Dissolved Sulfate Concentrations Research Articles

A lime-augmented electro-coagulation (LATEC) process for treatment of sulfate-bearing wastewater with simultaneous recovery of commercially relevant end-product

Treatment of industrial effluents bearing high dissolved sulfate concentration presents a major environmental challenge. A resource recovery-based approach was adopted in the present study. A two-stage electro-coagulation process developed using aluminum anode and stainless-steel cathode to treat a real industrial wastewater containing about 18,000 mg/L dissolved sulfate. EC process operating parameters like electrode arrangement, electrode-spacing, current density, and lime dosing were optimized. The treatment resulted in nearly 99 % overall sulfate removal efficiency with a final concentration of 191 mg/L meeting the maximum acceptable limit of 200 mg/L prevalent in India. Advanced analysis techniques like Field-Emission Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis, Brunauer-Emmett-Teller surface area, X-Ray Diffraction Analysis, thermal analysis, and X-Ray Photoelectron Spectroscopy were applied to characterize the end product to ascertain the presence and purity of ettringite. Further, setting time analysis, compressive strength test, and heat of hydration analysis were performed to evaluate the performance of the EC residue in ordinary portlandite cement as a seed-based hardening accelerator. A dose of 10 % ECEP by weight of concrete reduced the setting time significantly. The heat of hydration test showed that ECEP was rich in ettringite which reduced the induction period for the setting of concrete. Addition of the ECEP to concrete showed about 15 % and 5 % higher strength than the normal concrete after 1 and 3 days, respectively. Therefore, the process provides an EC-based treatment scheme with end product valorization potential in the precast concrete industry as a seed-based hardening accelerator for the early setting of concrete.

The role of saline-related species in the electrochemical treatment of produced water using Ti/IrO2-Ta2O5 anode

Large volumes of produced water (PW) are daily generated at oil and gas exploration plants, and it has to be efficiently treated before final disposal, avoiding the sending of recalcitrant organic and inorganic constituents to the ecosystem. Anodic oxidation (AO) was employed in the treatment of a PW using a Ti/IrO2-Ta2O5 dimensionally stable anode (DSA). The role of the high concentration of chloride (13 g L-1) and sulfate (0.8 g L-1) in the effluent was investigated regarding the quantification of oxidizing species, chemical oxygen demand (COD) removal, different applied current densities (j), total current efficiency (TCE), and a partial costs analysis. The better efficiency, in terms of organic removal (79% of COD elimination), was achieved by applying 30 mA cm−2 during 240 min of electrolysis, at room temperature, without pH adjustment before or after the treatment. Under these conditions, lower concentrations of undesirable chlorinated species were produced, as well as lower energy requirements were also achieved regarding those achieved at 60 and 90 mA cm−2, and consequently, reduced costs were estimated. It seems that no significant influence on the oxidation of the organic matter was observed due to the higher dissolved sulfate concentration in the effluent. Conversely, the high concentration of chloride in the effluent plays an important role to favor the electrochemical production of oxidant species which act in the bulk of the solution to remove the organic matter as well as the chloride ions help in the maintenance of the conductivity.

Chemical evolution and evaporation of shallow groundwaters discharging from a gold mine, southern New Zealand

Waste rock piles at the Macraes gold mine in southern New Zealand expose abundant fresh schist to alteration reactions that principally involve pyrite, chlorite, and calcite, yielding alkaline waters (pH~8) with elevated dissolved loads. Pyrite oxidation has resulted in dissolved sulfate concentrations rising up to ~3000 mg/L over 10 years, and this high sulfate is a potential environmental issue in waters leaving the site. Up to 600 mg/L dissolved Mg2+ results from chlorite dissolution. The site has a semi-arid climate and evaporative precipitation of sulfate minerals could contribute to an engineered water treatment system. However, the evolved waters are saturated or supersaturated with respect to carbonate minerals and spontaneous precipitation of aragonite is widespread, thereby limiting Ca-sulfate (gypsum) precipitation. Localised precipitation of Mg-sulfate (epsomite) also occurs, but this redissolves during rain events. Compositionally different waters seeping from the mine tailings impoundment have >2500 mg/L dissolved sulfate, and elevated Na+, K+ and Cl− from addition of reagents in the processing system. These seepage waters have also undergone interaction with waste rocks in the impoundment walls. The seep waters are approximately saturated with respect to carbonate minerals, but no spontaneous carbonate precipitation has occurred over two years of observations. Ephemeral evaporative epsomite and Na–Mg sulfate (bloedite) precipitates form locally. Geochemical modelling suggests that evaporative precipitation of gypsum has potential to remove sulfate from both types of water, especially if carbonate precipitation was suppressed by lowering the water pH to ~6 in an engineered treatment system that involves substantial evaporation.

