Transport and thermodynamics constrain belowground carbon turnover in a northern peatland

Rates of anaerobic respiration are of central importance for the long-term burial of carbon (C) in peatlands, which are a relevant sink in the global C cycle. To identify constraints on anaerobic peat decomposition, we determined detailed concentration depth profiles of decomposition end-products, i.e. methane (CH₄) and dissolved inorganic carbon (DIC), along with concentrations of relevant decomposition intermediates at an ombrotrophic Canadian peat bog. The magnitude of in situ net production rates of DIC and CH₄ was estimated by inverse pore-water modeling. Vertical transport in the peat was slow and dominated by diffusion leading to the buildup of DIC and CH₄ with depth (5500 μmol L⁻¹ DIC, 500 μmol L⁻¹ CH₄). Highest DIC and CH₄ production rates occurred close to the water table (decomposition constant k_d ∼ 10⁻³-10⁻⁴ a⁻¹) or in some distinct zones at depth (k_d ∼ 10⁻⁴ a⁻¹). Deeper into the peat, decomposition proceeded very slowly at about k_d = 10⁻⁷ a⁻¹. This pattern could be related to thermodynamic and transport constraints. The accumulation of metabolic end-products diminished in situ energy yields of acetoclastic methanogenesis to the threshold for microbially mediated processes (−20 to −25 kJ mol⁻¹ CH₄). The methanogenic precursor acetate also accumulated (150 μmol L⁻¹). In line with these findings, CH₄ was formed by hydrogenotrophic methanogenesis at Gibbs free energies of −35 to −40 kJ mol⁻¹ CH₄. This was indicated by an isotopic fractionation α_CO2-CH4 of 1.069-1.079. Fermentative degradation of acetate, propionate and butyrate attained Gibbs free energies close to 0 kJ mol⁻¹ substrate. Although methanogenesis was apparently limited by some other factor in some peat layers, transport and thermodynamic constraints likely impeded respiratory processes in the deeper peat. Constraints on the removal of DIC and CH₄ may thus slow decomposition and contribute to the sustained burial of C in northern peatlands.     

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.     

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.     

Hydro-geochemical processes in an area with saline groundwater in lower Shire River valley, Malawi: An integrated application of hierarchical cluster and principal component analyses

Our ability to adapt to changes in groundwater quality, arising from a changing climate and/or local pressures, is dependent on comprehension of the governing controls of spatial variation in groundwater chemistry. This paper presents results of an assessment of dominant hydro-geochemical processes controlling groundwater chemical composition, using an integrated application of hierarchical cluster analysis (HCA) and principal component analysis (PCA) of a major ion dataset of groundwater from lower Shire River valley, Malawi. The area is in the southernmost part of the western section of the East African Rift System (EARS) and has localised occurrence of saline groundwater. HCA classified samples into three main clusters (C1-C3) according to their dominant chemical composition: C1 (dominant composition: Na-Cl; median TDS: 3436 mg L⁻¹), C2 (dominant composition: Na-HCO₃; median TDS: 966 mg L⁻¹) and C3 (dominant composition: alkali earths-HCO₃; median TDS: 528 mg L⁻¹). These clusters were in turn described by the principal components PC1, PC3 and PC2, respectively, resulting from the PCA. The results of the PCA and geochemical interpretation suggest that the spatial variation of groundwater quality in the area is influenced by the following processes: C3 samples result mainly from H₂CO₃ weathering of aluminosilicate minerals by percolating water supersaturated with CO₂. In addition to aluminosilicate weathering, C2 samples are influenced by the processes of cation exchange of Ca²⁺ and Mg²⁺ in the water for Na⁺ on clay minerals, and carbonate precipitation. The increase in ionic strength of C2 samples is attributed to mixing with high TDS groundwater in proximity with C2 samples. The saline/brackish C1 groundwater results from the processes of evaporation (for samples with high water table close to the Shire marshes) and dissolution of Cl⁻ and SO₄-evaporative salts followed by mineralised seep from sedimentary Karoo and Cretaceous Lupata sandstones.     

