Amazonian former gold mined soils as a source of methylmercury: evidence from a small scale watershed in French Guiana

Mercury migration to surface water from remediated mine waste and impacts of rainfall in a karst area - Evidence from Hg isotopes

Mine waste (MW) in historical mercury (Hg) mining areas continuously emits Hg into local environment, including aquatic ecosystems. Tracing Hg migration process from MW and determining its relative contribution to Hg pollution is critical for understanding the environmental impact of MW remediation. In this study, we combined data of Hg concentration, speciation, and isotope to address this issue in the Wanshan Hg mining area in southwest China. We found that rainfall can elevate Hg concentrations in river water and control the partitioning and transport of Hg in karst fissure zones through changing the hydrological conditions. A consistently large offset of δ²⁰²Hg (1.24‰) was observed between dissolved Hg (DHg) and particulate Hg (PHg) in surface water during the low-flow period (LFP), which may have been related to the relatively stable hydrologic conditions and unique geological background (karst fissure zones) of the karst region (KR). Results from the ternary Hg isotopic mixing model showed that, despite an order of magnitude reduction in Hg concentration and flux in river water after remediation, the remediated MW is still a significant source of Hg pollution to local aquatic ecosystems, accounting for 49.3 ± 11.9% and 37.8 ± 11.8% of river DHg in high flow period (HFP) and LFP, respectively. This study provides new insights into Hg migration and transportation in aquatic ecosystem and pollution source apportionment in Hg polluted area, which can be used for making polices for future remediation actions.

Positive Effect of Ecological Restoration with Fabaceous Species on Microbial Activities of Former Guyanese Mining Sites

Understanding ecological trajectories after mine site rehabilitation is essential to develop relevant protocols adapted for gold mining sites. This study describes the influence of a range of mine site rehabilitation and revegetation protocols on soil physicochemical parameters and microbial activities related to carbon, nitrogen and phosphorus cycles. We sampled soil from six rehabilitated mining sites in French Guiana with different plant cover (herbaceous, Cyperaceous, monoculture of Clitoria racemosa and Acacia mangium and association of C. racemosa and A. mangium). We measured the mineralization potential of organic matter by estimating the mineralization of carbon, nitrogen and phosphorus and the microbial catabolic diversity balance. The results showed an improvement in the quality of organic matter on revegetated sites with tree cover. On restored sites with fabaceous species, the microbial biomass is three times higher than non-restored sites, improving the rates of organic matter mineralization and restoring the catabolic diversity to the level of natural Guyanese soils. These results confirm that the establishment of fabaceous species under controlled conditions significantly improves the restoration of microbial communities in mining soils.

In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano

Photochemical reactions are major pathways for the removal of Hg species from aquatic ecosystems, lowering the concentration of monomethylmercury (MMHg) and its bioaccumulation in foodwebs. Here, we investigated the rates and environmental drivers of MMHg photodegradation and inorganic Hg (IHg) photoreduction in waters of two high-altitude lakes from the Bolivian Altiplano representing meso- to eutrophic conditions. We incubated three contrasting waters in situ at two depths after adding Hg-enriched isotopic species to derive rate constants. We found that transformations mostly occurred in subsurface waters exposed to UV radiation and were mainly modulated by the dissolved organic matter (DOM) level. In parallel, we incubated the same waters after the addition of low concentrations of natural MMHg and followed the stable isotope composition of the remaining Hg species by compound-specific isotope analysis allowing the determination of enrichment factors and mass-independent fractionation (MIF) slopes (Δ¹⁹⁹Hg/Δ²⁰¹Hg) during in situ MMHg photodegradation in natural waters. We found that MIF enrichment factors potentially range from -11 to -19‰ and average -14.3 ± 0.6‰ (1 SE). The MIF slope diverged depending on the DOM level, ranging from 1.24 ± 0.03 to 1.34 ± 0.02 for the low and high DOM waters, respectively, and matched the MMHg MIF slope recorded in fish from the same lake. Our in situ results thus reveal (i) a relatively similar extent of Hg isotopic fractionation during MMHg photodegradation among contrasted natural waters and compared to previous laboratory experiments and (ii) that the MMHg MIF recorded in fish is characteristic for the MMHg bonding environment. They will enable a better assessment of the extent and conditions conducive to MMHg photodegradation in aquatic ecosystems.