Stable isotopes (H, O, S) signatures evidencing evolutionary trends of Brazilian spas groundwaters

This paper reports a stable isotope (H, O, S) study of groundwater samples (25) in 10 spas from southeast Brazil. They are cold (25 °C, 36%), hypothermal (25–33 °C, 60%) and mesothermal (33–36 °C, 4%) waters; and are predominantly composed of bicarbonate, carbonate and sodium. Dissolved sulfate is the dominant anion in one water sample and its concentration increased with increase in pH, electrical conductivity (EC) and total dissolved solids (TDS) values in the samples. 2H and 18O data reported in the literature for circa 700 rainwater samples collected in São Paulo State and Brasília airport, Brazil, were used to construct the following regional meteoric waterline (RMWL): δ2H V-SMOW (‰) = 8.06 δ18Owater V-SMOW (‰) + 12.85. The HO isotopes of the studied groundwater samples show a δ18Owater variation of −14.1 to −5.3‰ V-SMOW, whereas the δ2H range was −66.5 to −31.7‰ V-SMOW, which was plotted on or near the RMWL, suggesting a meteoric origin for them. However, one hypothermal water sample (29 °C) plotted away from this RMWL, and is from a deep fractured aquifer which mainly occurs in fenitization aureole rocks surrounding the carbonatite complex at Araxá spa, Minas Gerais State, and a magmatic hydrothermal water may contribute to this water sample. The use of both δ34Ssulfate and δ18Osulfate values in the selected sites permitted the plotting of a mixture line of the dissolved sulfate in the different aquifer systems of this study, considering two major sources as endmembers: sulfide oxidation, SOX (δ34Ssulfate = +1‰ and δ18Osulfate = +1‰) and sulfates possessing δ34Ssulfate = +9‰ and δ18Osulfate = +21‰ (OSW). This is an unusual isotopic composition opposite from that of sea water. The δ34Ssulfate and δ18Osulfate values tend to increase with increase in dissolved sulfate concentration. The OS isotopes signatures that deviated from the general mixture line could be due to several processes, including atmospheric deposition, SO2 oxidation in the atmosphere, soil-derived sulfate, sulfide oxidation, evaporites dissolution and the presence of dissimilatory sulfate-reducing bacteria. Three sulfate isotope composition of spring waters from Águas da Prata spa in the Poços de Caldas alkaline massif (PCAM) represented endmember sources that defined a triangle around the remaining PCAM samples, as well as the samples from the crystalline basement, São Francisco craton, and Alto Paranaíba igneous province (APIP). Such triangle allows for estimation of the relative isotopic contribution in the different spas groundwaters, which is a new approach that focuses on the use of δ34Ssulfate-δ18Osulfate pair in environmental studies, and shows its usefulness in addition to other conventional hydrogeochemical fingerprints.

Shallow water anoxia in the Mesoproterozoic ocean: Evidence from the Bashkir Meganticlinorium, Southern Urals

The apparent lag between the first permanent rise of atmospheric oxygen to appreciable levels and oxygenation of the deep ocean has focused efforts in deciphering the evolution of seawater chemistry across the Proterozoic Eon (2.5–0.542 Ga). It is generally accepted that from ∼1.85 Ga oxic shallow marine waters were widespread while the deep ocean remained dominantly ferruginous (Fe(II)-rich), with episodic euxinia confined to productive continental margins and intracontinental basins. The geochemical record that informs this picture, however, is currently sparse, and further studies are required to adequately evaluate temporal and spatial variability in ocean redox conditions across this vast expanse of time. Here, we report Fe-S-C systematics, alongside major (Al, Mn) and trace metal (Mo, U) data for ∼1.6–0.8 Ga marine sediments from the Bashkir Meganticlinorium (BMA), Southern Urals, Russia. Our Fe speciation data reveal a water column dominated by ferruginous conditions, no evidence for euxinia, and oxygenated waters confined to only the shallowest, near-coastal settings. Trace metal data support these findings, with Mo and U enrichment factors in anoxic sediments implying the operation of a particulate Fe (oxyhydr)oxide shuttle under ferruginous conditions as the main mechanism for generating moderate authigenic Mo enrichments. Sulfur isotope systematics imply that predominantly low dissolved sulfate concentrations prevailed throughout the deposition of the BMA succession, which, in combination with a low organic carbon loading, promoted the development of ferruginous, rather than euxinic, water column conditions. The restriction of oxic conditions to the shallowest, near-coastal depositional settings within the Bashkir basin contrasts to other redox studies from this period. Such discrepancies highlight major uncertainty in our understanding of the temporal and spatial evolution of water column redox chemistry across the Mesoproterozoic Era, and signify the need for further detailed work to constrain the chemical evolution of the oceans during this crucial time period.

