[Show abstract][Hide abstract] ABSTRACT: Pore-scale models are useful for studying relationships between fundamental processes and phenomena at larger (i.e., Darcy) scales. However, the size of domains that can be simulated with explicit pore-scale resolution is limited by computational and observational constraints. Direct numerical simulation of pore-scale flow and transport is typically performed on millimeter-scale volumes at which X-ray computed tomography (XCT), often used to characterize pore geometry, can achieve micrometer resolution. In contrast, laboratory experiments that measure continuum properties are typically performed on decimeter-scale columns. At this scale, XCT resolution is coarse (tens to hundreds of micrometers) and prohibits characterization of small pores and grains. We performed simulations of pore-scale processes over a decimeter-scale volume of natural porous media with a wide range of grain sizes, and compared to results of column experiments using the same sample. Simulations were conducted using high-performance codes executed on a supercomputer. Two approaches to XCT image segmentation were evaluated, a binary (pores and solids) segmentation and a ternary segmentation that resolved a third category (porous solids with pores smaller than the imaged resolution). We used a multiscale Stokes-Darcy simulation method to simulate the combination of Stokes flow in large open pores and Darcy-like flow in porous solid regions. Flow and transport simulations based on the binary segmentation were inconsistent with experimental observations because of overestimation of large connected pores. Simulations based on the ternary segmentation provided results that were consistent with experimental observations, demonstrating our ability to successfully model pore-scale flow over a column-scale domain. This article is protected by copyright. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: The use of non-Newtonian shear thinning fluids (STFs) containing xanthan is a potential enhancement for emplacing a solute amendment near the water table and within the capillary fringe. Most research to date related to STF behavior has involved saturated and confined conditions. A series of flow cell experiments were conducted to investigate STF emplacement in variable saturated homogeneous and layered heterogeneous systems. Besides flow visualization using dyes, amendment concentrations and pressure data were obtained at several locations. The experiments showed that injection of STFs considerably improved the subsurface distribution near the water table by mitigating preferential flow through higher permeability zones compared to no-polymer injections. The phosphate amendment migrated with the xanthan STF without retardation. Despite the high viscosity of the STF, no excessive mounding or preferential flow were observed in the unsaturated zone. The STOMP simulator was able to predict the experimentally observed fluid displacement and amendment concentrations well. Based on the observed pressure gradients and concentration data in layers of differing hydraulic conductivity, cross flow between layers was identified as the main mechanism transporting STFs into lower permeability layers.
[Show abstract][Hide abstract] ABSTRACT: As a remedial approach, vacuum-induced pore-water extraction offers the possibility of contaminant and water removal from the vadose zone, which may be beneficial in reducing the flux of vadose zone contaminants to groundwater. Vadose zone water extraction is being considered at the Hanford Site in Washington State as a means to remove technetium-99 contamination from low permeability sediments with relatively high water contents. A series of intermediate-scale laboratory experiments have been conducted to improve the fundamental understanding and to recognize the limitations of the technique. Column experiments were designed to investigate the relations between imposed suctions, water saturations, and water extraction. Flow cell experiments were conducted to investigate the effects of high-permeability layers and near-well compaction on pore-water extraction efficiency. Results show that water extraction from unsaturated systems can be achieved in low permeability sediments, provided that the initial water saturations are relatively high. The presence of a high-permeability layer decreased the yield, and compaction near the well screen had a limited effect on overall performance. In all experiments, large pressure gradients were observed near the extraction screen. Minimum requirements for water extraction include an imposed suction larger than the initial sediment capillary pressure in combination with a fully saturated seepage-face boundary. A numerical multiphase simulator with a coupled seepage-face boundary condition was used to simulate the experiments. Reasonable matches were obtained between measured and simulated results for both water extraction and capillary pressures, suggesting that numerical simulations may be used as a design tool for field-scale applications of pore-water extraction.