Impact of Old and Recent Gold Mining Sites on Mercury Fluxes in Suspended Particulate Matter, Water and Sediment in French Guiana

In 2006 the use of mercury (Hg) was banned for gold mining in French Guiana. However, mining of old placers could mobilize Hg accumulated in soils and sediment. This study aimed to measure the current impact of a mining concession (Boulanger site) on the Hg load in the watershed. Turbidity, Total Mercury (THg), and Monomethylmercury (MMHg) were measured in water, suspended particulate matter (SPM), river sediment and sediments from old tailing ponds along a river section of 30 km up and downstream from a mining concession in French Guiana during a dry and a rainy season. Total dissolved Hg (THgD) concentrations varied little from up- to down-stream but were all higher (fourfold on average) during the rainy season (3.2 to 4.4 ng L−1), than during the dry season and consistent with previous data known for the Amazonian area. Dissolved MMHg (MMHgD) represented up to 30% of THgD during the dry season, which is higher than previous results (typically around 2%). Mercury concentrations in sediments were highest in the vicinity of areas affected by old (before 2006) rather than new gold mining practices. Even though Hg was banned in 2006, present gold mining practices still release natural Hg and Hg inherited from older mining practices into the watershed.

Total mercury and methylmercury in the soil and vegetation of a riparian zone along a mercury-impacted reservoir

The Baihua Reservoir (Guizhou Province, Southwest China) has a history of mercury contamination associated with past acetic acid production activities at the Guizhou Organic Chemical Plant (GOCP). Soil and plant samples collected from riparian zones were analyzed for total Hg (THg) and methylmercury (MeHg) using cold vapor atomic fluorescence spectroscopy. The concentrations of THg and MeHg in soil samples were in the range of 109-371 ng g^-1 and 0.32-1.80 ng g^-1, respectively. Soils in the riparian zones close to the pollution source (the GOCP) presented higher Hg contamination, with relatively light Hg contamination in remote areas. This suggests a decreasing trend of THg concentrations along the riparian zones, with higher concentrations closer to the pollution source. Significant correlations were found between MeHg and soil organic matter (n = 24, p = 0.01). THg concentrations varied 11.3-161 ng g^-1 in aboveground areas and 11.3-193 ng g^-1 in underground areas. MeHg concentrations ranged from 0.23 to 1.06 ng g^-1 in aboveground areas to 0.13-1.51 ng g^-1 in the below ground areas. The vegetation studied showed different concentrations of THg and MeHg and can be considered to be impacted by Hg contamination. Different concentrations of total and methyl mercury were found among the different plant species. The high Hg concentrations in soils and vegetation suggests that the ability of Hg to bioaccumulate in riparian plants is affected by plant physiological characteristics and soil mercury concentrations. Although the bioaccumulation factors (BCFs) of the studied plants were low, their transfer factors (TFs) were >1. Our findings suggest that vegetation exhibiting TFs for THg >1 have the potential for phytoextraction in Hg-impacted riparian zones.

Diagenetic production, accumulation and sediment-water exchanges of methylmercury in contrasted sediment facies of Lake Titicaca (Bolivia)

Monomethylmercury (MMHg) concentrations in aquatic biota from Lake Titicaca are elevated although the mercury (Hg) contamination level of the lake is low. The contribution of sediments to the lake MMHg pool remained however unclear. In this work, seven cores representative of the contrasted sediments and aquatic ecotopes of Lake Titicaca were sliced and analyzed for Hg and redox-sensitive elements (Mn, Fe, N and S) speciation in pore-water (PW) and sediment to document early diagenetic processes responsible for MMHg production and accumulation in PW during organic matter (OM) oxidation. The highest MMHg concentrations (up to 12.2 ng L^-1 and 90% of THg) were found in subsurface PWs of the carbonate-rich sediments which cover 75% of the small basin and 20% of the large one. In other sediment facies, the larger content of OM restricted MMHg production and accumulation in PW by sequestering Hg in the solid phase and potentially also by decreasing its bioavailability in the PW. Diagenetically reduced S and Fe played a dual role either favoring or restricting the availability of Hg for biomethylation. The calculation of theoretical diffusive fluxes suggests that Lake Titicaca bottom sediments are a net source of MMHg, accounting for more than one third of the daily MMHg accumulated in the water column of the Lago Menor. We suggest that in the context of rising anthropogenic pressure, the enhancement of eutrophication in high altitude Altiplano lakes may increase these MMHg effluxes into the water column and favor its accumulation in water and biota.

