Wetland Porewater Research Articles

Evaluation of sulfate reduction at experimentally induced mixing interfaces using small-scale push–pull tests in an aquifer–wetland system

This paper presents small-scale push–pull tests designed to evaluate the kinetic controls on SO 4 2 - reduction in situ at mixing interfaces between a wetland and aquifer impacted by landfill leachate at the Norman Landfill research site, Norman, OK. Quantifying the rates of redox reactions initiated at interfaces is of great interest because interfaces have been shown to be zones of increased biogeochemical transformations and thus may play an important role in natural attenuation. To mimic the aquifer–wetland interface and evaluate reaction rates, SO 4 2 - -rich anaerobic aquifer water ( ∼ 100 mg / L SO 4 2 - ) was introduced into SO 4 2 - -depleted wetland porewater via push–pull tests. Results showed SO 4 2 - reduction was stimulated by the mixing of these waters and first-order rate coefficients were comparable to those measured in other push–pull studies. However, rate data were complex involving either multiple first-order rate coefficients or a more complex rate order. In addition, a lag phase was observed prior to SO 4 2 - reduction that persisted until the mixing interface between test solution and native water was recovered, irrespective of temporal and spatial constraints. The lag phase was not eliminated by the addition of electron donor (acetate) to the injected test solution. Subsequent push–pull tests designed to elucidate the nature of the lag phase support the importance of the mixing interface in controlling terminal electron accepting processes. These data suggest redox reactions may occur rapidly at the mixing interface between injected and native waters but not in the injected bulk water mass. Under these circumstances, push–pull test data should be evaluated to ensure the apparent rate is actually a function of time and that complexities in rate data be considered.

Anaerobic Biodegradation and Hydrogeochemical Controls on Natural Attenuation of Trichloroethene in an Inland Forested Wetland

ABSTRACT A field and laboratory investigation of natural attenuation, focusing on anaerobic biodegradation, was conducted in a forested wetland where a plume of trichloroethene discharges from a sand aquifer through organic-rich wetland and stream-bottom sediments. The rapid response of the wetland hydrology to precipitation events altered groundwater flow and geochemistry during wet conditions in the spring compared to the drier conditions in the summer and fall. During dry conditions, partial reductive dechlorination of trichloroethene to cis-1,2-dichloroethene occurred in methanogenic wetland porewater. Influx of oxygenated recharge during wet conditions resulted in a change from methanogenic to iron-reducing conditions and a lack of 1,2-dichloroethene production in the wet spring conditions. During these wet conditions, dilution was the primary attenuation mechanism evident for trichloroethene in the wetland porewater. Trichloroethene degradation was insignificant in anaerobic microcosms constructed with the shallow wetland sediment, and microbiological analyses showed a low microbial biomass and absence of known dehalorespiring microorganisms. Despite the typically organic-rich characteristic of wetland sediments, natural attenuation by anaerobic degradation may not be an effective groundwater remediation for chlorinated solvents at all sites.

Degradation of 1,1,2,2-tetrachloroethane and accumulation of vinyl chloride in wetland sediment microcosms and in situ porewater: biogeochemical controls and associations with microbial communities

The biodegradation pathways of 1,1,2,2-tetrachloroethane (TeCA) and 1,1,2-trichloroethane (112TCA) and the associated microbial communities in anaerobic wetland sediments were evaluated using concurrent geochemical and genetic analyses over time in laboratory microcosm experiments. Experimental results were compared to in situ porewater data in the wetland to better understand the factors controlling daughter product distributions in a chlorinated solvent plume discharging to a freshwater tidal wetland at Aberdeen Proving Ground, Maryland. Microcosms constructed with wetland sediment from two sites showed little difference in the initial degradation steps of TeCA, which included simultaneous hydrogenolysis to 112TCA and dichloroelimination to 1,2-dichloroethene (12DCE). The microcosms from the two sites showed a substantial difference, however, in the relative dominance of subsequent dichloroelimination of 112TCA. A greater dominance of 112TCA dichloroelimination in microcosms constructed with sediment that was initially iron-reducing and subsequently simultaneously iron-reducing and methanogenic caused approximately twice as much vinyl chloride (VC) production as microcosms constructed with sediment that was methanogenic only throughout the incubation. The microcosms with higher VC production also showed substantially more rapid VC degradation. Field measurements of redox-sensitive constituents, TeCA, and its anaerobic degradation products along flowpaths in the wetland porewater also showed greater production and degradation of VC with concurrent methanogenesis and iron reduction. Molecular fingerprinting indicated that bacterial species [represented by a peak at a fragment size of 198 base pairs (bp) by MnlI digest] are associated with VC production from 112TCA dichloroelimination, whereas methanogens (190 and 307 bp) from the Methanococcales or Methanobacteriales family are associated with VC production from 12DCE hydrogenolysis. Acetate-utilizing methanogens (acetotrophs) appear to be involved in the biodegradation of VC. The relative abundance of Methanosarcinaceae, the only methanogen group with acetotrophic members, doubled in microcosms in which degradation of VC was observed. In addition, molecular analyses using primers specific for known dehalorespiring bacteria in the Dehalococcoides and Desulfuromonas groups showed the presence of these bacteria in microcosm slurry from the site that showed the highest VC production and degradation. Determination of biogeochemical controls and microbial consortia involved in TeCA degradation is leading to a better understanding of the heterogeneity in biodegradation rates and daughter product distribution in the wetland, improving capabilities for developing remediation and monitoring plans.

