Influence of waterfall aeration and seasonal temperature variation on the iron and arsenic attenuation rates in an acid mine drainage system

Dramatic seasonal changes in water chemistry and precipitate mineralogy associated with acid-mine drainage (AMD) in the waterfall and creek sections of the Chinkuashih area, northern Taiwan were investigated. Special attention has been paid to the kinetic effects of seasonal temperature variation and waterfall aeration. Precipitation of schwertmannite associated with removal of metals and As are indicated by delicate growth microstructures on precipitate surfaces, X-ray diffraction data, and downstream reductions of metal and As concentrations. Geochemical modeling suggested a downstream increase of the degree of saturation/supersaturation with respect to schwertmannite in the waterfall section, which can be attributed to high Fe²⁺ oxidation rates. The waterfall section was characterized by high rates and model rate constants of Fe²⁺ oxidation (6.1–6.7 × 10⁻⁶ mol L⁻¹ s⁻¹ and 2.7–2.9 × 10⁻² s⁻¹) and Fe (schwertmannite) precipitation (1.7–2.1 × 10⁻⁶ mol L⁻¹ s⁻¹ and 3.5–4.1 × 10⁻⁷ mol L⁻¹ s⁻¹). A high As sorption rate (4.7–6.3 × 10⁻⁹ mol L⁻¹ s⁻¹) and low As distribution coefficient (7.9–11.8 × 10⁻⁹ mol⁻¹ L) were observed. The creek section showed up to 1–2 orders of magnitude slower rates and lower rate constants than the waterfall section and had seasonal variations comparable to those in areas polluted by AMD elsewhere. The summer rates were 4–5 times higher than the winter rates in the creek section, and are largely attributed to a temperature effect. In contrast, the seasonal differences in rate and rate constant were small in the waterfall section. Several factors associated with the waterfall aeration in addition to elevated temperature and As concentration enhanced Fe and As attenuation in the waterfall section. The waterfall effects on Fe precipitation rate were enhanced when the flow rate was large in the winter. Despite the remarkable removal of metals and As by the rapid precipitation of As-bearing schwertmannite, large effluent loads of potentially hazardous contaminants including As, Cu and Zn discharged to the sea in the Chinkuashih area.     

Streamwater chemistry in three contrasting monolithologic Czech catchments

Hydrochemical patterns resulting from differing bedrock geochemistry were ascertained by concurrent streamwater sampling in three small catchments, each underlain by geochemically contrasting silicate rock types in the western Czech Republic, Central Europe in 2001–2010. The catchments are situated 5–7 km apart in the Slavkov Forest and are occupied by Norway spruce (Picea abies) plantations. They have similar altitude, area, topography, mean annual air temperature, and atmospheric deposition fluxes. The amount of base cations oxides (Ca + Mg + Na + K) is markedly different among the three studied rocks (leucogranite 8%, amphibolite 22%, serpentinite 36%). The leucogranite contains a very small amount of MgO, while the serpentinite contains extremely large amounts of MgO. The amphibolite contains an intermediate amount of MgO and elevated CaO. The Lysina, on leucogranite, exhibited very small concentrations of Mg (median 0.4 mg L⁻¹) in streamwater; Pluh?v Bor, on serpentinite, contained extremely high concentrations of streamwater Mg (18 mg L⁻¹). Streamwater in the Na Zeleném catchment, on amphibolite, contained an intermediate amount of Mg and an elevated Ca. Very low pH (4.2), negative alkalinity (−60 μeq L⁻¹) and high inorganic monomeric Al concentrations (0.3 mg L⁻¹) were found in the stream draining leucogranite. Serpentinite streamwater exhibited the highest pH (7.6), alkalinity (+940 μeq L⁻¹) and Ni concentrations (100 μg L⁻¹). Aquatic chemistry reflected the composition of the underlying rocks within the studied catchments. Contrasting streamwater compositions of the studied catchments were generated according to the MAGIC model simulations mainly by differences in chemical weathering rates of base cations (65 meq m⁻² a⁻¹ at Lysina, 198 meq m⁻² a⁻¹ at Na Zeleném, and 241 meq m⁻² a⁻¹ at Pluh?v Bor).     

