Mercury pollution from artisanal mercury mining in Tongren, Guizhou, China

Concentrations of total Hg (T-Hg) were measured in mine waste, stream water, soil and moss samples collected from the Tongren area, Guizhou, China to identify potential Hg contamination to local environments, which has resulted from artisanal Hg mining. Mine waste contained high T-Hg concentrations, ranging from 1.8 to 900 mg/kg. High concentrations of Hg were also found in the leachates of mine waste, confirming that mine waste contains significant water-soluble Hg compounds. Total Hg distribution patterns in soil profiles showed that top soil is contaminated with Hg, which has been derived from atmospheric deposition. Data suggest that organic matter plays an important role in the binding and transport of Hg in soil. Elevated T-Hg concentrations (5.9–44 mg/kg) in moss samples suggest that atmospheric deposition is the dominant source of Hg to local terrestrial ecosystems. Concentrations of T-Hg were highly elevated in stream water samples, varying from 92 to 2300 ng/L. Particulate Hg in water constituted a large proportion of the T-Hg and played a major role in Hg transport. Methyl–Hg (Me–Hg) concentrations in the water samples was as high as 7.9 ng/L. Data indicate that Hg contamination is dominantly from artisanal Hg mining in the study area, but the extent of Hg contamination is dependent on the mining history and the scale of artisanal Hg mining.     

Gold mining related mercury contamination in Tongguan,Shaanxi Province,PR China

Elemental Hg–Au amalgamation mining practices are used widely in many developing countries resulting in significant Hg contamination of surrounding ecosystems. The authors examined for the first time Hg contamination in air, water, sediment, soil and crops in the Tongguan Au mining area, China, where elemental Hg has been used to extract Au for many years. Total gaseous Hg (TGM) concentrations in ambient air in the Tongguan area were significantly elevated compared to regional background concentrations. The average TGM concentrations in ambient air in a Au mill reached 18,000 ng m⁻³, which exceeds the maximum allowable occupational standard for TGM of 10,000 ng m⁻³ in China. Both total and methyl-Hg concentrations in stream water, stream sediment, and soil samples collected in the Tongguan area were elevated compared to methyl-Hg reported in artisanal Au mining areas in Suriname and the Amazon River basin. Total Hg concentrations in vegetable and wheat samples ranged from 42 to 640 μg kg⁻¹, all of which significantly exceed the Chinese guidance limit for vegetables (10 μg kg⁻¹) and foodstuffs other than fish (20 μg kg⁻¹). Fortunately, methyl-Hg was not significantly accumulated in the crops sampled in this study, where concentrations varied from 0.2 to 7.7 μg kg⁻¹.     

Environmental assessment of copper–gold–mercury mining in the Andacollo and Punitaqui districts,northern Chile

The Coquimbo region has been one of the richest producers of Cu, Au and Hg in Chile, and some of the deposits have been mined almost continuously since the 16th century. To assess the potential environmental contamination in this region, the authors measured the concentration of Cu, As, Cd, Zn and Hg in samples of stream and mine waters, stream sediments, soils, flotation tailings, and mine wastes in the Andacollo (Cu, Au, Hg) and Punitaqui (Cu–Au, Hg) districts. The concentration of Hg in the atmosphere in these districts were also measured. Although contamination is strongly controlled by the ore in each district, metal dispersion is modified by the degree of metallurgical processing efficiency as shown by the outdated Cu flotation system at Andacollo (stream sediments Cu 75–2200 μg/g). Conversely, more efficient procedures at Punitaqui resulted in less stream contamination, where stream sediments contained Cu ranging from 110–260 μg/g. However, efficient concentration by flotation of a given metal (e.g. Cu) may lead to the loss of another (e.g. Hg up to 190 μg/g in the tailings at Punitaqui), and therefore, to contamination via erosion of the tailings (downstream sediments Hg concentrations up to 5.3 μg/g). Continued use of Hg for Au amalgamation at Andacollo has led to significant contamination in stream sediments (0.2–3.8 μg/g Hg) and soils (2.4–47 μg/g Hg). Communities in this region are underdeveloped, and decades of inefficient treatment of flotation tailings and waste-rock stock piles has resulted in significant contamination of the surrounding landscape.     