Vadose Zone Journal 08/2014; 13(8). DOI:10.2136/vzj2014.04.0044 · 1.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Four sets of nonreactive solute transport experiments were conducted with micromodels. Each set consisted of three experiments with one variable, i.e., flow velocity, grain diameter, pore-aspect ratio, and flow-focusing heterogeneity. The data sets were offered to pore-scale modeling groups to test their numerical simulators. Each set consisted of two learning experiments, for which all results were made available, and one challenge experiment, for which only the experimental description and base input parameters were provided. The experimental results showed a nonlinear dependence of the transverse dispersion coefficient on the Peclet number, a negligible effect of the pore-aspect ratio on transverse mixing, and considerably enhanced mixing due to flow focusing. Five pore-scale models and one continuum-scale model were used to simulate the experiments. Of the pore-scale models, two used a pore-network (PN) method, two others are based on a lattice Boltzmann (LB) approach, and one used a computational fluid dynamics (CFD) technique. The learning experiments were used by the PN models to modify the standard perfect mixing approach in pore bodies into approaches to simulate the observed incomplete mixing. The LB and CFD models used the learning experiments to appropriately discretize the spatial grid representations. For the continuum modeling, the required dispersivity input values were estimated based on published nonlinear relations between transverse dispersion coefficients and Peclet number. Comparisons between experimental and numerical results for the four challenge experiments show that all pore-scale models were all able to satisfactorily simulate the experiments. The continuum model underestimated the required dispersivity values, resulting in reduced dispersion. The PN models were able to complete the simulations in a few minutes, whereas the direct models, which account for the micromodel geometry and underlying flow and transport physics, needed up to several days on supercomputers to resolve the more complex problems.
[Show abstract][Hide abstract] ABSTRACT: Wettability is a key parameter influencing capillary pressures,
permeabilities, fingering mechanisms, and saturations in multiphase flow
processes within porous media. Glass-covered silicon micromodels provide
precise structures in which pore-scale displacement processes can be
visualized. The wettability of silicon and glass surfaces can be
modified by silanization. However, similar treatments of glass and
silica surfaces using the same silane do not necessarily yield the same
wettability as determined by the oil-water contact angle. In this study,
surface cleaning pretreatments were investigated to determine conditions
that yield oil-wet surfaces on glass with similar wettability to silica
surfaces treated with the same silane, and both air-water and oil-water
contact angles were determined. Borosilicate glass surfaces cleaned with
standard cleaning solution 1 (SC1) yield intermediate-wet surfaces when
silanized with hexamethyldisilazane (HMDS), while the same cleaning and
silanization yields oil-wet surfaces on silica. However, cleaning glass
in boiling concentrated nitric acid creates a surface that can be
silanized to obtain oil-wet surfaces using HMDS. Moreover, this method
is effective on glass with prior thermal treatment at an elevated
temperature of 400°C. In this way, silica and glass can be silanized
to obtain equally oil-wet surfaces using HMDS. It is demonstrated that
pretreatment and silanization is feasible in silicon-silica/glass
micromodels previously assembled by anodic bonding, and that the change
in wettability has a significant observable effect on immiscible fluid
displacements in the pore network.
Water Resources Research 08/2013; 49(8):4724-4729. DOI:10.1002/wrcr.20367 · 3.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Water saturation is an important indicator of contaminant distribution and plays a governing role in contaminant transport within the vadose zone. Understanding the water saturation distribution is critical for both remediation and contaminant flux monitoring in unsaturated environments. In this work, we propose and demonstrate a method of remotely determining water saturation levels using gas phase partitioning tracers and time-lapse bulk electrical conductivity measurements. The theoretical development includes the partitioning chemistry for the tracers we demonstrate (ammonia and carbon dioxide), as well as a review of the petrophysical relationship governing how these tracers influence bulk conductivity. We also investigate methods of utilizing secondary information provided by electrical conductivity breakthrough magnitudes induced by the tracers. We test the method on clean, well characterized, intermediate-scale sand columns under controlled conditions. Results demonstrate the capability to accurately monitor gas breakthrough curves along the length of the column according to the corresponding electrical conductivity response, and to adequately determine partitioning coefficients, leading to accurate water saturation estimates. This work is motivated by the need to develop effective characterization and monitoring techniques for contaminated deep vadose zone environments, and provides a proof-of-concept toward uniquely characterizing and monitoring water saturation levels at the field scale and in three-dimensions using electrical resistivity tomography.