Irrigation produces elevated arsenic in the underlying groundwater of a semi-arid basin in Southwestern Idaho

The shallow aquifer beneath the Western Snake River Plain (Idaho, USA) exhibits widespread elevated arsenic concentrations (up to 120 μg L⁻¹). While semi-arid, crop irrigation has increased annual recharge to the aquifer from approximately 1 cm prior to a current rate of >50 cm year⁻¹. The highest aqueous arsenic concentrations are found in proximity to the water table (all values >50 μg L⁻¹ within 50 m) and concentrations decline with depth. Despite strong vertical redox stratification within the aquifer, spatial distribution of aqueous species indicates that redox processes are not primary drivers of arsenic mobilization. Arsenic release and transport occur under oxidizing conditions; groundwater wells containing dissolved arsenic at >50 μg L⁻¹ exhibit elevated concentrations of O₂ (average 4 mg L⁻¹) and NO₃ (average 8 mg L⁻¹) and low concentrations of dissolved Fe (<20 μg L⁻¹). Sequential extractions and spectroscopic analysis of surficial soils and sediments indicate solid phase arsenic is primarily arsenate and is present at elevated concentrations (4–45 mg kg⁻¹, average: 17 mg kg⁻¹) relative to global sedimentary abundances. The highest concentrations of easily mobilized arsenic (up to 7 mg kg⁻¹) are associated with surficial soils and sediments visibly stained with iron oxides. Batch leaching experiments on these materials using irrigation waters produce pore water arsenic concentrations approximating those observed in the shallow aquifer (up to 152 μg L⁻¹). While As:Cl aqueous phase relationships suggest minor evaporative enrichment, this appears to be a relic of the pre-irrigation environment. Collectively, these data indicate that infiltrating irrigation waters leach arsenic from surficial sediments to the underlying aquifer.     

Hydrogeological and geochemical control of the variations of Rn concentrations in a hard rock aquifer: Insights into the possible role of fracture-matrix exchanges

To investigate the possible variations of Rn concentration in crystalline rocks as a function of flow conditions, a field study was carried out of a fractured aquifer in granite. The method is based on the in situ measurement of Rn in groundwater, aquifer tests for the determination of hydraulic characteristics of the aquifer and laboratory measurement of Rn exhalation rate from rocks. A simple crack model that simulates the Rn concentration in waters circulating in a fracture intersecting a borehole was also tested. The Rn concentrations in groundwaters from boreholes of the study site ranged from 192 to 1597 Bq L⁻¹. The Rn exhalation rates of selected samples of granite and micaschist were determined from laboratory experiments. The results yielded fluxes varying from 0.5 to 1.3 mBq m⁻² s⁻¹ in granite and from 0.5 to 0.9 mBq m⁻² s⁻¹ in micaschists. Pumping tests were performed in the studied boreholes to estimate the transmissivity and calculate the equivalent hydraulic aperture of the fractures. Transmissivities ranged from 10⁻⁵ to 10⁻³ m² s⁻¹. Using the cubic law, hydraulic equivalent fracture apertures were calculated to be in the range of 0.5–2.3 mm.     

The distribution of total mercury and methyl mercury in a shallow hypereutrophic lake (Lake Taihu) in two seasons

To understand the geochemical cycle of Hg in hypereutrophic freshwater lake, two sampling campaigns were conducted in Lake Taihu in China during May and September of 2009. The concentrations of unfiltered total Hg (unfTHg) were in the range of 6.8–83 ng L⁻¹ (28 ± 18 ng L⁻¹) in the lake water and total Hg in the sediment was 12–470 ng g⁻¹, both of which are higher than in other background lakes. The concentration of unfTHg in ∼11% of the lake water samples exceeded the second class of the Chinese environmental standards for surface water of 50 ng L⁻¹ (GB 3838-2002), indicating that a high ecological risk is posed by the Hg in Lake Taihu. However, the concentrations of unfiltered total MeHg (unfMeHg) were relatively low in the lake water (0.14 ± 0.05 ng L⁻¹, excluding two samples with 0.81 and 1.0 ng L⁻¹). Lake sediment MeHg varied from 0.2–0.96 ng g⁻¹, with generally low ratios of MeHg/THg of <1%. The low concentrations of TMeHg in the lake water may have resulted from a strong uptake by the high primary productivity and the demethylation of MeHg in oxic conditions. In addition, contrary to the results of previous research conducted in deep-water lakes and reservoirs, the low concentrations of MeHg and low ratio of MeHg/THg in the lake sediment indicates that the net methylation of Hg was not accelerated by the elevated organic matter load created by the eutrophication of Lake Taihu. The results also showed that sediments were a source of THg and MeHg in the water. Higher diffusion fluxes of THg and MeHg may be partly responsible for the higher concentrations of THg in the lake water in May, 2009.     

