Redox-sensitive Species Research Articles

Redox states of underground brine system along the southern coast of the Laizhou Bay

Underground brine samples were collected along the southern coast of the Laizhou Bay, Shangdong, China in two field investigations in 2003. The brines are confined in the Quaternary sediment and underwent a series of geochemical changes. The redox states of these brines were assessed qualitatively based on the measurements of Eh and redox-sensitive species such as DO, NO NO3−, Mn2+, Fe2+, SO42− in the brines. The redox condition of the underground brine is anoxic, and the redox reactions that controlled the redox potential of brines should be Fe(III) reduction and sulfate reduction.

Spatial variation of redox and trace metal geochemistry in a minerotrophic fen

Pore water and solid phase samples were collected from the upper 50 cm of a peat profile at four sites within a 10 m2 area in Kleinstuck Marsh, a minerotropic fen located in Kalamazoo, MI. Although the chosen sites are in close proximity to each other, they differ with respect to vegetation species and density. Pore water analyses for a suite of redox sensitive species (pH, alkalinity, dissolved Mn(II), Fe(II), Fe(III), sulfide, sulfate), together with Fe and Mn distributions inferred from operationally-defined sequential extractions, demonstrate that Fe(III) and Mn(IV) reduction occurs in the shallow peat at three of the four sites. At the fourth site, the only site containing the invasive purple loosestrife (Lythrum salicaria), accumulation of dissolved sulfide in the pore waters and increased levels of oxidizable phases in the shallow peat point to increased net sulfate reduction relative to the other three sites. Speciation calculations indicate that pore water concentrations of phosphate, especially below ∼10 cm depth, are largely controlled by the solubility of phases such as strengite or hydroxylapatite, and that at all but the loosestrife site, dissolved Ca and Mg are likely determined by carbonate solubility. Fe and Mn distribution among operationally defined solid phase fractions are consistent with reductive dissolution of FMO in the uppermost peat, leading to precipitation of Fe sulfides and Mn carbonates deeper in the peat profile. Zn, Co, Cr and Ni distributions are consistent with release from FMO to form sulfides or organic associations deeper in the peat. Pb and Cu may also be released by reductive dissolution of FMO, or more likely, shift from primary association with organic matter to increased association with sulfides under more sulfidic conditions. This study highlights the existence of extreme lateral variations in peat pore water and solid phase geochemical profiles, even over quite small areas.

Benthic response to shellfish farming in Thau lagoon: Pore water signature

Vertical distributions of dissolved species across the sediment–water interface (SWI), including major cations (sodium, potassium, magnesium, calcium), minor cations (lithium, strontium, barium), redox sensitive species (dissolved manganese, iron, sulfate, sulfide, ammonium) and other chemical parameters (pH, alkalinity, soluble reactive phosphorous, dissolved silica) were studied in a Mediterranean lagoon used for intensive shellfish farming. In order to quantify the impact of this activity on diagenetic processes and the influence of seasonal changes, two stations contrasted with respect to organic carbon fluxes were sampled in Thau lagoon from March 2001 to August 2002 during four field campaigns in winter, spring, summer and fall. Well-defined layers enriched with redox sensitive species were observed following the conventional sequence of early diagenetic reactions. However, differences were observed between both stations in depths and thickness layers. Concentration gradients extended down to more than 92 cm depth at the central position of the lagoon (station C4 – 8 m depth) and down to 40 cm depth inside shellfish farming zones (station C5 – 9 m depth). Station C4 showed an unusual diagenetic signature: sharp dissolved oxygen, iron, nitrate and manganese gradients existed at the SWI but gradients of dissolved sulfide and alkalinity as well as other parameters (dissolved silica, Ba, etc.) were recorded only from 25 to 30 cm depth downward. Seasonal changes were observed in pore water composition as deep as 30–50 cm in station C4 (only 15 cm in station C5). The center of the lagoon is not directly subjected to biodeposits deriving from shellfish activity. Isotopic and bioturbation data allowed to rule out a reworking of the sediment deeper than a few centimeters. In addition to organic content of the sediment, physical parameters were likely to induce the 10–20 cm gap between dissolved iron and sulfide profile as well as the higher vertical extent of diagenetic sequence observed at station C4. Conversely to station C5, station C4 underwent stronger currents and wave effect probably generating advective transport of water through the sediment, but no permeability data were available to confirm this hypothesis. During summer, climatic conditions generated vertical stratification of the water column and transient suboxic conditions at the bottom. Such conditions drove the upward shift of redox fronts, compacting the diagenetic sequence. These effects were reinforced at station C5 by shellfish and its farm structures (mainly attenuation of current and increased heat absorption).

