Wetland Porewater Research Articles

Remediation of Sulfides in Produced Waters of the Oil and Gas Industry Using Hydrogen Peroxide

Produced waters are often treated in open lagoons where hydrogen sulfide (H2S) can off gas, posing a risk to human health and the environment. The aim of this study was to optimize a treatment process using hydrogen peroxide (H2O2) to oxidize H2S while minimizing off gassing. Samples of produced water from West Texas and laboratory-prepared waters utilizing sodium sulfide (Na2S) or biogenic polysulfides were oxidized with H2O2 alone or in combination with copper or iron catalysts, sodium hydroxide (NaOH), or a commercial sulfide oxidizer, HydroPower Green™. Sulfur speciation was measured using Hach test kits for sulfide/sulfate/sulfite and Dräger tubes for headspace H2S. HydroPower Green™ (HPG) helped to reduce H2S in the headspace of water samples; some of this was pH related as NaOH also worked, but not as well as HPG. The dose of peroxide necessary to oxidize sulfides to sulfate is a function of the oxidation-reduction potential (Eh) of the water and total sulfide concentration as well as pH; approximately a 1–4:1 ratio of peroxide to sulfide concentration was needed to oxidize sulfidic waters of pH 7–10 with half-lives under 30 min. Both copper and iron catalysts reduce H2O2 demand and the half-life of H2S. Peracetic acid (PAA) and copper (II) sulfate pentahydrate (CuSO4, 5H2O) were explored as biocides for controlling sulfate-reducing bacteria (SRBs) that produce H2S. An AquaSnap (Hygenia) test kit was employed to monitor relative microbial activity in a wetland porewater containing H2S. Microbial regrowth occurred after a few days using the highest dose of PAA; these results showed that PAA was being used by bacteria as a carbon source even after the initial substantial reduction in the microbial activity. CuSO4, 5H2O at a dose of 1 ppm prevented microbial regrowth. The recommended treatment process from this research is determined by jar testing with H2O2, a base for pH control, a biocide, and possibly a metal catalyst or other co-oxidants in order to achieve oxidation of sulfides without H2S release or the precipitation of metal carbonates or oxides.

Porewater microbiomes in buried wetland soils: Synergic effects of water chemistry and redox gradients associated with hydrological processes

Abstract Interstitial water or pore water occupies the space between soil particles and provides “hotspots” and “fluvial networks” for microbial activities in floodplain soil. However, to date, we know very little about the microorganisms living in pore water and how they respond to environmental changes. This study aimed to understand microbial distribution and assemblage in riparian pore waters, and how they respond to water chemistry and redox gradients associated with hydrological processes. We analysed the annual changes of porewater microbial communities from the east and west banks of the White Clay Creek, a site at the Christina River Basin – Critical Zone Observatory, Pennsylvania, USA. Microbial abundances were quantified by epifluorescence microscopy and detailed community structures were characterised by high‐throughput sequencing. Water chemistry and redox gradients were also monitored and recorded, and their interactions with porewater microbiomes were analysed using correlations and multivariate analyses. Abundance of microbial cells increased during summer and late autumn. Wetland porewater microbiomes mainly contained Acidobacteria, Bacteroidetes, Nitrospirae and Proteobacteria, and microbiome structures were easily distinguishable from those in the underlying hyporheic gravel layer. Seasonal dynamics of bacterial community structure in the east and west wetlands were distinct, responding to floodplain topography and associated hydrological/geochemical processes. Iron (Fe)‐cycling bacteria (mainly Gallionellaceae and Rhodoferax spp.) dominated the porewater microbiome, and their relative abundance was significantly higher in the east than the west wetland. Furthermore, Fe‐oxidising bacteria (Gallionellaceae) were negatively correlated with Fe‐reducing bacteria (Rhodoferax spp.) at the east wetland. Microbial abundances (cell density) in pore waters showed similar seasonal patterns across stream banks, but microbial community structure did not. Microbiome assembly in pore water is correlated with water chemistry and redox gradients primarily associated with local hydrological processes. As a consequence of their significance for carbon (C) mineralisation and Fe reduction at terrestrial–aquatic interfaces, microbiomes in riparian pore waters and associated microbial activity play an essential role in C and mineral dynamics. These findings will inform future studies of the response of freshwater ecosystems to hydrological dynamics influenced by global climate change.

