Models to determine first-order rate coefficients from single-well push-pull tests

A modified and rapid method for the single-well push-pull (SWPP) test using SF6, Kr, and uranine tracers

In the present study, a single-well push-pull (SWPP) test was conducted with multi-component tracers, including inert gas (SF₆ and Kr) and uranine (conservative), to understand the volatile/semi-volatile component transport characteristics in the groundwater system. In an SWPP test, it is essential to obtain an initial breakthrough curve (BTC) of the inert gas concentration at the beginning of the pulling stage to analyze the hydraulic properties of the groundwater system. As a result of the SWPP test using a proposed method in this study, physicochemical parameters of the groundwater and BTC of gas tracers and uranine were acquired simultaneously and successfully. In addition, on-site measurements of uranine, pCO₂, and water quality data, such as electrical conductivity (EC), temperature, pH, and dissolved oxygen, were undertaken. Modification of an existing pCO₂ measuring system allowed the gas samples to be collected, transported, and analyzed for inert gas components within a few hours. As a result, reliable and interpretable data with a recovery ratio of 26%, 85%, and 95% for SF_6, Kr, and uranine, respectively, were obtained. The differences in the recovery ratio were utilized to identify the environmental system, whether it contains gas inside the isolated system (closed) or not (open), and to understand plume behavior characteristics in the experimental zone. By applying a two-dimensional advection-dispersion model to the acquired tracer test data and comparing the observed and computed tracer concentrations, helpful information was obtained on the hydraulic and transport characteristics of the targeted zone. This method can be extended to the design of dissolved CO₂ transport monitoring of an aquifer above a CCS site.

Termite mounds contain soil-derived methanotroph communities kinetically adapted to elevated methane concentrations

Termite mounds have recently been confirmed to mitigate approximately half of termite methane (CH4) emissions, but the aerobic CH4 oxidising bacteria (methanotrophs) responsible for this consumption have not been resolved. Here, we describe the abundance, composition and CH4 oxidation kinetics of the methanotroph communities in the mounds of three distinct termite species sampled from Northern Australia. Results from three independent methods employed show that methanotrophs are rare members of microbial communities in termite mounds, with a comparable abundance but distinct composition to those of adjoining soil samples. Across all mounds, the most abundant and prevalent methane monooxygenase sequences were affiliated with upland soil cluster α (USCα), with sequences homologous to Methylocystis and tropical upland soil cluster (TUSC) also detected. The reconstruction of a metagenome-assembled genome of a mound USCα representative highlighted the metabolic capabilities of this group of methanotrophs. The apparent Michaelis-Menten kinetics of CH4 oxidation in mounds were estimated from in situ reaction rates. Methane affinities of the communities were in the low micromolar range, which is one to two orders of magnitude higher than those of upland soils, but significantly lower than those measured in soils with a large CH4 source such as landfill cover soils. The rate constant of CH4 oxidation, as well as the porosity of the mound material, were significantly positively correlated with the abundance of methanotroph communities of termite mounds. We conclude that termite-derived CH4 emissions have selected for distinct methanotroph communities that are kinetically adapted to elevated CH4 concentrations. However, factors other than substrate concentration appear to limit methanotroph abundance and hence these bacteria only partially mitigate termite-derived CH4 emissions. Our results also highlight the predominant role of USCα in an environment with elevated CH4 concentrations and suggest a higher functional diversity within this group than previously recognised.