Sulfate reduction and sulfide oxidation in anoxic confined aquifers in the northeastern Osaka Basin, Japan

Summary We used δ34S values and chemical compositions of groundwater to investigate the influence of sulfate reduction and sulfide oxidation processes in a confined aquifer system in the northeastern Osaka Basin (NEOB), central Japan. Shallow confined groundwater ( HCO 3 - throughout the basin. However, concentrations of redox-sensitive species such as dissolved oxygen, NO 3 - , Fe2+, and Mn2+ show that groundwater conditions become increasingly anoxic along the flowpaths. Moreover, SO4/(SO4 + 2HCO3) molar ratios decrease with increasing δ34S along the flowpaths, which demonstrates that sulfate is reduced in the NEOB aquifer system. Groundwater with both high total dissolved solids and high SO4 content was observed mainly in areas down-flow from faults. Because of the high SO4 content, the anion composition of this groundwater cannot be adequately explained by a simple sulfate reduction process. A Rayleigh distillation model with e = −20‰, which allows for sulfide oxidation within marine sediments in proximity to faults, explains the chemistry of all groundwater samples, including those with high SO4 content. The model demonstrates that 76% of initial sulfate is reduced at maximum, and that reduced SO 4 2 - content is up to 48.5 mg/L after sulfide oxidation in the NEOB aquifer system.

Chemical characterization of the Neogene Aquifer, Belgium

The evolution of groundwater chemistry along the direction of groundwater flow was studied using hydrochemical data from samples collected along a flow line in the Neogene Aquifer, Belgium. Infiltrating water was found to have a very low mineral content and low pH because the sediments are strongly decalcified. Increasing SiO2 and cation concentrations along the groundwater flow line indicate silicate-weathering processes, confirmed with the aid of saturation indices, calculated with PHREEQC, and stability diagrams. A classification system based on redox sensitive species was developed and shows that an extensive redox sequence is present in the aquifer. At a shallow depth, pyrite oxidation has caused an increase in sulphate, while iron is precipitated as hydroxides. Elevated arsenic concentrations are related to the reduction of these iron hydroxides at a relatively shallow depth and to the dissolution of siderite at greater depth. Dissolution of carbonate in the aquifer material, present in deep layers and to the north, has lead to increased Ca2+ and HCO3 − concentrations. The Ca2+ from the groundwater is exchanged for Na+, Mg2+ and K+ adsorbed to the clay surfaces at the bottom of the groundwater reservoir. Although the Neogene Aquifer is well flushed, there are still some marine influences present in the deepest parts.

Determination of Interstitial Water Chemistry and Porosity in Consolidated Aquifer Materials by Diffusion Equilibrium-Exchange

Diffusion equilibrium exchange (DEE) is presented as a novel, practical alternative to centrifugation for the recovery and chemical analysis of interstitial water in contaminated core samples from consolidated rocks and aquifers. The methodology is suitable for sampling organic and inorganic compounds, including redox sensitive species such as SO4(2-), NO3-, NO2-, Mn(II), Fe(II), and sulfide (HS-). DEE also permits analyte extraction from kilogram quantities of core, which avoids extended centrifugation or sample amalgamation and provides analyte masses appropriate for stable isotope analysis. The procedure involves simple and rapid on-site sectioning of representative core samples, which are preserved in the field by storage in airtight bottles filled with deoxygenated deionized water containing a conservative tracer (Br-). Equilibration times for individual solutes can be estimated in advance to reduce the need for time-series analysis; for an effective diffusion coefficient of 2.5 x 10(-10) m2 s(-1) (Br- in chalk rock) equilibration was >90% completed after 30 h, consistent with the predicted equilibration time. The DEE method presented minimizes sampling errors from temperature changes, oxidation of reduced chemical species, and loss of volatile compounds, which can occur with other interstitial water sampling techniques. It also gives superior resolution of in situ solute distributions and geochemical processes in consolidated sediments than centrifugation and can provide estimates of aquifer porosity in core samples. Laboratory experiments using chalk rock core and simulated extraction procedures confirm the superior performance of the DEE method over centrifugation for a range of solutes. The method has been used to generate detailed interstitial water profiles of electron acceptor and contaminant concentrations along the flow path of a petroleum hydrocarbon plume in the U.K. Upper Chalk aquifer as part of a natural attenuation assessment.