Trees in the Stream: Determining Patterns of Terrestrial Dissolved Organic Matter Contributions to the Northeast Pacific Coastal Temperate Rainforest

AbstractDissolved organic matter (DOM) composition in small watersheds depends on complex antecedent conditions that ultimately influence DOM generation, processing, and stability downstream. Here, we used ultrahigh resolution Fourier‐transform ion cyclotron resonance mass spectrometry and total dissolved nitrogen and dissolved organic carbon concentrations to investigate how DOM is produced in distinct sub‐catchment types (poor fen, forested wetland, and upland forest) and transported through a watershed in the northeast Pacific coastal temperate rainforest (NPCTR). We traced a suite of previously identified source‐specific marker formulae from vegetation and soil downstream and used them to test models of terrestrial DOM inputs. Marker formulae escaped microbial degradation and were exported from the watershed, demonstrating strong land‐to‐ocean connectivity through the transfer of unmodified tree DOM from specific tree species into the marine environment. Simple hydrologic and temperature variables were better able to predict inputs of soil‐sourced DOM into the stream network than tree‐sourced DOM, highlighting the role of antecedent conditions (e.g., plant growth stage) in DOM source availability and hydrologic flow connectivity, particularly for plant‐derived material. Forested wetland pore waters featured thousands of nitrogen‐containing molecular formulae that potentially provide a path of direct organic nitrogen uptake to organisms. The modified aromaticity index peaked in midsummer (up to 0.55 for fen headwaters) suggesting DOM inputs from freshly produced vegetation provide a strong summertime terrestrial signal. As the climate changes, new watershed‐scale conditions may further complicate predictions of DOM source availability, flow connectivity, and downstream fate in NPCTR watersheds.

Effects of organic sulfur and arsenite/dissolved organic matter ratios on arsenite complexation with dissolved organic matter.

The speciation and fate of arsenic (As) in soil-water systems is a topic of great interest, in part due to growing awareness of As uptake into rice as an important human exposure pathway to As. Rice paddy and other wetland soils are rich in dissolved organic matter (DOM), leading to As/DOM ratios that are typically lower than those in groundwater aquifers or that have been used in many laboratory studies of As-DOM interactions. In this contribution, we evaluate arsenite (As(III)) binding to seven different DOM samples at As/DOM ratios relevant for wetland pore waters, and explore the chemical properties of the DOM samples associated with high levels of As(III)-DOM complexation. We integrate data from wet chemical analysis of DOM chemical properties, dialysis equilibrium experiments, and two-site ligand binding models to show that in some DOM samples, 15-60% of As(III) can be bound to DOM at environmentally-relevant As/DOM ratios of 0.0032-0.016μmol As/mmol C. Binding decreases as the As(III)/DOM ratio increases. The organic sulfur (Sorg) content of the DOM samples was strongly correlated with levels of As(III)-DOM complexation and "strong" binding site densities, consistent with theories that thiols are strong binding ligands for As(III) in natural organic matter. Finally, a whole-cell E. coli biosensor assay was used to show that DOM samples most effective at complexing As(III) also led to decreased microbial As(III) uptake at low As/DOC ratios. This work demonstrates that naturally-occurring variations in the Sorg content of DOM has a significant impact on As(III) binding to DOM, and has implications for As(III) availability to microorganisms.

Abiotic soil properties affecting interpretation of IRIS sensors in tidal and freshwater soils

