Temporal dynamics of AVS and SEM in sediment of shallow freshwater floodplain lakes

Acid volatile sulfide (AVS) is an operationally defined sulfide fraction, which is considered important for trace metal fate in reduced sediments. Understanding AVS formation rates is important for the management of metal polluted sediment. However, little is known about the fate and dynamics of AVS in spatially and seasonally variable freshwater environments. The authors monitored in situ AVS formation and degradation and simultaneously extracted metals (SEM) in two floodplain lakes and compared this to AVS formation rates in laboratory experiments with the same sediment. In the laboratory experiments, the formation rates of AVS were studied at 20 °C for initially oxidized sediments that were: (a) untreated; (b) enriched with extra ${\mathrm{SO}}_4^{2-}$; and (c) treated with sodium-azide (biocide). In the field, AVS concentrations were highly variable and were significantly correlated to surface water temperature and O₂ concentrations as well as to sediment composition. Between February and August, AVS formation was approximately linear at a rate of 0.07 μmol g⁻¹ d⁻¹. Degradation rates differed drastically between the lakes due to different degradation mechanisms. In one lake AVS removal was caused by reworking and oxygenation of the sediments by bream (Abrami brama), at a rate of 0.25 μmol g⁻¹ d⁻¹. In the other lake AVS removal was caused by desiccation, at a rate of ±2.6 μmol g⁻¹ d⁻¹. This illustrates the large differences that can be found between similar lakes, and the importance of biological processes. In the laboratory, concentrations of AVS with and without ${\mathrm{SO}}_4^{2-}$ addition were similar during the first weeks, and increased at a rate of 0.15 μmol g⁻¹ d⁻¹. However, ${\mathrm{SO}}_4^{2-}$ addition increased the AVS concentration at the end of the experiment, whereas sodium-azide eliminated AVS formation, as expected. This suggests that AVS formation was ${\mathrm{SO}}_4^{2-}$-limited in the laboratory as well as in these shallow freshwater lakes.     

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.     

Arsenic associations in sediments from the loess aquifer of La Pampa,Argentina

Groundwater from the Quaternary loess aquifer of La Pampa, central Argentina, has significant problems with high concentrations of As (up to 5300 μg L⁻¹) as well as other potentially toxic trace elements such as F, B, Mo, U, Se and V. Total As concentrations in 45 loess samples collected from the aquifer have a range of 3–18 mg kg⁻¹ with a mean of 8 mg kg⁻¹. These values are comparable to world-average sediment As concentrations. Five samples of rhyolitic ash from the area have As concentrations of 7–12 mg kg⁻¹. Chemical analysis included loess sediments and extracted porewaters from two specially cored boreholes. Results reveal a large range of porewater As concentrations, being generally higher in the horizons with highest sediment As concentrations. The displaced porewaters have As concentrations ranging up to 7500 μg L⁻¹ as well as exceptionally high concentrations of some other oxyanion species, including V up to 12 mg L⁻¹. The highest concentrations are found in a borehole located in a topographic depression, which is a zone of likely groundwater discharge and enhanced residence time. Comparison of sediment and porewater data does not reveal unequivocally the sources of the As, but selective extract data (acid-ammonium oxalate and hydroxylamine hydrochloride) suggest that much of the As (and V) is associated with Fe oxides. Primary oxides such as magnetite and ilmenite may be partial sources but given the weathered nature of many of the sediments, secondary oxide minerals are probably more important. Extract compositions also suggest that Mn oxide may be an As source. The groundwaters of the region are oxidising, with dissolved O₂, NO₃ and SO₄ normally present and As(V) usually the dominant dissolved As species. Under such conditions, the solubility of Fe and Mn oxides is low and As mobilisation is strongly controlled by sorption–desorption reactions. Desorption may be facilitated by the relatively high-pH conditions of the groundwaters in the region (7.0–8.8) and high concentrations of potential competitors (e.g. V, P, HCO₃). PHREEQC modelling suggests that the presence of V at the concentrations observed in the Pampean porewaters can suppress the sorption of As to hydrous Fe(III) oxide (HFO) by up to an order of magnitude. Bicarbonate had a comparatively small competitive effect. Oxalate extract concentrations have been used to provide an upper estimate of the amount of labile As in the sediments. A near-linear correlation between oxalate-extractable and porewater As in one of the cored boreholes investigated has been used to estimate an approximate K_d value for the sediments of 0.94 L kg⁻¹. This low value indicates that the sediments have an unusually low affinity for As.     