Form and distribution of gold mobilized into surface waters and sediments from a gossan tailings pile,Murray Brook massive sulphide deposit,New Brunswick,Canada

Stream waters and sediments draining a gossan tailings pile at the Murray Brook massive sulphide deposit were collected to investigate Au mobility. Weathering of the massive sulphides at Murray Brook during the Late Tertiary period resulted in the concentration of Au in the gossan cap overlying the supergene Cu and unoxidized massive sulphide zones of the deposit. The gossan was mined between 1989 and 1992, and Au and Ag were extracted using a cyanide vat leach process. Although stream sediments prior to mining had Au<5 ppb (the detection limit), sediments collected in 1997 had Au contents ranging up to 256 ppm with values up to 6 ppm more than 3 km downstream from the deposit. Dissolved Au contents were similarly anomalous, up to 19 μg/L and in excess of 3 μg/L 3 km downstream. The elevated Au contents in the waters and sediments are interpreted to reflect complexation of Au (as Au(CN)₂⁻) by cyanide hosted within the gossan tailings pile. Precipitation recharges through the tailings pile with groundwater flow exiting to Gossan Creek. Degradation of cyanide along the flow path and within Gossan Creek allows colloidal Au to form via reduction of Au(I) by Fe²⁺, consistent with SEM observations of Au as <1 μm subrounded particles. In the surface waters, the majority of the Au must be in a form <0.45 μm in size to account for the similarity in Au contents between the <0.45 μm and unfiltered samples. The very elevated stream sediment Au values close to the headwaters of Gossan Creek near the tailings indicate that upon exiting to the surface environment, Au(CN)₂⁻ complexes are rapidly destroyed and Au removed from solution. However, the high Au_{<0.004 μm}/Au_total in the headwaters and the extended Au dispersion in Gossan Creek waters and sediments suggest that Au(CN)₂⁻ complexes persist for the full length of Gossan Creek. The decrease in aqueous Au which is less than 0.004 μm indicates that Au is converted from a complexed form to a colloidal form with increasing distance downstream, consistent with dissolved NO₃⁻ contents which decrease from 5210 μg/L near the headwaters to 1350 μg/L at the lower end of the stream.     

Mercury contamination of coastal sediments as the result of long-term cinnabar mining activity (Gulf of Trieste,northern Adriatic sea)

The Isonzo river mouth has been the source of Hg in the Gulf of Trieste (northern Adriatic sea) since the sixteenth century, making this shallow basin one of the most contaminated marine areas in the length of time and amount of metal accumulated. The occurrence and behaviour of total Hg (range 0.064–30.38 μg g⁻¹; average 5.04 μg g⁻¹; median 3.10 μg g⁻¹, n=80) and related size fractions in sediments of this coastal area were investigated in detail. The relationship between total Hg and the fine silt-clay (< 16 μm) fraction has provided information on the hydrological and mineralogical fractionation process affecting this element, when compared to other heavy metals associated with fluvial inputs. Mercury contents are very high along the littoral zone of the northern (Italian) sector where this metal is present in detrital form (cinnabar) in sandy-silty sediments near the river mouth and the surrounding beaches. Within the sediments belonging to the Gulf area, Hg is bound either to fine particles or adsorbed onto the surface of clay minerals and/or partially complexed by colloids and organic matter. Recent accumulation of Hg in a 70 cm long ²¹⁰Pb dated core, collected in the central part of the Gulf, was also compared to other heavy metals (Fe, Cr, Cu, Mn, Ni and Zn). A preliminary estimate of Hg enrichment shows that the first 50 cm of sediment in the central sector of the Gulf of Trieste are noticeably contaminated, reaching a maximum of up to 25-fold above the proposed natural regional background of 0.17 μg g⁻¹. The vertical trend is well correlated to historical data of Hg extraction activity at the Idrija mine.     