Volatile earliest Triassic sulfur cycle: A consequence of persistent low seawater sulfate concentrations and a high sulfur cycle turnover rate?

Marine biodiversity decreases and ecosystem destruction during the end-Permian mass extinction (EPME) have been linked to widespread marine euxinic conditions. Changes in the biogeochemical sulfur cycle, microbial sulfate reduction (MSR), and marine dissolved sulfate concentrations during the Permian-Triassic transition can provide insights into the role of ocean chemistry change in the largest mass extinction in Earth history. In this study, we constrain marine dissolved sulfate concentrations using the MSR-trend method of Algeo et al. [Algeo, T.J., Luo, G.M., Song, H.Y., Lyons, T.W., Canfield, D.E., 2015. Reconstruction of secular variation in seawater sulfate concentrations. Biogeosciences 12, 2131–2151] on sulfur (S) isotope records from Iran (the Kuh-e-Ali Bashi and Zal sections) and Hungary (the Bálvány North and Bálvány East sections). This empirically derived transfer function is based on the S isotope fractionation between sulfate and sulfide associated with MSR in natural aquatic environments. This fractionation is proxied by the difference in S isotope compositions between chromium-reducible sulfur (CRS) and carbonate-associated sulfate (CAS), i.e., Δ34SCAS-CRS. We show that, despite region-specific redox conditions, Δ34SCAS-CRS exhibits a nearly invariant value of 15–16‰ in both study sections. By comparing our record with a Δ34Ssulfate-sulfide density distribution for modern marine sediments, we deduce that porewater Rayleigh distillation, carbonate diagenesis, and other effects are unlikely to have appreciably altered the S isotope offset between CRS and CAS in the study sections. In addition, differences in sedimentary regimes and organic carbon (OC) fluxes between the Iranian and Hungarian sections exclude major influence of the electron donor on MSR-S isotope fractionation and point to a more universal control, i.e., contemporaneous seawater sulfate concentration.The MSR-trend transfer function yielded estimates of seawater sulfate of 0.6–2.8mM for the latest Permian to earliest Triassic, suggesting a balanced oceanic S-cycle with equal S inputs and outputs and no major changes in sulfate concentrations during this interval. However, a secular trend toward heavier δ34SCAS (by >5‰) in the earliest Triassic can be explained only by increasing the turnover rate of the S-cycle (by ca. one order of magnitude) and a concomitant change in terrestrial S sources in a box model experiment. Exposure of evaporite deposits having a high δ34S may account for the source change, with a possible role for the Siberian Traps volcanism by magmatic remobilization of Cambrian rock salt. A high sulfur cycle turnover rate would have left the ocean system vulnerable to development of widespread euxinic conditions, posing a sustained threat to marine life during the Early Triassic.

S reactivity of an oil sands composite tailings deposit undergoing reclamation wetland construction