Termite mounds mitigate half of termite methane emissions

Termites are responsible for ∼1 to 3% of global methane (CH₄) emissions. However, estimates of global termite CH₄ emissions span two orders of magnitude, suggesting that fundamental knowledge of CH₄ turnover processes in termite colonies is missing. In particular, there is little reliable information on the extent and location of microbial CH₄ oxidation in termite mounds. Here, we use a one-box model to unify three independent field methods-a gas-tracer test, an inhibitor approach, and a stable-isotope technique-and quantify CH₄ production, oxidation, and transport in three North Australian termite species with different feeding habits and mound architectures. We present systematic in situ evidence of widespread CH₄ oxidation in termite mounds, with 20 to 80% of termite-produced CH₄ being mitigated before emission to the atmosphere. Furthermore, closing the CH₄ mass balance in mounds allows us to estimate in situ termite biomass from CH₄ turnover, with mean biomass ranging between 22 and 86 g of termites per kilogram of mound for the three species. Field tests with excavated mounds show that the predominant location of CH₄ oxidation is either in the mound material or the soil beneath and is related to species-specific mound porosities. Regardless of termite species, however, our data and model suggest that the fraction of oxidized CH₄ (f _ox) remains well buffered due to links among consumption, oxidation, and transport processes via mound CH₄ concentration. The mean f _ox of 0.50 ± 0.11 (95% CI) from in situ measurements therefore presents a valid oxidation factor for future global estimates of termite CH₄ emissions.

Oxidation of Reduced Peat Particulate Organic Matter by Dissolved Oxygen: Quantification of Apparent Rate Constants in the Field

Peat particulate organic matter (POM) is an important terminal electron acceptor for anaerobic respiration in northern peatlands provided that the electron-accepting capacity of POM is periodically restored by oxidation with O₂ during peat oxygenation events. We employed push-pull tests with dissolved O₂ as reactant to determine pseudo-first-order rate constants of O₂ consumption ( k_obs) in anoxic peat soil of an unperturbed Swedish ombrotrophic bog. Dissolved O₂ was rapidly consumed in anoxic peat with a mean k_obs of 2.91 ± 0.60 h^-1, corresponding to an O₂ half-life of ∼14 min. POM dominated O₂ consumption, as evidenced from approximately 50-fold smaller k_obs in POM-free control tests. Inhibiting microbial activity with formaldehyde did not appreciably slow O₂ consumption, supporting abiotic O₂ reduction by POM moieties, not aerobic respiration, as the primary route of O₂ consumption. Peat preoxygenation with dissolved O₂ lowered k_obs in subsequent oxygen consumption tests, consistent with depletion of reduced moieties in POM. Finally, repeated oxygen consumption tests demonstrated that anoxic peat POM has a high reduction capacity, in excess to 20 μmol electrons donated per gram POM. This work demonstrates rapid abiotic oxidation of reduced POM by O₂, supporting that short-term oxygenation events can restore the capacity of POM to accept electrons from anaerobic respiration in temporarily anoxic parts of peatlands.

Field-scale tracking of active methane-oxidizing communities in a landfill cover soil reveals spatial and seasonal variability

Aerobic methane-oxidizing bacteria (MOB) in soils mitigate methane (CH4 ) emissions. We assessed spatial and seasonal differences in active MOB communities in a landfill cover soil characterized by highly variable environmental conditions. Field-based measurements of CH4 oxidation activity and stable-isotope probing of polar lipid-derived fatty acids (PLFA-SIP) were complemented by microarray analysis of pmoA genes and transcripts, linking diversity and function at the field scale. In situ CH4 oxidation rates varied between sites and were generally one order of magnitude lower in winter compared with summer. Results from PLFA-SIP and pmoA transcripts were largely congruent, revealing distinct spatial and seasonal clustering. Overall, active MOB communities were highly diverse. Type Ia MOB, specifically Methylomonas and Methylobacter, were key drivers for CH4 oxidation, particularly at a high-activity site. Type II MOB were mainly active at a site showing substantial fluctuations in CH4 loading and soil moisture content. Notably, Upland Soil Cluster-gamma-related pmoA transcripts were also detected, indicating concurrent oxidation of atmospheric CH4 . Spatial separation was less distinct in winter, with Methylobacter and uncultured MOB mediating CH4 oxidation. We propose that high diversity of active MOB communities in this soil is promoted by high variability in environmental conditions, facilitating substantial removal of CH4 generated in the waste body.

