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A Method to Infer In Situ Reaction Rates from Push‐Pull Experiments
Abstract
A method to evaluate first-order and zero-order in situ reaction rates from a push-pull test is presented. A single-well push-pull test starts with the rapid injection of a well-mixed slug containing a known quantity of a conservative tracer and a reactive solute into the saturated zone. The slug is then periodically extracted and sampled from the same well. For zero- or first-order reactions, in the absence of sorption and assuming negligible background concentrations, these measurements can be used to evaluate reaction rate coefficients directly. The method does not involve computer-based solute transport models and requires no knowledge of regional ground water flow or hydraulic parameters. The method performs well when the dominate processes are advection, dispersion, and zero- or first-order irreversible reactions. Regional flow velocities must be sufficiently low such that the slug stays within the area of the well during the sampling phase. In the case of zero-order reactions, results using the method proposed here are compared with those obtained through the traditional method of calibrating a computer-based transport model. The two methods give similar estimates of the reaction rate coefficient. The method is general enough to work with a broad range of push-pull experiment designs and sampling techniques.
... As for the analytical solutions of the SWPP test, they have been widely used for applications, due to the high efficiency and great accuracy of the solutions, like the model of Gelhar and Collins (1971) for a fully penetrating well, the model of Schroth and Istok (2005) for a point source/sink well, and the model of Huang et al. (2010) for a partially penetrating well, assuming that the advection, the dispersion and the first-order reaction were involved in the transport processes. Haggerty et al. (1998) and Snodgrass and Kitanidis (1998) presented a simplified method based on a well-mixed reactor to estimate the first-order and zero-order reaction rate, without involving complex numerical modeling. Schroth and Istok (2006) provided two alternative models: one of them was a plug-flow model and the other was a variably mixed reactor model. ...
... Schroth et al. (2000) presented a simplified method for estimating retardation factors, based on the model of Gelhar and Collins (1971). Istok et al. (2001) extended the models of Haggerty et al. (1998) and Snodgrass and Kitanidis (1998) to estimate the Michaelis-Menten kinetic parameters which were used to describe the microbial respiration in the aquifer. Jung and Pruess (2012) presented a closed-form analytical solution for heat transport in a fractured aquifer involving a push-and-pull procedure. ...
... For the purpose of simplicity, we only present the reactive processes of the chemicals as described by Eq. (1), while the expressions of the transport (e.g., dispersion, diffusion, and advection) could be seen in Phanikumar and McGuire (2010), who used it to describe biogeochemical reactive transport of an arbitrary number of species including Monod/Michaelis-Menten kinetics, and the sorption models could be isotherm (Freundlich, Langmuir and linear sorption), one-site kinetic and two-site kinetic. As for the studies of Gelhar and Collins (1971), Schroth and Istok (2005), Huang et al. (2010), Haggerty et al. (1998), Snodgrass and Kitanidis (1998), Schroth and Istok (2006), Schroth et al. (2000), Istok et al. (2001), Jung and Pruess (2012), Wang et al. (2017), and so on, the governing equation is a special case of Eq. (1). ...
Using the single-well
push–pull (SWPP) test to determine the in situ biogeochemical reaction
kinetics, a chase phase and a rest phase were recommended to increase the
duration of reaction, besides the injection and extraction phases. In this
study, we presented multi-species reactive models of the four-phase SWPP test
considering the wellbore storages for both groundwater flow and solute
transport and a finite aquifer hydraulic diffusivity, which were ignored in
previous studies. The models of the wellbore storage for solute transport
were proposed based on the mass balance, and the sensitivity analysis and
uniqueness analysis were employed to investigate the assumptions used in
previous studies on the parameter estimation. The results showed that
ignoring it might produce great errors in the SWPP test. In the injection and
chase phases, the influence of the wellbore storage increased with the
decreasing aquifer hydraulic diffusivity. The peak values of the breakthrough
curves (BTCs) increased with the increasing aquifer hydraulic diffusivity in
the extraction phase, and the arrival time of the peak value became shorter
with a greater aquifer hydraulic diffusivity. Meanwhile, the Robin condition
performed well at the rest phase only when the chase concentration was zero
and the solute in the injection phase was completely flushed out of the
borehole into the aquifer. The Danckwerts condition was better than the Robin
condition even when the chase concentration was not zero. The reaction
parameters could be determined by directly best fitting the observed data
when the nonlinear reactions were described by piece-wise linear functions,
while such an approach might not work if one attempted to use nonlinear
functions to describe such nonlinear reactions. The field application
demonstrated that the new model of this study performed well in interpreting
BTCs of a SWPP test.
... Single-well push-pull (SWPP) test is a popular technique to characterize the in situ geological formations and the biogeochemical reaction kinetics of the aquifer (Istok, 2012;Phanikumar and McGuire, 2010;Schroth and Istok, 2006), through which the parameters of concern could be estimated by best fitting the observed breakthrough curves (BTCs) with analytical/numerical models in the well (Snodgrass and Kitanidis, 1998;Haggerty et al., 1998). As a result, the accuracy of 5 the parameter estimation is not only dependent on the experimental operation, but also on the conceptual model which is expected to represent the proper physical and biogeochemical processes. ...
... As for the analytical solutions of the SWPP test, they have been widely used for applications, due to the high efficiency 10 and great accuracy of the solutions, e.g. the model of Gelhar and Collins (1971) for a fully penetrating well, the model of Schroth and Istok (2005) for a point source/sink well, the model of Huang et al. (2010) for a partially penetrating well, assuming that advection, dispersion and the first-order reaction were involved in the transport processes. Haggerty et al. (1998) and Snodgrass and Kitanidis (1998) presented a simplified method based on a well-mixed reactor to estimate the firstorder and zero-order reaction rate coefficients, without involving complex numerical modeling. Schroth and Istok (2006) 15 provided two alternative models, one of them was a plug-flow model and the other was a variably mixed reactor model. ...
... Schroth et al. (2000) presented a simplified method for estimating retardation factors, based on the model of Gelhar and Collins (1971). Istok et al. (2001) extended the models of Haggerty et al. (1998) and Snodgrass and Kitanidis (1998) to estimate the Michaelis-Menten kinetic parameters which were used to describe the microbial respiration in the aquifer. ...
Using the single-well push-pull (SWPP) test to determine the in situ biogeochemical reaction kinetics, a chase phase and a rest phase were recommended to increase the duration of reaction, besides the injection and extraction phases. In this study, we presented multi-species reactive models of the four-phase SWPP test considering the wellbore storages for both groundwater flow and solute transport and a finite aquifer hydraulic diffusivity, including three isotherm-based models (Freundlich, Langmuir and linear sorption models), one-site kinetic sorption model, two-site sorption model, which were also capable of describing the biogeochemical reactive transport processes, e.g. Monod or Michaelis-Menten kinetics. The models of the wellbore storage for solute transport were derived based on the mass balance, and the results showed that ignoring it could produce great errors in the SWPP test. In the injection and chase phases, the influence of the wellbore storage increased with the decreasing aquifer hydraulic diffusivity. The peak values of the breakthrough curves (BTCs) increased with the increasing aquifer hydraulic diffusivity in the extraction phase, and the arrival time of the peak value became shorter with a greater aquifer hydraulic diffusivity. Meanwhile, the Robin condition performed well at the rest phase only when the chase concentration was zero and the solute in the injection phase was completely flushed out of the borehole into the aquifer. The Danckwerts condition was better than the Robin condition even when the chase concentration was not zero. The reaction parameters could be determined by directly best fitting the observed data when the non-linear reactions were described by piece-wise linear functions, while such an approach might not work if one attempted to use non-linear functions to describe such non-linear reactions. The field application demonstrated that the new model of this study performed well in interpreting BTCs of a SWPP test.
... SF 6 and He were chosen because they are biochemically conservative and nonsorbing in organic sediments [Wilson and Mackay, 1996;Busenberg and Plummer, 2008]. Due to their different sizes ( Bromide (Br -) was used as a nonreactive and nonvolatile tracer to account for mixing between the injected tracer solution and ambient pore water [Istok et al., 1997;Haggerty et al., 1998;Snodgrass and Kitanidis, 1998]. The effects of infiltration, leaching, pore water advection, and other hydrodynamic transport processes on reactive tracer concentrations are accounted for with Br --normalization: ...
... In Br --normalization PPTs, hydrodynamic mixing is accounted for [Haggerty et al., 1998;Snodgrass and Kitanidis, 1998], and in the case of inert gas tracers the biochemical source/sink term k rxn is zero. Equations (1) and (3) thus simplify to the first-order kinetic equation: ...
... The rate constant k i v describes the removal of dissolved gases from pore water via mass transfer to root aerenchyma. While Br --normalization allows advection-dispersion effects on the tracer plume to be neglected, pore water advection, m, must be sufficiently slow such that the tracer plume remains within the vicinity of the injection/extraction well point [Snodgrass and Kitanidis, 1998]. k i v is determined by fitting the following model to the log-transformed, dilution-adjusted volatile tracer concentrations C 0 i (Figure 1c) [Haggerty et al., 1998]: ...
Gas transfer processes are fundamental to the biogeochemical and water quality functions of wetlands, yet there is limited knowledge of the rates and pathways of soil - atmosphere exchange for gases other than oxygen and methane (CH4). In this study we use a novel push-pull technique with sulfur hexafluoride (SF6) and helium (He) as dissolved gas tracers to quantify the kinetics of root-mediated gas transfer, which is a critical efflux pathway for gases from wetland soils. This tracer approach disentangles the effects of physical transport from simultaneous reaction in saturated, vegetated wetland soils. We measured significant seasonal variation in first-order gas exchange rate constants, with smaller spatial variations between different soil depths and vegetation zones in a New Jersey tidal marsh. Gas transfer rates for most biogeochemical trace gases are expected to be bracketed by the rate constants for SF6 and He, which ranged from ∼10−2 to 2x10−1 h−1 at our site. A modified Damköhler number analysis is used to evaluate the balance between biochemical reaction and root-driven gas exchange in governing the fate of environmental trace gases in rooted, anaerobic soils. This approach confirmed the importance of plant gas transport for CH4, and showed that root-driven transport may affect nitrous oxide (N2O) balances in settings where N2O reduction rates are slow This article is protected by copyright. All rights reserved.
... Single-well push-pull tests (PPTs) have been used to determine in situ physical, geochemical, and microbiological properties of aquifers (Drever and Mckee, 1980;Istok et al., 1997;Reinhard et al., 1997;Haggerty et al., 1998;Snodgrass and Kitanidis, 1998;Field et al., 1999;Istok et al., 1999;Schroth et al., 2001;Davis et al., 2002;Hageman et al., 2003;Schurmann et al., 2003;Istok et al., 2004;Kim et al., 2004;Cassiani et al., 2005;Davis et al., 2005;Urmann et al., 2005;Gonzalez-Gil et al., 2006;Kim et al., 2006;and Schroth and Istok, 2006). A PPT consists of the controlled injection of a prepared test solution into an aquifer, followed by the extraction of the test solution/groundwater mixture from the same location. ...