Prediction of ground water quality affected by acid mine drainage to accompany in situ remediation

Most of the former East-German open cast lignite mines were shut down. To mine the lignite, the ground water table had been lowered up to 100 m. Today, the ground water flow regimen is changing due to the refilling of the depression cones and subsurface acid mine drainage is endangering surface waters and drinking water supply facilities outside the mining areas. Because of the large discharge area, only in situ technology can be used to reduce acid mine drainage. It has been developed on a laboratory scale and is currently being tested in an aquifer which already is contaminated by acid mine drainage. Sulphate reduction and the formation of sulphide minerals are being enhanced by adding bioreactive liquids in the aquifer. To apply the remediation technology to other areas which will soon be endangered, their future ground water quality is being assessed by predictive modelling. A 1-D transport and reaction model based on a hydraulic 3-D model of the study area is being developed. The dump material is described as a two component system consisting of low permeable silt aggregates with a large surface area, with high sulphide and organic contents and a matrix of highly permeable sands with a small surface area and low sulphide and organic contents. The transport and reaction model reflects the process of dump ground water genesis and includes the processes: (i) diffusive interaction between mobile and immobile pore water; (ii) dissolution of minerals according to the dissolution product and the redox state of the solution; (iii) control of the concentration of redox-sensitive species by addition of biodegradable organic material as electron donor (this part of the model is used to represent the remediation technology); (iv) weathering of feldspars; and (v) cation exchange reactions. Most parameters used to describe the processes are based on laboratory experiments with sediments and waters from the site.

Groundwater As mobilization in the Bengal Delta Plain, the use of ferralite as a possible remedial measure—a case study

High As groundwater (50–1600 μg l −1) poses the greatest threat to human health in the Holocene alluvial aquifers of the Bengal Delta Plain (BDP) with increasing global concern in recent years. This study deals with groundwater quality and As mobilization vis-à-vis employing ferralite as a remedial option for removal of As from groundwater. The investigation suggests that Fe-rich As traps undergo degeneration to produce Fe oxyhydroxide (HFO) as coating/precipitation on the fine-grained sediment surface and release redox sensitive species (As, Fe and Mn) as well as PO 4 3− into the groundwater under local reducing conditions. Sediment analysis reveals the presence of As T (average 17.2 mg/kg), Fe T (average 0.93 g/kg) and organic matter (average 7.6 g/kg). Sediment As T and Fe T content cannot validate the presence of high groundwater As/Fe. Fe II catalysed Fe III reduction, induced by dissimilatory Fe reducing bacteria liberate the more toxic As III than As V. The release of redox sensitive species (As, Fe and Mn) are the functions of bio-available forms of Fe oxides, concentration and distribution of fresh organic matter and availability of electron donors within the sediment. Further attempt is made to establish the role of ferralite, enriched with natural HFO as an As scavenger. Batch studies demonstrate the competency of the material over the natural/commonly used chemical coagulants generally used for water treatment. The high pHpzc value, 8.5 of ferralite along with the adsorption studies over a wide range of pH elucidate the effectiveness of the material in adsorbing both As III and As V from the well-buffered groundwater. The presence of Fe II in the system enhances the As removal process. The Langmuir adsorption isotherm further confirms the merit of ferralite as an efficient As scavenger. The material has been shaped for a fixed bed filter medium to remove As from groundwater (both laboratory and field scale). Ferralite is also cost effective (US$ 8/metric ton of ferralite with a density 1.17 kg/dm 3). Transportation cost for ferralite (from ferralitic bed to the affected area) is US$ 16/ton/1000 km whereas US$ 0.6/100 l is required for treatment of contaminated water.