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.     

Environmental assessment of abandoned mine tailings in Adak,Västerbotten district (northern Sweden)

Sulfide-rich mine tailings in Adak that are exposed to weathering cause acid mine drainage characterized by low pH (2–4) and high SO₄ (up to 800 mg L⁻¹). Surface water, sediment and soil samples collected in this study contain higher concentrations of As, Cu, Fe and Zn, compared to the target and/or intervention limits set by international regulatory agencies. In particular, high As concentrations in water (up to 2900 μg L⁻¹) and sediment (up to 900 mg kg⁻¹) are of concern. There is large variability in trace element concentrations, implying that both physical (grain size) and chemical factors (pH, secondary phases as sulfides, Al-oxides or clay minerals) play an important role in their distribution. The low pH keeps the trace elements dissolved, and they are transported farther downstream. Trace element partition coefficients are low (log K_d = 0.3–4.3), and saturation indices calculated with PHREEQC are <0 for common oxide and sulfidic minerals. The sediment and soil samples indicate an enhanced pollution index (up to 17), and high enrichment factors for trace elements (As up to 38,300; Zn up to 800). Finally, leaves collected from different plant types indicate bioaccumulation of several elements (As, Al, Cu, Fe and Zn). However, some of the plants growing in this area (e.g., Salix, Equisétum) are generally resistant to metal toxicity, and hence, liming and phytoremediation could be considered as potential on-site remediation methods.     

Natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water: the example of West Bengal and its worldwide implications

In order to investigate the mechanism of As release to anoxic ground water in alluvial aquifers, the authors sampled ground waters from 3 piezometer nests, 79 shallow (<45 m) wells, and 6 deep (>80 m) wells, in an area 750 m by 450 m, just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of As (200–1180 μg L⁻¹) are accompanied by high concentrations of Fe (3–13.7 mg L⁻¹) and PO₄ (1–6.5 mg L⁻¹). Ground water that is rich in Mn (1–5.3 mg L⁻¹) contains <50 μg L⁻¹ of As. The composition of shallow ground water varies at the 100-m scale laterally and the metre-scale vertically, with vertical gradients in As concentration reaching 200 μg L⁻¹ m⁻¹. The As is supplied by reductive dissolution of FeOOH and release of the sorbed As to solution. The process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermo-tolerant coliforms, a proxy for faecal contamination, are not present in high numbers (<10 cfu/100 ml in 85% of wells) showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so does not release As to ground water.Arsenic concentrations are high (≫50 μg L⁻¹) where reduction of FeOOH is complete and its entire load of sorbed As is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where reduction is incomplete, the sediments are brown in colour and resorption of As to residual FeOOH keeps As concentrations below 10 μg L⁻¹ in the presence of dissolved Fe. Sorbed As released by reduction of Mn oxides does not increase As in ground water because the As resorbs to FeOOH. High concentrations of As are common in alluvial aquifers of the Bengal Basin arise because Himalayan erosion supplies immature sediments, with low surface-loadings of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of FeOOH is common.     

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.     

Detection of coenzyme F430 in deep sea sediments: A key molecule for biological methanogenesis

We report the presence of coenzyme factor 430 (F430), a prosthetic group of methyl coenzyme M reductase for archaeal methanogenesis, in the deep sub-seafloor biosphere. At 106.7 m depth in sediment collected off Shimokita Peninsula, northwestern Pacific, its concentration was estimated to be at least 40 fmol g sediment⁻¹ (i.e. 36 pg g⁻¹ wet sediment). This is about three orders of magnitude lower than typical concentrations of archaeal intact polar lipids in similar sub-seafloor sediments. On the basis of the concentration of F430 in methanogens and conversion to biomass composed of typical sub-seafloor microbial cells, we estimated that ca. 2 × 10⁶ cells g⁻¹ could be methanogens in the deeply buried marine sediment.     