... The test solution usually consists of a conservative tracer (hereafter called tracer) and one or more reactive solutes selected to investigate specific processes of interest. Haggerty and others (1998) and Snodgrass and Kitanidis (1998) derived ''simplified'' methods for analyzing PPT data in terms of the zeroth-or first-order reaction rates by using the breakthrough curves of tracer and reactive solutes without knowing (or assuming) aquifer properties such as porosity, permeability, dispersivity, and hydraulic conductivity. The methods perform well if 1) dominant processes are advection, dispersion, and zeroth-or first-order reactions for which the reaction rates are constant in both time and space within the tested portion of the aquifer; 2) sorption is negligible; 3) there is complete and instantaneous mixing of injected test solution in the portion of the aquifer invested by the test (i.e., the system is treated like a well-mixed reactor); and 4) background concentrations of reactive solutes and tracer are zero or negligible. ...
... The methods perform well if 1) dominant processes are advection, dispersion, and zeroth-or first-order reactions for which the reaction rates are constant in both time and space within the tested portion of the aquifer; 2) sorption is negligible; 3) there is complete and instantaneous mixing of injected test solution in the portion of the aquifer invested by the test (i.e., the system is treated like a well-mixed reactor); and 4) background concentrations of reactive solutes and tracer are zero or negligible. In addition, the method of Snodgrass and Kitanidis (1998) assumes also that the injection time is short compared to the total time of the experiment. Snodgrass and Kitanidis (1998) and Haggerty and others (1998) discussed that the heterogeneity of the aquifer may lead to inaccuracy in the estimated kinetic rates. ...
This paper presents a general method and the mathematical equations for estimating in situ reaction rates from the tracer and reactive solute breakthrough curves obtained from single-well push-pull tests (PPTs).
The in situ zeroth-order reaction rates can be obtained through the linear regression of the net mass transfer of reactive solutes versus
time. The method was first tested for scenarios of various concentrations of reactive constituents and conservative tracer
in background and injection solution using a numerical reactive transport model and was then applied to a set of field biostimulation
data from PPTs performed at the U.S. Department of Energy's Natural and Accelerated Bioremediation Research Program's Field
Research Center (Oak Ridge, TN). The results show that the method is general and can be applied to each of the scenarios as
long as the concentrations in background water are known. While the method presents practitioners with a simplified and economic
tool for a first approximation analysis of in situ reaction rates from PPT data, the derived reaction orders and rates are apparent and bulk properties by nature, masking the
complexity of competing reactions regarding the reactive solute of interest.
... The push-pull test is a powerful site characterization method and has been applied in a wide range of hydrological settings including saturated and unsaturated soils, sediments and surface water bodies (Istok 2013). Push-pull tests are particularly useful for estimating reaction rates of solutes (Haggerty et al. 1998;Snodgrass and Kitanidis 1998). In a groundwater setting, a push-pull test is conducted by injecting a volume of water containing one or more nonreactive and reactive solutes into a single well (push phase), followed by a nonpumping period (drift phase), and subsequent extraction of groundwater from the same well (pull phase). ...
... Suppose, further that the dilution of the injection fluid and the reaction of ethanol obey first-order kinetics, with rate constants of k d and k r , respectively (Table 1). Dilution and reaction rates are typically modeled using zero-, first-, or Michaelis-Menton kinetics (Istok et al. 1997;Haggerty et al. 1998;Snodgrass and Kitanidis 1998;Hageman et al. 2003;Yang et al. 2007). For example 1, the dilution rates of bromide and ethanol are equal (−0.2/h) but ethanol has a reaction rate (−0.2/h) (Table 1). ...
The breakthrough curve obtained from a single-well push-pull test can be adjusted to account for dilution of the injection fluid in the aquifer fluid. The dilution-adjusted breakthrough curve can be analyzed to estimate the reaction rate of a solute. The conventional dilution-adjusted method assumes that the ratio of the concentrations of the non-reactive and reactive solutes in the injection fluid versus the aquifer fluid are equal. If this assumption is invalid, the conventional method will generate inaccurate breakthrough curves and may lead to erroneous conclusions regarding the reactivity of a solute. In this study, a new method that generates a dilution-adjusted breakthrough curve was theoretically developed to account for any possible combination of non-reactive and reactive solute concentrations in the injection and aquifer fluids. The newly developed method was applied to a field-based data set and was shown to generate more accurate dilution-adjusted breakthrough curves. The improved dilution-adjusted method presented here is simple, makes no assumptions regarding the concentrations of the non-reactive and reactive solutes in the injection and aquifer fluids, and easily allows for estimating reaction rates during push-pull tests.
... If the concentration of Cl − decreases throughout the course of the experiment, then it is assumed that the 15 N-NO 3 − and N 2 concentrations would also have been affected by advection or dispersion. Snodgrass and Kitanidis (1998) derived an equation to allow concentrations of reactive tracers (e.g., NO 3 − ) in push-pull tests to be corrected by changes in a conservative tracer (e.g., Cl − ) for use in determining reaction kinetics. We have used this method to correct the concentrations of 29 N 2 and 30 N 2 before use in the IPT calculations described above; however, the absolute concentrations of 29 N 2 and 30 N 2 in the samples must first be calculated. ...
... We generally use He over air-equilibrated water (see Trimmer and Sanders, 2006) where x = 29 or 30 (as in N 2 ), the m/z signal is the area under the curve obtained from the CF-IRMS, and [N 2 ] is the concentration of N 2 in the aqueous phase of the calibration standard. The calculated N 2 concentrations are then corrected for the effects of advection or dispersion (Snodgrass and Kitanidis, 1998) ...
... The slope of a line fit to the natural logarithm transformed ratio of either NO 3 − /Br − or 15 NO 3 − /Br − plotted against time corresponds to the first-order rate constant (k, T −1 ) for the reaction (Snodgrass and Kitanidis, 1998). In the example of Fig. 28-2b, the k for NO 3 − removal is −0.0049 min −1 . ...
... First-order kinetics are often most appropriate for calculating NO 3 − rate constants in wetland sediments, but on occasion a zero-order fit is superior. Details on the calculation of zero-order kinetics were described by Snodgrass and Kitanidis (1998). ...
... If the concentration of Cl − decreases throughout the course of the experiment, then it is assumed that the 15 N-NO 3 − and N 2 concentrations would also have been affected by advection or dispersion. Snodgrass and Kitanidis (1998) derived an equation to allow concentrations of reactive tracers (e.g., NO 3 − ) in push-pull tests to be corrected by changes in a conservative tracer (e.g., Cl − ) for use in determining reaction kinetics. We have used this method to correct the concentrations of 29 N 2 and 30 N 2 before use in the IPT calculations described above; however, the absolute concentrations of 29 N 2 and 30 N 2 in the samples must first be calculated. ...
... We generally use He over air-equilibrated water (see Trimmer and Sanders, 2006) where x = 29 or 30 (as in N 2 ), the m/z signal is the area under the curve obtained from the CF-IRMS, and [N 2 ] is the concentration of N 2 in the aqueous phase of the calibration standard. The calculated N 2 concentrations are then corrected for the effects of advection or dispersion (Snodgrass and Kitanidis, 1998) ...
... If the concentration of Cl − decreases throughout the course of the experiment, then it is assumed that the 15 N-NO 3 − and N 2 concentrations would also have been affected by advection or dispersion. Snodgrass and Kitanidis (1998) derived an equation to allow concentrations of reactive tracers (e.g., NO 3 − ) in push-pull tests to be corrected by changes in a conservative tracer (e.g., Cl − ) for use in determining reaction kinetics. We have used this method to correct the concentrations of 29 N 2 and 30 N 2 before use in the IPT calculations described above; however, the absolute concentrations of 29 N 2 and 30 N 2 in the samples must first be calculated. ...
... We generally use He over air-equilibrated water (see Trimmer and Sanders, 2006) where x = 29 or 30 (as in N 2 ), the m/z signal is the area under the curve obtained from the CF-IRMS, and [N 2 ] is the concentration of N 2 in the aqueous phase of the calibration standard. The calculated N 2 concentrations are then corrected for the effects of advection or dispersion (Snodgrass and Kitanidis, 1998) ...
... Several analytical methods have been used to estimate in situ reaction rates from PPT BTCs. Snodgrass and Kitanidis (1998), and Haggerty et. al (1998) each developed simplified analytical methods to determine zero-and first-order reaction rate coefficients. ...
... The underlying assumptions in these methods include: (1) the injected tracers are simultaneously introduced as well-mixed slugs, (2) the dominating processes are advection and dispersion in a homogeneous, isotropic aquifer with spatially and temporally, uniformly distributed zero-or first-order irreversible reactions, (3) the tracer and reactants exhibit identical retardation (sorption is negligible), and (4) the background concentration of the conservative tracer and reactive tracers are negligible. Yang et al. (2007) elaborated on the method of Snodgrass and Kitanidis (1998) to include the case where the background concentrations are not negligible. Hageman (2003) presented an alternative analytical method (forced mass balance technique) that identifies first-order reaction rate coefficients for the case where sorption is not neglected. ...
... Push-pull tests have originally been used to determine reaction rates for pollutant degradation in groundwater aquifers (Istok et al., 1997;Haggerty et al., 1998;Snodgrass and Kitanidis, 1998;Pitterle et al., 2005). Urmann et al. (2005) have amended the method for the use with gas mixtures in unsaturated soil above a petroleum-contaminated aquifer. ...
... Obviously, the validity of the evaluation method is limited when the concentration of the reactive gas approaches zero. Actually, it was noted by both Yang et al. (2007) and Snodgrass and Kitanidis (1998), on whose model the former work is based, that the transformations are only valid for concentrations of the reactive compound greater than zero. ...
: In order to quantify and optimise the amount of methane oxidised in the landfill cover, methods for in situ measurement of the methanotrophic activity are needed. A possible approach may be the method of gas push-pull tests (GPPTs). 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. In the next step, the mixture of injected gas and soil air is extracted from the same location and periodically sampled. From the differences in the breakthrough curves, kinetic parameters for the biological oxidation taking place in the soil can be derived. The original method of Urmann et al. (2005) was optimised for the situation in landfill cover soils and the reduction of analytical effort. Optimised parameters included the flow rate during the injection phase and the duration of the experiment. So far, 48 successful GPPTs have been conducted at different landfills in Germany during different seasons. Generally, methane oxidation rates ranged between 0 and 150 g m -3 soil air h -1 . At one location, rates up to 440 g m -3 soil air h -1 were measured under particularly favourable conditions. The method proved to be simple and reliable and has the potential of becoming a standard for the measurement and comparison of the performance of landfill biocovers.
... For this reason, it is desirable to gather additional insights from biodegradation experiments conducted within natural systems. In the work reported here, we utilize a field method called the push-pull test (PPT) (Leap and Kaplan 1988;Istok et al. 1997;Snodgrass and Kitanidis 1998) to study in situ biodegradation of ethanol in a shallow aquifer in which sulfate is the dominant dissolved electron acceptor. Most fuel-contaminated sites in the United States overlie aquifers in which sulfate presents the highest capacity for fuel component degradation compared to other dissolved electron acceptors (Wiedemeier et al. 1999). ...
