Effects Of Water Table Fluctuations Research Articles

Deterministic and probabilistic analysis of the response of shallow footings on unsaturated soils due to rainfall

Due to rainfall, footings are usually subjected to effects of water table fluctuations which results in the variation of ultimate bearing capacity in unsaturated soils. However, the conventional bearing capacity theories don’t account for the variation of suction as the water table position changes. In this paper, an equation based on Terzaghi theory is proposed to evaluate the variation of bearing capacity due to suction variations due to rainfall. The proposed equation results in higher bearing capacity values compared to the conventional Terzaghi equation. Further, a comparison of the bearing capacity obtained using the proposed equation is made with those obtained from Barcelona Basic model (BBM), Mohr-Coulomb model and Terzaghi equation. It is observed that BBM results in lower values of bearing capacity compared to Mohr-Coulomb model as the former considers the loss of suction upon infiltration. Soil properties like porosity, permeability and soil water characteristic (SWCC) influence the bearing capacity in unsaturated soils. Thus, probabilistic analysis is carried out by using an efficient kriging surrogate model in order to analyze the impact of variation of the associated parameters on the bearing capacity. The surrogate model can predict the surface displacements of footing upon infiltration with enhanced accuracy and computational efficiency. Considering the uncertainty associated with rainfall loads, it is observed that as the return period of rainfall increases, failure probability increases.

Effects of Water Table Fluctuation on Greenhouse Gas Emissions from Wetland Soils in the Peruvian Amazon

Amazonian swamp forests remove large amounts of carbon dioxide (CO2) but produce methane (CH4). Both are important greenhouse gases (GHG). Drought and cultivation cut the CH4 emissions but may release CO2. Varying oxygen content in nitrogen-rich soil produces nitrous oxide (N2O), which is the third most important GHG. Despite the potentially tremendous changes, GHG emissions from wetland soils under different land uses and environmental conditions have rarely been compared in the Amazon. We measured environmental characteristics, and CO2, CH4 and N2O emissions from the soil surface with manual opaque chambers in three sites near Iquitos, Peru from September 2019 to March 2020: a pristine peat swamp forest, a young forest and a slash-and-burn manioc field. The manioc field showed moderate soil respiration and N2O emission. The peat swamp forests under slight water table drawdown emitted large amounts of CO2 and CH4. A heavy post-drought shower created a hot moment of N2O in the pristine swamp forest, likely produced by nitrifiers. All in all, even small changes in soil moisture can create hot moments of GHG emissions from Amazonian wetland soils, and should therefore be carefully monitored.

The effects of water table fluctuation on LNAPL deposit in highly permeable porous media: A coupled numerical and experimental study

Light Non-Aqueous Phase Liquid (LNAPL) flow on the water table is highly mobile and is sensitive to the fluctuation of groundwater. This process is highly complex and involves the migration of three immiscible phases (i.e. water, LNAPL and air) which need the explicit definition of multiple parameters. A coupled experimental and numerical simulation methodology is performed by using Time Domain Reflectrometer (TDR) and multiphase simulation of a controlled environment to mimic the water table fluctuation and its effect on the LNAPL residual saturation. TDR probes are installed in different locations of a 2D tank (i.e. a cuboid box with relatively low off-plane thickness) and the bulk permittivity of the phases are measured through artificially imposed boundary conditions. The bulk permittivity is then translated into saturation of the three different phases. The translated residual saturations along with the previously measured porous media properties (e.g. porosity and saturated permeability) are then inserted into the numerical simulator (i.e. COMSOL Multiphysics®) and the migration of the three phase in porous media is simulated. The numerical exponents and entry pressures needed for the simulation of the multiphase flow are estimated using the temporal experimental values. The exponents of water LNAPL relative permeability were estimated to be around 2 while the exponents gas LNAPL relative permeability were estimated to be closer to 3. The results, simulated with the optimized parameters, are then evaluated with pictures taken from the transparent face of the 2D tank different stages of the experiment. The temporal evolution of different phase saturation has been compared and validated between the experimental results obtained and interpreted by the TDR probe measurements and the simulations. The relative error stays in the 5 % confidence level for most reported points and only in the highly dynamic flow time steps the error reaches around 12% which are discussed in the text and is accepted due to the highly nonlinear nature of the problem.