Mercury in sediments from gold and copper exploitation areas in the Camaquã River Basin,Southern Brazil

Mercury concentrations were determined in stream sediments from the Camaquã River Basin, located in the shield region of the state of Rio Grande do Sul, southern Brazil. The resulting geochemical data show that overbank floodplain deposits exhibit higher concentrations than sediments collected from the active channel bed. In addition, higher Hg concentrations were measured in the fine(<63 μm) sediment fraction of the samples. Total Hg concentrations in the fine fraction of active stream sediments from Lavras do Sul County, which have been influenced by past gold mining activities, have decreased during the last five years to values ≤142 ng g⁻¹. However, in a settling pond containing abandoned mine wastes, the Hg concentration of a bulk sample remained exceptionally high (5220 ng g⁻¹). Preliminary speciation results show that Hg⁰ is the predominant species in most of the samples. This was the form of Hg released by the gold amalgamation activities in the area, and appears to be relatively stable under the existing E_h and pH conditions.     

Mercury interactions in a simulated gold placer

A simulated stream sediment bed was constructed in a laboratory to determine whether dissolved Hg could be transported through sediment and deposited as amalgam on Au grains. Metallic Hg was placed in a sump at one end of a tank filled with gravel (quartz sand, granules, and pebbles), and Au grains were buried in the gravel at the other end. Water was circulated in a continuous closed loop over the Hg and through the gravel that included the Au grains for more than 850 days. The Hg content of the water increased from nil at the beginning to approximately 0.5 μg/l after approximately 22 days. The Hg content on the rims of Au grains went from nil to approximately 0.2 wt.% over 22 days reaching a maximum 0.48 wt.% Hg after 14 days. Subsequent measurements indicated a persistent decrease of the maximum Hg on Au grains to ⩽0.19 wt.% Hg at 552 days and to ⩽0.05 wt.% at 851 days. Deposition of Hg on the Au grains indicates that amalgams can form without actual contact of Hg on Au in stream sediments. Why Hg first deposited on Au and then dissolved from the Au is unknown, but a paucity of microbiota early in the experiment and subsequent development of microbiota that could facilitate dissolution of Hg is suspected. The simulated Au placer, with its coarse sediments and free water flow, is analogous to streams that have measurable (>0.2 μg/l) Hg in the water and no amalgams on Au grains within the sediments. An example of a Au mining region with similar water concentrations of Hg would be the Amazon Basin, although information on the presence of amalgam rims on Au grains is lacking, as for most regions. Lower but still measurable (⩽0.55 μg/l Hg) concentrations of Hg in stream water and a lack of amalgams on Au grains occur in Au placers near Talladega, Alabama. The opposite case would be streams with less-than-measurable (<0.2 μg/l) Hg in the water but amalgams on Au grains, where conditions are less aerated and Hg would be more likely to remain in the substrate. This situation is analogous to Au placers in the North Carolina Piedmont (South Mountains, Robbins, and High Point), where Hg is not detected in stream water (<0.2 μg/l) and Au grains possess amalgamated rims. Mercury concentrations in the air over the tank (41–465 ng/m³) varied inversely with barometric pressure (1012–1033 mb @ SL), with a positive response to light, which is consistent with the work of other researchers. The positive photo effect on Hg concentration in the air was obvious at lower barometric pressures (∼ave. 1015 mb @ SL) but subdued or nonexistent at higher barometric pressures (∼ave 1025 mb @ SL). Mercury concentrations were as much as 3 times as high during daylight hours compared to nighttime concentrations over the tank at relatively low barometric pressures. The Hg content of the water remained relatively low (<1 μg/l) through the first 200 days and then abruptly increased where it oscillated between 4 and 17 μg/l to the end of the experiment (851 days). Meanwhile, the rate of loss of Hg from the tank averaged approximately 1 μg/cm² day with a high of 1.54 μg/cm² day. Apparently the release of Hg is little affected by the Hg content of the water, as long as a minimum amount of Hg is maintained in the water. The release rate of Hg from the tank experiment is approximately 10 times higher than those reported by other workers but probably represents a maximum due to ideal, oxidizing, high water flow conditions in the tank.     