AbstractIndicator of Reduction in Soils (IRIS) films are visual sensors used to document weakly or moderately reducing conditions in soils based on the reduction and removal of brown manganese (Mn) oxide or orange iron (Fe) oxide paints, respectively, from underlying white polyvinyl chloride (PVC) films. Paint removal is largely assumed to result from anaerobic microbial reduction using metal oxides on the PVC films as electron acceptors. If true, IRIS films could indicate conditions favorable to other biogeochemical processes that occur at similar redox potentials to those facilitating paint removal. Our objective here was to assess the effects of selected abiotic soil properties on IRIS film paint removal to determine whether removal can accurately be attributed to biotic processes alone. Through field deployments and laboratory incubation experiments using IRIS films, we investigated relative sulfide concentration and dissolved organic matter as two abiotic factors potentially capable of removing paint from Mn IRIS films. Our results showed that abundance of reactive soluble sulfides cause rapid and extensive abiotic paint removal from Mn films, whereas ambient concentrations of dissolved organic matter in freshwater wetland porewater does not drive abiotic removal. Furthermore, we found that the visible formation of black iron monosulfides on Fe films can be used to detect sulfide concentrations that will remove paint from Mn films. This study suggests that whereas some abiotic soil properties (e.g., sulfide concentration) can cause paint removal, IRIS films may be a viable tool to approximate biotic process rates where abiotic paint removal can be ruled out.

Methylmercury formation in boreal wetlands in relation to chemical speciation of mercury(II) and concentration of low molecular mass thiols

Methylmercury (MeHg) is a neurotoxin formed from inorganic divalent mercury (HgII) via microbial methylation, and boreal wetlands have been identified as major sources of MeHg. There is however a lack of studies investigating the relationship between the chemical speciation of HgII and MeHg formation in such environments, in particular regarding to role of thiol compounds. We determined HgII methylation potentials, kmeth, in boreal wetland soils using two HgII isotope tracers: 198Hg(OH)2(aq) and HgII bonded to thiol groups in natural organic matter, 200HgII–NOM(ads), representing HgII sources with high and low availability for methylation. The 198Hg(OH)2(aq) tracer was consistently methylated to a 5-fold higher extent than 200HgII–NOM(ads), independent of environmental conditions. This suggests that the concentration of HgII in porewater was a decisive factor for HgII methylation. A comprehensive thermodynamic speciation model (including HgII complexes with inorganic sulfide (H2S), polysulfides (H2Sn), thiols associated with natural organic matter (NOM–RSH) and specific low molecular mass thiols (LMM–RSH) provided new insights on the speciation of HgII in boreal wetland porewaters, but did not demonstrate any clear relationship between kmeth and the calculated chemical speciation. In contrast, significant positive relationships were observed between kmeth and the sum of LMM thiol compounds of biological origin. We suggest two possible mechanisms underlying these correlations: 1) LMM thiols kinetically control the size and composition of the HgII pool available for microbial uptake, and/or 2) LMM thiols are produced by microbes such that the correlation reflects a relation between microbial activity and MeHg formation.

Quantification of total concentration of thiol functional groups in environmental samples by titration with monobromo(trimethylammonio)bimane and determination with tandem mass spectrometry

Thiol compounds (R–SH) have many important biological functions and are principal controls of the speciation of several toxic metals in the environment. However, determining the concentration of thiols associated with environmental matrices is challenging due to the compounds’ low abundance and interferences from non-thiol compounds for many available methods. Here a novel method has been developed and validated to quantify the total concentration of thiol functional groups in aqueous samples using derivatization with monobromo(trimethylammonio)bimane (qBBr) and quantification with tandem mass spectrometry. The thiol concentration was determined by titration of the sample with qBBr, which reacts selectively with thiols, and quantification of the residual qBBr. We systematically evaluated potential interferences from various organic compounds, inorganic ions (including sea water matrices), sulfide and mercury (Hg) species, and demonstrate that the method is highly sensitive, selective and robust. The limit of detection (LOD) for total thiols is in the nanomolar concentration range (~6 nM). The method performance was also demonstrated by determination of the total thiol concentration in different natural samples including boreal stream water (1.16 μM), wetland porewater (0.96 μM) and the Suwanee River natural organic matter (NOM) reference material SR101 N (7.9 μmol g−1). The developed method represents a combination of low LOD and high selectivity and robustness that is unsurpassed for total thiol concentration measurements.