... The recovery curves of ethanol and bromide were similar in shape (Figure 3) but the recovery of ethanol was much less than that of bromide (27% vs 68%; Table 1). The results from PPT5 show a decrease in recovery of ethanol relative to bromide from the beginning of the extraction and closely fit the theoretical line of Snodgrass and Kitanidis (1998) for first order degradation ( Figure 3). Therefore, the data could be used to calculate the first order reaction rate, which was 0.31 ± 0.02/h (Table 1). ...
Single well push‐pull tests (PPTs) were used to characterize in situ biodegradation rates of ethanol in groundwater at a leaking underground fuel tank (LUFT) site at Site 60, Vandenberg Air Force Base (VAFB), CA. For the tests, local groundwater was spiked with bromide and ethanol and injected at different times into three different wells throughout the experimental area. The spiked water was allowed to remain in the aquifer for 1 to 15.9 h prior to extraction. Biodegradation of ethanol was not observed within 15 h of the aquifer’s first exposure to ethanol near any test well; the ethanol/Br ratio was nearly constant in the extraction samples. Biostimulation treatments (ethanol injections) over the course of 1 to 2 weeks resulted in a linear decrease in ln(ethanol/Br) with time in the extraction samples indicating that ethanol was biodegrading with a first order rate constant of about 0.3/h. After exposing an area to ethanol for 3 months, the biodegradation rate increased further by about a factor of 2. Ethanol degradation rates in the aquifer at this site were temporally variable based on the ethanol exposure history. Our results suggest that PPTs were an effective tool for examining such variability. PPT investigations should be valuable at other locations because ethanol degradation rates in groundwater should vary spatially and temporally depending on the type and timing of fuel releases as well as other factors that control the history of ethanol exposure to an aquifer.
... Compare these colloidal Al/Fe mole ratios to the Al/Fe mole ratio determined for the bulk matrix (3.2 Ϯ 0.2) from digestion of the Ͻ63 m size fraction [Swartz, 1998]. but slightly more complicated, constructs which account for dispersion have recently been presented to obtain zero-and first-order reaction rate coefficients from injection-withdrawal experimental data [Haggerty et al., 1998;Snodgrass and Kitanidis, 1998]. Our experimental methodology most closely matches the methodology described by Snodgrass and Kitanidis [1998], whose rate expressions simplify to general zero-and first-order rate expressions when the conservative tracer concentration at a given time equals that of the initial concentration. ...
... but slightly more complicated, constructs which account for dispersion have recently been presented to obtain zero-and first-order reaction rate coefficients from injection-withdrawal experimental data [Haggerty et al., 1998;Snodgrass and Kitanidis, 1998]. Our experimental methodology most closely matches the methodology described by Snodgrass and Kitanidis [1998], whose rate expressions simplify to general zero-and first-order rate expressions when the conservative tracer concentration at a given time equals that of the initial concentration. ...
The release of colloids to groundwater was investigated in situ in an iron-oxyhydroxide-rich, sandy aquifer. Groundwater amended with various solutes was injected into and immediately withdrawn from the shallow aquifer. Turbidity and colloid composition were monitored in the retrieved injectate. The response of the aquifer material to the amendments generally mimicked that observed in an earlier study using packed columns containing the sediment, demonstrating the viability of the single-well method for testing colloid mobilization in situ. The decline of turbidity in the retrieved injectates with increasing withdrawal volume was analyzed to determine a ``reaction order'' n, describing the redeposition of mobilized colloids to the immobile matrix. Differences in the reaction order for the amendments tested presumably indicated the effectiveness of these amendments to generate repulsive colloid-immobile matrix interactions.
... TCFE transformation products were also treated with the FMB technique so that the distribution of transformation products formed in situ could be readily compared between tests. FMB was selected over other available data processing techniques (Haggerty et al., 1998; Snodgrass and Kitanidis, 1998) because it was designed for use with sorbing solutes. TCFE and its products were expected to undergo sorption during push–pull tests in both A-and C-zones based on results from short-term transport tests conducted at the selected site (Hageman et al., 2001) as well as estimated retardation factor (R) values. ...
... Since the error analysis indicated that the in situ rate for the reductive dechlorination of TCFE was underestimated relative to the actual in situ rate by 10%, all rates reported herein are expected to be underestimated by a similar magnitude. The effects of transport processes were removed from measured aqueous concentrations of fumarate, succinate and formate using a data processing technique developed for use with nonsorbing solutes that is referred to herein as btracer-normalizationQ (Haggerty et al., 1998; Snodgrass and Kitanidis, 1998). Sorption was assumed to have a minimal effect on fumarate, succinate and formate concentrations since these solutes are negatively charged and highly water-soluble. ...
... Istok et al. (1997) described the application of such injection-withdrawal tracer tests for the quantification of microbial activities in aquifers. Haggerty et al. (1998) and Snodgrass and Kitanidis (1998) subsequently developed a simple and robust framework for the derivation of reaction rates from push-pull test experimental data, requiring three simple computational steps: First, the log-normalized ratio of reactant and tracer concentration vs. time elapsed since injection is plotted. Second, a linear regression is fitted to this plot, whose slope corresponds to the first-order-reaction rate constant k. ...
... The data were analyzed according to Haggerty et al. (1998) and Snodgrass and Kitanidis (1998) assuming that BSR followed first-order reaction kinetics: ...
Bacterial sulfate reduction (BSR) is a key process in anaerobic respiration in wetlands and may have considerable impacts on methane emissions. A method to determine sulfate production and consumption in situ is lacking to date. We applied a single-well, injection-withdrawal tracer test for the in situ determination of potential sulfate turnover in a northern temperate peatland. Piezometers were installed in three peat depth levels (20, 30, and 50 cm) during summer 2004, and three series of injection-withdrawal cycles were carried out over a period of several days. Turnover rates of sulfate, calculated from first-order-reaction constant k (–0.097 to 0.053 h–1) and pore-water sulfate concentrations (approx. 10 µmol L–1), ranged from –1.3 to –9.0 nmol cm–3 d–1 for reduction and from +0.7 to +25.4 nmol cm–2 d–1 for production, which occurred after infiltration of water following a heavy rainstorm. Analysis of stable isotopes in peat-water sulfate revealed slightly increasing δ34S values and decreasing sulfate concentrations indicating the presence of BSR. The calculated low sulfur-fractionation factors of <2‰ are in line with high sulfate-reduction rates during BSR. Routine application will require technical optimization, but the method seems a promising addition to common ex situ techniques, as the investigated soil is not structurally altered. The method can furthermore be applied at low expense even in remote locations.
... Though using breakthrough curves in push-pull tests imply more flexibility in characterizing observational data, in practice the shape of the breakthrough curve is often fixed a-priori by researchers (e.g. Davis et al., 2002;Schroth et al., 2000;Snodgrass and Kitanidis, 1998). With only dispersivity to be fitted for a designated flow geometry, breakthrough curves lose this advantage over recovery efficiency. ...
... Though using breakthrough curves in push-pull tests imply more flexibility in characterizing observational data, in practice the shape of the breakthrough curve is often fixed a-priori by researchers (e.g., Davis et al., 2002;Schroth et al., 2000;Snodgrass & Kitanidis, 1998). With only dispersivity to be fitted for a designated flow geometry, breakthrough curves lose this advantage over recovery efficiency. ...
The recovery efficiency of aquifer storage systems with radial flow fields are studied for heterogeneous aquifers. Macrodispersion, arising from spatially heterogeneous hydraulic conductivity, is modeled as a scale‐dependent mechanical dispersion process. Approximate solutions for the recovery efficiency as a function of local dispersion and macrodispersion parameters, the injection‐extraction rate Q $Q$ and duration T $T$, and storage cycle count, are derived and validated against numerical simulations. If macrodispersion dominates and the macrodispersion coefficient scales linearly with distance, the recovery efficiency is independent of both Q,T $Q,T$. For sublinear and superlinear scalings, recovery increases and decreases respectively if Q,T $Q,T$ increases. However, if local dispersion dominates, increasing Q,T $Q,T$ always increases recovery. As macrodispersion becomes increasingly dominant with scale, the recovery efficiency may be a nonmonotonic function of Q,T $Q,T$, with a maximum. In homogeneous aquifers, nonmonotonicity does not occur for 1D and 2D radial flow, but occurs for 3D radial flow fields only as a function of T $T$, not Q $Q$. These methods may also be used for fitting local dispersion and macrodispersion parameters with push‐pull tests using recovery data, with advantages in scope of applicability and ease of data acquisition and interpretation, compared to existing push‐pull test methods, which fit to breakthrough curves and do not consider macrodispersion. Furthermore, characterizing macrodispersion with push‐pull tests may be advantageous over methods that use observation wells, as observation well placement may be challenging in highly heterogeneous aquifers. The results show that the macrodispersion parameters are not innate aquifer hydraulic properties, as their values vary with flow field geometry.
... Hydrogeologically, SWIW tracer tests were initially used to measure aquifer flow velocities (Leap and Kaplan, 1988) and effective porosities (Hall, 1993). In recent years, SWIW tests have been used more to look at groundwater remediation i.e. microbial activity (Istok et al., 1997), in-situ remediation rates (Snodgrass andKitanidis, 1998, Haggerty et al., 1998), solute retardation rates (Schroth et al., 2000), sorption (Cassiani et al., 2005) and DNAPL detection (Istok et al., 2002). ...