Landscape variations in stream water SO4 and δSSO4 in a boreal stream network

Despite reduced anthropogenic deposition during the last decades, deposition sulphate may still play an important role in the biogeochemical cycles of S and many catchments may act as net sources of S that may remain for several decades. The aim of this study is to elucidate the temporal and spatial dynamics of both SO₄²⁻ and δ³⁴S_SO4 in stream water from catchments with varying percentage of wetland and forest coverage and to determine their relative importance for catchment losses of S. Stream water samples were collected from 15 subcatchments ranging in size from 3 to 6780 ha, in a boreal stream network, northern Sweden. In forested catchments (<2% wetland cover) S-SO₄²⁻ concentrations in stream water averaged 1.7 mg L⁻¹ whereas in wetland dominated catchments (>30% wetland cover) the concentrations averaged 0.3 mg L⁻¹. A significant negative relationship was observed between S-SO₄²⁻ and percentage wetland coverage (r² = 0.77, p < 0.001) and the annual export of stream water SO₄²⁻ and wetland coverage (r² = 0.76, p < 0.001). The percentage forest coverage was on the other hand positively related to stream water SO₄²⁻ concentrations and the annual export of stream water SO₄²⁻ (r² = 0.77 and r² = 0.79, respectively). The annual average δ³⁴S_SO4 value in wetland dominated streams was +7.6‰ and in streams of forested catchments +6.7‰. At spring flood the δ³⁴S_SO4 values decreased in all streams by 1‰ to 5‰. The δ³⁴S_SO4 values in all streams were higher than the δ³⁴S_SO4 value of +4.7‰ in precipitation (snow). The export of S ranged from 0.5 kg S ha⁻¹ yr⁻¹ (wetland headwater stream) to 3.8 kg S ha⁻¹ yr⁻¹ (forested headwater stream). With an average S deposition in open field of 1.3 kg S ha⁻¹ yr⁻¹ (2002-2006) the mass balance results in a net export of S from all catchments, except in catchments with >30% wetland. The high temporal and spatial resolution of this study demonstrates that the reducing environments of wetlands play a key role for the biogeochemistry of S in boreal landscapes and are net sinks of S. Forested areas, on the other hand were net sources of S.     

Enhancement of in situ biodegradation of organic compounds in groundwater by targeted pump and treat intervention

This study demonstrates the value of targeted pump and treatment (PAT) to enhance the in situ biodegradation of organic contaminants in groundwater for improved restoration. The approach is illustrated for a plume of phenolic compounds in a sandstone aquifer, where PAT is used for hydraulic containment and removal of dissolved phase contaminants from specific depth intervals. Time-series analysis of the plume hydrochemistry and stable isotope composition of dissolved species (δ³⁴S-SO₄, δ¹³C-CH₄, δ¹³C-TDIC (TDIC = Total Dissolved Inorganic Carbon)) in groundwater samples from high-resolution multilevel samplers were used to deduce changes in the relative significance of biodegradation processes and microbial activity in the plume, induced by the PAT system over 3 years. The PAT system has reduced the maximum contaminant concentrations (up to 6800 mg L⁻¹ total phenols) in the plume by 50% to ∼70% at different locations. This intervention has (i) stimulated in situ biodegradation in general, with an approximate doubling of contaminant turnover based on TDIC concentration, which has increased from <200 mg L⁻¹ to >350 mg L⁻¹, (ii) enhanced the activity of SO₄-reducing microorganisms (marked by a declining SO₄ concentration with corresponding increase in SO₄-δ³⁴S to values >7–14‰_V-CDT relative to background values of 1.9–6.5‰_V-CDT), and (iii) where the TDIC increase is greatest, has changed TDIC-δ¹³C from values of −10 to −15‰_V-PDB to ∼−20‰_V-PDB. This indicates an increase in the relative importance of respiration processes (including denitrification and anaerobic methane oxidation, AMO) that yield ¹³C-depleted TDIC over fermentation and acetoclastic methanogenesis that yield ¹³C-enriched TDIC in the plume, leading to higher contaminant turnover. The plume fringe was found to be a zone of enhanced biodegradation by SO₄-reduction and methanogenesis. Isotopically heavy methane compositions (up to −47.8‰_V-PDB) and trends between δ¹³C-TDIC and δ¹³C-CH₄ suggest that AMO occurs at the plume fringe where the contaminant concentrations have been reduced by the PAT system. Mass and isotope balances for inorganic carbon in the plume confirm the shift in spatial dominance of different biodegradation processes and significant increase in contribution of anaerobic respiration for contaminant biodegradation in zones targeted by the PAT system. The enhanced in situ biodegradation results from a reduction in organic contaminant concentrations in the plume to levels below those that formerly suppressed microbial activity, combined with increased supply of soluble electron acceptors (e.g. nitrate) into the plume by dispersion. An interruption of the PAT system and recovery of the dissolved organic contaminant concentrations towards former values highlights the dynamic nature of this enhancement on restoration and relatively rapid response of the aquifer microorganisms to changing conditions induced by the PAT system. In situ restoration using this combined engineered and passive approach has the potential to manage plumes of biodegradable contaminants over shorter timescales than would be possible using these methods independently. The application of PAT in this way strongly depends on the ability to ensure an adequate flux of dissolved electron acceptors into the plume by advection and dispersion, particularly in heterogeneous aquifers.     