Laboratory study of calcite–gypsum sludge–water interactions in a flooded tailings impoundment at the Kristineberg Zn–Cu mine,northern Sweden

Due to liming of acid mine drainage, a calcite–gypsum sludge with high concentrations of Zn (24,400 ± 6900 μg g⁻¹), Cu (2840 ± 680 μg g⁻¹) and Cd (59 ± 20 μg g⁻¹) has formed in a flooded tailings impoundment at the Kristineberg mine site. The potential metal release from the sludge during resuspension events and in a long-term perspective was investigated by performing a shake flask test and sequential extraction of the sludge. The sequentially extracted carbonate and oxide fractions together contained ⩾97% of the total amount of Cd, Co, Cu, Ni, Pb and Zn in the sludge. The association of these metals with carbonates and oxides appears to result from sorption and/or coprecipitation reactions at the surfaces of calcite and Fe, Al and Mn oxyhydroxides forming in the impoundment. If stream water is diverted into the flooded impoundment, dissolution of calcite, gypsum and presumably also Al oxyhydroxides can be expected during resuspension events. In the shake flask test (performed at a pH of 7–9), remobilisation of Zn, Cu, Cd and Co from the sludge resulted in dissolved concentrations of these metals that were significantly lower than those predicted to result from dissolution of the carbonate fraction of the sludge. This may suggest that cationic Zn, Cu, Cd and Co remobilised from dissolving calcite, gypsum and Al oxyhydroxides were readsorbed onto Fe oxyhydroxides remaining stable under oxic conditions. In a long-term perspective (≳10² a), ⩾97% of the Cd, Co, Cu, Ni, Pb and Zn content of the sludge potentially is available for release by dissolution of calcite and reductive dissolution of Fe oxyhydroxides if the sludge is subject to a soil environment with lower dissolved Ca concentrations, pH and redox than in the impoundment.     

Mercury speciation in soils of the industrialised Thur River catchment (Alsace,France)

Methylmercury (MeHg) and total Hg (THg) concentrations in soil profiles were monitored in the Thur River basin (Alsace, France), where a chlor-alkali plant has been located in the city of Vieux-Thann since the 1930s. Three soil types were studied according to their characteristics and location in the catchment: industrial soil, grassland soil and alluvial soil. Contamination of MeHg and THg in soil was important in the vicinity of the plant, especially in industrial and alluvial soil. Concentrations of MeHg reached 27 ng g⁻¹ and 29,000 ng g⁻¹ for THg, exceeding the predictable no effect concentration. Significant ecotoxicological risk exists in this area and remedial actions on several soil types are suggested. In each type of soil, MeHg concentrations were highest in topsoil, which decreased with depth. Concentrations of MeHg were negatively correlated with soil organic matter and total S, particularly when MeHg concentrations exceeded 8 ng g⁻¹. Under these conditions, MeHg concentrations in soil seemed to be influenced by THg, soil organic matter and total S concentrations. It was found that high MeHg/THg ratios (near 2%) in soil were mainly related to the combined soil environmental conditions such as low THg concentrations, low organic C/N ratios (<11) and relatively low pH (5–5.5). Nevertheless, even when the MeHg/THg ratio was low (∼0.04%), MeHg and THg concentrations were elevated, up to 13 ng g⁻¹ and to 29,000 ng g⁻¹, respectively. Thus, both THg and MeHg concentrations should be taken into account to assess potential environmental risks of Hg.     