Disposal of waste (landfilling) is considered to be the least desirable waste management option in terms of sustainability. However, it is clear that landfills are required to dispose of waste that cannot be reused or recycled, and for residues of waste incineration and waste treatment. Landfills have changed considerably throughout time and across the world. The UK has seen landfills that allowed natural processes of dilution and attenuation be replaced with landfills that hydraulically contain the waste mass. These landfills have covers to reduce the infiltration of water and therefore leachate production, leachate collection systems to collect any leachate created and basal liners to minimise the release of leachate. These measured are effectively creating a ‘dry tomb’ with the main aim being shortterm environmental protection. In the US, Canada and Australia, more sophisticated landfills have been envisaged, these ‘bioreactor landfills’ operate with optimum water content to promote biodegradation and production of methane, an exploitable resource. The concept of a sustainable landfills has received significant discussion. Such landfills must exploit the methane resource (as per the bioreactor landfill), protect the environment (as per the hydraulic containment landfill) and not pass the management burden to future generations. Flushing has been 3 considered as one way to meet the last requirement, whereby the circulation of treated water removes inorganic contaminants after the biodegradation phase reduces the organics. This concept of landfill, termed the high rate flushing bioreactor, is currently considered the only viable option to meet sustainable landfill criteria. Irrespective of current landfill practice, the UK has a backlog of 1,500 older closed landfill sites. Most of these sites do not currently pose an environmental risk, however many have the potential to in the future. A minority of these sites are currently polluting the environment (mainly controlled waters) at unacceptable levels. For most closed sites, in-situ remediation would be the only financially viable option to reduce the pollutant levels. When considering the inorganic fraction of the pollutant load, arguably the most effective in-situ remediation technique is flushing. For older landfill sites, or sites where the basal drainage layer has failed, the use of vertical injection wells may be the only viable option for the introduction and removal of liquid. For an effective flushing strategy, either as an on-going management technique (i.e. a high rate flushing bioreactor) or as a post-closure remediation measure, it is important to consider the hydraulic and contaminant transport properties of the waste. Considerable literature has been published regarding the hydraulic properties of saturated landfilled municipal solid waste, however much of this data is from small scale laboratory and variable size test cells and not field studies. Contaminant transport in waste is very much less understood with preliminary literature suggesting that the transport is dominated by a dual-porosity nature of the waste. This thesis presents two adapted hydrogeological field tracer tests, which take advantage of the single leachate wells present within most landfill cells, to determine the hydrogeological (hydraulic and contaminant) properties of saturated landfilled waste. Hydraulic data is presented from 32 dilution tests undertaken at four UK landfill sites. Hydraulic and contaminant transport data is also presented from 4 12 scales of single well injection-withdrawal tracer tests undertaken at three UK sites. Hydraulic observations show that hydraulic conductivity ranges over four orders of magnitude. The results also confirm the heterogeneous nature of the waste mass, and generally dispel the theory whereby hydraulic conductivity and therefore leachate flow decreases with burial depth within the sites. Contaminant transport observations support the dual-porosity theories and data has provided a good fit when applied to an advection-diffusion dual porosity model. The general conclusion, that waste is a heterogeneous medium with a dualporosity nature, has severe implications for flushing. Injected water will tend to follow preferential flow paths and/or zones and areas will be flushed better than others. The characteristics of the system are such that the contaminant removal time will be affected by the slow release (diffusion) of contaminants from negligible flow zones.
... Aliquots of the tracer solution are periodically extracted ('pulled') from the same well over time, and the disappearance of nitrate relative to the conservative reference tracer is measured, sometimes in conjunction with accrual of products of denitrification and other biogeochemical indicators. Comparison of reactive tracers to the conservative tracer permits some estimation of reaction rates (e.g., Snodgrass and Kitanidis, 1998;Schroth and Istok, 2006). To confirm denitrification reactions are responsible for any observed nitrate losses, tests often employ 15 N enrichment methods, microbiological analyses and/or the acetylene block method that involves measurement of N 2 O gas amassing in the tracer solution as it is recovered from the test-well (Jahangir et al., 2013;Baldwin et al., 2008;Sánchez-Pérez et al., 2003). ...
Nitrate attenuation factors are becoming an important and routine consideration in applied water resource management within New Zealand (NZ), as nitrogen load limits are being set for hydrological catchments. The few nitrate attenuation factors so far reported in the NZ scientific literature have been calculated from comparing landscape nitrogen yield with catchment nitrogen yield. An important limitation of the nutrient budgeting approaches so far employed is that they do not consider the fate and transport of nitrate in the vadose zone-groundwater continuum that links the base of the soil zone with surface waters. There have been few objective scientific studies made targeting measurement of nitrate reactivity below the root zone in NZ. The few in situ measurements that have been made have mainly been focussed in the Waikato and Manawatu-Wanganui regions. Owing to the paucity of available data on nitrate reactivity below the root zone, much discussion pertaining to nitrate attenuation in NZ groundwater remains speculative.
The theme of this paper is a review of the topic of denitrification in groundwater, written from a NZ perspective, although anammox and dissimilatory nitrate reduction to ammonium reactions are covered also, for they represent alternative potential natural attenuation processes for nitrate in groundwater. A role of this review is to describe the various practical methods by which nitrate reactivity can be determined through in situ field measurement, to inform design of future applied research in NZ. Molecular bioassay and stable isotope chemical analytical techniques for diagnosing denitrification are highlighted. Example cases are provided of where the practical methods have been applied in NZ and the findings from those studies are documented.
Potential for nitrate attenuation via heterotrophic and autotrophic denitrification pathways has so far been demonstrated in anoxic groundwater systems of the Waikato region, including on the Volcanic Plateau where the surficial geology comprises sandy volcanic deposits interbedded with palaeosols that formed as a consequence of episodic volcanic eruption events. Active nitrate reduction has also been confirmed in anoxic portions of the alluvial aquifer systems distributed across the Manawatu-Wanganui region. There is very low potential however for any effective nitrate attenuation in aerobic alluvial gravel aquifer systems, such as constitute the most significant groundwater systems in NZ.
Whilst the method itself has yet to be applied in NZ, excess-N2 measurement from analysis of Ar/Ne/N2 ratios carried out in conjunction with groundwater dating conceivably represents a useful method for determining nitrate attenuation rates at a scale commensurate with NZ resource management applications. To be effective, nitrate reaction rates ideally need to be determined on a site specific basis and reconciled with local groundwater flow information, from which nitrogen fluxes can be quantified. The problems of up-scaling point observational data, constraining groundwater ages, flow paths and historic nitrogen loadings, are identified as continuing on-going technical challenges in this field.
... In the anoxic site, zero-order nitrate loss rates ranged from 0.03 ± 0.02 mmol L À1 h À1 (AN) to 0.13 ± 0.02 mmol L À1 h À1 (ANC) ( Table 2). While first-order rate PPTT BTC analysis methods have been developed for homogenous aquifers Snodgrass and Kitanidis, 1998], the karst aquifer BTCs were highly influenced by preferential flow paths that violate the homogenous aquifer assumption of the methods. The resulting skewed cumulative mass recovery (Figure 2) did not fit the models; therefore zero-order rates were used to assess relative magnitude of responses among treatments at the two sites. ...
This study integrates push-pull tracer tests (PPTT) with microbial characterization of extracted water via quantitative polymerase chain reaction (qPCR) and reverse transcriptase qPCR (RT-qPCR) of selected functional N transformation genes to quantify nitrate reduction mechanisms and rates in sites with different redox potential in a karst aquifer. PPTT treatments with nitrate (AN) and nitrate-fumarate (ANC) were executed in two wells representing anoxic and oxic geochemical end members. Oxic aquifer zero order nitrate loss rates (mmol L-1 hr-1) were similar for AN and ANC treatment, ranging from 0.03 ± 0.01 to 0.05 ± 0.01. Anoxic aquifer zero order nitrate loss rates ranged from 0.03 ± 0.02 (AN) to 0.13 ± 0.02 (ANC). Microbial characterization indicates mechanisms influencing these rates were dissimilatory nitrate reduction to ammonium (DNRA) at the anoxic site with AN treatment, assimilatory reduction of nitrate to ammonium (ANRA) with ANC treatment in the water column at both sites, and additional documented nitrate reduction that occurred in unsampled biofilms. With carbon treatment, total numbers of microbes (16S rRNA genes) significantly increased (14-30 fold), supporting stimulated growth with resulting ANRA. Decreased DNRA gene concentrations (nrfA DNA) and increased DNRA activity ratio (nrfA-cDNA/DNA) supported the assertion that DNRA occurred in the anoxic zone with AN and ANC treatment. Furthermore decreased DNRA gene copy numbers at the anoxic site with ANC treatment suggests DNRA microbes in the anoxic site are chemolithoautotrophic. Increased RT-qPCR denitrification gene expression (nirK and nirS) was not observed in water samples, supporting that any observed NO3-N loss due to denitrification may be occurring in unsampled microbial biofilms.
... Kitanidis. 44 Although the Monod model is considered the most robust description of biodegradation kinetics, Schreiber and Bahr 45 showed that first-order kinetics accurately describe BTEX biodegradation rates, especially at low concentrations. Bekins et al. 46 discusses the conditions when it is appropriate to use first-order kinetics in lieu of the Monod model and shows that first-order kinetics are appropriate and accurately approximate "true" Monod values under conditions when the substrate concentration is much less than the half-saturation constant (K s ) for the substrate. ...
Biodegradation of organic matter, including petroleum-based fuels and biofuels, can create undesired secondary water-quality effects. Trace elements, especially arsenic (As), have strong adsorption affinities for Fe(III) (oxyhydr)-oxides and can be released to groundwater during Fe-reducing biodegradation. We investigated the mobilization of naturally occurring As, cobalt (Co), chromium (Cr), and nickel (Ni) from wetland sediments caused by the introduction of benzene, toluene, ethylbenzene, and xylenes (BTEX) and ethanol mixtures under iron- and nitrate-reducing conditions using in-situ push-pull tests. When BTEX alone was added, results showed simultaneous onset and similar rates of Fe-reduction and As mobilization. In the presence of ethanol, the maximum rates of As release and Fe-reduction were higher, the time to onset of reaction was decreased, and the rates occurred in multiple stages reflecting additional processes. The concentration of As increased from <1 μg/L to a maximum of 99 μg/L, exceeding the 10 μg/L limit for drinking water. Mobilization of Co, Cr, and Ni was observed in association with ethanol biodegradation, but not with BTEX. These results demonstrate the potential for trace element contamination of drinking water during biodegradation and highlight the importance of monitoring trace elements at natural and enhanced attenuation sites.
... Des essais de push pull ont été réalisés en laboratoire dans des milieux homogènes (Istok et al., 2001) pour confirmer les interprétations réalisées in situ. Ces mesures ont permis de développer des solutions analytiques permettant d'obtenir de ces essais les coefficients de réaction de premier ordre (Haggerty et al., 1998;Schroth and Istok, 2006) et d'ordre zéro (Snodgrass and Kitanidis, 1998). Des modèles numériques ont aussi été développés pour affiner les interprétations (Phanikumar and McGuire, 2010;Vandenbohede et al., 2008) Nous souhaitons utiliser cette méthode de push-pull pour démontrer les possibilités de dénitrification sur le site de Ploemeur, pour identifier les cinétiques de consommation des nitrates ainsi que les cinétiques de formation des sous-produits de réaction tels que les nitrites et le protoxyde d'azote. ...
Impact of physical heterogeneity on solute transport in groundwater is well known. However, the links with biogeochemical processes are more complex and create non linear processes dependent on the media's structure and the reaction kinetics. Some reactions such as denitrification are controlled by biological activity. The influence of transport properties on these processes is not clearly known. This study aims at characterizing the influence of flow velocity on reactivity at different scales. At the scale of a 2 mm diameter tube, where a denitrification reaction occurs, experiments shows a biologically controlled reaction which becomes with time physically controlled. Diffusion properties inside the developed biofilm seem to explain the observed concentrations. In porous media, biotite-related denitrification has been observed. At field scale, push-pull test allowed to quantify the effect of reaction kinetics on nitrate degradation and nitrous oxide formation. This field information allows assessing the influence of this reaction at field scale. This work improves the knowledge on reactivity in groundwater at multi-scales.