Determination of Hydroxyl Radical Production from Sulfide Oxidation Relevant to Sulfidic Porewaters

Hydroxyl radical (·OH) production from the reaction between aqueous total sulfide ([H2S]T = [H2S] + [HS–] + [S2–]) and dissolved oxygen is potentially an environmentally important reaction when anoxic, sulfidic water is exposed to oxygen. Using hydroxyterephthalate (hTPA) formation from the reaction of terephthalic acid (TPA) with ·OH as a probe for ·OH production, hydrogen peroxide was verified as an essential intermediate, and the production of free ·OH, versus lower energy hydroxylating agents, was established. The optimal conditions for the quantification of ·OH production kinetics and yield were determined by varying TPA and total sulfide concentrations. An initial total sulfide concentration of 10 μM and TPA concentration of 2 mM was used to find a yield of 15 mmol of ·OH per mole [H2S]T. Additionally, a pseudo-first-order model elucidated a maximum rate of production of 1.04 (±0.05) × 10–4 moles of ·OH per hour per mole of [H2S]T. Experiments with sulfidic wetland porewaters containing up to 294 mg/L of dissolved organic carbon (DOC) revealed that [H2S]T, and not reduced DOC, was the dominant source of ·OH. A simple model considering a water containing [H2S]T, DOC, and methane exposed to a constant concentration of oxygen (∼50 μM) gave steady state values of [·OH] ranging from 5.7 × 10–19 to 3.2 × 10–18 M. The results indicate that [H2S]T should be considered a source of dark formation of ·OH in addition to ferrous iron and reduced DOC.

Geochemical Factors Controlling Dissolved Elemental Mercury and Methylmercury Formation in Alaskan Wetlands of Varying Trophic Status.

The transformations of aqueous inorganic divalent mercury (Hg(II)i) to volatile dissolved gaseous mercury (Hg(0)(aq)) and toxic methylmercury (MeHg) govern mercury bioavailability and fate in northern ecosystems. This study quantified concentrations of aqueous mercury species (Hg(II)i, Hg(0)(aq), MeHg) and relevant geochemical constituents in pore waters of eight Alaskan wetlands that differ in trophic status (i.e., bog-to-fen gradient) to gain insight on processes controlling dark Hg(II)i reduction and Hg(II)i methylation. Regardless of wetland trophic status, positive correlations were observed between pore water Hg(II)i and dissolved organic carbon (DOC) concentrations. The concentration ratio of Hg(0)(aq) to Hg(II)i exhibited an inverse relationship to Hg(II)i concentration. A ubiquitous pathway for Hg(0)(aq) formation was not identified based on geochemical data, but we surmise that dissolved organic matter (DOM) influences mercury retention in wetland pore waters by complexing Hg(II)i and decreasing the concentration of volatile Hg(0)(aq) relative to Hg(II)i. There was no evidence of Hg(0)(aq) abundance directly limiting mercury methylation. The concentration of MeHg relative to Hg(II)i was greatest in wetlands of intermediate trophic status, and geochemical data suggest mercury methylation pathways vary between wetlands. Our insights on geochemical factors influencing aqueous mercury speciation should be considered in context of the long-term fate of mercury in northern wetlands.

Determination of picomolar concentrations of thiol compounds in natural waters and biological samples by tandem mass spectrometry with online preconcentration and isotope-labeling derivatization

We present a sensitive, selective and robust method for the determination of 14 thiol compounds in aqueous samples. Thiols were derivatized with ω-bromoacetonylquinolinium bromide (BQB) and its deuterium labeled equivalent D7-ω-bromoacetonylquinolinium bromide (D7). Derivatized thiols were preconcentrated by online solid-phase extraction (SPE) followed by liquid chromatography separation and electrospray ionization tandem mass spectrometry determination (SPE/LC-ESI-MS/MS). The robustness of the method was validated for wide ranges in pH, salinity, and concentrations of sulfide and dissolved organic carbon (DOC) to cover contrasting natural water types. The limits of detection (LODs) for the thiols were 3.1–66 pM. Between 6 and 14 of the thiols were detected in different natural sample types at variable concentrations: boreal wetland porewater (0.7–51 nM), estuarine sediment porewater (50 pM–11 nM), coastal sea water (60 pM–16 nM), and sulfate reducing bacterium cultures (80 pM–4 nM). MS/MS fragmentation of the compounds produces two pairs of common product ions, m/z 130.2/137.1 and 218.1/225.1, which enables scanning for unknown thiols in precursor ion scan mode. Using this approach, we identified cysteine, mercaptoacetic acid, N-acetyl-L-cysteine and sulfurothioic S-acid in boreal wetland porewater. The performance of the developed method sets a new state of the art for the determination of thiol compounds in environmental and biological samples.