Seasonal variations of arsenic at the sediment–water interface of Poyang Lake,China

Arsenic species including arsenite, arsenate, and organic arsenic were measured in the porewaters collected from Poyang Lake, the largest freshwater lake of China. The vertical distributions of dissolved arsenic species and some diagenetic constituents [Fe(II), Mn(II), S(−II)] were also obtained in the same porewater samples in summer and winter. In sediments the concentration profiles of total As and As species bound to Fe–Mn oxyhydroxides and to organic matter were also determined along with the concentrations of Fe, Mn and S in different extractable fractions. Results indicate that, in the summer season, the concentrations of total dissolved As varying from 3.9 to 55.8 μg/L in sediments were higher than those (5.3–15.7 μg/L) measured in the winter season, while the concentrations of total As species in the solid phase varied between 10.97 and 25.32 mg/kg and between 7.84 and 30.52 mg/kg on a dry weight basis in summer and winter, respectively. Seasonal profiles of dissolved As suggest downward and upward diffusion, and the flux of dissolved As across the sediment–water interface (SWI) in summer and winter were estimated at 3.88 mg/m² a and 0.79 mg/m² a, respectively. Based on porewater profiles and sediment phase data, the main geochemical behavior of As was controlled by adsorption/desorption, precipitation and molecular diffusion. The solubility and migration of inorganic As are controlled by Fe–Mn oxyhydroxides in summer whereas they appear to be more likely controlled by both amorphous Fe–Mn oxyhydroxides and sulfides in winter. A better knowledge of the cycle of As in Poyang Lake is essential to a better management of its hydrology and for the environmental protection of biota in the lake.     

Chlor-alkali industrial contamination and riverine transport of mercury: Distribution and partitioning of mercury between water,suspended matter,and bottom sediment of the Thur River,France

Total dissolved and total particulate Hg mass balances were estimated during one hydrological period (July 2001–June 2002) in the Thur River basin, which is heavily polluted by chlor-alkali industrial activity. The seasonal variations of the Hg dynamics in the aquatic environment were assessed using total Hg concentrations in bottom sediment and suspended matter, and total and reactive dissolved Hg concentrations in the water. The impact of the chlor-alkali plant (CAP) remains the largest concern for Hg contamination of this river system. Upstream from the CAP, the Hg partitioning between dissolved and particulate phases was principally controlled by the dissolved fraction due to snow melting during spring high flow, while during low flow, Hg was primarily adsorbed onto particulates. Downstream from the CAP, the Hg partitioning is controlled by the concentration of dissolved organic and inorganic ligands and by the total suspended sediment (TSS) concentrations. Nevertheless, the particulate fluxes were five times higher than the dissolved ones. Most of the total annual flux of Hg supplied by the CAP to the river is transported to the outlet of the catchment (total Hg flux: 70 μg m⁻² a⁻¹). Downstream from the CAP, the bottom sediment, mainly composed of coarse sediment (>63 μm) and depleted in organic matter, has a weak capacity to trap Hg in the river channel and the stock of Hg is low (4 mg m⁻²) showing that the residence time of Hg in this river is short.     

Mobilization of arsenic from acid deposition – The Elbe River catchment,Czech Republic

Temporal changes of As concentration in surface waters were observed in some areas of the Czech Republic. Mobilized As originates from past atmospheric deposition. To understand the factors influencing As aqueous concentration and mobility the chemistry and runoff generation of a number of brooks, springs and rivers in the central part of the Elbe River catchment, Czech Republic, were monitored. Seasonal variations of As (from 0.5 to 10.5 μg L⁻¹), Fe (from 0.05 to 3.9 mg L⁻¹) and DOC (dissolved organic C – from 1.2 to 17.5 mg L⁻¹) were observed in monitored stream waters with maximum values of As and Fe in the summer months at pH values 7.6–7.8. The concentration of As in particles with a diameter < 60 μm correlates with the Fe concentration. There is no correlation between Fe and As in filtered samples (<0.45 μm). The As concentration in stream water colloids depends on an increase in DOC concentration and a decrease in ionic strength. The DOC stabilizes As in solution and reduces its re-adsorption on Fe colloids and consequently As concentration in the stream increases.     