Natural attenuation of xenobiotic organic compounds in a landfill leachate plume (Vejen, Denmark)

Demonstration of natural attenuation of xenobiotic organic compounds (XOCs) in landfill leachate plumes is a difficult task and still an emerging discipline within groundwater remediation. One of the early studies was made at the Vejen Landfill in Denmark in the late 1980s, which suggested that natural attenuation of XOCs took place under strongly anaerobic conditions within the first 150 m of the leachate plume. This paper reports on a revisit to the same plume 10 years later. Within the strongly anaerobic part of the plume, 49 groundwater samples were characterized with respect to redox-sensitive species and XOCs. The analytical procedures have been developed further and more compounds and lower detection limits were observed this time. In addition, the samples were screened for degradation intermediates and for toxicity. The plume showed fairly stationary features over the 10-year period except that the XOC level as well as the level of chloride and nonvolatile organic carbon (NVOC) in the plume had decreased somewhat. Most of the compounds studied were subject to degradation in addition to dilution. Exceptions were benzene, the herbicide Mecoprop (MCPP), and NVOC. In the early study, NVOC seemed to degrade in the first part of the plume, but this was no longer the case. Benzyl succinic acid (BSA) was for the first time identified in a leachate plume as a direct indicator, and as the only intermediate of toluene degradation. Toxicity measurements on solid phase-extracted (SPE) samples revealed that toxic compounds not analytically identified were still present in the plume, suggesting that toxicity measurements could be helpful in assessing natural attenuation in leachate plumes.

Mobilization of arsenic in sedimentary aquifer vis-à-vis subsurface iron reduction processes

Groundwater of the arsenic affected areas demonstrates that the water is Ca-bicarbonate type and typical anoxic in nature. High arsenic groundwater is distributed in patches and hot spots are identified in palaeo-meander belt. Iron rich arsenic traps undergo reduction and release redox sensitive species (As and Fe) under local reducing conditions. Releases of redox species are often depends on land-use pattern and the abundance of bioavailable forms of iron oxides/minerals.

Modelling americium sorption onto colloids: effect of redox potential

An analytical expression that evaluates the effect of the pH and the redox potential (Eh) was developed for studying the sorption of actinides onto colloids. It includes surface complexation with one type of surface site and ion exchange processes, and its formulation yields to a distribution coefficient (Kd) as function of the pH (hydrolysis) and Eh (redox sensitive species). The formulation considers also the values of the stability and hydrolysis constants for all species in solution and sorbed at the surface, and makes use of semi-empirical correlations between hydrolysis and surface complexation constants, for each colloid surface. The presence of complexing ligands in solution (such as carbonates) was also taken into account. The model was applied to the sorption of americium onto Al2O3, FeOOH and SiO2 colloids, in the presence and in the absence of carbonates in solution. From model testing it was shown that Am(III) sorption does not undergo redox change under natural conditions, as was expected. When carbonates are present in solution the calculated values of the distribution coefficient were lower than those calculated in the absence of carbonates, and no redox effect was detected. The distribution coefficient (Kd) values obtained with the developed model are in agreement with values reported for the sorption of americium onto colloids. It is known that in the water stability region Am(III) and their hydroxides are the only stable species; however, for the model application, all the possible redox couples of americium were considered. Moreover, this model is applicable to study the sorption of other redox sensitive elements.

Distribution and reactivity of O2-reducing components in sediments from a layered aquifer.

The redox status of subsurface aqueous systems is controlled by the reactivity of solid redox-sensitive species and by the inflow of such species dissolved in groundwater. The reactivity toward molecular oxygen (O2) of solid reductants present in three particle size fractions of sediments from a pristine aquifer was characterized during 54 days. The stoichiometric relationships between carbon dioxide (CO2) production and O2 consumption was used in combination with sulfate production to discriminate between the contributions of sedimentary organic matter (0-87%), pyrite (6-100%), and siderite (0-43%) as the dominant reductants. The observed simultaneous oxidation of these reductants indicates that they are reactive on the same time scales. The measured reduction capacity 18-84 micromol O2/g) ranged from 8 to 42% of the total reduction capacity present as pyrite and organic carbon in the total sediment fraction (<2 mm). Fine fractions (<63 microm) were 10-250 times more reactive than their corresponding total fractions. Oxygen consumption rates decreased continuously during carbonate buffered conditions, due to a decreasing reactivity of reductants. Acidification accelerated pyrite oxidation but impeded SOM respiration. Our findings indicate that the geological history of aquifer sediments affects the amounts of organic matter, pyrite and siderite present, while environmental conditions, such as pH and microbial activity, are important in controlling the reactivity of these reductants. These controls should be considered when assessing the natural reduction activity of aquifer sediments in either natural or polluted systems.