In situ aerobic cometabolism of chlorinated solvents: a review

The possible approaches for in situ aerobic cometabolism of aquifers and vadose zones contaminated by chlorinated solvents are critically evaluated. Bioaugmentation of resting-cells previously grown in a fermenter and in-well addition of oxygen and growth substrate appear to be the most promising approaches for aquifer bioremediation. Other solutions involving the sparging of air lead to satisfactory pollutant removals, but must be integrated by the extraction and subsequent treatment of vapors to avoid the dispersion of volatile chlorinated solvents in the atmosphere. Cometabolic bioventing is the only possible approach for the aerobic cometabolic bioremediation of the vadose zone. The examined studies indicate that in situ aerobic cometabolism leads to the biodegradation of a wide range of chlorinated solvents within remediation times that vary between 1 and 17 months. Numerous studies include a simulation of the experimental field data. The modeling of the process attained a high reliability, and represents a crucial tool for the elaboration of field data obtained in pilot tests and for the design of the full-scale systems. Further research is needed to attain higher concentrations of chlorinated solvent degrading microbes and more reliable cost estimates. Lastly, a procedure for the design of full-scale in situ aerobic cometabolic bioremediation processes is proposed.

Degradation of polar organic micropollutants during riverbank filtration: complementary results from spatiotemporal sampling and push-pull tests

The fate of polar organic micropollutants (logDOW (pH 7) between -4.2 and +3.5) during riverbank filtration (RBF) at the river Thur was studied using both spatiotemporally resolved sampling and single-well push-pull tests (PPT), followed by LC-MS/MS analysis. The Thur is a dynamic prealpine river with an alluvial sandy-gravel aquifer, which is characterized by short groundwater travel times (a few days) from surface water infiltration to groundwater extraction. The spatiotemporal sampling allowed tracing concentration dynamics in the river and the groundwater and revealed persistence for the drug carbamazepine, while the herbicide MCPA (2-methyl-4-chloro-phenoxyacetic acid) and the drug 4-acetamidoantipyrine were very quickly degraded under the prevalent aerobic conditions. The corrosion inhibitor 1H-benzotriazole was degraded slightly, particularly in a transect influenced by river restoration measures. For the first time in situ first-order degradation rate constants for three pesticides and two pharmaceuticals were determined by PPTs, which confirmed the results of the spatiotemporal sampling. Atenolol was transformed almost completely to atenolol acid. Rate constants of 0.1-1.3 h(-1) for MCPA, 2,4-D, mecoprop, atenolol, and diclofenac, corresponding to half-lives of 0.6-6.3 h, demonstrated the great potential of RBF systems to degrade organic micropollutants and simultaneously the applicability of PPTs for micropollutants in such dynamic systems.

A push-pull test to measure root uptake of volatile chemicals from wetland soils

This paper introduces a novel modification of the single-well "push-pull" test that uses nonvolatile and multiple volatile tracers to investigate the transport and root uptake kinetics of volatile chemicals in saturated soils. This technique provides an estimate of potential volatilization fluxes without relying on enclosure-based measurements. The new push-pull methodology was validated with mesocosm experiments, and bench-scale hydroponic measurements were performed to develop an empirical relationship for scaling root uptake rates between chemicals. A new modeling approach to interpret data using sulfur hexafluoride and helium as dual volatile tracers was developed and shown to decrease errors relative to existing analytical techniques that utilize bromide as a conservative tracer. Root uptake of the volatile tracers was diffusion-limited, and uptake rate constants (kv) in vegetated experimental mesocosms ranged from 0.021 ± 9.0 × 10(-4) h(-1) for CFC-12 to 2.41 ± 0.98 h(-1) for helium. Hydroponic and mesocosm experiments demonstrate that the molecular diameter is a robust empirical predictor of kv.