Vadose Zone Journal 05/2013; 12(2). DOI:10.2136/vzj2012.0142 · 1.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The abiotic precipitation of uranium (U(VI)) was evaluated in a microfluidic pore network (i.e. micromodel) to assess the efficacy of using a phosphate amendment to immobilize uranium in groundwater. U(VI) was mixed transverse to the direction of flow with hydrogen phosphate (HPO42-), in the presence or absence of calcium (Ca2+) or sulfate (SO42-), in order to identify precipitation rates, morphology and types of minerals formed, and effects of mineral precipitates on pore blockage. Precipitation occurred over the time scale of hours to days. Relative to when only U(VI) and HPO42- were present, precipitation rates were 2.3X slower when SO42- was present, and 1.4X faster when Ca2+ was present; larger crystals formed in the presence of SO42-. Raman backscattering spectroscopy and micro X-ray diffraction (μ-XRD) results both showed that the only mineral precipitated was chernikovite, UO2HPO4•4H2O; energy dispersive x-ray spectroscopy results indicate that Ca and S are not incorporated into the chernikovite lattice. A pore scale model was developed, and simulation results of saturation ratio (SR=Q/Ksp) suggest that chernikovite is the least thermodynamically favored mineral to precipitate (0<SR<1) compared to uranyl hydrogen phosphate and Na-Autunite (13<SR<40), and uranyl orthophosphate and Ca-autunite (when Ca2+ is present) (SR>105). Fluorescent tracer studies and laser confocal microscopy images showed that densely aggregated precipitates blocked pores and reduced permeability. The results suggest that uranium precipitation with phosphate as chernikovite is rapid on the time scale of remediation for the conditions considered, and can block pores, alter fluid flow paths, and potentially limit mixing and precipitation.
Water Resources Research 02/2013; 49(2). DOI:10.1002/wrcr.20088 · 3.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Super-absorbent polymers (SAPs) have the potential to remove water and associated contaminants from unsaturated sediments in the field. Column and flow cell experiment were conducted to test the ability of four types of SAPs to remove water from unsaturated porous media. Column experiments, with emplacement of a layer of polymer on top of unsaturated porous media, showed the ability of the SAPs to extract up to 80% of the initially emplaced water against gravity into the sorbent over periods up to four weeks. In column experiments where the sorbent was emplaced between layers of unsaturated porous media, gel formation was observed at both the sorbent-porous medium interfaces. The extraction percentages over four weeks of contact time were similar for both column configurations and no obvious differences were observed for the four tested SAPs. Two different flow cells were used to test the wicking behavior of SAPs in two dimensions using three configurations. The largest removal percentages occurred for the horizontal sorbent layer configuration which has the largest sorbent-porous medium interfacial area. In a larger flow cell, a woven nylon “sock” was packed with sorbent and subsequently placed between perforated metal plates, mimicking a well configuration. After one week of contact time the sock was removed and replaced by a fresh sock. The results of this experiment showed that the sorbent was able to continuously extract water from the porous media, although the rate decreased over time. The declining yield during both periods is associated with the sharp reduction in water saturation and relative permeability near the sorbent. It was also observed that the capillary pressure continued to increase over the total contact time of 14 days, indicating that the sorbent remained active over that period. This work has demonstrated the potential of soil moisture wicking using SAPs at the proof-of-principle level.