Heavy metal concentrations in shallow marine sediments affected by submarine tailings disposal and artisanal gold mining,Buyat-Ratototok district,North Sulawesi,Indonesia

Trace element concentrations in shallow marine sediments of the Buyat-Ratototok district of North Sulawesi, Indonesia, are affected by submarine disposal of industrial gold mine tailings and unregulated dumping of tailings and wastewater from small-scale gold mining using mercury amalgamation. Industrial mine tailings contained 590–690 ppm arsenic, 490–580 ppm antimony, and 0.8–5.8 ppm mercury. Tailings-affected sediment As and Sb concentrations were 20–30 times higher than in muddy sediments not contaminated with tailings, and 50–60 times higher than pre-mining average. Highest mercury concentrations were observed in sediments affected by small-scale mining using mercury amalgamation (5–29 ppm). Concentrations of most other trace elements were comparable in sediments affected by both types of mining and were slightly higher than regional averages for sediments collected before the onset of industrial mining. Elevated concentrations of both As and Sb in approximately equal proportions suggest tailings dispersal of at least 3.5 km. Mercury released from artisanal gold mining dispersed up to 4 km from river mouths. Slight increases in concentrations of non-mercury trace elements in areas affected by artisanal mining over pre-industrial mining concentrations were probably caused by increased rates of erosion. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

The distribution of total and methylmercury concentrations in soils near the Idrija mercury mine, Slovenia, and the dependence of the mercury concentrations on the chemical composition and organic carbon levels of the soil

Although the mining activity of the Idrija mine in Slovenia ceased in 1995, a large amount of mining dregs containing high concentrations of mercury remains in the area. The mining dregs were transported with river flow and deposition along the Idrija River. To estimate the dispersion and change in the chemical form of mercury, a total of 28 soil core samples were taken around the river. The individual core samples were separated into layers for the analysis of their chemical composition, carbon contents, total mercury (T-Hg) and methylmercury (MeHg) concentrations. The chemical composition measured by X-ray fluorescence spectrometry was useful to estimate the dispersion of tailings: the fluvial terrace soil had a chemical composition similar to that of the tailings and could be distinguished clearly from the forest soil. The highest T-Hg concentration, 1,100 mg kg⁻¹, was observed in the fluvial terrace soil near the mine. Although the concentration decreased gradually along with distance from the mine, concentrations higher than 200 mg kg⁻¹ of T-Hg were still observed in the fluvial terrace soil approximately 20 km downstream from the mine. In the vertical distribution of T-Hg in the hillslope soil, a higher value was observed in the upper layers, which suggests the recent atmospheric deposition of mercury. The concentration of MeHg was the lowest at the riverside and higher in the hillslope soil, which was the opposite of the T-Hg distribution. The total organic carbon content tracked similarly with the distribution of MeHg and a linear relation with a significantly high correlation coefficient was obtained. The distinction may be related to the different dispersion process of mercury, and the organic carbon contents may be an important factor for MeHg formation.     

Mineralogy and trace element relative solubility patterns of shallow marine sediments affected by submarine tailings disposal and artisanal gold mining,Buyat-Ratototok district,North Sulawesi,Indonesia

Shallow marine sediments of the Buyat-Ratototok district of North Sulawesi, Indonesia, are affected by submarine disposal of industrial gold mine tailings and small-scale gold mining using mercury amalgamation. Industrial mine tailings contained 590–660 ppm arsenic, 490–580 ppm antimony, and 0.8–5.8 ppm mercury. Electron microprobe survey found both colloidal iron–arsenic-phases without sulphur and arsenian pyrite in tailings and sites to which tailings had dispersed, but only arsenopyrite in sediments affected by artisanal mining. Antimony in tailings was present as antimony oxides, colloidal iron–antimony phases, colloidal iron–antimony phases, and stibnite in sediments affected by both types of mining. A sequential extraction found that 2% of arsenic held in tailings and tailings-contaminated sediments was exchangeable, 20–30% was labile, including weakly adsorbed, carbonate- and arsenate bound, 20–30% was metastable, probably incorporated into iron or manganese oxyhydroxides, or strongly adsorbed to silicate minerals, and 40–48% was relatively insoluble, probably incorporated into sulphides or silicates. Arsenic in sediments affected by artisanal gold mining was 75–95% relatively insoluble. Antimony in all sediments was >90% relatively insoluble. Relative solubility patterns of most other metals did not differ between industrial tailings-affected, artisanal-mining affected areas, and fluvial sediments. Results suggest that submarine tailings disposal is not suitable for refractory Carlin-like gold deposits because ore processing converts arsenic to forms unstable in anoxic marine sediments. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Mercury and methylmercury in riparian soil,sediments, mine-waste calcines,and moss from abandoned Hg mines in east Guizhou province,southwestern China