Tracing industrial sulfur contributions to atmospheric sulfate deposition in the Athabasca oil sands region,Alberta, Canada

Anthropogenic S emissions in the Athabasca oil sands region (AOSR) in Alberta, Canada, affect SO₄ deposition in close vicinity of industrial emitters. Between May 2008 and May 2009, SO₄-S deposition was monitored using open field bulk collectors at 15 sites and throughfall collectors at 14 sites at distances between 3 and 113 km from one of the major emission stacks in the AOSR. At forested plots >90 km from the operations, SO₄ deposition was ∼1.4 kg SO₄-S ha⁻¹ yr⁻¹ for bulk deposition and ∼3.3 kg SO₄-S ha⁻¹ yr⁻¹ for throughfall deposition. Throughfall SO₄ deposition rates in the AOSR exceeded bulk deposition rates at all sites by a factor of 2–3, indicating significant inputs of dry deposition especially under forest canopies. Both bulk and throughfall SO₄ deposition rates were elevated within 29 km distance of the industrial operations with deposition rates as high as 11.7 kg SO₄-S ha⁻¹ yr⁻¹ for bulk deposition and 39.2 kg SO₄-S ha⁻¹ yr⁻¹ for throughfall at industrial sites. Sulfur isotope ratio measurements of atmospheric SO₄ deposited in the AOSR revealed that at a few selected locations ³⁴S-depleted SO₄, likely derived from H₂S emissions from tailing ponds contributes to local atmospheric SO₄ deposition. In general, however, δ³⁴S values of SO₄ deposition at distant forested plots (>74 km) with low deposition rates were not isotopically different from δ³⁴S values at sites with high deposition rates in the AOSR and are, therefore, not suitable to determine industrial S contributions. However, O isotope ratios of atmospheric SO₄ in bulk and throughfall deposition in the AOSR showed a distinct trend of decreasing δ¹⁸O-SO₄ values with increasing SO₄ deposition rates allowing quantification of industrial contributions to atmospheric SO₄ deposition. Two-end-member mixing calculations revealed that open field bulk SO₄ deposition especially at industrial sites in close proximity (<29 km) to the operations is significantly (17–59%) affected by industrial S emissions and that throughfall generally contained 49–100% SO₄ of industrial origin. Hence, it is suggested that δ¹⁸O values of SO₄ may constitute a suitable tracer for quantifying industrial contributions to atmospheric SO₄ deposition in the AOSR.     

Temporal trends of dissolved weathering products released from a high Arctic coal mine waste rock pile in Svalbard (78°N)