Vadose Zone Journal 11/2012; 11(4). DOI:10.2136/vzj2011.0200 · 1.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Soil desiccation (drying), involving water evaporation induced by dry gas injection, is a potentially robust vadose zone remediation process to limit contaminant transport through the vadose zone. A series of four intermediate-scale flow cell experiments was conducted in homogeneous and simple layered heterogeneous porous medium systems to investigate the effects of heterogeneity on desiccation of unsaturated porous media. The permeability ratios of porous medium layers ranged from about five to almost two orders of magnitude. The insulated flow cell was equipped with twenty humidity and temperature sensors and a dual-energy gamma system was used to determine water saturations at various times. The multiphase code STOMP was used to simulate the desiccation process. Results show that injected dry gas flowed predominantly in the higher permeability layer and delayed water removal from the lower permeability material. For the configurations tested, water vapor diffusion from the lower to the higher permeability zone was considerable over the duration of the experiments, resulting in much larger relative humidity values of the outgoing air than based on permeability ratios alone. Acceptable numerical matches with the experimental data were obtained when an extension of the saturation-capillary pressure relation below the residual water saturation was used. The agreements between numerical and experimental results suggest that the correct physics are implemented in the simulator and that the thermal and hydraulic properties of the porous media, flow cell wall and insulation materials were properly represented.
Vadose Zone Journal 11/2012; 11(4). DOI:10.2136/vzj2011.0168 · 1.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Carbon sequestration in saline aquifers involves displacing brine from the pore space by supercritical CO(2) (scCO(2)). The displacement process is considered unstable due to the unfavorable viscosity ratio between the invading scCO(2) and the resident brine. The mechanisms that affect scCO(2)-water displacement under reservoir conditions (41 °C, 9 MPa) were investigated in a homogeneous micromodel. A large range of injection rates, expressed as the dimensionless capillary number (Ca), was studied in two sets of experiments: discontinuous-rate injection, where the micromodel was saturated with water before each injection rate was imposed, and continuous-rate injection, where the rate was increased after quasi-steady conditions were reached for a certain rate. For the discontinuous-rate experiments, capillary fingering and viscous fingering are the dominant mechanisms for low (logCa ≤ -6.61) and high injection rates (logCa ≥ -5.21), respectively. Crossover from capillary to viscous fingering was observed for logCa = -5.91 to -5.21, resulting in a large decrease in scCO(2) saturation. The discontinuous-rate experimental results confirmed the decrease in nonwetting fluid saturation during crossover from capillary to viscous fingering predicted by numerical simulations by Lenormand et al. (J. Fluid Mech.1988, 189, 165-187). Capillary fingering was the dominant mechanism for all injection rates in the continuous-rate experiment, resulting in monotonic increase in scCO(2) saturation.
[Show abstract][Hide abstract] ABSTRACT: The use of air-water, θ(wa), or air-liquid contact angles is customary in surface science, while oil-water contact angles, θ(ow), are of paramount importance in subsurface multiphase flow phenomena including petroleum recovery, nonaqueous phase liquid fate and transport, and geological carbon sequestration. In this paper we determine both the air-water and oil-water contact angles of silica surfaces modified with a diverse selection of silanes, using hexadecane as the oil. The silanes included alkylsilanes, alkylarylsilanes, and silanes with alkyl or aryl groups that are functionalized with heteroatoms such as N, O, and S. These silanes yielded surfaces with wettabilities from water wet to oil wet, including specific silanized surfaces functionalized with heteroatoms that yield intermediate wet surfaces. The oil-water contact angles for clean and silanized surfaces, excluding one partially fluorinated surface, correlate linearly with air-water contact angles with a slope of 1.41 (R = 0.981, n = 13). These data were used to examine a previously untested theoretical treatment relating air-water and oil-water contact angles in terms of fluid interfacial energies. Plotting the cosines of these contact angles against one another, we obtain the relationship cos θ(wa) = 0.667 cos θ(ow) + 0.384 (R = 0.981, n = 13), intercepting cos θ(ow) = -1 at -0.284, which is in excellent agreement with the linear assumption of the theory. The theoretical slope, based on the fluid interfacial tensions σ(wa), σ(ow), and σ(oa), is 0.67. We also demonstrate how silanes can be used to alter the wettability of the interior of a pore network micromodel device constructed in silicon/silica with a glass cover plate. Such micromodels are used to study multiphase flow phenomena. The contact angle of the resulting interior was determined in situ. An intermediate wet micromodel gave a contact angle in excellent agreement with that obtained on an open planar silica surface using the same silane.