Early diagenetic processes in the muddy sediments of the Bay of Biscay

Abstract In order to understand the early diagenesis processes occurring in continental margin environment, modern sediments collected in six different sites from the Bay of Biscay have been studied. These sites can be separated into two groups. In the shallowest stations, where sediments are highly bioturbated, organic carbon levels are higher than 2%. In the deepest stations, sediments are much less bioturbated, and organic carbon levels are lower. In all sites, the vertical distribution of redox sensitive species can be explained by the well-established depth sequence of redox reactions, based on the bacterially mediated oxidation of organic matter. We have considered some alternative reaction pathways to explain the profiles of Fe, Mn, and N species. These reactions deal with the ammonia oxidation by manganese oxide, the aerobic denitrification and the oxidation of dissolved iron (II) by nitrate or Mn-oxides. Vertical flux calculations with a simple diffusion model indicate that these reactions could account for the reduction of all the Mn-oxides and the oxidation of all the upward diffusing Fe(II). They may also be responsible for a significant part of the total dinitrogen production. The relative importance of these pathways on early diagenetic processes and benthic fluxes has not been determined and must be examined with additional experimental works. Our study suggests, however, that the coupling between the benthic cycles of iron, manganese and nitrogen could strongly influence the carbon cycling at the ocean floor.

Dating and Heavy Metal Contents of Sediment Cores of a High-Alpine, Remote Lake: Jörisee (Switzerland)

Sediment cores were collected at the MOLAR (MOuntain LAke Research) site Jörisee. The goal of the analysis was to date the sediment cores in order to correlate actual input conditions of nutrients and pollutants at this high-alpine, remote site, to historical situations. The sediment cores were dated by determining gamma rays of Pb-210 and Cs-137. The calculated sedimentation rate is 0.1 cm/year. Leaching techniques were used to investigate redox sensitive species. The results give evidence for post depositional recycling in case of Fe and Mn. In contrast, the heavy metals Pb, Zn, Cr and Ni are not affected by existing redox gradients. Sediment samples were leached with diluted nitric acid to determine anthropogenic input of Pb, Zn, Cr and Ni. Concentrations of these heavy metals reflect changes in the input conditions since the beginning of industrialization, which is the examined time period.

An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark: 2. Deduction of anaerobic (methanogenic, sulfate‐, and Fe (III)‐reducing) redox conditions

Redox conditions may be environmental factors which affect the fate of the xenobiotic organic compounds. Therefore the redox conditions were characterized in an anaerobic, leachate‐contaminated aquifer 15–60 m downgradient from the Grindsted Landfill, Denmark, where an field injection experiment was carried out. Furthermore, the stability of the redox conditions spatially and over time were investigated, and different approaches to deduce the redox conditions were evaluated. The redox conditions were evaluated in a set of 20 sediment and groundwater samples taken from locations adjacent to the sediment samples. Samples were investigated with respect to groundwater chemistry, including hydrogen and volatile fatty acids (VFAs) and sediment geochemistry, and bioassays were performed. The groundwater chemistry, including redox sensitive species for a large number of samples, varied over time during the experimental period of 924 days owing to variations in the leachate from the landfill. However, no indication of change in the redox environment resulting from the field injection experiment or natural variation was observed in the individual sampling points. The methane, Fe(II), hydrogen, and VFA groundwater chemistry parameters strongly indicated a Fe(III)‐reducing environment. This was further supported by the bioassays, although methane production and sulfate‐reduction were also observed in a few samples close to the landfill. On the basis of the calculated carbon conversion, Fe(III) was the dominant electron acceptor in the region of the aquifer, which was investigated. Because of the complexity of a landfill leachate plume, several redox processes may occur simultaneously, and an array of methods must be applied for redox characterization in such multicomponent systems.