It is well known that oxidation of sulphide-containing coal mine waste has considerable environmental impacts due to generation of acid mine drainage (AMD) containing high dissolved metal concentrations. This study is the first to evaluate seasonal trends in the release of AMD from high arctic coal mine waste rock. Runoff from an abandoned coal mine waste pile in Svalbard (78°N) was studied during the entire 3–4 month period with running water in 2005. Temporal variation in concentrations and fluxes of dissolved elements were quantified based on daily water sampling and used to evaluate weathering processes and estimate element budgets on a daily, seasonal and annual basis. Apart from alkali- and alkaline earth metals; Fe, Al, Mn, Zn and Ni were found to be the most abundant metals in the runoff. Element concentrations were highly correlated and suggest that the processes of sulphide oxidation, ion exchange and silicate weathering occurring within the waste pile were linked throughout the measuring period. Observed pH values varied from 2.8 to 5.2 and SO₄ concentrations from 21 to 1463 mg L⁻¹. Manganese and Al concentrations were observed above phytotoxic levels (up to 4 and 23 mg L⁻¹, respectively) and were considered the most critical elements in terms of environmental impact. Throughout the summer a total dissolved quantity of 58 kg Mn, 238 kg Al and 13,700 kg SO₄ was released from the pile containing approximately 200,000 m³ of pyritic waste material (<1% FeS₂). The highest concentrations of metals, lowest pH values and a very high daily release of H₂SO₄ (up to twice as high as the following month) were observed during the first week of thaw. This is considered a result of an accumulation of weathering products, generated within the waste pile during winter and released as a pollution-flush during early spring. Similar accumulation/flush sequences were observed later in the summer where rain events following relatively long dry periods caused high daily metal fluxes and on some occasions also elevated dissolved metal concentrations. Despite highly variable weather/climate conditions during the rest of the summer the investigated waste rock pile acted like a relative constant pollution-source during this period. Future investigations regarding the environmental impact of mine waste in the region should include measurements of bioavailable metals in order to provide further details on the seasonal trends in environmental impact.     

Chemical and isotopic tracers of the contribution of microbial gas in Devonian organic-rich shales and reservoir sandstones,northern Appalachian Basin

In this study, the geochemistry and origin of natural gas and formation waters in Devonian age organic-rich shales and reservoir sandstones across the northern Appalachian Basin margin (western New York, eastern Ohio, northwestern Pennsylvania, and eastern Kentucky) were investigated. Additional samples were collected from Mississippian Berea Sandstone, Silurian Medina Sandstone and Ordovician Trenton/Black River Group oil and gas wells for comparison. Dissolved gases in shallow groundwaters in Devonian organic-rich shales along Lake Erie contain detectable CH₄ (0.01–50.55 mol%) with low δ¹³C–CH₄ values (−74.68 to −57.86‰) and no higher chain hydrocarbons, characteristics typical of microbial gas. Nevertheless, these groundwaters have only moderate alkalinity (1.14–8.72 meq/kg) and relatively low δ¹³C values of dissolved inorganic C (DIC) (−24.8 to −0.6‰), suggesting that microbial methanogenesis is limited. The majority of natural gases in Devonian organic-rich shales and sandstones at depth (>168 m) in the northern Appalachian Basin have a low CH₄ to ethane and propane ratios (3–35 mol%; C₁/C₂ + C₃) and high δ¹³C and δD values of CH₄ (−53.35 to −40.24‰, and −315.0 to −174.6‰, respectively), which increase in depth, reservoir age and thermal maturity; the molecular and isotopic signature of these gases show that CH₄ was generated via thermogenic processes. Despite this, the geochemistry of co-produced brines shows evidence for microbial activity. High δ¹³C values of DIC (>+10‰), slightly elevated alkalinity (up to 12.01 meq/kg) and low SO₄ values (<1 mmole/L) in select Devonian organic-rich shale and sandstone formation water samples suggest the presence of methanogenesis, while low δ¹³C–DIC values (<−22‰) and relatively high SO₄ concentrations (up to 12.31 mmole/L) in many brine samples point to SO₄ reduction, which likely limits microbial CH₄ generation in the Appalachian Basin. Together the formation water and gas results suggest that the vast majority of CH₄ in the Devonian organic-rich shales and sandstones across the northern Appalachian Basin margin is thermogenic in origin. Small accumulations of microbial CH₄ are present at shallow depths along Lake Erie and in western NY.     