This study is the first to characterize the S stability of a composite tailings (CT) deposit undergoing pilot wetland reclamation in the Athabasca Oil Sands Region (AOSR, Alberta, Canada). As CT is sulfur, organic carbon and bacterially rich, the goal of this study was to characterize the in situ aqueous distribution of sulfur compounds across the wetland, sand cap and underlying CT zones of the deposit, in an effort to establish the potential for microbial sulfur cycling and generation of H2S, an explosive, corrosive and toxicity risk. Porewater samples from three depths spanning the different layers of the deposit, as well as wetland surface ponded water samples were collected for geochemical analyses (July and Sept 2013), and for microbial enrichments (both S reducing and S oxidizing bacteria) in June 2014. While porewater ΣH2S(aq) was detected at all depths across the three zones of the deposit, results identify that the sand cap layer required for construction, acts as a mixing zone generating the highest solution H2S concentrations (>500 uM or 18 mg/L) and H2S gas levels (over 100 and up to 180 ppm) observed. Porewater dissolved sulfate concentrations (0.14–6.97 mM) were orders of magnitude higher and did not correlate to the observed distribution of ΣH2S concentrations throughout the deposit. Unique to the sandcap, dissolved organic carbon positively correlated with the observed maxima of ΣH2S(aq) seen in this layer. The water management of the deposit is a critical factor in the observed S trends. Active dewatering of the CT resulted in migration of S rich water up into the sandcap, while downwelling labile organic carbon from the developing wetland acted in concert to stimulate microbial generation of the H2S in this structural layer to the highest levels observed. Functional enrichments identified that diverse S reducing and oxidizing microbial metabolisms are widespread throughout the deposit, indicating that these waste materials are biogeochemically reactive with implications for longterm stability. These results are of relevance to both the oil sands region, as well as other mine contexts where S rich wastes occur, identifying the need to consider the potential bacterially driven cycling of S and C in the generation of constituents of concern, as well as the water management of such waste deposits to minimize risk.

Does Disposing of Construction and Demolition Debris in Unlined Landfills Impact Groundwater Quality? Evidence from 91 Landfill Sites in Florida.

More than 1,500 construction and demolition debris (CDD) landfills operate in the United States (U.S.), and U.S. federal regulations do not require containment features such as low-permeability liners and leachate collection systems for these facilities. Here we evaluate groundwater quality from samples collected in groundwater monitoring networks at 91 unlined, permitted CDD landfills in Florida, U.S. A total of 460,504 groundwater sample results were analyzed, with a median of 10 years of quarterly or semiannual monitoring data per site including more than 400 different chemical constituents. Downgradient concentrations of total dissolved solids, sulfate, chloride, iron, ammonia-nitrogen, and aluminum were greater than upgradient concentrations (p < 0.05). At downgradient wells where sulfate concentrations were greater than 150 mg/L (approximately 10% of the maximum dissolved sulfate concentration in water, which suggests the presence of leachate from the landfill), iron and arsenic were detected in 91% and 43% of samples, with median concentrations of 1,900 μg/L and 11 μg/L, respectively. These results show that although health-based standards can be exceeded at unlined CDD landfills, the magnitude of detected chemical concentrations is generally small and reflective of leached minerals from components (wood, concrete, and gypsum drywall) that comprise the bulk of discarded CDD by mass.

Analysis of the potential impact of capillarity on long-term geochemical processes in sulphidic waste-rock dumps

Assessing long-term production of acid rock drainage (ARD) from waste-rock dumps (WRDs) requires a careful analysis of the processes controlling acid-generating geochemical reactions under unsaturated conditions. In this work, we focus on the potential control of capillarity on these reactions, as previous studies showed that capillarity affects the activity of water and solutes in the unsaturated zone through the pore water pressure. We used capillarity-corrected thermodynamic databases and compared calculated speciation and solubility results with those from databases that do not account for capillarity. We developed a simple dynamic model with reduced geochemical components to analyze in detail the effect of capillarity. Results indicate that under low pH conditions simulations with capillarity-controlled reactions generate relatively larger dissolved sulfate concentrations from the WRDs over longer time scales, when compared against simulations without capillarity control. This occurs because capillarity strongly controls the formation of secondary sulfate-bearing minerals such as gypsum. When sufficient oxygen and carbon dioxide partial pressures are maintained within WRDs (such as in well-ventilated systems) and calcite content is insufficient to buffer acidity, the amount of secondary gypsum was calculated to be much larger in capillarity-corrected models. No appreciable effects of capillarity were observed under conditions where gypsum was not generated. Model results are also insensitive to temperature changes in typical climatic ranges. These results indicate some of the conditions by which capillarity should be accounted for as a key component to make accurate long-term predictions concerning with ARD from WRDs, and for the correct interpretation of potential processes such as calcite passivation.