Novel Insights into the Distribution of Reduced Sulfur Species in Prairie Pothole Wetland Pore Waters Provided by Bismuth Film Electrodes

Mercury/gold amalgam and bismuth film electrodes (BiFEs) were used to make the first centimeter-scale measurements of redox species in benthic pore waters of prairie pothole wetlands across a hydrologic gradient. Sulfide in pore waters increased across this system from negligible sulfide in hydrologically up-gradient recharge wetlands to thousands of micromolar in down-gradient discharge wetlands. Field measurements of sulfides using the BiFE were tested against an established colorimetric assay. Sulfide measured with the BiFE agreed well with colorimetric measurements but is not subject to analytical artifacts associated with methods needed to extract the pore waters. Use of Hg/Au and BiFE electrodes should allow for rapid in situ detection of redox active species, especially sulfide concentrations of >500 μM, in pore waters over seasonal to decadal time scales. Such measurements are needed to understand important biogeochemical and environmental processes such as carbon cycling and contaminant attenuati...

Transformation of chlorpyrifos and chlorpyrifos-methyl in prairie pothole pore waters.

Non-point source pesticide pollution is a concern for wetlands in the prairie pothole region (PPR). Recent studies have demonstrated that reduced sulfur species (e.g., bisulfide and polysulfides) in PPR wetland pore waters directly undergo reactions with chloroacetanilide and dinitroaniline compounds. In this paper, the abiotic transformation of two organophosphate compounds, chlorpyrifos and chlorpyrifos-methyl, was studied in PPR wetland pore waters. Chlorpyrifos-methyl reacted significantly faster (up to 4 times) in pore water with reduced sulfur species relative to hydrolysis. No rate enhancement was observed in the transformation of chlorpyrifos in pore water with reduced sulfur species. The lack of reactivity was most likely caused by steric hindrance from the ethyl groups and partitioning to dissolved organic matter (DOM), thereby shielding chlorpyrifos from nucleophilic attack. Significant decreases in reaction rates were observed for chlorpyrifos in pore water with high concentrations of DOM. Rate enhancement due to other reactive species (e.g., organo-sulfur compounds) in pore water was minor for both compounds relative to the influence of bisulfide and DOM.

Behaviour of pharmaceuticals and personal care products in constructed wetland compartments: Influent, effluent, pore water, substrate and plant roots.

Seven mesocosm-scale constructed wetlands (CWs) with different design configurations, dealing with primary-treated urban wastewater, were assessed for the concentration, distribution and fate of ten pharmaceutical and personal care products (PPCPs) [ibuprofen, ketoprofen, naproxen, diclofenac, salicylic acid, caffeine, carbamazepine, methyl dihydrojasmonate, galaxolide and tonalide] and eight of their transformation products (TPs). Apart from influent and effluent, various CW compartments were analysed, namely, substrate, plant roots and pore water. PPCP content in pore water depended on the specific CW configuration. Macrophytes can take up PPCPs through their roots. Ibuprofen, salicylic acid, caffeine, methyl dihydrojasmonate, galaxolide and tonalide were present on the root surface with a predominance of galaxolide and caffeine in all the planted systems. Naproxen, ibuprofen, salicylic acid, methyl dihydrojasmonate, galaxolide and tonalide were uptaken by the roots. In order to better understand the removal processes, biomass measurement and biodegradability studies through the characterization of internal-external isomeric linear alkylbenzenes present on the gravel bed were performed. Three TPs namely, ibuprofen-amide, 3-ethylbenzophenone and 4-hydroxy-diclofenac were identified for the first time in wetland pore water and effluent water, which suggests de novo formation (they were not present in the influent). Conversely, O-desmethyl-naproxen was degraded through the wetland passage since it was detected in the influent but not in the subsequent treatment stages. Biodegradation pathways are therefore suggested for most of the studied PPCPs in the assessed CWs.