... Some authors use SWPP to quantify fluid phase saturation in two-phase systems (highly inspiring contribution by Tomich et al. 1973 on reactive/partitioning tracer-based quantification of residual oil saturation). Some authors use SWPP to characterize matrix diffusion (Kocabas and Horne 1987, Pruess and Doughty 2010, Jung and Pruess 2012; while many other authors use SWPP to characterize in-situ reaction processes, relying precisely on the idea that all fluid transport-related processes (AD, matrix diffusion etc.) are either negligible, or can be 'calibrated away' by means of a reference tracer (Istok et al. 1997, Haggerty et al. 1998, Snodgrass and Kitanidis 1998, Schroth et al. 2001, Lee et al. 2010. A useful literature overview on various experiences made with SWPP is given by Neretnieks (2007), with a focus on applications in the realm of contaminant hydrology. ...
... [13] Finally, to show a practical application of the proposed semianalytical solution to a real field problem, a single-well push-pull test (PPT) example was designed. PPTs have been employed to quantify various microbially mediated processes in the subsurface [Snodgrass and Kitanidis, 1998;Kim et al., 2004;Istok et al., 1997]. The method consists of the pulse-type injection of a test solution into the saturated zone of an aquifer through a well followed by extraction of the test solution and groundwater mixture from the same well, thus obtaining measurements of the concentrations of the tracer, the reactants, and possible reaction products. ...
Key Points
semianalytical method for solving a multispecies reactive‐transport equation
sequential first‐order reaction under spatially or temporally varying flow
GITT and general linear‐transformation method by Clement (2001)
... This method is based on the injection of a test solution that contains a conservative tracer and one or more reactants in an aquifer through a well. After an incubation period, the mixture of groundwater and test solution is extracted from the same well and analyzed to determine reactant consumption [21,22]. So far, PPTs have been employed to quantify several microbial processes in PHC-contaminated aquifers including aerobic respiration, denitri¢cation, sulfate reduction and methanogenesis [20,23,24], and degradation of PHC constituents under nitrate-and sulfate-reducing conditions [25]. ...
Stable isotope analysis of phospholipid-derived fatty acids (PLFA) is a novel tool to trace assimilation of organic carbon in microbial communities. The 13C-labeling of biomarker fatty acids allows the identification of specific microbial populations involved in the metabolism of particular substrates, supplemented in 13C-labeled form. The goal of this study was to investigate the feasibility of 13C-labeling of PLFA and produced dissolved inorganic carbon (DIC) in a petroleum hydrocarbon (PHC)-contaminated aquifer during an in-situ experiment. To this end, we performed a single-well "push-pull" test in a monitoring well located in the denitrifying zone of a PHC-contaminated aquifer in Studen, Switzerland. During the experiment, we injected 500 L of site groundwater that was amended with 13C-labeled acetate (50% [2-13C]) and nitrate as reactants, and bromide as conservative tracer. Following the injection, we extracted a total of 1000 L of test solution/groundwater mixture after 4, 23 and 46 h from the same location. Concentrations of anions were measured in samples collected during the extraction. From these data, we computed first order rate coefficients for consumption of acetate (0.70 +/- 0.05 1/d) and nitrate (0.63 +/- 0.08 1/d). In addition, we extracted and identified PLFA, and measured \delta13C values of PLFA and DIC. After only 4 h of incubation, we detected 13C-enrichment of certain PLFA in suspended biomass of extracted groundwater. After 46 h, we measured enrichments of up to 5000 per mil in certain PLFA (e.g. 16:1omega 7c), and up to 1500 per mil in the produced DIC. Our results demonstrate the feasibility of in-situ 13C-labeling of PLFA and DIC using push-pull tests to determine microbial activities in-situ in a natural ecosystem.
... Several studies have used push-pull tests to describe in situ microbial reaction kinetics. Studies by Haggerty et al. (1998) and Snodgrass and Kitanidis (1998) provide simplified methods of calculating first and zero-order in situ microbial reaction rate coefficients. These studies account for decreases in solute concentration as a result of dilution from diffusion and dispersion and require no knowledge of aquifer porosity, dispersivity, or hydraulic conductivity, nor the use of flow and transport models. ...
A series of modified in situ push-pull tests were used to quantify rates of SO42- reduction at the mixing interface between wetland porewater and groundwater from the underlying anaerobic aquifer at the Norman Landfill research site, Norman, OK. At small (cm) scale mixing interfaces, steep geochemical gradients and enhanced biogeochemical cycling have been observed. Quantifying the role of these interfaces on system level biogeochemical cycling, including the rates of sulfate reduction is poorly studied due to the small transient nature of mixing interfaces. In this study, the kinetic controls on sulfate reduction were evaluated using a series of modified ``mini'' push-pull tests to introduce electron-acceptor (SO42-) limited wetland porewater to anaerobic groundwater containing abundant electron acceptor (SO42-), thus simulating the aquifer-wetland interface. A relatively well-sorted, fine-grained sand lens within the reducing wetland sediments was targeted using small-diameter (2.54 cm, O.D.) ``drive-point'' wells with a discrete, internally packed 4.5 cm well screen. A series of push-pull tests were then performed using these wells by injecting the sulfate-rich aquifer water into the targeted zone. Results indicated that 1) SO42- reduction was the dominant terminal electron accepting process (TEAP) initiated by the mixing event 2) in all tests in which sulfate reduction was observed, a lag period was present until mixing of water began and 3) tests indicate that the lag period is neither a function of electron donor (acetate) concentration nor directly related to time in situ. These results indicate that, data retrieved from push-pull tests should be interpreted with caution to ensure that the ``rate'' is actually a function of time and not another parameter such as degree of mixing.
... Most studies of the kinetics of fluid-rock interaction carried out to date have assumed flow and reaction through a homogeneous, well-mixed system, as would be the case with a single continuously stirred flowthrough reactor (Haggerty et al. 1998;Snodgrass and Kitanidis 1998;White and Brantley 2003;Zhu 2005). This is perhaps somewhat strange given the enormous number of studies on macrodispersion in the hydrologic literature indicating that travel times, and thus residence times, in the subsurface are not all the same. ...
Fluid-rock interaction (or water-rock interaction, as it was more commonly known) is a subject that has evolved considerably in its scope over the years. Initially its focus was primarily on interactions between subsurface fluids of various temperatures and mostly crystalline rocks, but the scope has broadened now to include fluid interaction with all forms of subsurface materials, whether they are unconsolidated or crystalline ('fluid-solid interaction' is perhaps less euphonious). Disciplines that previously carried their own distinct names, for example, basin diagenesis, early diagenesis, metamorphic petrology, reactive contaminant transport, chemical weathering, are now considered to fall under the broader rubric of fluid-rock interaction, although certainly some of the key research questions differ depending on the environment considered. Beyond the broadening of the environments considered in the study of fluid-rock interaction, the discipline has evolved in perhaps an even more important way. The study of water-rock interaction began by focusing on geochemical interactions in the absence of transport processes, although a few notable exceptions exist (Thompson 1959; Weare et al. 1976). Moreover, these analyses began by adopting a primarily thermodynamic approach, with the implicit or explicit assumption of equilibrium between the fluid and rock. As a result, these early models were fundamentally static rather than dynamic in nature. This all changed with the seminal papers by Helgeson and his co-workers (Helgeson 1968; Helgeson et al. 1969) wherein the concept of an irreversible reaction path was formally introduced into the geochemical literature. In addition to treating the reaction network as a dynamically evolving system, the Helgeson studies introduced an approach that allowed for the consideration of a multicomponent geochemical system, with multiple minerals and species appearing as both reactants and products, at least one of which could be irreversible. Helgeson's pioneering approach was given a more formal kinetic basis (including the introduction of real time rather than reaction progress as the independent variable) in subsequent studies (Lasaga 1981; Aagaard and Helgeson 1982; Lasaga 1984). The reaction path approach can be used to describe chemical processes in a batch or closed system (e.g., a laboratory beaker), but such systems are of limited interest in the Earth sciences where the driving force for most reactions is transport. Lichtner (1988) clarified the application of the reaction path models to water-rock interaction involving transport by demonstrating that they could be used to describe pure advective transport through porous media. By adopting a reference frame which followed the fluid packet as it moved through the medium, the reaction progress variable could be thought of as travel time instead. Multi-component reactive transport models that could treat any combination of transport and biogeochemical processes date back to the early 1980s. Berner and his students applied continuum reactive transport models to describe processes taking place during the early diagenesis of marine sediments (Berner 1980). Lichtner (1985) outlined much of the basic theory for a continuum model for multicomponent reactive transport. Yeh and Tripathi (1989) also presented the theoretical and numerical basis for the treatment of reactive contaminant transport. Steefel and Lasaga (1994) presented a reactive flow and transport model for nonisothermal, kinetically-controlled water-rock interaction and fracture sealing in hydrothermal systems based on simultaneous numerical solution of both reaction and transport This chapter begins with a review of the important transport processes that affect or even control fluid-rock interaction. This is followed by a general introduction to the governing equations for reactive transport, which are broadly applicable to both qualitative and quantitative interpretations of fluid-rock interactions. This framework is expanded through a discussion of specific topics that are the focus of current research, or are either incompletely understood or not fully appreciated. At this point, the focus shifts to a brief discussion of the three major approaches to modeling multi-scale porous media (1) continuum models, (2) pore scale and pore network models, and (3) hybrid or multi-continuum models. From here, the chapter proceeds to investigate some case studies which illuminate the power of modern numerical reactive transport modeling in deciphering fluid-rock interaction.
... [3] Push-pull tests have been used to measure aquifer physical characteristics like longitudinal dispersivity [Gelhar and Collins, 1971] and effective porosity [Hall et al., 1991]. They have also been used to quantify solute sorption [Istok et al., 1999;Schroth et al., 2000;Cassiani et al., 2005], the rate of solute degradation [Istok et al., 1997;Haggerty et al., 1998;Snodgrass and Kitanidis, 1998;Istok et al., 2001;Schroth and Istok, 2006], non-aqueous phase liquid (NAPL) saturation Istok et al., 2002;Davis et al., 2003Davis et al., , 2005, and rate of mass transfer into zones of immobile water [Haggerty et al., 2001]. In all cases, the desired parameter is obtained through the interpretation of concentration versus time breakthrough data collected during the extraction phase of the push-pull test. ...
Single-well push-pull tracer tests have been used to characterize the
extent, fate, and transport of subsurface contamination. Analytical
solutions provide one alternative for interpreting test results. In this
work, an exact analytical solution to two-dimensional equations
describing the governing processes acting on a dissolved compound during
a modified push-pull test (advection, longitudinal and transverse
dispersion, first-order decay, and rate-limited sorption/partitioning in
steady, divergent, and convergent flow fields) is developed. The
coupling of this solution with inverse modeling to estimate aquifer
parameters provides an efficient methodology for subsurface
characterization. Synthetic data for single-well push-pull tests are
employed to demonstrate the utility of the solution for determining (1)
estimates of aquifer longitudinal and transverse dispersivities, (2)
sorption distribution coefficients and rate constants, and (3)
non-aqueous phase liquid (NAPL) saturations. Employment of the solution
to estimate NAPL saturations based on partitioning and non-partitioning
tracers is designed to overcome limitations of previous efforts by
including rate-limited mass transfer. This solution provides a new tool
for use by practitioners when interpreting single-well push-pull test
results.