Geochemistry of waters associated with the old mine workings at Fonte Santa (NE of Portugal)

The quartz veins containing scheelite from Fonte Santa mine cut the Lower Ordovician quartzites. A muscovite–biotite granite (G1) and a muscovite granite (G2), both S-type, crop out close to the Fonte Santa mine and are related to the Moncorvo–Bemposta shear zone. The most altered samples of G2 show intense muscovitization and microclinization and contain chlorite, columbite–tantalite, wolframite, W-ixiolite and Fe-oxides. The tin-bearing granites contain 18 ppm (G1) and 73 ppm (G2) Sn. The most altered samples of G2 correspond to a tungsten granite. The quartz veins contain muscovite, chlorite, tourmaline, scheelite, pyrrhotite, pyrite, sphalerite, chalcopyrite, galena, arsenopyrite, iron oxides, Fe sulfates, phosphates of Pb, Fe and Al. The Fonte Santa mine area was exploited for W between 1942 and 1982. At the end of November 2006, a flood event damaged the tailings dam of Fonte Santa mine, releasing contaminated material and increasing contaminant levels in water within the area of influence of the mine. The waters related to the Fonte Santa mine are poorly mineralized, with electrical conductivity of < 965 µS/cm, and of a mixed type or HCO₃⁻ and SO₄²⁻ types. Most pH values (5.0−8.5) indicate that there is no significant acidic drainage in the region, as found in other areas. More acidic values (pH = 3.4) were found in the mine's lagoon. Waters associated with mineralized veins and old mine activities have Fe and Mn concentrations that forbid their use for human consumption and agriculture. Natural Na, Mg and K water contents are associated with the alteration of albite, chlorite and muscovite of country rock, while Ca with the W-bearing quartz veins. Weathering agents are carbonic and sulphuric acids and the latter has a strong influence in areas draining fine-grained mine tailings.     

Genesis of arsenic/fluoride-enriched soda water: A case study at Datong,northern China

The high As and F⁻ groundwaters from Datong Basin are mostly soda waters with a Na/(Cl+SO₄) (meq) ratio greater than unity, As and F⁻ up to 1550 μg/L and 10.4 mg/L, respectively, and with pH between 7.6 and 9.1. Geochemical modeling indicates that the waters are oversaturated with respect to calcite and clay minerals such as kaolinite, and undersaturated with respect to primary rock-forming minerals such as anorthite and albite. The water chemistry also is affected by evapotranspiration. The degree of evaporative enrichment is up to 85 in terms of Cl⁻. Results of the hydrogeochemical studies indicate that the occurrence of soda water at Datong is the result of incongruent dissolution of aluminosilicates at one stage of their interaction with groundwater when the water is oversaturated with respect to calcite and evapotranspiration-related salt accumulation is not too strong. Studying the genesis of soda waters provides new insights into mechanism of As and F⁻ enrichment in the aquifer system. Due to CaF₂ solubility control and OH⁻–F⁻ exchange reactions, F⁻ can be enriched in soda water. And the high pH condition of soda water favors As desorption from oxyhydroxide surfaces, thereby increasing the concentration of As in the aqueous phase.     

Dissolved metals and associated constituents in abandoned coal-mine discharges,Pennsylvania, USA. Part 1: Constituent quantities and correlations