Determination of sub-nanomolar levels of low molecular mass thiols in natural waters by liquid chromatography tandem mass spectrometry after derivatization with p-(hydroxymercuri) benzoate and online preconcentration.

Low molecular mass (LMM) thiols is a diverse group of compounds, which play several important roles in aquatic ecosystems, even though they typically occur at low concentrations. Comprehensive studies of LMM thiols in natural waters have so far been hampered by selectivity and limit of detection constraints of previous analytical methods. Here, we describe a selective and robust method for the quantification of 16 LMM thiols in natural waters. Thiols were derivatized with 4-(hydroxymercuri)benzoate (PHMB) and preconcentrated online by solid-phase extraction (SPE) before separation by liquid chromatography and determination by electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). Their quantification was performed by selective reaction monitoring (SRM), while the presence of a product ion at m/z 355, specific for thiols and common for the investigated compounds, also allows to screen samples for unknown thiols by a precursor ion scan approach. The robustness of the method was validated for aqueous matrices with different pH, sulfide, and dissolved organic carbon (DOC) concentrations. The limits of detection for the thiols were in the sub-nanomolar range (0.06-0.5 nM) and the methodology allowed determination of both reduced and total thiol concentrations (using tris(2-carboxyethyl)phosphine (TCEP) as reducing agent). Six thiols (mercaptoacetic acid, cysteine, homocysteine, N-acetyl-cysteine, mercaptoethane-sulfonate, and glutathione) were detected with total concentrations of 7-153 nM in boreal lake or wetland pore waters while four thiols (mercaptoacetic acid, cysteine, homocysteine, and N-acetyl-cysteine) were detected in their reduced form at concentrations of 5-80 nM.

Sulfur, iron and carbon cycling following hydrological restoration of acidic freshwater wetlands

Freshwater re-flooding is a relatively novel approach to remediate drained acid sulfate soil (ASS) wetlands. This study documents the geochemical consequences of restoring freshwater re-flooding for contemporary reduced inorganic sulfur (RIS) and iron species in two coastal floodplain ASS wetlands. Re-flooding has established predominantly reducing/suboxic conditions and encouraged organic carbon accumulation in surface sediments (~20–30%). The pH of former sulfuric horizons has increased by ~2–3 units, partly in response to alkalinity generation from anaerobic metabolism of organic carbon coupled with Fe(III) and SO42− reduction. Despite considerable sulfidisation, reactive Fe (FeR; sum of 1M HCl and citrate–dithionite extracts) and non-sulfidic Fe(II) remain abundant in both wetlands. High concentrations of Fe2+ (up to ~5mM) in wetland porewaters represent a considerable pool of labile net acidity and is partly a result of insufficient S(−II) to sequester excess Fe2+. Accumulation of iron sulfides appears to be constrained more by SO42− and carbon availability rather than FeR. Reformation and accumulation of RIS species is greatest in organic-rich surface horizons (~40–500μmolg−1), where time integrated RIS accumulation rates approximate 10–100nmolg−1 d−1. While pyrite is the dominant RIS species to have formed since re-flooding, there is anomalous accumulation of S(0) (up to 80μmolg−1), accounting for ~50% of the RIS pool in some samples. Greigite (Fe3S4) has formed in near-surface sediments and while AVS-S is a minor component of the RIS pool overall, at some locations maximum concentrations exceed 300μmolg−1. Contemporary near-surface pyrite is characterised by abundant small (200–300nm) crystals, in contrast to relic sedimentary pyrite of estuarine origin that is dominated by larger crystals with diverse habit. Although Fe and SO42− reduction are partly responsible for wetland-scale recovery from acute acidification, the resultant accumulation of diverse RIS species in surficial sediments indicates an oxidative component to the S-cycle and represents a hysteresis in S-cycling that contrasts markedly with the drained conditions existing before remediation. Analysis of seasonal climate fluctuations suggests that near-surface sediments containing contemporary RIS are vulnerable to oxidation and possible temporary re-acidification during future drought episodes. This study underscores the long-term legacy of ASS wetland drainage and highlights the need for both considered hydrological management of re-flooded wetlands and further study to quantify possible re-acidification risks associated with seasonal drought.