Application of the re-circulating tracer well test method to determine nitrate reaction rates in shallow unconfined aquifers

Five re-circulating tracer well tests (RCTWTs) have been conducted in a variety of aquifer settings, at four sites across New Zealand. The tests constitute the first practical assessment of the two-well RCTWT methodology described by Burbery and Wang (Journal of Hydrology, 2010; 382:163-173) and were aimed at evaluating nitrate reaction rates in situ. The performance of the RCTWTs differed significantly at the different sites. The RCTWT method performed well when it was applied to determine potential nitrate reaction rates in anoxic, electro-chemically reductive, nitrate-free aquifers of volcanic lithology, on the North Island, New Zealand. Regional groundwater flow was not fast-flowing in this setting. An effective first-order nitrate reaction rate in the region of 0.09 d(-1) to 0.26 d(-1) was determined from two RCTWTs applied at one site where a reaction rate of 0.37 d(-1) had previously been estimated from a push-pull test. The RCTWT method performed poorly, however, in a fast-flowing, nitrate-impacted fluvio-glacial gravel aquifer that was examined on the South Island, New Zealand. This setting was more akin to the hypothetical physiochemical problem described by Burbery and Wang (2010). Although aerobic conditions were identified as the primary reason for failure to measure any nitrate reaction in the gravel aquifer, failure to establish significant interflow in the re-circulation cell due to the heterogeneous nature of the aquifer structure, and natural variability exhibited in nitrate contaminant levels of the ambient groundwater further contributed to the poor performance of the test. Our findings suggest that in practice, environmental conditions are more complex than assumed by the RCTWT methodology, which compromises the practicability of the method as one for determining attenuation rates in groundwater based on tracing ambient contaminant levels. Although limited, there appears to be a scope for RCTWTs to provide useful information on potential attenuation rates when reactants are supplemented to the aquifer system under examination.

Above- and below-ground methane fluxes and methanotrophic activity in a landfill-cover soil

Landfills are a major anthropogenic source of the greenhouse gas methane (CH(4)). However, much of the CH(4) produced during the anaerobic degradation of organic waste is consumed by methanotrophic microorganisms during passage through the landfill-cover soil. On a section of a closed landfill near Liestal, Switzerland, we performed experiments to compare CH(4) fluxes obtained by different methods at or above the cover-soil surface with below-ground fluxes, and to link methanotrophic activity to estimates of CH(4) ingress (loading) from the waste body at selected locations. Fluxes of CH(4) into or out of the cover soil were quantified by eddy-covariance and static flux-chamber measurements. In addition, CH(4) concentrations at the soil surface were monitored using a field-portable FID detector. Near-surface CH(4) fluxes and CH(4) loading were estimated from soil-gas concentration profiles in conjunction with radon measurements, and gas push-pull tests (GPPTs) were performed to quantify rates of microbial CH(4) oxidation. Eddy-covariance measurements yielded by far the largest and probably most representative estimates of overall CH(4) emissions from the test section (daily mean up to ∼91,500μmolm(-2)d(-1)), whereas flux-chamber measurements and CH(4) concentration profiles indicated that at the majority of locations the cover soil was a net sink for atmospheric CH(4) (uptake up to -380μmolm(-2)d(-1)) during the experimental period. Methane concentration profiles also indicated strong variability in CH(4) loading over short distances in the cover soil, while potential methanotrophic activity derived from GPPTs was high (v(max)∼13mmolL(-1)(soil air)h(-1)) at a location with substantial CH(4) loading. Our results provide a basis to assess spatial and temporal variability of CH(4) dynamics in the complex terrain of a landfill-cover soil.

Structure and function of methanotrophic communities in a landfill-cover soil

In landfill-cover soils, aerobic methane-oxidizing bacteria (MOB) convert CH(4) to CO(2), mitigating emissions of the greenhouse gas CH(4) to the atmosphere. We investigated overall MOB community structure and assessed spatial differences in MOB diversity, abundance and activity in a Swiss landfill-cover soil. Molecular cloning, terminal restriction-fragment length polymorphism (T-RFLP) and quantitative PCR of pmoA genes were applied to soil collected from 16 locations at three different depths to study MOB community structure, diversity and abundance; MOB activity was measured in the field using gas push-pull tests. The MOB community was highly diverse but dominated by Type Ia MOB, with novel pmoA sequences present. Type II MOB were detected mainly in deeper soil with lower nutrient and higher CH(4) concentrations. Substantial differences in MOB community structure were observed between one high- and one low-activity location. MOB abundance was highly variable across the site [4.0 × 10(4) to 1.1 × 10(7) (g soil dry weight)(-1)]. Potential CH(4) oxidation rates were high [1.8-58.2 mmol CH(4) (L soil air)(-1) day(-1) ] but showed significant lateral variation and were positively correlated with mean CH(4) concentrations (P < 0.01), MOB abundance (P < 0.05) and MOB diversity (weak correlation, P < 0.17). Our findings indicate that Methylosarcina and closely related MOB are key players and that MOB abundance and community structure are driving factors in CH(4) oxidation at this landfill.