... Push-pull tests are relatively easy to perform and less expensive than well-to-well tests. Solute recovery curves from push-pull tests have been used to estimate aquifer dispersivity [Hoopes and Harleman, 1967;Gelhar and Collins, 1971;Pickens and Grisak, 1981;Molz et al., 1985;Lessoff and Konikow, 1997;Novakowski et al., 1998], chemical and microbiological parameters Istok et al., 1997;Haggerty et al., 1998;Schroth et al., 1998;Snodgrass and Kitanidis, 1998;Istok et al., 1999;Swartz and Gschwend, 1999;Johnson et al., 2001;Harvey et al., 2002], and recently mobile-immobile domain mass transfer parameters in a fractured dolomite [Meigs and Beauheim, 2001;Haggerty et al., 2001]. ...
In order to better understand factors controlling the migration of inorganic colloids through heterogeneous porous media, the transport of colloidal silica through columns containing a single preferred flow path was investigated. The preferred flow path, which consisted of a tubule of coarse-grained sand, was embedded in a matrix of fine-grained sand. The relative importance of advection, dispersion, deposition (removal of particles from suspension), entrainment (resuspension of deposited particles), and mass exchange between the two pore water regions was evaluated by comparing experimental data to calculations of a two-dimensional advection-dispersion model. Results of this comparison indicate that silica movement could be understood in terms of advective-dispersive transport in the preferred flow path and surrounding matrix with small interaction between the adjacent regions. Deposition of silica was found to follow a first-order kinetic process and was at least partially reversible.
... Several studies have used push-pull tests to describe in situ microbial reaction kinetics. Studies by Haggerty et al. (1998) and Snodgrass and Kitanidis (1998) provide simplified methods of calculating first and zero-order in situ microbial reaction rate coefficients. These studies account for decreases in solute concentration as a result of dilution from diffusion and dispersion and require no knowledge of aquifer porosity, dispersivity, or hydraulic conductivity, nor the use of flow and transport models. ...
A wide variety of methods have been used to quantify the subsurface
activities of microorganisms in response to geochemical perturbations
but identifying representative reaction rates has proven challenging.
This study was conducted at the Norman Landfill, Norman, OK, to evaluate
kinetic controls on SO42- reduction at simulated in situ mixing
interfaces between wetland porewater and groundwater impacted by
landfill leachate using a series of modified push-pull tests. Small
(cm) scale mixing zones exhibiting steep geochemical gradients were
targeted because quantifying the role of these poorly understood
reaction zones is important for understanding system level
biogeochemical cycling and in turn the fate and transport of
contaminants. This study was designed to isolate rates of reactions
from push-pull tests in an effort to evaluate the kinetic controls on
rates within these dynamic mixing zones. To obtain in situ rates we
evaluated kinetic controls on SO42- reduction using geochemical data
collected from small-scale push-pull tests used to introduce electron-
acceptor (SO42-) limited wetland porewater to anaerobic groundwater
containing abundant electron acceptor (SO42-), thus simulating the
aquifer-wetland interface. A relatively well-sorted, fine-grained sand
lens within the reducing wetland sediments was targeted using
small-diameter (2.54 cm, O.D.) drive-point wells with a discrete,
internally packed 4.5 cm well screen. A series of push-pull tests were
performed in these wells by injecting the SO42--rich aquifer water into
the targeted zone. The SO42--rich water used for the push phase of the
tests was pumped from the anaerobic aquifer at the site and amended with
100 mg/L bromide (as NaBr) which served as a conservative tracer to
track dilution from mixing, advection, and dispersion. Geochemical
results revealed that 1) SO42- reduction was the dominant terminal
electron accepting process initiated by the mixing event 2) in all tests
in which sulfate reduction was observed, a lag period was present until
mixing of water began 3) the lag period is neither a function of
electron donor (acetate) concentration nor directly related to time in
situ. These results demonstrate that the presence of the mixing
interface (fringe water at the contact between end member waters) is
important for initiating SO42- reduction and that the degree of mixing
between end member waters may need to be accounted for when interpreting
the lag time often seen with these tests and when calculating rates.
... They showed that our predictability is limited only to the bulk plume behaviour that is controlled by some significant heterogeneities. Snodgrass and Kitanidis (1998) have proposed a method to evaluate first-order and zero-order in situ reaction rates from push-pull test. The method does not involve computer-based solute transport models. ...
Simulation of a field tracer experiment from an injection well in an axially symmetrical flow field in homogenous, stratified and slightly tilted aquifers is presented. The simulation has been verified by analytical solutions of the evolution of the first and second radial spatial moments of the tracer displacements derived in the current study under pure advective transport in case of layered formation. The study focused also on the discrepancies in the transport mechanisms between uniform (linear) and axially symmetrical radial flow fields in homogenous, layered and slightly tilted formations. Excellent agreement exists between analytical solution and the numerical simulation for the case of pure advection in both first and second moments supporting the validity of the numerical simulations. A subdiffusive dispersion regime in case of transport by advection and dispersion in homogeneous aquifer is observed due to the decline of the velocity field. N K;) is the representative effective medium of the layered aquifer in case of pure advection under axially symmetrical flow field.
... They showed that our predictability is limited only to the bulk plume behaviour that is controlled by some significant heterogeneities. Snodgrass and Kitanidis (1998) have proposed a method to evaluate first-order and zero-order in situ reaction rates from push-pull test. The method does not involve computer-based solute transport models. ...
Simulation of a field tracer experiment from an injection well in an axially symmetrical flow field in homogenous, stratified and slightly tilted aquifers is presented. The simulation has been verified by analytical solutions of the evolution of the first and second radial spatial moments of the tracer displacements derived in the current study under pure advective transport in case of layered formation. The study focused also on the discrepancies in the transport mechanisms between uniform (linear) and axially symmetrical radial flow fields in homogenous, layered and slightly tilted formations. Excellent agreement exists between analytical solution and the numerical simulation for the case of pure advection in both first and second moments supporting the validity of the numerical simulations. A subdiffusive dispersion regime in case of transport by advection and dispersion in homogeneous aquifer is observed due to the decline of the velocity field. (√ Ki) is the representative effective medium of the layered aquifer in case of pure advection under axially symmetrical flow field.
... Push-pull tests are relatively easy to perform and less expensive than well-to-well tests. Solute recovery curves from push-pull tests have been used to estimate aquifer dispersivity [Hoopes and Harleman, 1967; Gelhar and Collins, 1971; Pickens and Grisak, 1981; Molz et al., 1985; Lessoff and Konikow, 1997; Novakowski et al., 1998], chemical and microbiological parameters Istok et al., 1997; Haggerty et al., 1998; Schroth et al., 1998; Snodgrass and Kitanidis, 1998; Istok et al., 1999; Swartz and Gschwend, 1999; Johnson et al., 2001; Harvey et al., 2002], and recently mobile-immobile domain mass transfer parameters in a fractured dolomite [Meigs and Beauheim, 2001; Haggerty et al., 2001]. [4] In addition to fractured rock, the mobile-immobile domain mass transfer model has been applied to solute transport through a variety of clastic aquifers and soils [Nkedi-Kizza et al., 1984; Pang and Close, 1999; Haggerty et al., 1998]. ...
1] Bromide breakthrough curves from push-pull tests were obtained at two wells before, during, and after citrate injections to assess how citrate-induced colloid mobilization affected physical aquifer transport properties. Tailing and incomplete bromide recoveries (67–95%) could not be fit with a conservative advection/dispersion model, and the results of batch tests using aquifer solids implied bromide was not significantly sorbing. Thus we modeled the bromide returns considering advection, dispersion, and rate-limited diffusive mass transfer between mobile and immobile regions by fitting a r , the radial dispersivity; a, the rate-limited mass transfer coefficient; and b, the volumetric ratio of immobile-to-mobile domains. Statistical t-tests indicated that the changes in aquifer transport parameters at a well where colloid mobilization was limited were not significant at a 95% percent confidence level. However, the substantial colloid mobilization at a second well corresponded to significantly decreased a r and b, while increasing a between premobilization and both mobilization and postmobilization. The changes in aquifer parameters and their correlation to the recovered colloidal mass are consistent with the idea that pore-clogging colloids were mobilized and/or reorganized during citrate injections. The results suggest that flushing a site under the right conditions with citrate could open up immobile regions and substantially reduce remediation time and costs by liberating contaminants whose transport would otherwise be diffusion limited., Bromide transport before, during, and after colloid mobilization in push-pull tests and the implications for changes in aquifer properties, Water Resour. Res., 39(10), 1301, doi:10.1029/2003WR002112, 2003.
... This method is based on the injection of a test solution that contains a conservative tracer and one or more reactants in an aquifer through a well. After an incubation period, the mixture of groundwater and test solution is extracted from the same well and analyzed to determine reactant consumption [21,22]. So far, PPTs have been employed to quantify several microbial processes in PHC-contaminated aquifers including aerobic respiration, denitri¢cation, sulfate reduction and methanogenesis [20,23,24], and degradation of PHC constituents under nitrate-and sulfate-reducing conditions [25]. ...
This study was conducted to determine the feasibility of labeling phospholipid-derived fatty acids (PLFA) of an active microbial population with a 13C-labeled organic substrate in the denitrifying zone of a petroleum hydrocarbon-contaminated aquifer during a single-well push-pull test. Anoxic test solution was prepared from 500 l of groundwater with addition of 0.5 mM Br− as a conservative tracer, 0.5 mM NO3−, and 0.25 mM [2-13C]acetate. At 4, 23 and 46 h after injection, 1000 l of test solution/groundwater mixture were sequentially extracted. During injection and extraction phases we measured Br−, NO3− and acetate concentrations, characterized the microbial community structure by PLFA and fluorescent in situ hybridization (FISH) analyses, and determined 13C/12C ratios in dissolved inorganic carbon (DIC) and PLFA. Computed first-order rate coefficients were 0.63±0.08 day−1 for NO3− and 0.70±0.05 day−1 for acetate consumption. Significant 13C incorporation in DIC and PLFA was detected as early as 4 h after injection. At 46 h we measured δ13C values of up to 5614‰ in certain PLFA (especially monounsaturated fatty acids), and up to 59.8‰ in extracted DIC. Profiles of enriched PLFA and FISH analysis suggested the presence of active denitrifiers. Our results demonstrate the applicability of 13C labeling of PLFA and DIC in combination with FISH to link microbial structure and activities at the field scale during a push-pull test.