Concentrations of total Hg and methylmercury (MMHg) in riparian soil, mine-waste calcine, sediment, and moss samples collected from abandoned Hg mines in Wanshan district, Guizhou province, China, were measured to show regional dispersion of Hg-contamination. High total Hg and MMHg concentrations obtained in riparian soils from mined areas, ranged from 5.1 to 790 mg kg⁻¹ and 0.13 to 15 ng g⁻¹, respectively. However, total Hg and MMHg concentrations in the soils collected from control sites were significantly lower varying from 0.1 to 1.2 mg kg⁻¹ and 0.10 to 1.6 ng g⁻¹, respectively. Total Hg and MMHg concentrations in sediments varied from 90 to 930 mg kg⁻¹ and 3.0 to 20 ng g⁻¹, respectively. Total Hg concentrations in mine-waste calcines were highly elevated ranging from 5.7 to 4400 mg kg⁻¹, but MMHg concentrations were generally low ranging from 0.17 to 1.1 ng g⁻¹. Similar to the high Hg concentrations in soil and sediments, moss samples collected from rocks ranged from 1.0 to 95 mg kg⁻¹ in total Hg and from 0.21 to 20 ng g⁻¹ in MMHg. Elevated Hg concentrations in mosses suggest that atmospheric deposition might be an important pathway of Hg to the local terrestrial system. Moreover, the spatial distribution patterns of Hg contamination in the local environment suggest derivation from historic Hg mining sites in the Wanshan area.     

Heavy metal contamination of soils and waters in and around the Imcheon Au–Ag mine,Korea

The heavy metal contamination of soils and waters by metalliferous mining activities in an area of Korea was studied. In the study area of the Imcheon Au–Ag mine, soils and waters were sampled and analyzed using AAS for Cd, Cu, Pb and Zn. Analysis of HCO₃⁻, F⁻, NO₃⁻ and SO₄²⁻ in water samples was also undertaken by ion chromatography. Elevated concentrations of the metals were found in tailings. The maximum contents in the tailings were 9.4, 229, 6160 and 1640 mg/kg extracted by aqua regia and 1.35, 26.4, 70.3 and 410 mg/kg extracted by 0.1 N HCl solution for Cd, Cu, Pb and Zn, respectively. These metals are continuously dispersed downstream and downslope from the tailings by clastic movement through wind and water. Because of the existence of sulfides in the tailings, a water sample taken on the tailings site was very acidic with a pH of 2.2, with high total dissolved solids (TDS) of 1845 mg/l and electric conductivity (EC) of 3820 μS/cm. This sample also contained up to 0.27, 1.90, 2.80, 53.4, 4,700 mg/l of Cd, Cu, Pb, Zn and SO₄²⁻, respectively. TDS, EC and concentrations of metals in waters decreased with distance from the tailings. The total amount of pulverized limestone needed for neutralizing the acid tailings was estimated to be 46 metric tons, assuming its volume of 45,000 m³ and its bulk density of 1855 kg/m³.     