Water table fluctuations affect dichloromethane biodegradation in lab-scale aquifers contaminated with organohalides

Dichloromethane (DCM) is a toxic industrial solvent frequently detected in multi-contaminated aquifers. It can be degraded biotically or abiotically, and under oxic or anoxic conditions. The extent and pathways of DCM degradation in aquifers may thus depend on water table fluctuations and microbial responses to hydrochemical variations. Here, we examined the effect of water table fluctuations on DCM biodegradation in two laboratory aquifers fed with O2-depleted DCM-spiked groundwater from a well-characterized former industrial site. Hydrochemistry, stable isotopes of DCM (δ13C and δ37Cl), and bacterial community composition were examined to determine DCM mass removal and degradation pathways under steady-state (static water table) and transient (fluctuating water table) conditions. DCM mass removal was more pronounced under transient (95%) than under steady-state conditions (42%). C and Cl isotopic fractionation values were larger under steady-state (εbulkC = −23.6 ± 3.2‰, and εbulkCl= −8.7 ± 1.6‰) than under transient conditions (εbulkC = −11.8 ± 2.0‰, and εbulkCl = −3.1 ± 0.6‰). Dual C-Cl isotope analysis suggested the prevalence of distinct anaerobic DCM degradation pathways, with ΛC/Cl values of 1.92 ± 0.30 and 3.58 ± 0.42 under steady-state and transient conditions, respectively. Water table fluctuations caused changes in redox conditions and oxygen levels, resulting in a higher relative abundance of Desulfosporosinus (Peptococcaceae family). Taken together, our results show that water table fluctuations enhanced DCM biodegradation, and correlated with bacterial taxa associated with anaerobic DCM degradation. Our integrative approach allows to evaluate anaerobic DCM degradation under dynamic hydrogeological conditions, and may help improving bioremediation strategies at DCM contaminated sites.

Water Table Fluctuation and Methane Emission in Pineapples (Ananas comosus (L.) Merr.) Cultivated on a Tropical Peatland

Inappropriate drainage and agricultural development on tropical peatland may lead to an increase in methane (CH4) emission, thus expediting the rate of global warming and climate change. It was hypothesized that water table fluctuation affects CH4 emission in pineapple cultivation on tropical peat soils. The objectives of this study were to: (i) quantify CH4 emission from a tropical peat soil cultivated with pineapple and (ii) determine the effects of water table depth on CH4 emission from a peat soil under simulated water table fluctuation. Soil CH4 emissions from an open field pineapple cultivation system and field lysimeters were determined using the closed chamber method. High-density polyethylene field lysimeters were set up to simulate the natural condition of cultivated drained peat soils under different water table fluctuations. The soil CH4 flux was measured at five time intervals to obtain a 24 h CH4 emission in the dry and wet seasons during low- and high-water tables. Soil CH4 emissions from open field pineapple cultivation were significantly lower compared with field lysimeters under simulated water table fluctuation. Soil CH4 emissions throughout the dry and wet seasons irrespective of water table fluctuation were not affected by soil temperature but emissions were influenced by the balance between methanogenic and methanotrophic microorganisms controlling CH4 production and consumption, CH4 transportation through molecular diffusion via peat pore spaces, and non-microbial CH4 production in peat soils. Findings from the study suggest that water table fluctuation at the soil–water interface relatively controls the soil CH4 emission from lysimeters under simulated low- and high-water table fluctuation. The findings of this study provide an understanding of the effects of water table fluctuation on CH4 emission in a tropical peatland cultivated with pineapple.

Analyzing Trends in Water Table Elevations at the Marcell Experimental Forest, Minnesota, U.S.A.

Water table fluctuations in peatlands are closely coupled with the local climate setting and drive critical ecosystem processes such as nutrient cycling. In Minnesota, USA, peatlands cover ten percent of the surface area, approximately 2.5 million hectares, some of which are actively managed for forest products. To explore the relationship between peatland water tables and precipitation, long-term data (1961 to 2019) were used from the Marcell Experimental Forest in northern Minnesota. Starting in 1961, water table data from seven peatlands, including two types of peatlands (bogs and fens), were measured. We used the Theil-Sen estimator to test for monotonic trends in mean monthly water table elevations for individual peatlands and monthly precipitation. Water levels in bogs were both more variable and had mean water table elevations that were closer to the surface. Individual trends of water table elevations differed among peatlands. Water table elevations increased over time in three of the bogs studied and decreased over time in two of the bogs studied. Trends within fens were notably nonlinear across time. No significant linear trend was found for mean monthly precipitation between 1961 and 2019. These results highlight differences in peatlands types, local physiography, and the importance of understanding how changes in long-term dynamics coupled with changing current conditions will influence the effects of water table fluctuations on ecosystem services. The variability of water table elevations in bogs poses potential difficulties in modeling these ecosystems or creating adaptive management plans. KEYWORDS: Peatlands; Hydrology; Water tables; Bogs; Fens; Monitoring; Minnesota; Climate Change