This study demonstrates a combined field method accurately assessing the extent of trichloroethylene (TCE) reductive dechlorination activity and the mass fraction of its by-products. A combined method of injecting a known concentration of 1,1,2-trichloro-2-fluoroethene (TCFE) as a TCE bio-surrogate and a data processing technique of forced mass balance (FMB), considering the sorption effect on the mass fraction of chloroethene was evaluated by performing soil column and field bioaugmentation tests. In the soil column test, the FMB resulted in the mass fraction of 6% TCE, 48.3% cis-1,2-dichloroethene, 18.5% vinyl chloride and 27.2% ethylene. In the field bioaugmentation test, TCFE showed equivalent dechlorination pathways of TCE. The mass fractions estimated by FMB were very similar to those observed in the soil column bioaugmentation tests: 4.5% TCFE, 57.1% 1,2-dichloro-1-fluoroethene, 12% 1-chloro-1-fluoroethene and 26.4% fluoroethene (FE). The FMB method gave ∼50% higher mass fraction for more chlorinated ethenes (i.e., TCFE) and ∼10% lower mass fraction of less chlorinated ethenes (i.e., FE) than those considering only the aqueous concentrations of chlorofluoroethenes. A combined method of TCFE and FMB that could accurately estimate both the extent of dechlorination activities and mass distribution of TCE reductive dechlorination would be highly useful.
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 (kobs) in anoxic peat soil of an unperturbed Swedish ombrotrophic bog. Dissolved O2 was rapidly consumed in anoxic peat with a mean kobs of 2.91 ± 0.60 h⁻¹, corresponding to an O2 half-life of ~14 minutes. POM dominated O2 consumption, as evidenced from approximately 50-fold smaller kobs 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 pre-oxygenation with dissolved O2 lowered kobs 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.
Path reversibility and radial symmetry are often assumed in push-pull tracer test analysis. In reality, heterogeneous flow fields mean that both assumptions are idealizations. To understand their impact, we perform a parametric study which quantifies the scattering effects of ambient flow, local-scale dispersion and velocity field heterogeneity on push-pull breakthrough curves and compares them to the effects of mobile-immobile mass transfer (MIMT) processes including sorption and diffusion into secondary porosity. We identify specific circumstances in which MIMT overwhelmingly determines the breakthrough curve, which may then be considered uninformative about drift and local-scale dispersion. Assuming path reversibility, we develop a continuous time random walk-based interpretation framework which is flow-field agnostic and well suited to quantifying MIMT. Adopting this perspective, we show that the radial flow assumption is often harmless: to the extent that solute paths are reversible, the breakthrough curve is uninformative about velocity field heterogeneity. Our interpretation method determines a mapping function (i.e. subordinator) from travel time in the absence of MIMT to travel time in its presence. A mathematical theory allowing this function to be directly “plugged into” an existing Laplace-domain transport model to incorporate MIMT is presented and demonstrated. Algorithms implementing the calibration are presented and applied to interpretation of data from a push-pull test performed in a heterogeneous environment. A successful four-parameter fit is obtained, of comparable fidelity to one obtained using a million-node 3D numerical model. Finally, we demonstrate analytically and numerically how push-pull tests quantifying MIMT are sensitive to remobilization, but not immobilization, kinetics. This article is protected by copyright. All rights reserved.
One of the most promising and cost-effective options for control of low-level methane (CH4) emissions is the use of engineered bio-based systems. It has been proved that in landfill cover soil, micro-organisms remove CH4 from Landfill Gas (LFG) as the gas migrates through aerobic regions above the buried waste. However, one of the main issues delaying the field implementation of such techniques capable of reducing CH4 emissions from landfills is the lack of a proper field technique to assess the level of CH4 oxidation under field conditions. Only specialized Stable Isotope (SI) based methods are capable of such a task. Gas Push Pull Test (GPPT) method was developed and used to assess in-situ methane oxidation in two low-emitting closed landfills. Simple interpretations of the GPPT data was developed. Stable carbon isotope data was also obtained in conjunction with the GPPTs which provides an independent verification of CH4 oxidation. The GPPT analysis yielded an average percent oxidation of 40 and 52% for the two closed sites. The GPPT methane oxidation results agreed with the SI based values obtained from 20 above ground air samples from each of the closed sites. The study suggests that GPPT can be easily used to assess methane oxidation in in the field. Additionally, the results of the GPPT and the Isotopes analysis were lower than the overall oxidation rates estimated from the modeled generation and the measured tracer-based total emissions testing performed at the two landfills.
Leap and Kaplan (1988) and Hall et al. (1991) presented a type of push-pull test for determining regional groundwater velocity and effective porosity if the hydraulic conductivity of the aquifer and local the hydraulic gradient are known. The hydraulic conductivity could be determined by a pumping test conducted in the same well at the same time as the push-pull tracer test described here; the hydraulic gradient could be determined from water level measurements in a set of nearby wells surrounding and including the push-pull test well. The procedure involves injecting a constant-concentration test solution containing a nonreactive tracer into the aquifer using a single well, allowing the test solution to drift downgradient with the regional groundwater flow, and then extracting the tracer solution/groundwater mixture from the same well by continuous pumping to determine the temporal displacement of the tracer center of mass. The basic equations (using the notation of Hall et al. 1991) are:$$ {\hbox{q}} = \frac{\text{Qt}}{{{{\pi b}}{{\hbox{d}}^{{2}}}{\hbox{KI}}}} $$
$$ {\hbox{n}} = \frac{{\pi {\text{b}}{{\text{K}}^{{2}}}{{\text{I}}^{{2}}}{{\text{d}}^{{2}}}}}{\hbox{Qt}} $$where q is the apparent groundwater (Darcy) velocity, n is effective porosity, Q is the extraction pumping rate, t is the time elapsed from the start of extraction pumping until the centroid of the tracer mass has been extracted, b is the aquifer saturated thickness, d is the elapsed time from the end of tracer injection until the centroid of the tracer mass is extracted (drift time + t), K is the saturated hydraulic conductivity, and I is the local hydraulic gradient. Obviously, uncertainties in computed values of q and n will reflect uncertainties in values of K and I.
The paper presents a semianalytical method to solve the multispecies reactive solute-transport equation coupled with a sequential first-order reaction network under spatially or temporally varying flow velocities and dispersion coefficients. This method employs the generalized integral transform technique (GITT) and general linear transformation method by Clement [2001] to transform the set of coupled multispecies reactive transport equations into a set of independent uncoupled equations and to solve these independent equations for spatially or temporally varying flow velocities and dispersion coefficients, as well for temporally varying inlet concentration. The proposed semianalytical solution is compared against previously published analytical solutions of Srinivasan and Clement [2008] and van Genuchten [1985]. We show a practical implementation of the solution to an actual field, single-well push-pull test (PPT) example designed to obtain the concentration distribution of reactants consumed and products formed at the end of the injection phase.
Alteration of the global nitrogen cycle by man has increased nitrogen loading in waterways considerably, often with harmful consequences for aquatic ecosystems. Dynamic redox conditions within riverbeds support a variety of nitrogen transformations, some of which can attenuate this burden. In reality, however, assessing the importance of processes besides perhaps denitrification is difficult, due to a sparseness of data, especially in situ where sediment structure and hydrologic pathways are intact. Here we show in situ within a permeable riverbed, through injections of 15N-labelled substrates, that nitrate can either be consumed through denitrification, or produced through nitrification, at a previously unresolved fine-scale (cm). Nitrification and denitrification occupy different niches in the riverbed, with denitrification occurring across a broad chemical gradient whilst nitrification is restricted to more oxic sediments. The narrow niche width for nitrification is in effect a break point, with the switch from activity 'on' to activity 'off' regulated by interactions between subsurface chemistry and hydrology. Although maxima for denitrification and nitrification occur at opposing ends of a chemical gradient, high potential for both nitrate production and consumption can overlap when groundwater upwelling is strong.
The single-well push–pull test (SWPPT) was adapted to quantify in situ aerobic respiration and denitrification rates and to assess microbial population dynamics in a petroleum-contaminated fractured bedrock aquifer. Among three test wells, significant dissolved oxygen (DO) consumption was observed only in one well, with average zero- and first-order rate coefficients of 0.32 ± 0.63 and 7.07 ± 13.85 mmol L−1 day−1, respectively. Of the four test wells, significant NO3− consumption was noted in three wells. The average zero- and first-order rate coefficients were 2.87 ± 2.21 and 11.83 ± 7.99 mmol L−1 day−1, respectively. These results indicate that NO3− was more effectively consumed within this fractured bedrock aquifer. Significant DO or NO3− (electron acceptors (EAs)) consumption, the limited contribution of Fe(II) to overall EAs consumption, the production of dissolved CO2 during aerobic respiration and denitrification tests, and N2O production strongly suggest that the EAs consumption was largely due to microbial activity. Detection of Variovorax paradox, benzene-degrading culture, and 28 novel microbial species after the addition of O2 or NO3− suggests that EA injection into a fractured rock aquifer may stimulate aerobic or denitrifying petroleum-degrading microbes. Therefore, SWPPT may be useful for quantifying in situ aerobic respiration and denitrification rates and for assessing microbial population dynamics in petroleum-contaminated fractured bedrock aquifers.
As part of the Swedish site investigations and research associated with the disposal of spent nuclear fuel, tracer experiments in deep boreholes have been employed in order to characterize solute transport and hydraulic connectivity in the fractured bedrock at two sites—Forsmark and Laxemar-Simpevarp. Performance and analytical results are presented from a suite of tracer experiments in varying scales, from single-hole injection-withdrawal tests, intermediate-scale tests with sorbing tracers, to large-scale connectivity tests over distances up to several hundred meters in major fracture zones or networks of zones. In addition to demonstrating transport connectivity over large distances, a general result is that the single-hole tests, as well as the cross-hole tests with sorbing tracers, have clearly demonstrated the process of solute retention of water-conductive features in the rock at different scales.
The kinetics of geochemical and biogeochemical processes can be studied in iso- lation in the laboratory, but only rarely is it possible to separate these processes from those of transport when considering their importance in natural systems at the field scale. This is because the driving force for most reactions of interest in water-rock interaction is transport. The most intense geochemical and biogeochem- ical activity occurs at the interface between global compartments like the oceans, the atmosphere, and the Earth's crust where elemental and nutrient fluxes provide the maximum driving force for reactions to take place. The important role of trans- port in these settings makes it critical to consider these time-dependent processes in conjunction with those processes we think of as more purely biogeochemical. In other words, these global interfaces are open systems, where both mass and energy transfers must be accounted for. For the sake of convenience, geochemical and biogeochemical kinetics are of- ten studied in closed systems in the laboratory, but it is important to keep in mind how different the closed systems can be from open systems. In a closed system, the best analogue of which is the batch reactor in the laboratory (in its simplest form, just a laboratory beaker, Fig. 11.1a), the solution begins out of equilibrium initially and evolves over time towards either thermodynamic equilibrium, or to some other end point beyond which the reaction cannot proceed. Either equilib- rium is achieved (where net reaction rates are, by definition, = 0), or rates be- come so slow that the system effectively no longer changes. In an open system, a good laboratory analogue of which is the continuously stirred tank reactor (CSTR) or well-mixed flowthrough reactor (Fig. 11.1b), the continuous injection of solu- tion that is out of equilibrium (either with itself, or with a solid phase inside the reactor) causes the solution composition coming out of the reactor to reach a
Reactive groundwater tracer tests provide a practical means by which rates of reaction processes that constitute natural attenuation can be measured in aquifer systems. We present a two-well Re-Circulating Tracer Well Test (RCTWT) method that we conceive applicable to determining in situ reaction rates in fast-flowing alluvial aquifers where conventional small-scale single-well tracer tests, such as the push–pull test, might be impractical. The RCTWT concept was analysed mathematically and breakthrough curve datasets were generated using a numerical model to simulate hypothetical aquifer systems from which the sensitivity of the RCTWT method to governing mathematical variables was analysed. A simplified data interpretation method for determining reaction rates from observed RCTWT breakthrough data was developed. It was demonstrated that the efficiency of the RCTWT is strongly affected by the degree of aquifer heterogeneity and the performance can be optimised by applying a high pumping rate along with a short doublet well-spacing. The simplified method provided reasonably accurate estimates of first-order reaction rate coefficients when evaluated using the hypothetical datasets, from which it was concluded that the RCTWT is a valid concept for determining potential in situ reaction rates. The findings are to be incorporated in the design of practical RCTWTs aimed at measuring denitrification rates in fast-flowing alluvial aquifers, in New Zealand.