Complete hydrochemical data are rarely reported for coal-mine discharges (CMD). This report summarizes major and trace-element concentrations and loadings for CMD at 140 abandoned mines in the Anthracite and Bituminous Coalfields of Pennsylvania. Clean-sampling and low-level analytical methods were used in 1999 to collect data that could be useful to determine potential environmental effects, remediation strategies, and quantities of valuable constituents. A subset of 10 sites was resampled in 2003 to analyze both the CMD and associated ochreous precipitates; the hydrochemical data were similar in 2003 and 1999. In 1999, the flow at the 140 CMD sites ranged from 0.028 to 2210 L s⁻¹, with a median of 18.4 L s⁻¹. The pH ranged from 2.7 to 7.3; concentrations (range in mg/L) of dissolved (0.45-μm pore-size filter) SO₄ (34–2000), Fe (0.046–512), Mn (0.019–74), and Al (0.007–108) varied widely. Predominant metalloid elements were Si (2.7–31.3 mg L⁻¹), B (<1–260 μg L⁻¹), Ge (<0.01–0.57 μg L⁻¹), and As (<0.03–64 μg L⁻¹). The most abundant trace metals, in order of median concentrations (range in μg/L), were Zn (0.6–10,000), Ni (2.6–3200), Co (0.27–3100), Ti (0.65–28), Cu (0.4–190), Cr (<0.5–72), Pb (<0.05–11) and Cd (<0.01–16). Gold was detected at concentrations greater than 0.0005 μg L⁻¹ in 97% of the samples, with a maximum of 0.0175 μg L⁻¹. No samples had detectable concentrations of Hg, Os or Pt, and less than half of the samples had detectable Pd, Ag, Ru, Ta, Nb, Re or Sn. Predominant rare-earth elements, in order of median concentrations (range in μg/L), were Y (0.11–530), Ce (0.01–370), Sc (1.0–36), Nd (0.006–260), La (0.005–140), Gd (0.005–110), Dy (0.002–99) and Sm (<0.005–79). Although dissolved Fe was not correlated with pH, concentrations of Al, Mn, most trace metals, and rare earths were negatively correlated with pH, consistent with solubility or sorption controls. In contrast, As was positively correlated with pH.     

Benthic fluxes of mercury species in a lagoon environment (Grado Lagoon,Northern Adriatic Sea,Italy)

The role of the major biogeochemical processes in Hg cycling at the sediment–water interface was investigated in the Grado Lagoon (Northern Adriatic Sea). This wetland system has been extensively contaminated from the Idrija Hg Mine (Slovenia) through the Isonzo River suspended load carried by tidal fluxes. Three approaches were used to study the sediment–water exchange of total Hg (THg), methylmercury (MeHg), reactive Hg (RHg) and dissolved gaseous Hg (DGHg): (1) estimation of diffusive fluxes from porewater and overlying water concentrations, (2) measurements of benthic fluxes using a deployed light benthic chamber in situ and (3) measurements of benthic fluxes during oxic–anoxic transition with a laboratory incubation experiment. The THg solid phase, ranging between 9.5 and 14.4 μg g⁻¹, showed slight variability with depth and time. Conversely, MeHg contents were highest (up to 21.9 ng g⁻¹) at the surface; they tended to decrease to nearly zero concentration with depth, thus suggesting that MeHg production and accumulation occur predominantly just below the sediment–water interface. Porewater MeHg concentrations (0.9–7.9 ng L⁻¹, 0.15–15% of THg) varied seasonally; higher contents were observed in the warmer period. The MeHg diffusive fluxes (up to 17 ng m⁻² day⁻¹) were similar to those in the nearby Gulf of Trieste [Covelli, S., Horvat, M., Faganeli, J., Brambati, A., 1999. Porewater distribution and benthic flux of mercury and methylmercury in the Gulf of Trieste (Northern Adriatic Sea). Estuar. Coast. Shelf Sci. 48, 415–428], although the lagoon sediments contained four-fold higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (up to 110 ng m⁻² day⁻¹) were one- to two-fold higher than those previously estimated for the Gulf of Trieste. The diurnal MeHg benthic fluxes were highest in summer at both sites (41,000 and 33,000 ng m⁻² day⁻¹ at the fishfarm and in the open lagoon, respectively), thus indicating the influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with O₂ and inversely with DIC, suggesting an important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through the production of organic matter promptly available for methylation. The results from the dark chamber incubated in the laboratory showed that the regeneration of dissolved THg was slightly affected by the oxic–anoxic transition. Conversely, the benthic flux of MeHg was up to 15-fold higher in sediments overlain by O₂ depleted waters. In the anoxic phase, the MeHg fluxes proceeded in parallel with Fe fluxes and the methylated form reached approximately 100% of dissolved THg. The MeHg is mostly released into overlying water (mean recycling efficiency of 89%) until the occurrence of sulphide inhibition, due to scavenging of the available Hg substrate for methylation. The results suggest that sediments in the Grado Lagoon, especially during anoxic events, should be considered as a primary source of MeHg for the water column.     