Accumulation of contaminant metals in marine sediments of Halifax Harbour,Nova Scotia: environmental factors and historical trends

Sediments in Halifax Harbour have accumulated contaminant metals Hg, Cu, Zn, and Pb as a result of discharge of untreated sewage and industrial waste, leaching of land fill waste, and surface drainage. Concentrations of contaminants in²¹⁰Pb dated sediment cores became significant about 1880 and rose rapidly after 1900, reaching maximum concentrations in the decades between 1950 and 1980. Mean concentrations of Hg increased from 0.2 μg g⁻¹ in pre 1890 sediments to 1.6 μg g⁻¹ in the 20th century. Similar enrichments for Cu changed the concentration from 26 to 88 μg g⁻¹; for Zn from 90 to 250 μg g⁻¹, and for Pb the increase was from 12 to 206 μg g⁻¹ Statistical factor analyses of geochemical data have been used to identify: (1) primary contaminants directly associated with waste discharge, these include total and organic-bound forms of Cu, Zn, and Pb; (2) secondary contamination attributed to leaching and modification of primary contaminants include acid labile forms of Zn, Ni, and Cu; (3) diagenetic modification of buried sediments are identified by total and labile forms of Mn; (4) dispersion of contaminants from land surface drainage are characterized by fine-grained aluminosilicates. Historical trends in the changing dominance of these environmental factors reflect changes in industrial activity, urban growth, and changes in the use of metals in paints, domestic and industrial chemicals, and in the combustion of fuels.     

Geological analogue for circumneutral pH mine tailings: implications for long-term storage,Macraes Mine,Otago, New Zealand

Tailings from the Macraes Au mine cyanidation process are stored in an impoundment about 0.6 km² and 80 m deep whose pH is maintained near 8 by the neutralizing capacity of the gangue minerals. The tailings are sandy (>50 μm particles), have a hydraulic conductivity of about 10⁻² m/day, and contain 0.1–1.0 wt.% S and 0.1–1.5 wt.% graphitic C from the primary deposit. Concentrations of As in the pore water of the mixed tailings, which are a combination of various tailings types, range from 0.1 to 20 ppm, HCO₃^- is 100 to 200 ppm, and dissolved SO₄ is 100–1700 ppm. The mixed tailings will be stored in this impoundment in perpetuity after mining ceases. Confidence in the long-term pH stability of these tailings can be gained from examination of mineralogically and chemically similar geological analogues in the immediate vicinity. A sequence, typically about 5 m thick, of sands and gravels derived from the Macraes mineralized zone 12–28 ka ago contains rounded detrital sulfide mineral grains which are unoxidized despite their close proximity to the surface and the occasional incursion of oxygenated waters. These sediments have a hydraulic conductivity of about 10⁻⁴ m/day. Saturating water pH is currently 7–8. Sands with 0.2–0.8 wt.% organic C host SO₄-reducing bacteria (SRB), and local cementation by authigenic framboidal pyrite has occurred. SRB were found in water-saturated sediments with decreased hydraulic conductivity and alkaline and anoxic conditions. These bacteria are involved in the formation of authigenic framboidal pyrite, reducing the cycling of dissolved Fe in the sediments. Carbon is not a limiting factor in this process as organic matter is present in the sandstone and ground water contains up to 180 ppm HCO₃^-. Comparison of the 28 ka old sediments with the modern tailings suggests that the chemical behaviour of the two will be similar, possibly with the crystallization of authigenic pyrite in the tailings over the long term. As long as the present slightly anoxic and circumneutral pH environmental conditions are maintained in the mixed tailings impoundment, sulfide decomposition and acidification are unlikely.     

Mercury contamination in Marano Lagoon (Northern Adriatic sea,Italy): Source identification by analyses of Hg phases