Impacts of water table fluctuations on actual and perceived natural source zone depletion rates

A viable means of quantifying the rate of natural source zone depletion (NSZD) at hydrocarbon contaminated sites is by the measurement of carbon dioxide (CO2) and methane (CH4) effluxes at the surface. This methodology assumes that gas effluxes are reflective of actual contaminant degradation rates in the subsurface, which is only accurate for quasi-steady state conditions. However, in reality, subsurface systems are highly dynamic, often resulting in fluctuations of the water table. To quantify the effects of water table fluctuations on NSZD rates, a simulated biodiesel spill in a 400 cm long, 100 cm wide and 150 cm tall sandtank was subjected to lowering and raising the water table, while soil-gas chemistry and surface CO2 and CH4 effluxes were measured. Results show that water table fluctuations have both short-term (perceived) and long-term (actual) effects on NSZD rates, interpreted using surface efflux measurements. When the water table was lowered, surface effluxes immediately increased up to 3 and 344 times higher than baseline for CO2 and CH4 effluxes, respectively, due to the liberation of anaerobically produced gas accumulated below the water table. After this immediate release, the system then reached quasi-steady state conditions 1.4 to 1.6 times higher for CO2 than baseline conditions, attributed to increased aerobic degradation in the broadened and exposed smear zone. When the water table was raised, quasi-steady state CO2 and CH4 effluxes declined to values of 0.9 and 0.4 times baseline effluxes, respectively, implying that contaminant degradation rates were reduced due to submergence of the smear zone. The findings of this study show that the dynamic effects of water table fluctuations and redistribution of the contaminants affect surface effluxes as well as short-term (perceived) and long-term (actual) contaminant degradation rates. Therefore, water table fluctuations need to be considered when quantifying NSZD at contaminated sites using sparse temporal rate measurements to estimate NSZD rates for extended periods of time (e.g., annual rates).

Effects of water table fluctuation on diesel fuel migration in one-dimensional laboratory study

The effects of water table fluctuation on diesel fuel redistribution in unsaturated soil are investigated in the laboratory by one-dimensional column test. The simplified image analysis method is used to assess the saturation distributions of water and diesel in the flow domain under transient condition. The experiments are undertaken on two homogeneous sands to study the effects of different particle sizes. The two-phase experimental data illustrate soil–liquid characteristic curves (SLCCs) of water/air and diesel/air systems. The SLCCs of diesel/water systems can be predicted by scaling procedure. A descending order of matric suction at a given saturation of wetting phase is in the order of air/water, diesel/water and air/diesel systems. Coarser particle size gives smaller entry pressure, residual matric suction and residual degree of saturation. The three-phase (air/water/diesel) experimental data illustrate that, during imbibition stages, air and diesel are entrapped below water table with the entrapped air saturations of 18–24% and entrapped diesel saturations of 6–10% for fine sand. Coarser particle size yields smaller entrapped diesel saturations and larger entrapped air saturations. The residual diesel saturations during drainage stages and entrapped diesel saturations during imbibition stages do not show any systematic difference as these stages are repeated. During imbibition stages, 37–47% of the total amount of diesel fuel is entrapped under the water table. Also, there is no systematic difference in the amount of diesel fuel entrapped under the water table when imbibition stages are repeated.

Effects of groundwater level variations on the nitrate content of groundwater: a case study in Luoyang area, China

Most researchers usually adopt laboratory experimental methods when studying the effects of water level variations on the concentrations of pollutants. In this study, the data from routine monitoring sites in the city of Luoyang, China, are collected and analyzed to verify the results of previous laboratory experiments and to examine whether variations in the water level affect the concentration of pollutants in different locations, particularly that of nitrates. Statistical studies conducted between 2007 and 2011 show a significant variation in the groundwater depth in the Luoyang area. This depth variation clearly affected the groundwater environment in the soil system. This study uses field data to investigate the effects of water table fluctuation on the nitrate content of groundwater. Hydrogeological information and land management data are collected from five monitoring points in Luoyang. The significance of and correlation between the environmental indicators of groundwater depth and soil–water systems are then analyzed using SPSS and Origin software. The results show that the redox potential (Eh) and nitrate nitrogen content are strongly correlated with groundwater depth. Significantly negative correlations were found between nitrate nitrogen and ammonium nitrogen concentration, between Ehand ammonium nitrogen concentration, and between pH and nitrate nitrogen concentration. These results indicate that water table fluctuation affects the soil–water physicochemical properties and further exerts a significant effect on nitrate movement across soil sola.