Diffusion of HTO,Cl and I in Upper Toarcian argillite samples from Tournemire: Effects of initial iodide concentration and ionic strength

Diffusion parameters for HTO, ³⁶Cl⁻_, and ¹²⁵I⁻ were determined on Upper Toarcian argillite samples from the Tournemire Underground Research Laboratory (Aveyron, France) using the through diffusion technique. The direction of diffusion was parallel to the bedding plane. The purpose of the present study was 3-fold; it was intended (i) to confirm the I⁻ interaction with Upper Toarcian argillite and to verify the effects of initial I⁻ concentration on this affinity, as previously observed by means of radial diffusion experiments, (ii) to highlight any discrepancy between Cl⁻ and I⁻ diffusivity, and (iii) to investigate the effect of an increase of the ionic strength of the solution on the anionic tracers’ diffusive behaviour. The results show that the effective diffusion coefficient (D_e) and diffusion accessible porosity (ε_a) values obtained with an ionic strength (I.S.) synthetic pore water of 0.01 eq L⁻¹ are: D_e = 2.35–2.50 × 10⁻¹¹ m² s⁻¹ and ε_a = 12.0–15.0% for HTO, and D_e = 14.5–15.5 × 10⁻¹³ m² s⁻¹ and ε_a = 2.5–2.9% for ³⁶Cl⁻. Because of anionic exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than HTO, taken as a water tracer. The associated effective diffusion coefficient (D_e) and rock capacity factor (α) obtained for ¹²⁵I are: D_e = 7.00–8.60 × 10⁻¹³ m² s⁻¹ and α = 4.3–7.2%. Such values make it possible to calculate low ¹²⁵I distribution ratios (0.0057 < R_D < 0.0192 mL g⁻¹) which confirm the trend indicating that the ¹²⁵I rock capacity factor increases with the decrease of the initial I⁻ concentration. Additional through-diffusion experiments were carried out with a higher ionic strength synthetic pore water (I.S. = 0.11 eq L⁻¹). No evolution of HTO diffusion parameters was observed. The anionic tracers’ effective diffusion coefficient increased by a factor of two but no clear evolution of their accessible porosity was observed. Such a paradox could be related to the particularly small mean pore size of the Upper Toarcian argillite of Tournemire. The most significant finding of this study is the large discrepancy (factor of two) between the values of the effective diffusion coefficient for ¹²⁵I and ³⁶Cl. Whatever the ionic strength of the synthetic solution used, ¹²⁵I exhibited D_e values two times lower than those of ³⁶Cl. A detailed explanation for this difference cannot be given at present even if a hypothesis based on ion-pairing or on steric-exclusion cannot be excluded. This makes questionable the assumption usually made for quantifying ¹²⁵I sorption and postulating that ³⁶Cl and ¹²⁵I would diffuse in the same porosity. In other terms, at Tournemire, ¹²⁵I sorption could be more pronounced than previously indicated.     

Effect of groundwater pH and ionic strength on strontium sorption in aquifer sediments: Implications for Sr mobility at contaminated nuclear sites

Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of ⁹⁰Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments. The extent of sorption is controlled by the geochemistry of the groundwater and sediment mineralogy. Here, batch sorption experiments were used to examine the sorption behaviour of ⁹⁰Sr in sediment–water systems representative of the UK Sellafield nuclear site based on groundwater and contaminant fluid compositions. In experiments with low ionic strength groundwaters (<0.01 mol L⁻¹), pH variation is the main control on sorption. The sorption edge for ⁹⁰Sr was observed between pH 4 and 6 with maximum sorption occurring (K_d ∼ 10³ L kg⁻¹) at pH 6–8. At ionic strengths above 10 mmol L⁻¹, and at pH values between 6 and 8, cation exchange processes reduced ⁹⁰Sr uptake to the sediment. This exchange process explains the lower ⁹⁰Sr sorption (K_d ∼ 40 L kg⁻¹) in the presence of artificial Magnox tank liquor (IS = 29 mmol L⁻¹). Strontium K-edge EXAFS spectra collected from sediments incubated with Sr²⁺ in either HCO₃-buffered groundwater or artificial Magnox tank liquor, revealed a coordination environment of ∼9 O atoms at 2.58–2.61 Å after 10 days. This is equivalent to the Sr²⁺ hydration sphere for the aqueous ion and indicates that Sr occurs primarily in outer sphere sorption complexes. No change was observed in the Sr sorption environment with EXAFS analysis after 365 days incubation. Sequential extractions performed on sediments after 365 days also found that ∼80% of solid associated ⁹⁰Sr was exchangeable with 1 M MgCl₂ in all experiments. These results suggest that over long periods, ⁹⁰Sr in contaminated sediments will remain primarily in weakly bound surface complexes. Therefore, if groundwater ionic strength increases (e.g. by saline intrusion related to sea level rise or by design during site remediation) then substantial remobilisation of ⁹⁰Sr is to be expected.     