Paleo-environmental controls on cold seep carbonate authigenesis in the Sea of Marmara

The factors controlling fluid emission dynamics at ocean margins are poorly understood. In particular, there are significant uncertainties on how fluid seepage at cold seeps may have responded to abrupt environmental changes in the geological past. This study reports on a detailed geochemical investigation of seafloor carbonate crusts sampled at cold seeps along the submerged part of the North Anatolian Fault system in the Sea of Marmara – an inland sea, which has experienced major paleo-environmental changes over the last deglaciation period. We also analyzed a series of authigenic carbonate concretions recovered from two sediment cores at the Western-High ridge, an active fluid venting area.The ages of seafloor carbonate crusts derived from isochron U–Th dating cover the last 7 kyr, suggesting that fluid activity along the fault system remained continuous over that time interval. In the sediment cores, carbonate concretions are concentrated at the lacustrine-to-marine transition, which corresponds to the period when Mediterranean waters flowed into the Marmara Basin about 12–14 kyr ago. U–Th isotopic data indicate that most of these concretions formed later during the Holocene, around 9–10 kyr ago, a period coinciding with an important anoxic event that led to the deposition of a sapropel layer in the Sea of Marmara.Based upon these results, we suggest that the absence of carbonate concretions in the lacustrine sediment unit indicates that dissolved sulfate concentrations in the Marmara lake pore waters during glacial time were too low to promote significant anaerobic methane oxidation, thereby preventing sedimentary carbonate authigenesis. In contrast, the progressive inflow of Mediterranean waters into the glacial Marmara lake after 15 ka provided a source of dissolved sulfate that allowed anaerobic oxidation of methane to proceed within the anoxic sediment. Importantly, the synchronism between the main phase of authigenic carbonate precipitation at the studied sites (average 9.4±1.8 ka, n=16) and the regional anoxic sapropel event support the idea that the drop in bottom water dissolved oxygen content was probably a key factor to enhance microbial activity and associated carbonate precipitation at that time. Overall, these results provide straightforward evidence that fluid emission dynamics and hydrocarbon oxidation at cold seeps can be directly related to changing environmental conditions through time.

Identification of sulfate sources in groundwater using isotope analysis and modeling of flood irrigation with waters of different quality in the Jinghuiqu district of China

The main objective of this study was to identify the main sources and processes that control SO4 2− groundwater concentrations in the Jinghuiqu irrigation district of China using isotope analysis. Lysimeter irrigation experiments and numerical modeling were used to assess the impact of long-term irrigation practices on sulfate transport, when different sources of irrigation water were used. SO4 2− concentrations in the groundwater of the entire irrigation area increased significantly from the years 1990 (a mean value was 4.8 mmol L−1) to 2009 (a mean value was 9.84 mmol L−1). The δ34S-SO4 2− values (ranging from +5.27 to +10.69 ‰) indicated that sulfates in groundwater were initially predominantly derived from dissolution of minerals. However, no soluble sulfate minerals (gypsum and/or mirabilite) were detected after 1990. To better understand this seeming anomaly, water content and SO4 2− data were collected before and after the field irrigation experiment and analyzed using the HYDRUS-1D and HP1 software packages. The experimental data were also used to assess sulfate leaching when different sources of irrigation water were used under current irrigation practices. The dissolved sulfate concentrations in the soil profile increased significantly when groundwater was used for infiltration compared to the use of surface water. Irrigation water sources had a great impact on the increase of sulfate concentrations in the shallow groundwater, especially when groundwater with elevated concentrations was used for irrigation.

Dolomitization on an evaporitic Paleoproterozoic ramp: Widespread synsedimentary dolomite in the Denault Formation, Labrador Trough, Canada

The Denault Formation (2.1–1.9 Ga) crops out in the Labrador Trough, northeastern Québec and western Labrador. Rocks surrounding the town of Schefferville, Quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm-influenced shallow ramp. Mid-ramp facies consist of intraclastic grainstones with hummocky cross-stratification (HCS), swaley cross-stratification (SCS), current ripples, and graded event beds. Further outboard, grainstones grade into deeper-water laminites that are composed of even, mm-scale couplets of flat-lying organic and dolomudstone laminae. Scours within the laminites suggest periodic storm activity. Laminites gradually grade into outer ramp deep-water shales. An isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km (present day). This succession can be interpreted as the remnant of a near-continuous margin or may simply represent an isolated accumulation that developed on a pre-existing topographic high. The presence of gypsum pseudomorphs in all lithofacies indicates that the Denault margin was restricted and evaporitic. Four paragenetic stages are recognized in the diagenetic evolution of the Denault Formation: (1) carbonate deposition, contemporaneous marine cementation, authigenic gypsum growth, and precipitation of authigenic chert; (2) synsedimentary mimetic dolomite precipitation; (3) pore-rimming and pore-occluding shallow burial dolomite cement; and (4) fabric destructive, sutured, anhedral burial dolomite. Gypsum crystals occur in all lithofacies, form the nuclei of interstitial dolomite rhombs, average 10 μm in length, and often display swallowtail twinning. Paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum. Formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water Mg/Ca ratios and lowering its dissolved sulfate concentrations. Such an interpretation may explain why there is an abundance of synsedimentary dolostone in the Precambrian and the relative paucity of Phanerozoic analogs.