Mercury and methylmercury levels in soils associated with coal-fired power plants in central-northern Chile

Mercury pollution is a worldwide problem, and is associated with a number of natural and anthropogenic processes. The present work, conducted in Chile, a country that has traditionally depended heavily on fossil fuels for power generation, examines total mercury (THg) and monomethylmercury (MMHg) concentrations in soils across different sites exposed to coal fired power plant emissions. Samples from four selected (Renca, Laguna Verde, Las Ventanas, Huasco) and 1 control (Quintay) sites were analyzed using cold vapour and fluorescence spectroscopy (CV-AFS) for THg determination and chromatographic separation with atomic fluorescence detection (DI-GC-AFS) was followed for speciation analysis. From the sites analyzed, Renca and Las Ventanas showed high concentrations of total mercury, exhibiting ranges between 135 - 568 and 94-464 ng g^-1 respectively, while Laguna Verde and Huasco exhibited lower values ranged 5-27 and 9-44 ng g^-1 respectively. Conversely, analysis of MMHg concentrations showed that only Renca site possessed high values, ranging between 0.1 and 3.0 ng g^-1, resulting in this site being considered contaminated. Conversely, other sites showed minimal values comparable to the control site (0.024 ± 0.003 ng g^-1) in terms of MMHg concentrations. An analysis of the differences between MMHg and THg concentrations in contaminated sites, suggests an overall absence of methylation in soils of Las Ventanas, probably related to the very high levels of soil heavy metals, especially copper. Moreover, the influence of the composition and physicochemical properties of the different soils on the mobility of the species was assessed. Results obtained (as Log Kd) were 3.5 and 4.1 for Renca and Las Ventanas respectively, suggesting low mobility of mercury species in the environment for both sites. Finally, the data obtained allowed us to establish a first approximation of the differences in concentration and mobility of total and MMHg associated with coal fired power plants emission in central-northern Chile, an area previously understudied in a country heavily dependent on fossil-fuels.

Impact of recent artisanal small-scale gold mining in Senegal: Mercury and methylmercury contamination of terrestrial and aquatic ecosystems

In Senegal, the environmental impact of artisanal small-scale gold mining (ASGM) using mercury (Hg) is poorly documented despite its intensification over the past two decades. We report here a complete dataset including the distribution and speciation of Hg in soil, sediment, and water in pristine and ASGM impacted sites of the Gambia River ecosystem (Kedougou region - eastern Senegal). Selective extraction showed that soils surrounding ASGM activities were contaminated with elemental Hg [Hg(0)] at concentrations up to 3.9 mg kg^-1. In the Gambia River, high total Hg (THg: 1.16 ± 0.80 mg kg^-1) and methylmercury (MeHg: 3.2 ± 2.3 ng g^-1) were also measured in sediment samples collected at ASGM sites. Along the stream, THg concentrations in sediment decrease with distance from the ASGM sites, while those of methylmercury increase downstream. The study of THg and MeHg partitioning between filtered surface water and suspended particles demonstrate that particulate transport is responsible for the downstream dissemination of the Hg contamination from ASGM sites. Sedimentation of fine particles enriched in Hg downstream ASGM sites likely favors MeHg production and accumulation in sediment. Although elemental Hg is weakly labile, surface soil erosion may also provide important and long-term Hg inputs to downstream aquatic ecosystems, where it can be oxidized and methylated. Finally, the dissemination of THg and MeHg downstream from the ASGM sites in the Gambia River may constitute a long-term source of contamination and can have a large scale impact on the aquatic ecosystem through biomagnification.

Late Holocene volcanic and anthropogenic mercury deposition in the western Central Andes (Lake Chungará, Chile)