Total Hg concentrations and Hg speciation were determined in bottom sediments of Marano lagoon to investigate the consequences of Hg phases on fish farms and shellfish cultivation areas. Mercury phases were separated into cinnabar (HgS) and non-cinnabar compounds, via a thermo-desorption technique, in surface and core sediments; both of which had been contaminated by industrial wastes and mining activity residues. The former are due to an industrial complex, which has been producing cellulose, chlor-alkali and textile artificial fibres since 1940. Processing and seepage wastewaters, which were historically discharged into the Aussa-Corno river system and therefore into the lagoon, have been significantly reduced since 1984 due to the construction of wastewater treatment facilities. The second source is the Isonzo River, which has been the largest contributor of Hg into the northern Adriatic Sea since the 16th century due to Hg mining at the Idrija mine (western Slovenia). Red cinnabar (HgS) derived from the mining area is mostly stable and insoluble under current environmental conditions. In contrast, organically bound Hg, such as Hg bound to humic acids, has the potential to be transformed into bioavailable Hg compounds (for example, methylmercury). The presence of the two Hg forms permitted each Hg source to be quantified. It also allowed the areas with the highest risk of Hg contamination from Hg-rich sediment to be identified; thus potentially avoiding the transfer of Hg from the sediment into the water column and eventually into living biota. The results show that Hg Enrichment Factors in bottom sediments exceed values of 10 and cinnabar dominates the central sector near the main tidal channel where tidal flux is more effective. Non-cinnabar compounds were found to be enriched in fine grained material and organic matter. In fact, up to 98% of total Hg at the Aussa-Corno river mouth and in the inner margin of the basin occurred in an organic form. This evidence, combined with the high contents of total Hg (4.1–6.6 μg g⁻¹ and EF > 10) measured in surface sediments, suggest that Hg in Marano lagoon is involved in biogeochemical transformations (e.g., methylation).     

Weathering versus atmospheric contributions to mercury concentrations in French Guiana soils

This work focuses on two possible sources of Hg in tropical soils, (i) lithogenic Hg from in situ weathering of soil parental material, and (ii) exogenic Hg from natural long-term atmospheric inputs and anthropogenic input from past and present industrial activities. The concentration of lithogenic Hg [Hg]_lithogenic was based on comparison of measured Hg concentration with those of elements resistant to weathering such as Nb, U, Zn, Fe. Exogenic Hg was quantified by subtracting [Hg]_lithogenic from total Hg concentrations. This calculation was applied to 4 French Guiana soil profiles, 3 profiles on the same toposequence (ferralsol, acrisol, hydromorphic soil) and one acrisol close to a Au mine, where elemental Hg is used. In all profiles, [Hg]_lithogenic varied slightly and was always below 40 μg kg⁻¹, whereas [Hg]_exogenic varied considerably and reached 500 μg kg⁻¹. The highest [Hg]_exogenic was calculated for the upper horizon of the acrisol close to Au mining activity, but also in the ferralsol. Concentrations of Hg were insignificant in the compact alterite in acrisols. It was concluded that pedogenesis processes that affect the natural Hg supply, combined with anthropogenic sources, explain the Hg concentrations in these tropical soils.     

Methylmercury in tailings ponds of Amazonian gold mines (French Guiana): Field observations and an experimental flocculation method for in situ remediation

Sites of monomethylmercury (MMHg) production in Amazonian regions have been identified in hydraulic reservoirs, lake sediments and wetlands, but tailings ponds have not yet received sufficient attention for this purpose. This work evidenced high MMHg production within the water column and the interstitial water of two tailings ponds of French Guiana Au mines located; (i) in a small scale exploitation (Combat) where Hg was used for Au amalgamation, and (ii) in an industrial on-going Au mine (Yaoni) processing without Hg. The (MMHg)_D maximum (2.5 ng L⁻¹) occurred in the oxic water column above the sediment-water interface (SWI) of the most recent tailings pond (Combat), where the substrate was fresh, the redox transition was sharp and the pool of total Hg was large. In the Yaoni pond, the (MMHg)_D maximum concentration (1.4 ng L⁻¹) was located at the SWI where suboxic conditions prevailed. Using the (MMHg)_D concentration as a proxy for Hg methylation rates, the present results show that Hg methylation may occur in various redox conditions in tailings ponds, and are favored in areas where the organic matter regeneration is more active.A 3-month long laboratory experiment was performed in oxic and anoxic boxes filled with high turbidity waters from the Combat Au mine to simulate tailings ponds. Slaked lime was added in an experimental set (2 mg L⁻¹) and appeared to be very efficient for the reduction of suspended particulate matter (SPM) to environmentally acceptable concentrations. However, at the end of the experiment, large (MMHg)_D concentrations were monitored under treated anoxic conditions with the (MMHg)_D maximum located at the SWI above the Fe-reducing zones. No (MMHg)_D was detected in oxic experiments. The use of slaked lime for SPM decantation appears to be an efficient and non-onerous process for Au miners to avoid Hg methylation in tailings ponds when it is combined with rapid drainage of the mine waters. A subsequent human intervention is however necessary for the recovery of soil structure through the cover of dried ponds with organic rich materials and reforestation to avoid the stagnation of rain waters and the occurrence of anoxia.     