Aqueous- and Solid-Phase Biogeochemistry of a Calcareous Aquifer System Downgradient from a Municipal Solid Waste Landfill (Winterthur, Switzerland)

This study addresses the biogeochemical changes that take place in a calcareous aquifer system under and downgradient from a municipal solid waste landfill. Aqueous-phase chemical analysis of the redox-sensitive species indicates the presence of aerobic respiration, denitrification/NO3- reduction, and Fe(III), Mn(III/IV), and SO4 reduction processes under the landfill. Because available and released organic matter is limited, reduction processes downgradient from the landfill do not go far beyond aerobic respiration, denitrification, and Mn(III/IV) reduction. As suming steady-state conditions, STEADYQL computer program has been used to model the biogeochemical processes by taking into account the kinetics of the redox reactions, calcite precipitation and dilution. Dilution has the most significant influence on the concentrations of the dissolved organic and inorganic carbon. Dissolved Mn(II) concentrations in the entire anaerobic zone are controlled by the solubility of rhodocrocite [MnCO3(S)]. At selecte...

Chemical composition, pH, and redox state of sulfur and iron in complete vertical porewater profiles from two Sphagnum peat bogs, Jura Mountains, Switzerland

Complete porewater profiles from two peat bogs in the Jura Mountains were analysed for major and trace inorganic anions and cations. At La Tourbière des Genevez (TGe) and Etang de la Gruère (EGr), peat formation began approximately 5,000 and 10,000 years bp, respectively. The maximum depths of peat accumulation are 140 cm (TGe) and 650 cm (EGr); previous geochemical studies showed that the ombrogenic sections of the bogs extend to depths of approximately 20 cm (TGe) and 250 cm (EGr). Water samples were obtained using in situ diffusion equilibrium samplers (peepers), which allow filtered (0.2 μm) porewaters to be obtained while preventing degassing and oxidation. These samplers were found to be well suited to bog porewaters and allowed volatile (dissolved CO 2, acetate) and redox-sensitive species (HS −, Fe 2+) to be quantified without further sample preparation or treatment. Aqueous species concentrations were determined immediately afterwards using ion chromatography with either conductivity (acetate, HCO 3 −, Cl −, Br −, NO 3 −, HPO 4 2−, SO 4 2−, Na +, NH 4 +, K +, Mg 2+, Ca 2+), amperometry (HS −), or absorbance detection (Fe(III) and Fe(II)). The comprehensive analyses of anions and cations allowed humic substances to be calculated by the difference in electrical charge balance (i.e., the anion deficit). Concentrations of total dissolved CO 2 (2–12 mM) showed that carbonate equilibria play a significant role in the acid-base chemistry throughout the profiles. In near surface, ombrogenic porewaters with pH around 4, however, protons (approx. 160 μeq/L) are contributed mainly by the dissociation of humic substances (2–7 mM DOC). In the deepest, minerogenic layers H 2CO 3 is the predominant acid at both sites. At these depths, carbonate alkalinity (up to 3 meq/L at EGr, up to 8 meq/L at TGe) arises from reaction of the pore fluids with mineral matter in the underlying sediments. In the transition zone between the ombrogenic and minerogenic extremes, organic and inorganic acids are equal in importance. Unidentified organic S species accounted for 90–99% of total dissolved sulfur (S T) in the porewaters at TGe, with SO 4 2− and HS − the dominant inorganic species; S species with intermediate oxidation states such as SO 3 2− and S 2O 3 2− were always less than the detection limit of approximately 0.4 μM. At TGe the sulfate concentrations exceeded those of sulfide, with 1.25 and 0.25 μM, respectively, being typical. At EGr, S T and SO 4 2− were comparable to the waters at TGe, but HS − at EGr was always less than the detection limit of 0.15 μM. At both sites dissimilatory sulfate reduction is limited by the low concentrations of sulfate supplied to the bog surfaces (i.e., atmospheric deposition only), and the uptake of sulfate and its conversion to organic S compounds by the living plants. Despite the anoxic condition of the waters, the ratio of Fe(III) T to Fe(II) T was always high: at EGr this ratio was generally 1:1, and even in the sulfidic waters at TGe the ratio was 1:3. PHREEQE was used to calculate the effect of complex-forming organic ligands on {Fe 3+} and {Fe 2+} in these porewaters. The relatively high ratios of Fe(III) T compared to Fe(II) T can be explained in terms of the much greater thermodynamic stability of the organic complexes of Fe 3+ compared to those of Fe 2+.