Climatic and compositional controls on secondary arsenic mineral formation in high-arsenic mine wastes, South Island, New Zealand

Recent and historic gold mines and mine wastes in the South Island of New Zealand have been exposed under a wide range of climatic conditions for up to 100 yr. The studied sites are in rain shadow areas east of the Southern Alps, and in wet areas west of the mountains. Arsenic concentrations in mine wastes are up to 20 wt% locally. The arsenic is primarily derived from arsenopyrite in gold-bearing ore, although roasting and oxidation of ore occurred at some mines. Arsenolite was a byproduct of roasting at two sites, and this was also discharged into mine wastes. Five secondary As minerals, scorodite, kankite, pharmacosiderite, zykaite, and bukovskyite, have formed in different mines and mine wastes during surficial oxidation and remobilisation. The minerals formed under different chemical and climatic conditions. Ferrihydrite with combined arsenate (up to 20 wt% As) occurs at all localities. Scorodite is the most common secondary As mineral, replacing arsenopyrite and/or arsenolite, and forming cement in mine wastes. Dissolution and precipitation of scorodite, which is strongly pH dependent, controls maximum dissolved As concentrations on a regional scale. Kankite has formed instead of scorodite in a persistently moist microclimate east of the mountains. Pharmacosiderite formed instead of scorodite on the 50 yr time-scale in mine tunnels at pH 7–8. Pharmacosiderite is distinctly less soluble than scorodite in this groundwater setting at pH 7–8, and apparently restricts dissolved As to <25 mg/L locally. Bukovskyite and zykaite form where dissolved sulfate concentrations are elevated and the host mine wastes are perennially damp.

Isotopic tracing of clear water sources in an urban sewer: A combined water and dissolved sulfate stable isotope approach

This paper investigates the potential of stable isotopes of both water (δD and δOH2O18) and dissolved sulfate (δ34S and δOSO418) for determining the origin and the amount of clear waters entering an urban sewer. The dynamics of various hydrological processes that commonly occur within the sewer system such as groundwater infiltration, rainwater percolation, or stormwater release from retention basins, can be readily described using water isotope ratios. In particular, stable water isotopes indicate that the relative volumes of infiltrated groundwater and sewage remain approximately constant and independent of wastewater flow rate during the day, thus demonstrating that the usual quantification of parasitic discharge from minimal nocturnal flow measurements can lead to completely erroneous results. The isotopic signature of dissolved sulfate can also provide valuable information about the nature of water inputs to the sewage flow, but could not be used in our case to quantify the infiltrating water. Indeed, even though the microbial activity had a limited effect on the isotopic composition of dissolved sulfate at the sampling sites investigated, the dissolved sulfate concentration in sewage was regulated by the formation of barite and calcium-phosphate mineral species. Sulfate originating from urine was also detected as a source using the oxygen isotopic composition of sulfate, which suggests that δOSO418 might find use as a urine tracer.

Response of Sulfate Concentration and Isotope Composition in Icelandic Rivers to the Decline in Global Atmospheric SO2Emissions into the North Atlantic Region

This study presents the changes in dissolved sulfate concentration and isotope composition of Icelandic river waters between the peak of SO2 emissions in the United States and Europe and the present. Chloride concentration in Icelandic rivers has not changed much since 1972. The overall average change from 1972-1973 to 1996-2004 was -3%, indicating insignificant sea-salt contribution changes. More than 99% of the river-dissolved sulfur was in the form of sulfate. There are three main sources for dissolved sulfate in the rivers: rocks, sea-salts, and anthropogenic. Total dissolved sulfate, tdSO4(2-), and non-sea-salt sulfate, nssSO4(2-), decreased in all of the rivers from the early 1970s to 1996-2004. The percentage decrease varies from 13% to 65%. The decrease is smallest in rivers were there is considerable rock-derived dissolved SO4(2-). The overall average decrease was 39% for tdSO4(2-) and 46% for nssSO4(2-). The anthropogenic sulfate fraction has declined making most of the river waters delta34S values of sulfate higherthrough time. The overall decline in river sulfate and increase in delta34S, while SO2 emissions from Iceland has been increasing, demonstrates the response of river chemistry in the remote North Atlantic to the decline in man-made emissions of SO2 in North America and Europe.