U–Pb,Re–Os,and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronology of the Nambija Au-skarn and Pangui porphyry Cu deposits,Ecuador: implications for the Jurassic metallogenic belt of the Northern Andes

New U–Pb, Re–Os, and ⁴⁰Ar/³⁹Ar dates are presented for magmatic and hydrothermal mineral phases in skarn- and porphyry-related ores from the Nambija and Pangui districts of the Subandean zone, southeastern Ecuador. Nambija has been one of the main gold-producing centers of Ecuador since the 1980s due to exceptionally high-grade ores (average 15 g/t, but frequently up to 300 g/t Au). Pangui is a recently discovered porphyry Cu–Mo district. The geology of the Subandean zone in southeastern Ecuador is dominated by the I-type, subduction-related, Jurassic Zamora batholith, which intrudes Triassic volcanosedimentary rocks. The Zamora batholith is in turn cut by porphyritic stocks, which are commonly associated with skarn formation and/or porphyry-style mineralization. High precision U–Pb and Re–Os ages for porphyritic stocks (U–Pb, zircon), associated prograde skarn (U–Pb, hydrothermal titanite), and retrograde stage skarn (Re–Os, molybdenite from veins postdating gold deposition) of the Nambija district are all indistinguishable from each other within error (145 Ma) and indicate a Late Jurassic age for the gold mineralization. Previously, gold mineralization at Nambija was considered to be Early Tertiary based on K–Ar ages obtained on various hydrothermal minerals. The new Jurassic age for the Nambija district is slightly younger than the ⁴⁰Ar/³⁹Ar and Re–Os ages for magmatic–hydrothermal minerals from the Pangui district, which range between 157 and 152 Ma. Mineralization at Nambija and Pangui is associated with porphyritic stocks that represent the last known episodes of a long-lived Jurassic arc magmatism (∼190 to 145 Ma). A Jurassic age for mineralization at Nambija and Pangui suggests that the Northern Andean Jurassic metallogenic belt, which starts in Colombia at 3° N, extends down to 5° S in Ecuador. It also adds a new mineralization style (Au-skarn) to the metal endowment of this belt. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mercury contamination and its potential health effects in a lead-zinc mining area in the karst region of Guangxi, China

Mercury contamination of the environment is of worldwide concern because of its global presence and its potent neurotoxicity. Mining, smelting and the electronics industry are the main sources of Hg pollution. However, few studies have been performed to investigate systemic Hg contamination in metal mining regions. In this study, concentrations of Hg in air, farmland soil, and crops were measured in a Pb-Zn mining area in the karst region of Guangxi, China. Key factors that could affect Hg distribution, such as the fate of waste ore and waste residue, were analyzed. Geo-statistical methods were adopted to analyze the characteristics of spatial structure and distribution of Hg. The results show that Hg contamination in this region is serious. The total mercury (T-Hg) content is far higher than the Level II Limit Value of Chinese Soil Standards of 0.30 mg kg⁻¹, showing obvious directional characteristics from WNW to ESE. Highest Hg concentrations were found in the WNW portion of the study area. The contamination of paddy soil is higher than that in dry farmland soil. The vertical distribution of T-Hg and its decrease with depth suggest that the important sources are waste water irrigation and the improper disposal of the waste ore and waste rock. The T-Hg concentrations in the agricultural products examined exceed the Chinese tolerance value (0.02 mg kg⁻¹ for rice and 0.01 mg kg⁻¹ for vegetables), indicating the seriousness of the problem. The ecological environment and the safety of food grown in this mining area are being affected, with the result that human health is possibly being affected.