The response of trace element redox couples to suboxic conditions in the water column

Depth profiles for a series of redox-sensitive elements were determined in the productive eastern boundary of the tropical North Pacific. Suboxic conditions (regions with dissolved oxygen concentrations < 5 pmol kg −1 and evidence of nitrate reduction) was observed at depths between 140 and 600 m. The behavior of a suite of trace (≤1 μM) redox-sensitive species such as oxidized and reduced forms of iodine, manganese, chromium, selenium, and iron was examined, in order to better characterize redox conditions within such suboxic zones. Iodate was completely reduced to iodide, and leachable particulate manganese was completely reduced to dissolved manganese between the depths of 140 and 600 m. In contrast, no more than 30% of the chromate, selenate, and selenite were reduced, while there was no evidence of leachable particulate iron being reduced. The resulting distributions of trace redox elements in these suboxic waters are influenced both by the extent that they are reduced and by the particular biogeochemical characteristics of their oxidized and reduced species.

Sampling anoxic pore waters in peatlands using "peepers" for in situ-filtration.

Equilibrium diffusion chambers ("peepers") have been constructed to collect anoxic pore waters in bogs without degassing and/or oxidizing the samples. These samplers have been constructed of Plexiglass, either as a long board with a series of sampling chambers for close interval analyses near the surface of a bog, or as single chambers inserted at greater depths (1 to 6 m). Prior to installation, the chambers have been filled with deaerated, deionized water, and have been covered by a 0.2 microm membrane filter; this membrane allows the dissolved constituents in the waters to equilibrate with the deionized water in the chamber by diffusion. The samplers have been allowed to equilibrate in the bog for 4 to 6 weeks. Thereafter, they have been withdrawn into N(2)-filled glove bags. Individual chambers have been sampled in the field by inserting a syringe through the glove bag; these syringes have been used immediately upon the arrival in the lab to analyze volatile and redox-sensitive species by ion chromatography. The effectiveness of this sampling approach has been demonstrated by measuring the concentrations of the dominant volatile acids in these waters (H(2)CO(3)) and two redox sensitive species (Fe(2+) and HS(-)). The procedure described here should be applicable also to trace metal speciation studies, provided that appropriate checks are made for all possible sources of contamination.

In situ and laboratory studies on the fate of specific organic compounds in an anaerobic landfill leachate plume, 1. Experimental conditions and fate of phenolic compounds

The transformation of specific organic compounds was investigated by in situ and laboratory experiments in an anaerobic landfill leachate pollution plume at four different distances from the landfill. This paper presents the experimental conditions in the in situ microcosm and laboratory batch microcosm experiments performed and the results on the fate of 7 phenolic compounds. Part 2 of this series of papers, also published in this issue, presents the results on the fate of 8 aromatic compounds and 4 chlorinated aliphatic compounds. The redox conditions in the plume were characterized as methanogenic, Fe(III)-reducing and NO 3 −-reducing by the redox sensitive species present in groundwater and sediment and by bioassays. With a few exceptions the aquifer redox conditions were maintained throughout the experiments as monitored by redox sensitive species present in groundwater during the experiments, by redox sensitive species present in the sediment after the experiments and by bioassays performed after the experiments. Transformation of nitrophenol was very fast close to the landfill in strongly reducing conditions, while transformation was slower in the more oxidized part of the plume. Lag phases for the nitrophenols were short (maximum 10 days). Phenol was only transformed in the more distant part of the plume in experiments where NO 3 −, Fe(III) and Mn(IV) reduction was dominant. Lag phases for phenol were either absent or lasted up to 2 months. Dichlorophenols were only transformed in experiments representing strongly reducing, presumably methanogenic, redox conditions close to the landfill after lag phases of up to 3 months. Transformation of o-cresol was not observed in any of the experiments throughout the plume. Generally, there was good accordance between the results obtained by in situ and laboratory experiments, both concerning redox conditions and the fate of the phenolic compounds. However, for phenol and 2,4-dichlorophenol, transformation was observed in some in situ experiments but not in the corresponding laboratory experiments. In some experiments, this coul be explained by differences in the redox conditions developing during the experiments. Nitrophenols were apparently transformed abiotically in the most reduced part of the plume, at 2 m from the landfill.