Thermal-lifetime prediction is a traditional endeavor of inter-well tracer tests conducted in geothermal reservoirs. Early tracer test signals (detectable within first 5 years of operation) are expected to correlate with late-time production-temperature evolutions ("thermal breakthrough", supposed to not occur before 3 decades of operation) of a geothermal reservoir. Whenever a geothermal reservoir can be described as a single-fracture system, its thermal lifetime will, ideally, be determined by two parameters (fracture aperture and porosity), whose inversion from conservative-tracer test signals is straightforward and non-ambiguous (provided that the tracer tests, and their interpretation, are performed in accordance to the rules of the art). However, as soon as only “few more” fractures are considered (here five fracture), this clear-cut correlation is broken. A given geothermal reservoir can simultaneously exhibit a single-fracture behavior, in terms of heat transport, and a multiple-fracture behavior, in terms of solute tracer transport (or vice versa), whose effective values of fracture apertures, spacing, and porosities are essentially uncorrelated between heat and solute tracers. Solute transport parameters derived from conservative-tracer tests will no longer characterize the heat transport processes (and thus temperature evolutions) taking place in the same reservoir. Parameters determining its thermal lifetime will remain invisible to conservative tracers in inter-well tests. This study demonstrates this problem on the example of a five-fracture system, representing a deep-geothermal reservoir, with well-doublet placement inducing fluid flow both across and through the fractures. Thermal breakthrough in this system will be shown to strongly depend on fracture apertures, whereas solute-tracer signals from inter-well tests in this system will not show a clear-cut correlation with fracture apertures. Thus, inter-well tracer tests will no longer be useful for the goal of predicting thermal breakthrough, in such geothermal reservoirs whose hydrogeological structure resembles a system of "discrete" fractures with flow occurring "obliquely" to the fractures.
Next, dealing with a "continuum" of uniformly-distributed fractures of homogeneous aperture and spacing, we show that 3 characteristic regimes of tracer signal sensitivity w. r. to fracture aperture and w. r. to fracture spacing (fracture density, or fluid-rock interface area) can be identified during the pull phase of a single-well tracer push-pull test. From these regimes, some recommendations can be derived regarding dual-tracer test design and dimensioning for the specific purposes of geothermal reservoir characterization (using conservative solutes and heat as tracers).
Landfills are both a major anthropogenic source and a sink for the
greenhouse gas CH4. Methanogenic bacteria produce CH4 during the
anaerobic digestion of landfill waste, whereas, methanotrophic bacteria
consume CH4 as it is transported through a landfill cover soil.
Methanotrophs are thought to be ubiquitous in soils, but typically exist
in large numbers at oxic/anoxic interfaces, close to anaerobic methane
sources but exposed to oxygen required for metabolism. Accurate in-situ
quantification of the sink strength of methanotrophs in landfill cover
soils is needed for global carbon balances and for local emissions
mitigation strategies. We measured in-situ CH4 concentrations at 30,
60, and 100 cm depth at 18 evenly spaced locations across a landfill
cover soil. Furthermore, we performed Gas Push-Pull Tests (GPPTs) to
estimate in-situ rates of methanotrophic activity in the cover soil. The
GPPT is a gas-tracer test in which a gas mixture containing CH4, O2, and
non-reactive tracer gases is injected (pushed) into the soil followed by
extraction (pull) from the same location. Quantification of CH4
oxidation rates is based upon comparison of the breakthrough curves of
CH4 and tracer gases. We present the results of a series of GPPTs
conducted at two locations in the cover soil to assess the feasibility
and reproducibility of this technique to quantify methanotrophic
activity. Additional GPPTs were performed with a methanotrophic
inhibitor in the injection gas mixture to confirm the appropriate choice
of tracers to quantify CH4 oxidation. Estimated CH4 oxidation rate
constants indicate that the cover soil contains a highly active
methanotrophic community.
Review Report: on the characteristics and motion of pollution in subsurface layers from continuous and point sources
A simple, single-well push-pull test was conducted at a TCE-contaminated site to estimate the site-specific TCE degradation
and permanganate (MnO4−) consumption rate. Known quantities of a conservative tracer (Br−) and permanganate were rapidly injected into a saturated aquifer then periodically sampled during extraction from the same
well. Concentrations of Br−, TCE, and MnO4− were measured; breakthrough curves (BTCs) for all species of solute were determined. Data analysis of BTCs for bromide and
TCE showed that the first-order rate constant of TCE degradation by MnO4− is 1.67±0.152h−1. Further, the in situ MnO4− demand rate by TCE and aquifer materials is estimated to be 0.54±0.371h−1. This study demonstrates that in situ push-pull tests are useful and economical tools for field investigations to determine
contaminant reaction and oxidant consumption rates, which may then be used to optimize groundwater remediation designs.
The single-well, ``push-pull`` test method is useful for obtaining information on a wide variety of aquifer physical, chemical, and microbiological characteristics. A push-pull test consists of the pulse-type injection of a prepared test solution into a single monitoring well followed by the extraction of the test solution/ground water mixture from the same well. The test solution contains a conservative tracer and one or more reactants selected to investigate a particular process. During the extraction phase, the concentrations of tracer, reactants, and possible reaction products are measured to obtain breakthrough curves for all solutes. This paper presents a simplified method of data analysis that can be used to estimate a first-order reaction rate coefficient from these breakthrough curves. Rate coefficients are obtained by fitting a regression line to a plot of normalized concentrations versus elapsed time, requiring no knowledge of aquifer porosity, dispersivity, or hydraulic conductivity. A semi-analytical solution to the advective-dispersion equation is derived and used in a sensitivity analysis to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push-pull tests in a homogeneous, confined aquifer with a fully-penetrating injection/extraction well and varying porosity, dispersivity, test duration, and reaction rate. A numerical flow and transport code (SUTRA) is used to evaluate the ability of the simplified method to estimate reaction rate coefficients in simulated push-pull tests in a heterogeneous, unconfined aquifer with a partially penetrating well. In all cases the simplified method provides accurate estimates of reaction rate coefficients; estimation errors ranged from 0.1 to 8.9% with most errors less than 5%.
An equation is derived by which advective groundwater velocity in a confined aquifer may be estimated by a single-well tracer test in which a single tracer pulse is allowed to drift from the well and then pumped back to the well and sampled to obtain a breakthrough curve. Although similar in methodology to preexisting methods, this method differs in that it takes into account ambient groundwater movement during the pumpback phase. Using sodium chloride as a tracer solution, a series of small-scale tests were run in a laboratory sand tank model to test the theory. Results of linear flow tracer tests through the model, simulating unperturbed regional advective flow at known velocities, were compared to results of single-well drift-and-pumpback tests conducted during linear flow through the model. Advective velocities computed by both types of tests were identical, thus proving the validity of the equation.
The theoretical basis of a field technique for evaluating longitudinal dispersivity and effective porosity by a single-well test is presented. A measured quantity of an ideal tracer is released instantaneously into the well at rest and moves with the natural flow velocity. After a certain delay time the well is pumped at a constant rate and tracer concentrations in the pumped water are monitored. These serve as data for estimating the above parameters. The interpretation of the data is based on an approximate analytical solution of the direct problem by means of the method of small perturbations and superposition and on an iterative procedure for solving the inverse problem.Experimental procedures and field applications of the method are presented in a forthcoming paper.
In situ anaerobic biotransformation of BTEX (benzene, toluene, ethylbenzene, o-xylene, and m-xylene) was investigated under enhanced nitrate- and sulfate-reducing conditions. Controlled amounts of BTEX compounds added to slugs of treated groundwater were released into a gasoline-contaminated aquifer at Seal Beach, CA. In a series of studies, the slugs, 470−1700 L in volume, were released into the aquifer through a multi-port injection/extraction well and were subsequently withdrawn over a 2−3-month period. To evaluate unamended in situ conditions, the injectate was treated with granular activated carbon (GAC) and augmented with bromide as a tracer. To evaluate nitrate- and sulfate-reducing conditions, the injectate was also deionized and augmented with 200−300 μg/L BTEX, nitrate or sulfate, and background electrolytes. Under unamended conditions, transformation appeared to be limited to the slow removal of toluene and m,p-xylene (i.e., sum of m+p-xylene). Under nitrate-reducing conditions, toluene, ethylbenzene, and m-xylene were transformed without a lag phase in less than 10 days, and o-xylene was transformed in 72 days. Under sulfate-reducing conditions, toluene, m-xylene and o-xylene were completely transformed in less than 50 days, and ethylbenzene was removed in 60 days. Benzene appeared to be removed under sulfate-reducing conditions, but the trend was pronounced only at some levels. A two-dimensional model is presented for the evaluation of reactive solute behavior in such slug tests. For compounds that are transformed without a lag phase, zero-order kinetics appears to be more applicable than first-order kinetics.
A single-well, “push-pull” test method is proposed for the in situ determination of microbial metabolic activities in ground-water aquifers. The method consists of the pulse-type injection (“push”) of a test solution into the saturated zone of an aquifer through the screen of an existing monitoring well followed by the extraction (“pull”) of the test solution/ground-water mixture from the same well. The test solution contains a tracer and one or more reactive solutes selected to investigate specific microbial activities. During the injection phase, the test solution flows radially away from the monitoring well into the aquifer. Within the aquifer, biologically reactive components of the test solution are converted to various products by the indigenous microbial community. During the extraction phase, flow is reversed and solute concentrations are measured to obtain breakthrough curves, which are used to compute the quantities of reactant(s) consumed and/or product(s) formed during the test and reaction rates. Tests were performed to determine rates of aerobic respiration, denitrification, sulfate reduction, and methanogenesis in a petroleum contaminated aquifer in western Oregon. High rates of oxygen, nitrate, nitrite, and hydrogen utilization and nitrite, and carbon dioxide production support the hypothesis that petroleum contamination has resulted in an increase in microbial activity in the anaerobic portion of the site. The results suggest that the push-pull test method should be useful for obtaining quantitative information on a wide range of in situ microbial processes.