Selenium speciation,distribution, and transport in a river catchment affected by mercury mining and smelting in Wanshan,China

Selenium (Se) is an important co-existing elemental component of the mineral matrix of mercury (Hg) ore deposits. The hazards associated with Se contamination of the aquatic ecosystems in Hg mining areas; however, are often overlooked by environmental researchers due to a preoccupation with Hg. Selenium may also pose a long-term risk to the local ecosystem, and further complicate the situation as Se may also play an important antagonistic role against Hg. Furthermore, most studies on Se pollution have focused only on total Se, whereas the toxicity, bioavailability, and bioaccumulation of Se in aquatic ecosystems is primarily determined by its site-specific individual species. In this study, the concentrations of total Se, inorganic Se (tetravalent and hexavalent), and organic Se were determined in water samples collected from 41 typical sites selected in rivers, tributaries, and springs in Wanshan, China, where Hg and Se co-occur due to historic Hg mining and retorting activities. Se concentrations were observed to decrease with distance from mine-waste calcines, which indicated that mine-waste calcines may be significant sources of the elevated Se in the rivers, especially in downstream areas within 8 km from the mine-waste calcines. The concentration of total aqueous Se throughout the study area was highly variable (3.8 ± 6.0 μg L⁻¹) and on average was one order of magnitude greater than that in natural river systems worldwide (0.1–0.3 μg L⁻¹). The majority of the Se was hexavalent (3.1 ± 4.9 μg L⁻¹; 65%), followed by tetravalent (0.53 ± 0.86 μg L⁻¹; 15%) and organic forms (0.85 ± 1.5 μg L⁻¹; 20%), possibly due to the generally alkaline conditions. Se concentrations in some sampling sites exceeded certain recommended limit of values. However, the existing criteria for Se in aquatic system are mainly based on total Se and the recommended limit of values in different countries or organizations are inconsistent with one another. Therefore, the need to consider Se speciation rather than only total Se is highlighted for future studies.     

Dissolution kinetics of soil clays in sulfuric acid solutions: Ionic strength and temperature effects

Significant amounts of sulfuric acid (H₂SO₄) rich saline water can be produced by the oxidation of sulfide minerals contained in inland acid sulfate soils (IASS). In the absence of carbonate minerals, the dissolution of phyllosilicate minerals is one of very few processes that can provide long-term acid neutralisation. It is therefore important to understand the acid dissolution behavior of naturally occurring clay minerals from IASS under saline–acidic solutions. The objective of this study was to investigate the dissolution of a natural clay-rich sample under saline–acidic conditions (pH 1–4; ionic strengths = 0.01 and 0.25 M; 25 °C) and over a range of temperatures (25–45 °C; pH 1 and pH 4). The clay-rich sample referred to as Bottle Bend clay (BB clay) used was from an IASS (Bottle Bend lagoon) in south-western New South Wales (Australia) and contained smectite (40%), illite (27%), kaolinite (26%) and quartz (6%). Acid dissolution of the BB clay was initially rapid, as indicated by the fast release of cations (Si, Al, K, Fe, Mg). Relatively higher Al (pH 4) and K (pH 2–4) release was obtained from BB clay dissolution in higher ionic strength solutions compared to the lower ionic strength solutions. The steady state dissolution rate (as determined from Si, Al and Fe release rates; R_Si, R_Al, R_Fe) increased with decreasing solution pH and increasing temperature. For example, the highest log R_Si value was obtained at pH 1 and 45 °C (−9.07 mol g⁻¹ s⁻¹), while the lowest log R_Si value was obtained at pH 4 and 25 °C (−11.20 mol g⁻¹ s⁻¹). A comparison of these results with pure mineral dissolution rates from the literature suggests that the BB clay dissolved at a much faster rate compared to the pure mineral samples. Apparent activation energies calculated for the clay sample varied over the range 76.6 kJ mol⁻¹ (pH 1) to 37.7 kJ mol⁻¹ (pH 4) which compare very well with the activation energy values for acidic dissolution of monomineralic samples e.g. montmorillonite from previous studies. The acid neutralisation capacity (ANC) of the clay sample was calculated from the release of all structural cations except Si (i.e. Al, Fe, K, Mg). According to these calculations an ANC of 1.11 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 (I = 0.25 M, 25 °C) compared to an ANC of 0.21 kg H₂SO₄/tonne clay/day at pH 4 (I = 0.25 M, 25 °C). The highest ANC of 6.91 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 and at 45 °C (I = 0.25 M), which is more than three times higher than the ANC provided under the similar solution conditions at 25 °C. In wetlands with little solid phase buffering available apart from clay minerals, it is imperative to consider the potential ANC provided by the dissolution of abundantly occurring phyllosilicate minerals in devising rehabilitation schemes.     