Volcanism is one of the major natural processes emitting mercury (Hg) to the atmosphere, representing a significant component of the global Hg budget. The importance of volcanic eruptions for local-scale Hg deposition was investigated using analyses of Hg, inorganic elemental tracers, and organic biomarkers in a sediment sequence from Lake Chungará (4520 m a.s.l.). Environmental change and Hg deposition in the immediate vicinity of the Parinacota volcano were reconstructed over the last 2700 years, encompassing the pre-anthropogenic and anthropogenic periods. Twenty eruptions delivering large amounts of Hg (1 to 457 μg Hg m^-2 yr^-1 deposited at the timescale of the event) were locally recorded. Peaks of Hg concentration recorded after most of the eruptions were attributed to a decrease in sedimentation rate together with the rapid re-oxidation of gaseous elemental Hg and deposition with fine particles and incorporation into lake primary producers. Over the study period, the contribution of volcanic emissions has been estimated as 32% of the total Hg input to the lake. Sharp depletions in primary production occurred at each eruption, likely resulting from massive volcaniclastic inputs and changes in the lake-water physico-chemistry. Excluding the volcanic deposition periods, Hg accumulation rates rose from natural background values (1.9 ± 0.5 μg m^-2 yr^-1) by a factor of 2.3 during the pre-colonial mining period (1400-900 yr cal. BP), and by a factor of 6 and 7.6, respectively, during the Hispanic colonial epoch (400-150 yr cal. BP) and the industrial era (~140 yr cal. BP to present). Altogether, the dataset indicates that lake primary production has been the main, but not limiting, carrier for Hg to the sediment. Volcanic activity and climate change are only secondary drivers of local Hg deposition relative to the magnitude of regional and global anthropogenic emissions.

Mercury Isotopic Fractionation during Pedogenesis in a Tropical Forest Soil Catena (French Guiana): Deciphering the Impact of Historical Gold Mining

We used natural mercury (Hg) stable isotopes to investigate the Hg cycle in a rainforest soil catena (French Guiana) partially gold-mined during the early 1950s. Litterfall showed homogeneous Δ¹⁹⁹Hg values [-0.18 ± 0.05‰, i.e., a modern gaseous elemental Hg (GEM) isotopic signature]. After litter decomposition, Hg bound to organic matter (OM) is mixed with Hg from pristine (-0.55 ± 0.22‰) or gold-mined (-0.09 ± 0.16‰) mineral materials. Negative Δ¹⁹⁹Hg values in deep pristine mineral horizons (-0.60 ± 0.16‰) suggest the transfer of Hg bound to dissolved OM depleted in odd isotopes due to mass-independent fractionation during Hg abiotic reduction. Perennial palm tree leaves collected above gold-mined and pristine soil recorded contrasting Δ¹⁹⁹Hg signatures likely resulting from GEM re-emission processes from soils and leaf surfaces. Upslope, soil δ²⁰²Hg signatures showed a negative shift (ε ∼ -1‰) with depth attributed to mass-dependent fractionation during Hg sorption and complexation onto iron oxides and dissolved OM. Downslope, higher δ²⁰²Hg values in soils resulted from hydromorphy [lower humification, greater Hg(II) reduction, etc.]. The unique Hg isotopic signatures of Amazonian soils probably result in multistep fractionation processes during pedogenesis (millions of years) and in a potentially different Hg isotopic signature of preanthropogenic background GEM.

Oxoguanine glycosylase 1 (OGG1) protects cells from DNA double-strand break damage following methylmercury (MeHg) exposure

Methylmercury (MeHg) is a potent neurotoxin, teratogen, and probable carcinogen, but the underlying mechanisms of its actions remain unclear. Although MeHg causes several types of DNA damage, the toxicological consequences of this macromolecular damage are unknown. MeHg enhances oxidative stress, which can cause various oxidative DNA lesions that are primarily repaired by oxoguanine glycosylase 1 (OGG1). Herein, we compared the response of wild-type and OGG1 null (Ogg1(-/-)) murine embryonic fibroblasts to environmentally relevant, low micromolar concentrations of MeHg by measuring clonogenic efficiency, cell cycle arrest, DNA double-strand breaks (DSBs), and activation of the DNA damage response pathway.Ogg1(-/-) cells exhibited greater sensitivity to MeHg than wild-type controls, as measured by the clonogenic assay, and showed a greater propensity for MeHg-initiated apoptosis. Both wild-type and Ogg1(-/-) cells underwent cell cycle arrest when exposed to micromolar concentrations of MeHg; however, the extent of DSBs was exacerbated in Ogg1(-/-) cells compared with that in wild-type controls. Pretreatment with the antioxidative enzyme catalase reduced levels of DSBs in both wild-type and Ogg1(-/-) cells but failed to block MeHg-initiated apoptosis at micromolar concentrations. Our findings implicate reactive oxygen species mediated DNA damage in the mechanism of MeHg toxicity; and demonstrate for the first time that impaired DNA repair capacity enhances cellular sensitivity to MeHg. Accordingly, the genotoxic properties of MeHg may contribute to its neurotoxic and teratogenic effects, and an individual's response to oxidative stress and DNA damage may constitute an important determinant of risk.