Effect of water table fluctuations on phreatophytic root distribution

The vertical root distribution of riparian vegetation plays a relevant role in soil water balance, in the partition of water fluxes into evaporation and transpiration, in the biogeochemistry of hyporheic corridors, in river morphodynamics evolution, and in bioengineering applications. The aim of this work is to assess the effect of the stochastic variability of the river level on the root distribution of phreatophytic plants. A function describing the vertical root profile has been analytically obtained by coupling a white shot noise representation of the river level variability to a description of the dynamics of root growth and decay. The root profile depends on easily determined parameters, linked to stream dynamics, vegetation and soil characteristics. The riparian vegetation of a river characterized by a high variability turns out to have a rooting system spread over larger depths, but with shallower mean root depths. In contrast, a lower river variability determines root profiles with higher mean root depths.

Water Table Correction Factors for Settlements of Shallow Foundations in Granular Soils

Shallow foundations are designed to limit settlements within tolerable limits. Additional settlements produced by the rise of the water table due to rain or floods can jeopardize the integrity of the foundation. This paper proposes a rational method, based on strain influence factors, to predict the additional settlement produced by the rise of the water table on a footing resting on sands. The proposed method was validated using extensive laboratory test data, where model footings of five different shapes were loaded in sand placed at two relative densities, and the water level was raised from the bottom while the additional settlements were measured. The clean sands used in the tests were such that there were negligible capillary effects. Separate tests were carried out to demonstrate that the capillary effects were insignificant. The proposed method can be used as the basis for further studies that incorporate the effect of grain size distribution or the effects of water table fluctuations on additional settlement.

Effect of water-table fluctuations on the degradation of Sphagnum phenols in surficial peats

A much improved understanding of how water-table fluctuations near the surface affect decomposition and preservation of peat-forming plant litter and surficial peats is needed in order to predict possible feedbacks between the peatland carbon cycle and the global climate system. In this study peatland plants (bryophytes and vascular plants), their litter and peat cores were collected from the Ryggmossen peatland in the boreonemoral zone of central Sweden. The extracted insoluble residues from whole plant tissues were depolymerized using thermally assisted hydrolysis and methylation (THM) in the presence of both unlabelled and 13C-labelled tetramethylammonium hydroxide (TMAH) which yielded both vascular plant- and Sphagnum-derived phenols. Methylated 4-isopropenylphenol (IUPAC: 1-methoxy-4-(prop-1-en-2-yl)benzene), methylated cis- and trans-3-(4′-hydroxyphen-1-yl)but-2-enoic acid (IUPAC: (E/Z)-methyl 3-(4-methoxyphenyl)but-2-enoate), and methylated 3-(4′-hydroxyphen-1-yl)but-3-enoic acid (IUPAC: methyl 3-(4-methoxyphenyl)but-3-enoate) (van der Heijden et al., 1997) are confirmed as TMAH thermochemolysis products of “bound” sphagnum acid and also as being specific to Sphagnum mosses. These putative biomarkers were also significant components in the unlabelled TMAH thermochemolysis products from the depolymerization of ultrasonically extracted samples from eight peat cores, one from a hummock and one from a hollow at each of the four stages along the bog plateau-to-swamp forest gradient. We have proposed and measured two parameters namely (i) σ which is defined as the total amount of these four molecules normalised to 100mg of OC; and (ii) an index (SR%) which is the ratio of σ to the Λ parameter giving a measure of the relative amounts of “bound” sphagnum acid to the “bound” vascular plant phenols in peat moss and the surficial peat layers. Changes in σ and SR% down the bog plateau (BP), bog margin (BM) and fen lagg (FL) cores in the Ryggmossen mire indicates that the sphagnum acid bound into the peat is being degraded in the unsaturated and seasonally-saturated layers. There is then a stabilisation of Sphagnum-derived phenols in the deepest horizons of the seasonally-saturated layer and into the permanently-saturated layer. These results suggest that “bound” sphagnum acid will be stabilised in peatlands shifting to a wetter and more variable precipitation regime whereas it will be gradually stripped away (e.g. by hydrolysis/enzymatic activity) in surficial peats shifting to a drier climate, such that any subsequent rewetting of the peat could lead to anaerobic hydrolysis and fermentation of the newly exposed carbohydrates. This highlights the sensitivity of Sphagnum surficial peats to climate-induced changes in water levels albeit there may be differences in the extent of degradation along the bog-fen gradient.