Environmental geochemistry of an active Hg mine in Xunyang,Shaanxi Province,China

The Xunyang Hg mine (XMM) situated in Shaanxi Province is an active Hg mine in China. Gaseous elemental Hg (GEM) concentrations in ambient air were determined to evaluate its distribution pattern as a consequence of the active mining and retorting in the region. Total Hg (HgT) and methylmercury (MeHg) concentrations in riparian soil, sediment and rice grain samples (polished) as well as Hg speciation in surface water samples were measured to show local dispersion of Hg contamination. As expected, elevated concentrations of GEM were found, ranging from 7.4 to 410 ng m⁻³. High concentrations of HgT and MeHg were also obtained in riparian soils, ranged from 5.4 to 120 mg kg⁻¹ and 1.2 to 11 μg kg⁻¹, respectively. Concentrations of HgT and MeHg in sediment samples varied widely from 0.048 to 1600 mg kg⁻¹ and 1.0 to 39 μg kg⁻¹, respectively. Surface water samples showed elevated HgT concentrations, ranging from 6.2 to 23,500 ng L⁻¹, but low MeHg concentrations, ranging from 0.022 to 3.7 ng L⁻¹. Rice samples exhibited high concentrations of 50–200 μg kg⁻¹ in HgT and of 8.2–80 μg kg⁻¹ in MeHg. The spatial distribution patterns of Hg speciation in the local environmental compartments suggest that the XMM is the source of Hg contaminations in the study area.     

Methane microseepage from different sectors of the Yakela condensed gas field in Tarim Basin,Xinjiang, China

Methane microseepage is the result of natural gas migration from subsurface hydrocarbon accumulations to the Earth’s surface, and it is quite common in commercial petroleum fields. While the role of microseepage as a pathfinder in petroleum exploration has been known for about 80 a, its significance as an atmospheric CH₄ source has only recently been studied, and flux data are currently available only in the USA and Europe. With the aim of increasing the global data-set and better understanding flux magnitudes and variabilities, microseepage is now being extensively studied in China. A static flux chamber method was recently applied to study microseepage emissions into the atmosphere in four different sectors of the Yakela condensed gas field in Tarim Basin, Xinjiang, China, and specifically in: (a) a faulted sector, across the Luntai fault systems; (b) an oil–water interface sector, at the northern margin of the field; (c) an oil–gas interface sector, in the middle of the field; (d) an external area, outside the northern gas field boundaries. The results show that positive CH₄ fluxes are pervasive in all sectors and therefore, only part of the CH₄ migrating from the deep oil–gas reservoirs is consumed in the soil by methanotrophic oxidation. The intensity of gas seepage seems to be controlled by subsurface geologic settings and lateral variabilities of natural gas pressure in the condensed gas field. The highest CH₄ fluxes, up to ∼14 mg m⁻² d⁻¹ (mean of 7.55 mg m⁻² d⁻¹) with higher spatial variability (standard deviation, σ: 2.58 mg m⁻² d⁻¹), occur in the Luntai fault sector. Merhane flux was lower in the oil–water area (mean of 0.53 mg m⁻² d⁻¹) and the external area (mean of 1.55 mg m⁻² d⁻¹), and at the intermediate level in the gas–oil sector (mean of 2.89 mg m⁻² d⁻¹). These values are consistent with microseepage data reported for petroleum basins in the USA and Europe. The build-up of methane concentration in the flux chambers is always coupled with an enrichment of ¹³C, from δ¹³C₁ of −46‰ to −42.5‰ (VPDB), which demonstrates that seeping methane is thermogenic, as that occurring in the deep Yakela reservoir. Daily variations of microseepage are very low, with minima in the afternoon, corresponding to higher soil temperature (and higher methanotrophic consumption), and maxima in the early morning (when soil temperatures are lowest). A preliminary and rough estimate of the total amount of CH₄ exhaled from the Yakela field is in the order of 10² tonnes a⁻¹.     