Use of gas push-pull tests for the measurement of methane oxidation in different landfill cover soils

In order to optimise methane oxidation in landfill cover soils, it is important to be able to accurately quantify the amount of methane oxidised. This research considers the gas push-pull test (GPPT) as a possible method to quantify oxidation rates in situ. During a GPPT, a gas mixture consisting of one or more reactive gases (e.g., CH(4), O(2)) and one or more conservative tracers (e.g., argon), is injected into the soil. Following this, the mixture of injected gas and soil air is extracted from the same location and periodically sampled. The kinetic parameters for the biological oxidation taking place in the soil can be derived from the differences in the breakthrough curves. The original method of Urmann et al. (2005) was optimised for application in landfill cover soils and modified to reduce the analytical effort required. Optimised parameters included the flow rate during the injection phase and the duration of the experiment. 50 GPPTs have been conducted at different landfills in Germany during different seasons. Generally, methane oxidation rates ranged between 0 and 150 g m(soil air)(-3)h(-1). At one location, rates up to 440 g m(soil air)(-3)h(-1) were measured under particularly favourable conditions. The method is simple in operation and does not require expensive equipment besides standard laboratory gas chromatographs.

Quantifying methane oxidation in a landfill-cover soil by gas push-pull tests

Methane (CH(4)) oxidation by aerobic methanotrophs in landfill-cover soils decreases emissions of landfill-produced CH(4) to the atmosphere. To quantify in situ rates of CH(4) oxidation we performed five gas push-pull tests (GPPTs) at each of two locations in the cover soil of the Lindenstock landfill (Liestal, Switzerland) over a 4 week period. GPPTs consist of the injection of a gas mixture containing CH(4), O(2) and noble gas tracers followed by extraction from the same location. Quantification of first-order rate constants was based upon comparison of breakthrough curves of CH(4) with either Ar or CH(4) itself from a subsequent inactive GPPT containing acetylene as an inhibitor of CH(4) oxidation. The maximum calculated first-order rate constant was 24.8+/-0.8 h(-1) at location 1 and 18.9+/-0.6 h(-1) at location 2. In general, location 2 had higher background CH(4) concentrations in vertical profile samples than location 1. High background CH(4) concentrations in the cover soil during some experiments adversely affected GPPT breakthrough curves and data interpretation. Real-time PCR verified the presence of a large population of methanotrophs at the two GPPT locations and comparison of stable carbon isotope fractionation of CH(4) in an active GPPT and a subsequent inactive GPPT confirmed that microbial activity was responsible for the CH(4) oxidation. The GPPT was shown to be a useful tool to reproducibly estimate in situ rates of CH(4) oxidation in a landfill-cover soil when background CH(4) concentrations were low.

Modeling axisymmetric flow and transport

Unmodified versions of common computer programs such as MODFLOW, MT3DMS, and SEAWAT that use Cartesian geometry can accurately simulate axially symmetric ground water flow and solute transport. Axisymmetric flow and transport are simulated by adjusting several input parameters to account for the increase in flow area with radial distance from the injection or extraction well. Logarithmic weighting of interblock transmissivity, a standard option in MODFLOW, can be used for axisymmetric models to represent the linear change in hydraulic conductance within a single finite-difference cell. Results from three test problems (ground water extraction, an aquifer push-pull test, and upconing of saline water into an extraction well) show good agreement with analytical solutions or with results from other numerical models designed specifically to simulate the axisymmetric geometry. Axisymmetric models are not commonly used but can offer an efficient alternative to full three-dimensional models, provided the assumption of axial symmetry can be justified. For the upconing problem, the axisymmetric model was more than 1000 times faster than an equivalent three-dimensional model. Computational gains with the axisymmetric models may be useful for quickly determining appropriate levels of grid resolution for three-dimensional models and for estimating aquifer parameters from field tests.