Dissolved and particulate zinc and nickel in the Yangtze River (China): Distribution,sources and fluxes

Dissolved and particulate Zn and Ni concentrations were determined at 76 locations along the Yangtze River basin from the headwaters to the estuary during flood and dry seasons. Spatial and temporal variations of Zn and Ni were investigated and six major source zones were identified. The Three Gorges Dam (TGD) blocked most of the suspended loads and extremely low concentration of Zn and Ni were observed downstream of the dam. Dissolved (ranging from 0.062 to 8.0 μg L⁻¹) and particulate (ranging from 12 to 110 mg kg⁻¹) Ni showed similar levels of concentrations during flood and dry seasons, whereas dissolved (ranging from 0.43 to 49 μg L⁻¹) and particulate (ranging from 54 to 1100 mg kg⁻¹) Zn were slightly and much lower in the flood season than dry season, respectively. This was attributed to the increased water discharge during the flood season causing a dilution effect and sediment resuspension. In the flood season, average concentrations of Zn and Ni were higher in the main channel than in tributaries, due to soil erosion and mining activities providing the dominant inputs. The situation was opposite in the dry season, attributed to the contribution of municipal sewage, industrial activities, and waste disposal. During the flood season, dissolved Zn and Ni concentrations were negatively correlated with pH. Water and suspended particulate matter (SPM) from the upper reaches, middle reaches, and lower reaches of the Yangtze River were characterized by their Zn and Ni concentrations. The Panzhihua, Nanling and Tongling mining areas were considered as the most important source zones of particulate Zn and Ni. The Chongqing region, Wuhan region and the Yangtze River Delta provided most of the dissolved Zn and Ni inputs into the river. Annual net flux of Zn (10–72 × 10⁵ kg a⁻¹) and Ni (5.0–19 × 10⁵ kg a⁻¹) in each source zone were estimated according to their respective influent and effluent fluxes. Contributions of the source zones to Zn and Ni transport decreased from the upper reaches to the lower reaches.     

Identification and regional quantification of hydrochemical processes at the contact zone between anoxic groundwater and surface water in poldered floodplains (Oderbruch polder,Germany)

The hydrochemistry in the largest polder of the Oder River, named Oderbruch, is affected by long-term infiltration of water from the Oder into the aquifer below an alluvial loamy top layer of the polder. These exceptional hydraulic conditions are a result of dyke constructions which were built more than 250 a ago. The objective of this investigation is a better understanding and a characterisation of the contact zone between the anaerobic groundwater and the surface water of a vast drainage system. Induced by changing water levels, different hydraulic conditions occur, which strongly influence the hydrochemistry of the shallow aquifer and therefore the natural sink function of the polder area.Field investigations with a hydrochemical and hydraulic characterisation of selected drainage ditch locations show considerable chemical interactions between groundwater and surface water. Depending on the drainage ditch type, which is defined by the hydraulic situation, the redox processes create a chemical gradient combined with a distinct enrichment of Fe and Mn. The source of the high amounts of Fe and Mn in the groundwater are reduced Fe- and Mn-hydroxides from the aquifer sediments.Under exfiltrating conditions interrupted by dry phases, more than 50 g kg⁻¹ Fe and 0.25 g kg⁻¹ Mn have accumulated in the drainage ditch floor sediments since the construction of the drainage ditches 35 a ago. The results show a very effective fixation of trace metals in the drainage ditch sediments under these conditions. Under permanent exfiltration conditions, the enrichment of Fe and Mn is relatively low. The maximum Fe content was 4 g kg⁻¹ sediment and the Mn content reached only 0.4 g kg⁻¹. This is less than 10% of the mobile Fe²⁺ and less than 1% of the Mn²⁺ which migrates from the aquifer into the surface water.     