Solubility of cyclooctasulfur in pure water and sea water at different temperatures

The solubility of cyclooctasulfur in water and sea water at various temperatures in the range between 4 and 80 °C was determined. Cyclooctasulfur in equilibrium with rhombic sulfur reacted with hot acidic aqueous potassium cyanide to form thiocyanate anion which was measured by anion chromatography. Sulfur solubility in pure water was found to increase with temperature by more than 78 times: from 6.1 nM S₈ at 4 °C to 478 nM S₈ at 80 °C. The following thermodynamic values for solubilisation of S₈ in water were calculated from the experimental data: K° = 3.01 ± 1.04 × 10⁻⁸, ΔG_r° = 42.93 ± 0.73 kJ mol⁻¹, ΔH_r° = 47.4 ± 3.6 kJmol⁻¹, ΔS_r° = 15.0 ± 11.7 J mol⁻¹ K⁻¹). Solubility of cyclooctasulfur in sea water was found to be 61 ± 13% of the solubility in pure water regardless of the temperature.     

Mobility of metals in nickel mine spoil materials

An understanding of the biogeochemical behaviour of metals in mine spoil materials is a prerequisite to rehabilitate Ni mining sites. The objective of this study was to characterize the fate of metals in different Ni ore spoil materials as influenced by hydrological conditions and fertilisation practices. In tropical ultramafic complexes, the different stages of lateritic weathering lead to two types of ores, and therefore, to two spoil types. They are mainly either a clay-rich saprolite, so-called “garnierite”, enriched in phyllosilicates, or a limonitic material, enriched in Fe oxides. Lysimeter columns were designed to monitor leaching waters through both spoil materials. The garnieritic spoil released higher concentrations of Mg (mean = 2.25 mg L⁻¹), Ni (0.39 mg L⁻¹) and Cr (1.19 mg L⁻¹) than the limonitic spoil (Mg = 0.5 mg L⁻¹; Ni = 0.03 mg L⁻¹ and Cr = 0.25 mg L⁻¹). Chromium was mainly in an anionic form in leaching solutions. As exchangeable pools of Cr(VI) in limonite (980 mg kg⁻¹ of KH₂PO₄-extractable Cr) are considerable its release in water may still occur in the case of a pH increase. In mixed spoil, metal concentrations were almost as low as in the limonitic one. The effect of mineral-N fertilisation was a strong release of cations (Ni, Mg) into the leachate. Phosphate amendment did not affect the soil solution composition under experimental conditions.     

Monomethylmercury sources in a tropical artificial reservoir

The distribution and speciation of mercury (Hg) in the water column, the inputs (wet deposition and tributaries) and the outputs (atmospheric evasion and outlet) of an artificial partially anoxic tropical lake (Petit-Saut reservoir, French Guiana) were investigated on a seasonal basis in order to appraise the cycling and transformations of this metal. The total mercury (HgT) concentrations in the oxygenated epilimnetic waters averaged 5 ± 3 pmol L⁻¹ in the unfiltered samples (HgT_UNF) and 4 ± 2 pmol L⁻¹ in the dissolved (HgT_D) phase (<0.45 μm). On average, the monomethylmercury (MMHg) constituted 8%, 40% and 18% of the HgT in the dissolved phase, the particulate suspended matter and in the unfiltered samples, respectively. Covariant elevated concentrations of particulate MMHg and chlorophyll a in the epilimnion suggest that phytoplankton is an active component for the MMHg transfer in the lake. In the anoxic hypolimnion the HgT_UNF averages 13 ± 6 pmol L⁻¹ and the HgT_D 8 ± 4 pmol L⁻¹. The averages of MMHg_P and MMHg_D in hypolimnetic waters were two and three times the corresponding values of the epilimnion, 170 ± 90 pmol g⁻¹ and 0.9 ± 0.5 pmol L⁻¹, respectively. In the long dry and wet seasons, at the flooded forest and upstream dam sampling stations, the vertical profiles of MMHg_D concentrations accounted for two distinct maxima: one just below the oxycline and the other near the benthic interface. Direct wet atmospheric deposition accounted for 14 moles yr⁻¹ HgT_UNF, with 0.7 moles yr⁻¹ as MMHg_UNF, while circa 76 moles yr⁻¹ of HgT_UNF, with 4.7 moles yr⁻¹ as MMHg_UNF, coming from tributaries. Circa 78 moles (∼17% as MMHg) are annually exported through the dam, while 23 moles yr⁻¹ of Hg⁰ evolve in the atmosphere. A mass balance calculation suggests that the endogenic production of MMHg_UNF attained 8.1 moles yr⁻¹, corresponding to a methylation rate of 0.06% d⁻¹. As a result, the Petit-Saut reservoir is a large man-made reactor that has extensively altered mercury speciation in favor of methylated species.     

