permafrost thaw in Western Siberia on the global plantery processes, via the retroactive link between climate change and the thermokarst lakes’ geochemical activity, may be more significant than is currently expected.     

Atmospheric mercury exchange with a tallgrass prairie ecosystem housed in mesocosms

This study focused on characterizing air-surface mercury Hg exchange for individual surfaces (soil, litter-covered soil and plant shoots) and ecosystem-level flux associated with tallgrass prairie ecosystems housed inside large mesocosms over three years. The major objectives of this project were to determine if individual surface fluxes could be combined to predict ecosystem-level exchange and if this low-Hg containing ecosystem was a net source or sink for atmospheric Hg. Data collected in the field were used to validate fluxes obtained in the mesocosm setting. Because of the controlled experimental design and ease of access to the mesocosms, data collected allowed for assessment of factors controlling flux and comparison of models developed for soil Hg flux versus environmental conditions at different temporal resolution (hourly, daily and monthly). Evaluation of hourly data showed that relationships between soil Hg flux and environmental conditions changed over time, and that there were interactions between parameters controlling exchange. Data analyses demonstrated that to estimate soil flux over broad temporal scales (e.g. annual flux) coarse-resolution data (monthly averages) are needed. Plant foliage was a sink for atmospheric Hg with uptake influenced by plant functional type and age. Individual system component fluxes (bare soil and plant) could not be directly combined to predict the measured whole system flux (soil, litter and plant). Emissions of Hg from vegetated and litter-covered soil were lower than fluxes from adjacent bare soil and the difference between the two was seasonally dependent and greatest when canopy coverage was greatest. Thus, an index of plant canopy development (canopy greenness) was used to model Hg flux from vegetated soil. Accounting for ecosystem Hg inputs (precipitation, direct plant uptake of atmospheric Hg) and modeled net exchange between litter-and-plant covered soils, the tallgrass prairie was found to be a net annual sink of atmospheric Hg.     

Mercury budget of a small forested boreal catchment in southeast Norway

We present input and output fluxes of total mercury (Hg(tot)) and methyl mercury (MeHg) based on throughfall, litterfall and stream water samples from 2004/2005 from a small forested catchment (Langtjern) in Norway. Hg(tot) input via throughfall and litterfall was estimated to 6.7 and 2.7 microg m(-2) yr(-1), respectively, which is considerably lower than previously reported from other boreal catchments in Scandinavia. A likely cause for the low input flux is the sparseness and low productivity of the forest in the Langtjern catchment, with less atmospheric scavenging and lower litterfall fluxes than previously studied sites. In addition there has been a general decrease in mercury (Hg) in the atmosphere on the northern hemisphere in the last decade. The estimated output flux of Hg(tot) with surface water was 2.5 microg m(-2) yr(-1), which is comparable to what has been reported elsewhere. The ratio of Hg(tot) output flux to input flux was 26%, which is considerably higher than reported from other sites. This illustrates that catchment properties have greater importance for surface water export of Hg than the current atmospheric input. The estimated total soil pool of Hg(tot) in the catchment was 17.4 mg m(-2). This corresponds to roughly 8000 years of the current surface water output flux and 2000 years of the current input flux.     

Is the Negro River Basin (Amazon) impacted by naturally occurring mercury?

In order to investigate the major sources and cycling of mercury in the Negro River Basin (Amazon), total metal measurements were carried out in soils, in river and lake waters, in the atmosphere, and in bulk precipitation during the period 1995 throughout 1998. Median values of 1.3 ng m(-3) in the atmosphere, 172 microg kg(-1) in soils, 4.6-7.5 ng l(-1) in three different lakes, 4.5 ng l(-1) in 17 different Negro River tributaries and 20.3 microg m(-2) year(-1) in bulk precipitation were found. Mercury concentrations in rivers and lakes waters, as well as in soils and bulk precipitation were high, considering the scarcity of anthropogenic point sources in the region. Mercury leaching from soil, the largest regional reservoir of this metal, was considered to be the major pathway to mercury enrichment in the region.     

Mercury fluxes in a natural forested Amazonian catchment (Serra do Navio, Amapá State, Brazil)

Mercury (Hg total) fluxes were calculated for rainwater, throughfall and stream water in a small catchment located in the northeastern region of the Brazilian Amazon (Serra do Navio, Amapá State), whose upper part is covered by a natural rainforest and lower part was altered due to deforestation and activities related to manganese mining. The catchment area is 200 km from the nearest gold mining (garimpo). Minimum and maximum Hg concentrations were measured monthly from October 1996 to September 1997 and were 3.5-23.4 ng l(-1) for rainwater, 16.5-82.7 ng l(-1) for throughfall (March-August 1997) and 1.2-6.1 and 4.2-18.8 ng l(-1) for stream water, in natural and disturbed areas, respectively. In the natural area, the inputs were 18.2 microg m 2 year(-1) in rainwater and 72 microg m(-2) year(-1) in throughfall. This enrichment was attributed to dry deposition. The stream output of 2.9 microg m(-2) year(-1) indicates that Hg is being recycled within the forest as other chemical species or is being retained by the soil system, as confirmed by the cumulative Hg burden in the 0-10 cm surface layer, which was 36480 microg m(-2). When the disturbed area of the catchment was included, the stream output was 9.3 microg m(-2), clearly indicating the impact of the deforestation of the lower part of the basin on the release of mercury. The Hg burden in the disturbed area was 7560 microg m(-2) for the 0-10 cm surface layer.     

Effect of oxygen concentration on the combustion of horizontally-oriented slabs of PMMA

The aim of this study is to collect data on the combustion of horizontally-oriented poly(methyl methacrylate) (PMMA) samples in reduced oxygen atmospheres for CFD model validation. Experimental results relating the oxygen concentration to the burning behavior of 3-cm-thick clear PMMA slabs are discussed. Experiments are conducted in the controlled atmosphere calorimeter of IRSN called CADUCEE. Pyrolysis and combustion of 0.2×0.2 m² horizontally-oriented PMMA samples are studied varying the oxygen molar fraction from 0.210 to 0.180, extinction occurring at about 0.175. The measured quantities are the regression rate of the slab, mass loss rate, temperatures and total and radiative heat fluxes at the center of the slab. All experiments are carried out twice, showing a good repeatability. It is found that the slab regression rate, mass loss rate and heat fluxes at the slab center decrease significantly with the oxygen concentration, while the gas temperature is much less sensitive. Most notable is that the radiative and convective contributions to the total heat flux remain almost constant, respectively 0.65 and 0.35. It is also found that both heat fluxes and mass loss rate exhibit linear oxygen-concentration-dependent behavior. From an energy balance and current average values of the total heat flux and regression rate at the center of the slab, the present study obtains a heat of gasification value of 2.25 MJ kg⁻¹, in agreement with literature data.