A hot spring in granite of the Western Tianshan,China

The western Tianshan range is a major Cenozoic orogenic belt in central Asia exposing predominantly Paleozoic rocks including granite. Ongoing deformation is reflected by very rugged topography with peaks over 7000 m high. Active tectonic deformation is tied to an E–W trending fracture and fault system that sections the mountain chain into geologically diverse blocks that extend parallel to the orogen. In the Muzhaerte valley upwelling hot water follows such a fault system in the Muza granite. About 20 L min⁻¹ Na–SO₄–Cl water with a temperature of 55 °C having a total mineralization of about 1 g L⁻¹ discharge from the hot spring. The water is used in a local spa that is frequented by the people of the upper Ili river area. Its waters are used for balneological purposes and the spa serves as a therapeutic institution. The major element composition of the hot water is dominated by Na and by SO₄ and Cl, Ca is a minor component. Dissolved silica (1.04 mmol L⁻¹) corresponds to a quartz-saturation temperature of 116 °C and a corresponding depth of the source of the water of about 4600 m. This temperature is consistent with Na/K and Na/Li geothermometry. The water is saturated with respect to fluorite and contains 7.5 mg L⁻¹ F⁻ as a consequence of the low Ca-concentration. The water is undersaturated with respect to the primary minerals of the reservoir granite at reservoir temperature causing continued irreversible dissolution of granite. The waters are oversaturated with respect to Ca–zeolite minerals (such as stilbite and mesolite), and it is expected that zeolites precipitate in the fracture pore space and in alteration zones replacing primary granite.     

The use of O,H, B,Sr and S isotopes for tracing the origin of dissolved boron in groundwater in Central Macedonia,Greece

The groundwater B concentration in Mesozoic karst, Neogene and alluvial aquifers in the West part of Chalkidiki province in Central Macedonia, Greece reaches 6.45 mg L⁻¹, which exceeds the limit of 1 mg L⁻¹, set by the European Union for drinking water. The high B contents have been detected in this area, not only near the shoreline, where seawater intrusion occurs, but also in the inland part of the basin. Multi isotope (²H, ¹⁸O, ³⁴S, ¹⁸O_(SO4), ¹¹B, ⁸⁷Sr/⁸⁶Sr) data from borehole and thermal water springs allow identification of the possible B sources. The B dissolved in groundwater in the Chalkidiki area is mainly geogenic. The low δ¹¹B values, 0–1‰, similar to those of thermal fluids from continental geothermal fields, and the low Cl/B ratio compared to seawater both indicate a geothermal origin for B and reflect deep circulation and interaction with igneous rocks. The ⁸⁷Sr/⁸⁶Sr ratio also indicates that the deep-aquifer granodiorite is the predominant rock source of Sr, while the shallow limestone unit has negligible effects on the dissolved Sr budget in these thermal karst waters which O and H isotopes show to be of meteoric origin. The main source of high B in borehole water is mainly mixing with B-rich geothermal water. The mixing between geothermal water and water from the Neogene aquifer is also reflected by isotopic contents of SO₄.     

Rhizosphere and flooding regime as key factors for the mobilisation of arsenic and potentially harmful metals in basic,mining-polluted salt marsh soils

The occurrence of mining areas in the vicinities of salt marshes may affect their ecological functions and facilitate the transfer of pollutants into the food chain. The mobilisation of metals in salt marsh soils is controlled by abiotic (pH, redox potential) and biotic (influence of rhizosphere) factors. The effect of the rhizosphere of two plant species (Sarcocornia fruticosa and Phragmites australis) and different flooding regimes on potentially harmful metals and As mobilisation from salt marsh soil polluted by mining activities were investigated (total concentrations: 536 mg kg⁻¹ As, 37 mg kg⁻¹ Cd, 6746 mg kg⁻¹ Pb, 15,320 mg kg⁻¹ Zn). The results show that the changes in redox conditions (from 300 mV to −100 mV) and pH after flooding and rewetting periods may mobilise the contaminant elements into soil solution (e.g., 100 μg L⁻¹ Cd, 30 μg L⁻¹ Pb, 7 mg L⁻¹ Zn), where they are available for plants or may be leached from the soil. Drying periods generated peaks of concentrations in the soil solution (up to 120 μg L⁻¹ Cd and 50 μg L⁻¹ Pb). The risk assessment of As and metal-polluted salt marshes should take into account flood dynamics in order to prevent metal(loid) mobilisation.     