Legacy mercury and stoichiometry with C, N, and S in soil, pore water, and stream water across the upland‐wetland interface: The influence of hydrogeologic setting

Mechanisms influencing retention, biogeochemical cycling, and release of legacy mercury within soils of forests and wetlands remain poorly understood. We quantified mercury pool size and stoichiometry with carbon, nitrogen, and sulfur across forest‐wetland transects and among wetlands of different hydrogeologic settings in the Adirondack region of New York State. Average total mercury pool size in soils (to 50 cm depth) was greater in forests (17.5 mg/m2) than in wetlands (6.1 mg/m2; p < 0.010). The average mercury pool size (to 50 cm depth) in shallow‐peat riparian wetlands (9.3 mg/m2) was greater than in deep‐peat riparian (5.4 mg/m2; p = 0.099) or headwater wetlands (3.6 mg/m2; p = 0.046). Accumulation of mercury was enhanced at the forest‐wetland interface. In mineral horizons of the forest soil and in shallow‐peat riparian wetlands, mercury was positively correlated with carbon (r2 = 0.73‐0.96) and nitrogen (r2 = 0.82‐0.93), but not sulfur. In contrast, mercury and sulfur were strongly correlated in headwater wetland peat (r2 = 0.73). Dissolved mercury was correlated with dissolved organic carbon (DOC) in pore water and stream water of deep‐peat and shallow‐peat riparian wetlands (r2 = 0.46‐0.73), but not in headwater wetland pore water. In headwater outlet streams, dissolved mercury was correlated with DOC (r2 = 0.62), but the slope was only one third that in riparian streams. Hydrogeologic setting influences decomposition processes, biogeochemical cycling of mercury, and hydrologic transport that in turn, govern the size and stoichiometry of mercury pools across the upland‐wetland interface and among different wetland types. Ultimately, mobilization of legacy mercury into aquatic ecosystems from forest soils and wetlands likely depends upon decomposition dynamics and hydrologic flow paths.

DOC-dynamics in a small headwater catchment as driven by redox fluctuations and hydrological flow paths – are DOC exports mediated by iron reduction/oxidation cycles?

Abstract. Dissolved organic carbon (DOC) exports from many catchments in Europe and North-America are steadily increasing. Several studies have sought to explain this observation. As possible causes, a decrease in acid rain or sulfate deposition, concomitant reductions in ionic strength and increasing temperatures were identified. DOC often originates from riparian wetlands; but here, despite higher DOC concentrations, ionic strength in pore waters usually exceeds that in surface waters. In the catchment under study, DOC concentrations were synchronous with dissolved iron concentrations in pore and stream water. This study aims at testing the hypothesis that DOC exports are mediated by iron reduction/oxidation cycles. Following the observed hydrographs, δ18O of water and DOC fluorescence, the wetlands were identified as the main source of DOC. Antecedent biogeochemical conditions, i.e., water table levels in the wetlands, influenced the discharge patterns of nitrate, iron and DOC during an event. The correlation of DOC with pH was positive in pore waters, but negative in surface waters; it was negative for DOC with sulfate in pore waters, but only weak in surface waters. Though, the positive correlation of DOC with iron was universal for pore and surface water. The decline of DOC and iron concentrations in transition from anoxic wetland pore water to oxic stream water suggests a flocculation of DOC with oxidising iron, leading to a drop in pH in the stream during high DOC fluxes. The pore water did not per se differ in pH. There is, thus, a need to consider processes more thoroughly of DOC mobilisation in wetlands when interpreting DOC exports from catchments. The coupling of DOC with iron fluxes suggested that increased DOC exports could at least, in part, be caused by increasing activities in iron reduction, possibly due to increases in temperature, increasing wetness of riparian wetlands, or by a shift from sulfate dominated to iron reduction dominated biogeochemical regimes.