Transport of methane and noble gases during gas push-pull tests in variably saturated porous media

The gas push-pull test (GPPT) is a single-well gas-tracer method to quantify in situ rates of CH4 oxidation in soils. To improve the design and interpretation of GPPT field experiments, gas component transport during GPPTs was examined in abiotic porous media over a range of water saturations (0.0 < or = Sw < or = 0.61). A series of GPPTs using He, Ne, and Ar as tracers for CH4 were performed at two injection/extraction gas flow rates (approximately 200 and approximately 700 mL min(-1)) in a laboratory tank. Extraction phase breakthrough curves and mass recovery curves of the gaseous components became more similar at higher Sw as water in the pore space restricted diffusive gas-phase transport. Diffusional fractionation of the stable carbon isotopes of CH4 during the extraction period of GPPTs also decreased with increasing Sw (particularly when Sw > 0.42). Gas-component transport during GPPTs was numerically simulated using estimated hydraulic parameters for the porous media and no fitting of data for the GPPTs. Numerical simulations accurately predicted the relative decline of the gaseous components in the breakthrough curves, but slightly overestimated recoveries at low Sw (< or = 0.35) and underestimated recoveries at high Sw (> or = 0.49). Comparison of numerical simulations considering and not considering air-water partitioning indicated that removal of gaseous components through dissolution in pore water was not significant during GPPTs, even at Sw = 0.61. These data indicate that Ar is a good tracer for CH4 physical transport over the full range of Sw studied, whereas, at Sw > 0.61, any of the tracers could be used. Greater mass recovery at higher Sw raises the possibility to reduce gas flow rates, thereby extending GPPT times in environments such as tundra soils where low activity due to low temperatures may require longer test times to establish a quantifiable difference between reactant and tracer breakthrough curves.

Effects of stream restoration on denitrification in an urbanizing watershed

Increased delivery of nitrogen due to urbanization and stream ecosystem degradation is contributing to eutrophication in coastal regions of the eastern United States. We tested whether geomorphic restoration involving hydrologic "reconnection" of a stream to its floodplain could increase rates of denitrification at the riparian-zone-stream interface of an urban stream in Baltimore, Maryland. Rates of denitrification measured using in situ 15N tracer additions were spatially variable across sites and years and ranged from undetectable to >200 microg N x (kg sediment)(-1) x d(-1). Mean rates of denitrification were significantly greater in the restored reach of the stream at 77.4 +/- 12.6 microg N x kg(-1) x d(-1) (mean +/- SE) as compared to the unrestored reach at 34.8 +/- 8.0 microg N x kg(-1) x d(-1). Concentrations of nitrate-N in groundwater and stream water in the restored reach were also significantly lower than in the unrestored reach, but this may have also been associated with differences in sources and hydrologic flow paths. Riparian areas with low, hydrologically "connected" streambanks designed to promote flooding and dissipation of erosive force for storm water management had substantially higher rates of denitrification than restored high "nonconnected" banks and both unrestored low and high banks. Coupled measurements of hyporheic groundwater flow and in situ denitrification rates indicated that up to 1.16 mg NO3(-)-N could be removed per liter of groundwater flow through one cubic meter of sediment at the riparian-zone-stream interface over a mean residence time of 4.97 d in the unrestored reach, and estimates of mass removal of nitrate-N in the restored reach were also considerable. Mass removal of nitrate-N appeared to be strongly influenced by hydrologic residence time in unrestored and restored reaches. Our results suggest that stream restoration designed to "reconnect" stream channels with floodplains can increase denitrification rates, that there can be substantial variability in the efficacy of stream restoration designs, and that more work is necessary to elucidate which designs can be effective in conjunction with watershed strategies to reduce nitrate-N sources to streams.