Fractionation,distribution and transport of mercury in rivers and tributaries around Wanshan Hg mining district,Guizhou province,southwestern China: Part 1 – Total mercury

The Wanshan Hg mining area in Guizhou, China, was one of the world’s largest Hg producing regions. Numerous mine-waste and calcines still remain, leaching Hg to local rivers and streams and potentially impacting the local population. Several studies have been published on local environmental impacts of these mining and retorting residues, but a comprehensive, regional survey on the distribution of Hg in the rivers in the region, as presented in this paper, has not previously been conducted. This study focuses on the regional distribution and temporal variation of aqueous Hg fractions in the five main watercourses draining the Wanshan Hg mining and retorting area, covering more than 700 km². Three sampling campaigns were carried out in 2007 and 2008, covering high flow, normal flow and low flow periods. Total (THg), particulate (PHg), dissolved (DHg) and reactive (RHg) Hg fractions were determined. All rivers had the highest Hg concentrations at sample sites about 100–500 m downstream of the mine wastes. Total Hg concentrations ranged from extremely high (up to 12,000 ng L⁻¹) at the sample site just 100 m below mine wastes, to quite low in tributary streams (1.9 ng L⁻¹, about 14 km downstream of the mine wastes). Total Hg and PHg concentrations were usually highest during high flow periods in the Hg-contaminated areas (i.e. THg ? 50 ng L⁻¹), while in the less-impacted downstream areas (with THg < 50 ng L⁻¹) the Hg concentrations were usually lowest during high flow periods. Although highly elevated concentrations of Hg in water samples were found just downstream of the mine wastes, the concentrations decreased sharply to well below 50 ng L⁻¹ (US EPA Hg concentration standard for protection of fresh water), within only 6–8 km downstream. Concentrations of THg were highly dominated by and correlated with PHg (R² = 0.996–0.999, P < 0.001); PHg constituted more than 80% of THg in Hg-contaminated areas, and could account for 99.6% of the THg close to the mine wastes.     

Stable carbon isotope behaviour of natural seepage of deep underground C-rich methane detected along a fault zone and adsorbed in mudstone: Tokyo Bay area,Japan

Gas was sampled regionally, including by drilling into faults, in the South Kanto gas-field around Tokyo Bay, Japan. Gas samples were collected from cores in a gas sampling container immediately after drilling. A value of δ¹³C₁ = −44.3‰ was obtained for gas in the container and δ¹³C₁ = −36.3‰ for seeping gas in a fault zone. However, typical CH₄ in this dissolved-in-water gas-field is mainly depleted in ¹³C, and δ¹³C₁ values range from −66‰ to −68‰ owing to microbial degradation of organic material. ¹³C-rich CH₄ is so far uncommon in the South Kanto gas-field. Seepages were observed from the surface along the north–south fault zone. The natural gas is stored below the sandstone layer by impermeable mudstone underlying the boundary at a depth of 30 m. Gas seepages were not observed below a depth of 40 m. Gas rises along the fault zone dissolved-in-groundwater up to the shallow region and then separates from the groundwater. ¹³C-rich CH₄ (adsorbed CH₄) was found to have desorbed from drilled mudstone core samples taken at depths of 1400–1900 m in the main gas-production strata. Similarly, ¹³C-rich CH₄ was found in black shale overlying the oceanic crust forming part of a sedimentary accretionary prism underling the Tokyo region. It also appears in the spring-water of spa wells, originating at a depth of 1200–1500 m along a tectonic line. Methane generated by microbial degradation of organic material through CO₂ reduction in the South Kanto gas-field mainly originates as biogenic gas mixed with a small amount of ¹³C-rich CH₄, derived from thermogenic gas without oil components in strata. It is assumed that ¹²C-rich CH₄ is easily detached from core or pore water through gas production, whereas ¹³C-rich CH₄ is strongly adsorbed on the surfaces of particles. The ¹³C-rich CH₄ rises along the major tectonic line or up the 50 m wide normal fault zone from relatively deep sources in the Kanto region.