Source characterization using molecular distribution and stable carbon isotopic composition of n-alkanes in sediment cores from the tropical Mundaú–Manguaba estuarine–lagoon system,Brazil

The Mundaú–Manguaba estuarine–lagoon system (MMELS) constitutes one of the most representative ecosystems in the state of Alagoas, NE Brazil. Approximately 30% or 60,000 ha of the lower surrounding drainage basin of the MMELS are covered by sugar cane and a total of 250,000 inhabitants contribute untreated effluent to the system. Short sediment cores from MMELS were collected in 2007 at three sites: Manguaba Lagoon (C03), Mundaú Lagoon (C07) and Mundaú River (C08) in order to characterize the delivery and deposition of n-alkanes over the past 40 yr. The n-alkanes ranged from C₁₅ to C₃₅, with total aliphatic hydrocarbon (TAH) concentration in the range 27.8–139.5 μg g⁻¹. An unresolved complex mixture (UCM) was observed in all sediments. The terrigenous/aquatic ratio (TAR), carbon preference index (CPI) and average chain length (ACL) showed that the terrigenous input dominated. The (δ¹³C) values of individual n-alkanes (C₁₆–C₃₃) varied between −22.6‰ and −34.2‰, suggesting a dominance of ¹²C-enriched n-alkanes that originated from C₃ plants and lacustrine algae. The data reflect how anthropogenic input (via sewage, industrial pollution and agriculture) has influenced the organic content of the system through time.     

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.     

Arsenic mobility in mildly alkaline drainage from an orogenic lode gold deposit,Bralorne mine,British Columbia

The historical (1932–1971) Bralorne mine produced over 87 million grams of Au from an archetypal orogenic lode gold deposit in southwest British Columbia. High concentrations of As in mine drainage, however, represent an on-going environmental concern prompting a detailed study of effluent chemistry. The discharge rate at the mine portal was monitored continuously over a fourteen-month period during which effluent samples were collected on a quasi-weekly basis. Water samples were also collected on synoptic surveys of the adit between the portal and the main source of flow in the flooded workings. Total concentrations of As in the mildly alkaline (pH = 8.7) portal drainage average 3034 μg/L whereas at the source they average 5898 μg/L. As emergent waters from the flooded workings flow toward the portal, their dissolved oxygen content and pH increase from 0 to 10 mg/L and from 7.7 to 9, respectively. Near the emergence point, dissolved Fe precipitates rapidly, sorbing both As(III) and As(V). With increasing distance from the emergence point, dissolved As(III) concentrations drop to detection limits through sorption on hydrous ferric oxide and through oxidation to As(V). Concentrations of dissolved As(V), on the other hand, increase and stabilize, reflecting lower sorption at higher pH and the lack of available sorbent. Nonetheless, based on synoptic surveys, approximately 35% of the source As load is sequestered in the adit resulting in As sediment concentrations averaging 8.5 wt%. The remaining average As load of 1.34 kg/d is discharged from the portal. Partitioning of As(V) between dissolved and particulate phases in portal effluent is characterized by a sorption density of 0.37 mol As (mol Fe)⁻¹ and by a distribution coefficient (K_d) of 130 L/g HFO. The relatively high sorption density may reflect co-precipitation of As with Fe oxyhydroxides rather than a purely adsorption-controlled process. Results of this study show that the As self-mitigating capacity of drainage from orogenic lode gold deposits may be poor in high-pH and Fe-limited settings.