Oxygen isotope fractionation of dissolved oxygen during reduction by ferrous iron

The oxygen isotope fractionation factor of dissolved oxygen gas has been measured during inorganic reduction by aqueous FeSO₄ at 10−54 °C under neutral (pH 7) and acidic (pH 2) conditions, with Fe(II) concentrations ranging up to 0.67 mol L⁻¹, in order to better understand the geochemical behavior of oxygen in ferrous iron-rich groundwater and acidic mine pit lakes. The rate of oxygen reduction increased with increasing temperature and increasing Fe(II) concentration, with the pseudo-first-order rate constant k ranging from 2.3 to 82.9 × 10⁻⁶ s⁻¹ under neutral conditions and 2.1 to 37.4 × 10⁻⁷ s⁻¹ under acidic conditions. The activation energy of oxygen reduction was 30.9 ± 6.6 kJ mol⁻¹ and 49.7 ± 13.0 kJ mol⁻¹ under neutral and acidic conditions, respectively. Oxygen isotope enrichment factors (ε) become smaller with increasing temperature, increasing ferrous iron concentration, and increasing reaction rate under acidic conditions, with ε values ranging from −4.5‰ to −11.6‰. Under neutral conditions, ε does not show any systematic trends vs. temperature or ferrous iron concentration, with ε values ranging from −7.3 to −10.3‰. Characterization of the oxygen isotope fractionation factor associated with O₂ reduction by Fe(II) will have application to elucidating the process or processes responsible for oxygen consumption in environments such as groundwater and acidic mine pit lakes, where a number of possible processes (e.g. biological respiration, reduction by reduced species) may have taken place.     

Geochemical comparison of waters and stream sediments close to abandoned Sb-Au and As-Au mining areas,northern Portugal

Waters from abandoned Sb-Au mining areas have higher Sb (up to 2138 μg L⁻¹), As (up to 1252 μg L⁻¹) and lower Al, Zn, Li, Ni and Co concentrations than those of waters from the As-Au mining area of Banjas, which only contain up to 64 μg L⁻¹ As. In general, Sb occurs mainly as SbO₃⁻ and As H₂AsO₄⁻. In general, waters from old Sb-Au mining areas are contaminated in Sb, As, Al, Fe, Cd, Mn, Ni and NO₂⁻, whereas those from the abandoned As-Au mining area are contaminated in Al, Fe, Mn, Ni, Cd and rarely in NO₂⁻. Waters from the latter area, immediately downstream of mine dumps are also contaminated in As. In stream sediments from Sb-Au and As-Au mining areas, Sb (up to 5488 mg kg⁻¹) and As (up to 235 mg kg⁻¹) show a similar behaviour and are mainly associated with the residual fraction. In most stream sediments, the As and Sb are not associated with the oxidizable fraction, while Fe is associated with organic matter, indicating that sulphides (mainly arsenopyrite and pyrite) and sulphosalts containing those metalloids and metal are weathered. Arsenic and Sb are mainly associated with clay minerals (chlorite and mica; vermiculite in stream sediments from old Sb-Au mining areas) and probably also with insoluble Sb phases of stream sediments. In the most contaminated stream sediments, metalloids are also associated with Fe phases (hematite and goethite, and also lepidocrocite in stream sediments from Banjas). Moreover, the most contaminated stream sediments correspond to the most contaminated waters, reflecting the limited capacity of stream sediments to retain metals and metalloids.     

Landscape scale controls on the vascular plant component of dissolved organic carbon across a freshwater delta