Effects of streamflow variability on the vertical root density distribution of willow cutting experiments

The use of riparian tree cuttings for river rehabilitation relies on a good understanding of cutting survival and growth responses to environmental variables. A series of experiments aimed at studying the effects of water table fluctuation induced by streamflow variability on the development of below-ground biomass of riparian tree cuttings was performed on the Thur River (Switzerland). White willow (Salix alba) cuttings were installed in gravel bar plots at various locations to develop an understanding of how topography and flow variability control cutting root development strategies, which in turn can lead to observed different vertical root density distributions in response to varying forms of tropism. In particular, we show that the transient root growth dynamics, eventually create a distribution reflecting the hydrologic conditions experienced towards the end of the growing season. A scaling relationship is presented that is useful to predict the expected depth with the highest root density as a function of soil topography and river discharge statistics. The results have implications for predicting cutting survival and growth, along with a corresponding strengthening of the alluvial sediment, in different flow regimes. This approach is therefore useful to incorporate in morphodynamic models covering the ecological and hydraulics responses of below-ground biomass to floods.

Buried organic-rich horizons: their role as nitrogen sources in stream riparian zones

The effect of water table fluctuations on the interaction between nitrogen mineralization in buried organic-rich soil horizons and nitrate mobilization into groundwater was examined in the riparian zone of a small headwater agricultural catchment in southern Ontario, Canada. Riparian soils contained buried organic layers and lenses overlying a poorly sorted gravel layer at a depth of 0.8–1.0 m. The riparian water table in summer 2006 generally remained within 0.4 m of the surface, whereas during a major drought in 2007 the water table declined to >1.9 m in August and riparian soils above the gravel layer inland from the stream remained unsaturated from mid-June to early November. Mean daily net N mineralization and nitrification rates during 2–4 week in situ soil incubations were 0.2–1.35 mg N kg−1 day−1 in 0–10 cm surface soils in May–September 2006, whereas N mineralization and nitrification were negligible at 30–45 and 60–75 cm soil depths. In summer and fall 2007, high daily rates of N mineralization and nitrification of 0.3–0.8 mg N kg−1 day−1 occurred at 30–45 and 60–75 cm depths that were similar to surface soil rates. The soil nitrate pool at 60–75 cm depth was 16× larger in autumn 2007 in comparison to 2006. During high water tables in November 2006 groundwater in the gravel layer had low NO3 −–N concentrations of <0.1 mg l−1. In contrast, after the drought in 2007 nitrate was flushed into groundwater as the water table rose to within 30–50 cm of the surface in December. An extensive area of high NO3–N concentrations (3–18 mg l−1) occurred inland from the stream bank. This zone of high nitrates declined gradually by April 2008 probably as a result of denitrification. These results indicate that buried organic deposits at depth within riparian areas can be important nitrogen sources during major water table drawdowns. The influence of these episodes of mineralization at depth during droughts on riparian groundwater chemistry and the emissions of greenhouse gases to the atmosphere merit further research.

Fine-root growth, fine root mortality, and leaf morphological change of Populus alba in response to fluctuating water tables

Water table fluctuation in arid land regions may alter tree fine-root growth and mortality, thereby affecting leaf growth. To reveal the effects of water table fluctuation on fine-root growth and mortality and their relation to leaf growth, we exposed P. alba L. cuttings to various fluctuating water table depths. 1-year-old rooted cuttings were grown individually in pots containing sandy soil in a greenhouse in three water table depth treatments for 45 days: constant depth at 45 cm from the soil surface, fluctuating depths between 45 and 30 cm, and fluctuating depths between 45 and 15 cm. Fine-root biomass and mortality, biomass partitioning among plant parts, and whole-tree growth responses were determined in cuttings harvested every 15 days. Fluctuation of water tables increased the mortality of fine roots at the layers near the soil surface. Fine-root mortality increased during the shallower water table depth period. At the whole-root system level, although fine-root mortality increased when the water table was shallower, fine-root biomass was similar among the treatments, suggesting that P. alba cuttings would sustains its standing fine-root biomass under fluctuating water table depth conditions. Our structural equation modeling showed the fine-root proportion affects leaf morphological changes, suggesting that there would be a parallel relationship of morphological changes between roots and leaves with fluctuating water tables.