Sulfide Induced Mobilization of Wetland Phosphorus Depends Strongly on Redox and Iron Geochemistry

Constructed and restored wetland soils play a key role in the cycling and retention of nutrients from polluted surface waters. Elevated SO42- loading from irrigated agriculture, however, has been shown to affect both the availability and mobility of P in wetland soils. This study investigated the biogeochemical cycling of Fe, S, and P in wetland pore waters and surface soils, to determine the role of sulfides in inducing mobilization of P in a constructed wetland in the Central Valley of California (CVC). Water column, pore-water, and solid-phase soil chemistry were measured at three sites, representing a range in wetland environmental conditions (i.e., sediment and nutrient loading, redox environment, and Fe–S–P geochemistry). High SO42- concentration and active SO42- reduction in wetland pore waters contributed to elevated pore water PO43- concentration (0.42 –1.29 mg L−1) via SO42- induced P mobilization. Although high concentrations of acid volatile sulfide (AVS) were present in the surface (0–5 cm) layer (: 6.5–10.3 μmol g−1), only ∼8% of the labile Fe pool was S2- bound, leaving sufficient Fe to buffer S2−. Additionally, surface soils experienced a three- to four-fold increase in P sorption index (PSI) under anaerobic versus aerobic conditions. Results from solid-phase chemical extractions and chemical equilibrium modeling showed that this higher PSI may be due to the conversion of crystalline Fe(III) oxides to poorly crystalline forms, the preservation of Fe(III) oxides due to kinetic constraints on microbial Fe reduction, and the formation of Fe(II) oxides. This study demonstrates that mineral dominated wetlands with sufficient Fe to buffer both dissolved sulfide and PO43- concentrations, can limit the effects of high SO42- loading and SO42- reduction on the mobilization and flux of soil PO43- to the wetland water column.

Influence of Bedrock Geology on Water Chemistry of Slope Wetlands and Headwater Streams in the Southern Rocky Mountains

We characterized the water chemistry of nine slope wetlands and adjacent headwater streams in Colorado subalpine forests and compared sites in basins formed on crystalline bedrock with those formed in basins with a mixture of crystalline and sedimentary bedrock. The pH, Ca2+, Mg2+, NH4+, acid neutralizing capacity, and electrical conductivity of wetland porewater and streamwater were higher in the basins with mixed geology. Bryophyte cover was higher in lower pH, crystalline basins, and vascular plant cover was higher in the mixed bedrock basins. On average, wetland porewater had lower pH and higher concentrations of dissolved organic carbon (DOC) and nitrogen and several other ions than streamwater; however, because discharge from these small wetlands is low, their direct influence on stream solute concentrations was generally undetectable. Dilution altered stream solute concentrations during peak flow in both basin types, but had little effect on wetland chemistry. In contrast to other solutes, the concentration of DOC in streams increased marginally during peak runoff and its concentration in wetland porewater was stable throughout the year. These findings further knowledge of the influence of watershed characteristics on wetland and stream chemistry and will inform future decisions regarding conservation and management in headwater basins.

Phosphorus and Greenhouse Gas Dynamics in a Drained Calcareous Wetland Soil in Minnesota

Restoration of wetland hydrology can produce ecological benefits but may have unintended consequences. We examined effects of altered water level on release of dissolved reactive phosphorus (DRP) and greenhouse gases (GHG) in soil cores from a marsh being evaluated for restoration. We also measured field concentrations of DRP and other constituents in wetland porewater. Intact cores from a sampling location with higher Fe and lower calcium carbonate (CaCO(3)) contents released more DRP than another location, and displayed higher DRP under completely saturated compared to partly drained conditions. Porewater samples collected from the high-Fe location also contained higher DRP levels. Chemical data suggest that redox-driven reactions largely controlled DRP levels at the high-Fe site, while CaCO(3) adsorption was more important at the low-Fe site. Over the long term, water table elevation may attenuate P draining from the wetland due to decreased mineralization. However, such measures may increase P release in the short term. Raising the water level in soil cores resulted in decreased nitrous oxide (N(2)O) emissions, increased methane (CH(4)) emissions, and an overall increase in total global warming potential (GWP). The proportion of total GWP contributed by N(2)O decreased from 14% to < or = 1% as water level was raised, while the proportion contributed by CH(4) increased from 10 to 20% to 60 to 80%. Restoration of hydrology in the Rice Lake wetland has the potential to affect both local water quality and global air quality. These combined effects complicate the cost-to-benefit analysis of such wetland restoration efforts.