Lignin phenol concentrations and compositions were determined on dissolved organic carbon (DOC) extracts (XAD resins) within the Sacramento-San Joaquin River Delta (the Delta), the tidal freshwater portion of the San Francisco Bay Estuary, located in central California, USA. Fourteen stations were sampled, including the following habitats and land-use types: wetland, riverine, channelized waterway, open water, and island drains. Stations were sampled approximately seasonally from December, 1999 through May, 2001. DOC concentrations ranged from 1.3 mg L⁻¹ within the Sacramento River to 39.9 mg L⁻¹ at the outfall from an island drain (median 3.0 mg L⁻¹), while lignin concentrations ranged from 3.0 μg L⁻¹ within the Sacramento River to 111 μg L⁻¹ at the outfall from an island drain (median 11.6 μg L⁻¹). Both DOC and lignin concentrations varied significantly among habitat/land-use types and among sampling stations. Carbon-normalized lignin yields ranged from 0.07 mg (100 mg OC)⁻¹ at an island drain to 0.84 mg (100 mg OC)⁻¹ for a wetland (median 0.36 mg (100 mg OC)⁻¹), and also varied significantly among habitat/land-use types. A simple mass balance model indicated that the Delta acted as a source of lignin during late autumn through spring (10-83% increase) and a sink for lignin during summer and autumn (13-39% decrease). Endmember mixing models using S:V and C:V signatures of landscape scale features indicated strong temporal variation in sources of DOC export from the Delta, with riverine source signatures responsible for 50% of DOC in summer and winter, wetland signatures responsible for 40% of DOC in summer, winter, and late autumn, and island drains responsible for 40% of exported DOC in late autumn. A significant negative correlation was observed between carbon-normalized lignin yields and DOC bioavailability in two of the 14 sampling stations. This study is, to our knowledge, the first to describe organic vascular plant DOC sources at the level of localized landscape features, and is also the first to indicate a significant negative correlation between lignin and DOC bioavailability within environmental samples. Based upon observed trends: (1) Delta features exhibit significant spatial variability in organic chemical composition, and (2) localized Delta features appear to exert strong controls on terrigenous DOC as it passes through the Delta and is exported into the Pacific Ocean.     

Controls of δS and δO in dissolved sulphate: Learning from a detailed survey in the Llobregat River (Spain)

The S and O isotopic composition of dissolved SO₄, used as a tracer for SO₄ sources, was applied to the water of the Llobregat River system (NE Spain). The survey was carried out at 30 sites where surface water was sampled on a monthly basis over a period of 2a. The concentration of dissolved SO₄ varied from 20 to 1575 mg L⁻¹. Sulphur isotopic compositions clustered in two populations: one – 93% of the samples – had positive values with a mode of +9‰; the other had negative values and a mode of −5‰. Data for δ¹⁸O_SO4 showed a mean value of +11‰, with no bi-modal distribution, though lower values of δ¹⁸O corresponded to samples with negative δ³⁴S. These values can not be explained solely by the contribution of bedrock SO₄ sources: that is, sulphide oxidation and the weathering of outcrops of sulphates, though numerous chemical sediments exist in the basin. Even in a river with a high concentration of natural sources of dissolved SO₄, such as the Llobregat River, the δ³⁴S values suggest that dissolved SO₄ is controlled by a complex mix of both natural and anthropogenic sources. The main anthropogenic sources in this basin are fertilizers, sewage, potash mine effluent and power plant emissions. Detailed river water sampling, together with the chemical and isotopic characterisation of the main anthropogenic inputs, allowed determination of the influence of redox processes, as well as identification of the contribution of natural and anthropogenic SO₄ sources and detection of spatial variations and seasonal changes among these sources. For instance, in the Llobregat River the input of fertilisers is well marked seasonally. Minimum values of δ³⁴S are reported during fertilization periods – from January to March – indicating a higher contribution of this source. The dual isotope approach, δ³⁴S and δ¹⁸O, is useful to better constrain the sources of SO₄. Moreover, in small-scale studies, where the inputs are well known and limited, the mixing models can be enhanced and the contribution of the different sources can be quantified to some extent.     

Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite

To better understand reaction pathways of pyrite oxidation and biogeochemical controls on δ¹⁸O and δ³⁴S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O₂ as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III)_aq as an oxidant with varying δ¹⁸O_H2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ∼2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ∼2.7. The δ³⁴S_SO4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (∼−0.7‰) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O₂) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O₂ varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water (ε¹⁸O_SO4-H2O) of ∼3.5‰ was determined for the anaerobic (biological and abiotic) experiments. This measured value was then used to estimate the oxygen isotope fractionation effects between sulfate and dissolved oxygen in the aerobic experiments which were −10.0‰, −10.8‰, and −9.8‰ for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between δ¹⁸O_SO4 values in the biological and abiotic experiments, it is suggested that δ¹⁸O_SO4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III)_aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water-oxygen under both aerobic and anaerobic conditions.