Intense pyrite formation under low-sulfate conditions in the Achterwasser lagoon, SW Baltic Sea

In comparison to similar low-sulfate coastal environments with anoxic-sulfidic sediments, the Achterwasser lagoon, which is part of the Oder estuary in the SW Baltic Sea, reveals unexpectedly high pyrite concentrations of up to 7.5 wt%. Pyrite occurs mainly as framboidal grains variable in size with diameters between 1 and 20 μm. Pyritization is not uniform down to the investigated sediment depth of 50 cm. The consumption of reactive-Fe is most efficient in the upper 20 cm of the sediment column, leading to degrees of pyritization (DOP) as high as 80 to 95%. Sediment accumulation in the Achterwasser takes place in high productivity waters. The content of organic carbon reaches values of up to 10 wt%, indicating that pyrite formation is not limited by the availability of organic matter. Although dissolved sulfate concentration is relatively low (<2 mmol/L) in the Achterwasser, the presence of H 2S in the pore water suggests that sulfate is unlikely to limit pyrite authigenesis. The lack of free Fe(II) in the pore waters combined with the possibility of a very efficient transformation of Fe-monosulfides to pyrite near the sediment/water interface suggests that pyrite formation is rather controlled by (i) the availability of reactive-Fe, which limits the FeS formation, and by (ii) the availability of an oxidant, which limits the transformation of FeS into pyrite. The ultimate source for reactive-Fe is the river Oder, which provides a high portion of reactive-Fe (∼65% of the total-Fe) in the form of suspended particulate matter. The surficial sediments of the Achterwasser are reduced, but are subject to oxidation from the overlying water by resuspension. Oxidation of the sediments produces sulfur species with oxidation states intermediate between sulfide and sulfate (e.g., thiosulfate and polysulfides), which transform FeS to FeS 2 at a significant rate. This process of FeS-recycling is suggested to be responsible for the formation of pyrite in high concentrations near the sediment surface, with DOP values between 80 and 95% even under low sulfate conditions. A postdepositional sulfidization takes place in the deeper part of the sediment column, at ∼22 cm depth, where the downward diffusion of H 2S is balanced by the upward migration of Fe(II). The vertical fluctuation of the diffusion front intensifies the pyritization of sediments. We suggest that the processes described may occur preferentially in shallow water lagoons with average net-sedimentation rates close to zero. Such environments are prone to surficial sediment resuspension, initiating oxidation of Fe-sulfides near the sediment/water interface. Subsequent FeS 2 formation as well as postdepositional sulfidization leads to a major pyrite spike at depth within the sediment profile.

Total mercury and methylmercury accumulation in periphyton of Boreal Shield Lakes: Influence of watershed physiographic characteristics

Little is known about Hg accumulation in littoral communities, especially in periphyton biofilm of unperturbed lakes. The objectives of this study were to investigate and establish relationships between total mercury (THg) and methylmercury (MeHg) concentrations in periphyton communities of Boreal lakes and watershed physiographic and lake morphometric characteristics. This study was carried out on 23 Boreal Canadian Shield lakes located between 47–50°N and 73–77°W. Periphyton was sampled on rocks, the dominant periphyton substrate in the littoral zone of these lakes. Periphyton algal biomass (Chla) ranged from 12 to 164 mg m −2 whereas THg concentrations varied from 42 to 271 ng g −1 DW and MeHg levels varied from 3 to 55 ng Hg g −1 DW. Periphyton biomass was positively correlated to latitude, watershed wetland area and negatively correlated to watershed slope and depth of the lake. THg concentrations in periphyton were negatively correlated to watershed wetland area whereas MeHg concentrations were negatively correlated to latitude and positively correlated to watershed slope, dissolved sulfate concentration and the presence of beavers in the lake. This study confirms that periphyton can accumulate large amounts of Hg and the accumulation is strongly influenced by watershed characteristics and periphyton biomass.