Speciation of aqueous chromium by use of solid-phase extractions in the field.

Here the fate and mobility of chromium in an organic-rich wetland area was studied through a field investigation in which chromium speciation in aqueous samples was estimated by use of solid-phase extraction resins that removed the cationic, anionic, and hydrophobic organic species from solution. Results show that aqueous chromium concentrations in the surface and pore waters at this site are higher than would be predicted by simple thermodynamic equilibrium with solid chromium hydroxides. Hexavalent chromium was not observed and therefore could not explain these higher than expected chromium concentrations. Aqueous chromium in concentrations greater than 30 microg/L was found to be slightly correlated with total dissolved organic carbon (R2 = 0.66), thus suggesting enhanced chromium concentrations because of complexation with organic ligands. The results of the solid-phase extraction performed show that aqueous chromium in the wetland pore water and surface water exists primarily as an anion (96%). It is concluded that the solubility and mobility of chromium are controlled, at least in part, by complexation with dissolved organic carbon and that this may be a thermodynamically controlled process.

Pore water geochemistry near the sediment-water interface of a zoned, freshwater wetland in the southeastern United States

Sediment and its associated pore water were collected from a zoned, freshwater, riparian wetland, located in the Talladega National Forest, northeastern Hale County, Alabama, to study the porewater chemistry and its spatial changes within and between the wetland ecological zones. Obvious changes in pH, Eh and element concentrations were observed between the different wetland ecological zones. Major cations (Ca, Mg, and Na) and trace elements (B, Ba, Sr, and Mn) have very good spatial correlation with Fe and Mn distributions, both in the pore water and the sediment, suggesting that adsorption on, and desorption from, iron and manganese oxyhydroxides are important processes controlling the distributions of these elements in the wetland sediment. However, an equilibrium adsorption model is not able to explain the distribution of trace elements between the pore water and sediment. A redox kinetic model gives similar vertical profiles for iron and the correlated elements as those measured in the field and thus suggests that the relative rates of ferrous iron oxidation and the reductive dissolution of ferric iron in the sediment are important variables determining the distributions of these elements in the wetland pore waters.

Hydrologic Response of a Wetland to Changing Moisture Conditions: Modeling Effects of Soil Heterogeneity

Prediction of the effects of external influences such as climate change on wetland systems requires the prediction of hydrologic effects. Because wetland soils are typically heterogeneous, it is particularly important to understand the extent and connectedness of hydraulically conductive soil units, since water flow may be concentrated in such units while bypassing others of lower conductivity. However, subsurface hydrologic models typically do not represent heterogeneity adequately, being limited by sparse parameterization of soil properties. Conventional techniques for mapping units of soil within wetlands are highly laborious, requiring soil coring and laboratory testing. As an alternative, we developed a portable piezocone driver and highly sensitive piezocone designed to map wetland soil units with centimeter-scale resolution in the vertical and meter-scale resolution in the horizontal dimension. This system successfully delineated several different layers of peat, sand, and limnetic sediments, and their degree of interconnectedness in an eight-meter-thick peat deposit. Monitoring of wetland response to precipitation, changes in stream stage, and overbank flooding was then used in conjunction with the piezocone data and a two-dimensional flow model to constrain the hydraulic properties of the soil units. Thus parameterized, a standard subsurface flow model was able to realistically simulate a variety of hydrologic processes relevant to climate change, including wetland-stream water exchange, the movement of wetland porewaters to the root zone of plants, and wetland desaturation under dry conditions.