Field evaluation of the effects of water table fluctuations on soil radon gas emanations

Measurements of the depth of the water table and the concentration of soil gas radon at water wells in Virginia and Maryland show that at each well site, there is no correlation between the depths of the water table and the radon concentration. However, when comparing nearby water wells, there is a relationship between depth of the water table and the concentration of soil gas radon. Wells with a shallower water table tend to have less soil gas radon emanation. It may be that this relationship It may be that this relationship (higher water table with lower radon emanation) may explain seasonal changes in radon concentration, since changes in water table depth are caused by seasonal changes in precipitation.

Photosynthesis and stomatal conductance of Juncus effusus in a temperate wetland ecosystem

Net photosynthesis and stomatal conductance of the emergent macrophyte Juncus effusus L. was measured in six plots in the Talladega Wetland Ecosystem (Hale County, AL) over diel and annual periods to evaluate the effects of water table fluctuations on photosynthesis and water losses. Half of the plots were in the areas where sediments remained saturated annually (stable water table) and the remaining plots were in the areas where water table heights fluctuated and sediments (upper 20 cm) did not remain continuously saturated. Annual patterns of average net photosynthesis were similar among all sample plots at the wetland ( p > 0.05; repeated measures ANOVA), which suggested that no differences in net photosynthesis resulted from the observed differences in sediment saturation. A marked seasonal pattern of net photosynthesis occurred with the greatest average photosynthesis during winter, summer (1997), and spring (11.0, 10.2, and 9.03 μmol CO 2 m −2 s −1, respectively) compared to autumn and summer (1996) (6.64 and 5.93 μmol CO 2 m −2 s −1, respectively). Average apparent photosynthetic efficiency of J. effusus was 1.43% and varied seasonally with the highest values in winter (1.86%) followed by spring (1.69%), autumn (1.34%), and summer (0.80 and 1.13%; 1996 and 1997, respectively). Although significant differences ( p < 0.05) in stomatal conductance occurred among sample plots within the wetland, no correlation was found between stomatal conductance and sample plot location or sediment saturation within the wetland. Seasonally, greater transpiration by J. effusus occurred during the summer (1996) and autumn months (0.43 and 0.28 mol H 2O m −2 s −1, respectively) compared to winter, spring, and summer (1997) (0.22, 0.20, and 0.16 mol H 2O m −2 s −1, respectively). Seasonal measurements of diel photosynthesis and stomatal conductance showed that daily net photosynthesis was variable among sample plots and among the seasons at the wetland, with maximum photosynthesis from 1000 to 1400 hours. Diel measurements of stomatal conductance showed significantly higher transpirative water loss during evening hours compared to high light periods.

Effects of a fluctuating water table: column study on redox dynamics and fate of some organic pollutants

The development of the redox conditions has been studied in an initially aerobic column filled with quartz sand coated with ferrihydrite and subjected to a fluctuating water table. The purpose of this study was to evaluate the effect of water table fluctuations on the redox dynamics and the fate of selected organic pollutants. The column that was percolated continuously with electron acceptors (O 2, NO 3, SO 4) and electron donors (acetate and formate), was first operated under saturated conditions resulting in the classical redox zonation. After 4 months of operation, we started to fluctuate the water level and three drainage–imbibition cycles were run each with a total cycle length of 1 month. The pulse of oxygen introduced by lowering the water table caused a partial and temporal oxidation of previously reduced species. To investigate the effect of the changing redox environment on the transport and transformation of organic pollutants, breakthrough experiments were performed with 4-nitrobenzoate and toluene as model pollutants representative for nitro-substituted and volatile aromatics, respectively. The fate of 4-nitrobenzoate and toluene was studied under saturated conditions in short pulse breakthrough experiments and evaluated using the advection–dispersion model. 4-nitrobenzoate was transformed stoichiometrically into 4-aminobenzoate caused by the reduction of the nitrogroup. The transformation rate varied with depth and with time, dropping from 15.3 nmol g −1 h −1 after the first drainage–imbibition cycle to 1.5 nmol g −1 h −1 after 4 additional months of operation. Toluene was not degraded during the first breakthrough experiment and showed a retardation factor of 2.06 which was ascribed to diffusion into entrapped air, originating from drainage–imbibition cycles, and to sorption to biomass. After the 24-h pre-exposure to toluene, adaptation had occurred and in later experiments toluene was degraded within the first 6 cm. These data show that in an experiment that was well-described in terms of water flow, gas flow, and initial mineral phase composition, the microbial processes induced a chemical and physical heterogeneity. An additional heterogeneity in space and time was introduced by the fluctuating water table. The `history' of the column had consequences for the fate of organic pollutants and resulted in an unpredictable behaviour with respect to their transformation, transport and degradation.