Characterization of Redox Processes in Shallow Groundwater of Owens Dry Lake, California

Redox status of shallow groundwaters (1-3 m depths) at Owens Dry Lake was studied to help guide mitigation efforts for attenuating dust generation from the dry lakebed. Redox conditions were characterized by Eh, oxidative capacity (OXC), and terminal electron accepting processes (TEAPs) as well as examining the energetics of TEAPs. Groundwater chemistry related to redox status was determined by major solute concentrations, dissolved gases (O2, H2, CH4), aqueous redox species (NO3, Mn2+, Fe2+/ Fe3+, SO4(2-)/HS-, DOC), and major redox sensitive components in the solid phase (extractable Fe/Mn). All of these measures of redox status indicate that sulfate reduction is the major process regulating redox conditions in most shallow groundwaters of Owens Dry Lake. Dissolved sulfate concentrations were regulated primarily by evaporation resulting in increasing concentrations as water migrates from the shoreline (<1 mM) to the center (up to 417 mM) of the dry lakebed. Eh values were generally in the range of -240 to -170 mV. The oxidative capacity demonstrates the dominant contribution of sulfate to OXC. The dominance of sulfate restricts further redox development, such as methanogenesis. Dissolved H2 concentrations ranged from 0.5 to 7.8 nM. According to the empirically defined H2 ranges, sulfate reduction was the most predominant TEAP. Moreover, thermodynamic calculations of TEAPs for H2 utilization support favorable energetics for both sulfate reduction and methanogenesis. The calculated available energy yield for sulfate reduction in the shallow groundwater of Owens Dry Lake was higher than other systems due to the high sulfate concentration.

Bacterial production of anomalously high dissolved sulfate concentrations in Peru slope sediments: steady-state sulfur oxidation, or transient response to end of El Niño?

Concentrations of dissolved sulfate and sulfur isotopic ratios of dissolved sulfide in surface sediments of the Peru shelf and upper slope indicate that the sediments can be divided into two depth intervals based on the dominant biogeochemical reactions. Although rates of bacterial sulfate reduction are high throughout Peru surface sediments, chemistry of the upper interval (<10–20 cm) is dominated by chemoautotrophic oxidation of dissolved sulfide and elemental sulfur, while the lower interval (>10–20 cm) is dominated by dissimilatory sulfate reduction. In three of the four cores examined here, pore water concentrations of sulfate in the top 10 cm of the sediment are significantly higher than those of the overlying seawater. Peak sulfate concentrations in pore water (37–53 mmol/l) are ∼1.3–1.9 times that of seawater sulfate and are located 1–6 cm below the sediment/water interface (SWI). The excess sulfate is most likely produced by oxidation of elemental sulfur coupled to reduction of nitrate, a reaction mediated by a facultative chemoautotrophic sulfide-oxidizing bacterium, Thioploca spp. Numerical simulations demonstrate that the anomalously high concentrations of dissolved sulfate can be produced by steady-state or non-steady-state processes involving high rates of bacterial oxidation of elemental sulfur. If bacterial sulfur oxidation is a transient phenomenon, then it is probably triggered by seasonal or El Niño-induced changes in water-column chemistry of the Peru undercurrent.

Steady-state modelling of biogeochemical processes in columns with aquifer material:: 2. Dynamics of iron–sulfur interactions

Serial data from column experiments were used to develop a dynamic model to describe the evolution of concentration profiles of redox-sensitive chemical components such as iron(II), sulfate/sulfide, FeOOH(s), and FeS(s). The experimental results were gained from a study with a laboratory column filled with aquifer sand and operated in a water-saturated state. “Reduction of sulfate”, “reduction of FeOOH(s)”, “formation of FeS(s)” and “precipitation of FeS(s)” were identified as slow nonequilibrium reactions. The rate coefficients of these processes were determined by using an extended version of the program STEADYQL in combination with the parameter optimisation program SUFI. Based on this kinetic information it is possible to carry out prognostic modelling of the propagation of a sulfide plume in an anaerobic aquifer if the main characteristics of the solids' surfaces are known. The major sink for sulfide turned out to be FeOOH(s) that is gradually transformed into FeS(s). The Ti(III)EDTA-extractable fraction of FeOOH(s) appeared to be a good measure for the overall sulfide-immobilisation capacity. Under the prevailing experimental conditions with a flow velocity of approximately 1 m/day and a dissolved sulfate concentration in the inflow of 215 μM, the calculated sulfide front migrates in the order of 1 m/year. These model findings indicate that even in aquifers, where strongly reducible conditions are predominant, sulfide immobilisation by iron may lead to a sulfide-free groundwater for a long period.