Regulation of methane oxidation in a freshwater wetland by water table changes and anoxia

The effects of water table fluctuations and anoxia on methane emission and methane oxidation were studied in a freshwater marsh. Seasonal aerobic methane oxidation rates varied between 15% and 76% of the potential diffusive methane flux (diffusive flux in the absence of aerobic oxidation). On an annual basis, approximately 43% of the methane diffusing into the oxic zone was oxidized before reaching the atmosphere. The highest methane oxidation was observed when the water table was below the peat surface. This was confirmed in laboratory experiments where short-term decreases in water table levels increased methane oxidation but also net methane emission. Although methane emission was generally not observed during the winter, stems of soft rush ( Juncus effusus) emitted methane when the marsh was ice covered. Indigenous methanotrophic bacteria from the wetland studied were relatively anoxia tolerant. Surface peat incubated under anoxic conditions maintained 30% of the initial methane oxidation capacity after 32 days of anoxia. Methanotrophs from anoxic peat initiated aerobic methane oxidation relatively quickly after oxygen addition (1–7 hours). These results were supported by culture experiments with the methanotroph Methylosinus trichosporium OB3b. This organism maintained a greater capacity for aerobic methane oxidation when starved under anoxic compared to oxic conditions. Anoxic incubation of M. trichosporium OB3b in the presence ofsulfide (2 mM) and a low redox potential (− 110 mV) did not decrease the capacity for methane oxidation relative to anoxic cultures incubated without sulfide. The results suggest that aerobic methane oxidation was a major regulator of seasonal methane emission from the investigated wetland. The observed water table fluctuations affected net methane oxidation presumably due to associated changes in oxygen gradients. However, changes from oxic to anoxic conditions in situ had relatively little effect on survival of the methanotrophic bacteria and thus on methane oxidation potential per se.

Regulation of methane oxidation in a freshwater wetland by water table changes and anoxia

The effects of water table fluctuations and anoxia on methane emission and methane oxidation were studied in a freshwater marsh. Seasonal aerobic methane oxidation rates varied between 15% and 76% of the potential diffusive methane flux (diffusive flux in the absence of aerobic oxidation). On an annual basis, approximately 43% of the methane diffusing into the oxic zone was oxidized before reaching the atmosphere. The highest methane oxidation was observed when the water table was below the peat surface. This was confirmed in laboratory experiments where short-term decreases in water table levels increased methane oxidation but also net methane emission. Although methane emission was generally not observed during the winter, stems of soft rush (Juncus effusus) emitted methane when the marsh was ice covered. Indigenous methanotrophic bacteria from the wetland studied were relatively anoxia tolerant. Surface peat incubated under anoxic conditions maintained 30% of the initial methane oxidation capacity after 32 days of anoxia. Methanotrophs from anoxic peat initiated aerobic methane oxidation relatively quickly after oxygen addition (1–7 hours). These results were supported by culture experiments with the methanotroph Methylosinus trichosporium OB3b. This organism maintained a greater capacity for aerobic methane oxidation when starved under anoxic compared to oxic conditions. Anoxic incubation of M. trichosporium OB3b in the presence ofsulfide (2 mM) and a low redox potential (− 110 mV) did not decrease the capacity for methane oxidation relative to anoxic cultures incubated without sulfide. The results suggest that aerobic methane oxidation was a major regulator of seasonal methane emission from the investigated wetland. The observed water table fluctuations affected net methane oxidation presumably due to associated changes in oxygen gradients. However, changes from oxic to anoxic conditions in situ had relatively little effect on survival of the methanotrophic bacteria and thus on methane oxidation potential per se.