[Show abstract][Hide abstract] ABSTRACT: Soil desiccation, in conjunction with surface infiltration control, is considered at the Hanford Site as a potential technology to limit the flux of technetium and other contaminants in the vadose zone to the groundwater. An intermediate-scale experiment was conducted to test the response of a series of instruments to desiccation and subsequent rewetting of porous media. The instruments include thermistors, thermocouple psychrometers, dual-probe heat pulse sensors, heat dissipation units, and humidity probes. The experiment was simulated with the multifluid flow simulator STOMP, using independently obtained hydraulic and thermal porous medium properties. All instrument types used for this experiment were able to indicate when the desiccation front passed a certain location. In most cases the changes were sharp, indicating rapid changes in moisture content, water potential, or humidity. However, a response to the changing conditions was recorded only when the drying front was very close to a sensor. Of the tested instruments, only the heat dissipation unit and humidity probes were able to detect rewetting. The numerical simulation results reasonably match the experimental data, indicating that the simulator captures the pertinent gas flow and transport processes related to desiccation and rewetting and may be useful in the design and analysis of field tests.
Vadose Zone Journal 02/2012; 11(1). DOI:10.2136/vzj2011.0089 · 1.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The abiotic precipitation of uranium (U(VI)) was evaluated in a
microfluidic pore structure (i.e. micromodel) to assess the efficacy of
using a phosphate amendment to immobilize uranium in groundwater and
mitigate the risk of this contaminant to potential down-gradient
receptor sites. U(VI) was mixed transverse to the direction of flow with
hydrogen phosphate (HPO42-), in the presence or absence of calcium
(Ca2+) or sulfate (SO42-), in order to identify precipitation rates, the
morphology and types of minerals formed, and the stability of these
minerals to dissolution with and without bicarbonate (HCO3-) present.
Raman backscattering spectroscopy and micro X-ray diffraction (μ-XRD)
results both showed that the only mineral precipitated was chernikovite
(also known as hydrogen uranyl phosphate; UO2HPO4), even though the
formation of other minerals were thermodynamically favored depending on
the experimental conditions. Precipitation and dissolution rates varied
with influent conditions. Relative to when only U(VI) and HPO42- were
present, precipitation rates were 2.3 times slower when SO42- was
present, and 1.4 times faster when Ca2+ was present. These rates were
inversely related to the size of crystals formed during precipitation.
Dissolution rates for chernikovite increased with increasing HCO3-
concentrations, consistent with formation of uranyl carbonate complexes
in aqueous solution, and they were the fastest for chernikovite formed
in the presence of SO42-, and slowest for the chernikovite formed in the
presence of Ca2+. These rates are related to the ratios of mineral-water
interfacial area to mineral volume. Fluorescent tracer studies and laser
confocal microscopy images showed that densely aggregated precipitates
blocked pores and reduced permeability. The results suggest that changes
in the solute conditions evaluated affect precipitation rates, crystal
morphology, and crystal stability, but not mineral type.
[Show abstract][Hide abstract] ABSTRACT: Permeability contrasts exist in multilayer geological formations under
consideration for carbon sequestration. To improve our understanding of
immiscible displacements in heterogeneous formations at the pore-scale,
liquid and supercritical CO2 (LCO2 and scCO2) - water displacement was
imaged in real-time in a pore network micromodel with two distinct
permeability zones, under 9 MPa pressure, and temperatures up to 41
°C. Due to the low viscosity ratio (logM = -1.1, -1.2), unstable
displacement occurred at all injection rates over two orders of
magnitude. CO2 displaced water only in the high permeability zone at low
injection rates with the mechanism shifting from capillary fingering to
viscous fingering with increasing flow rate. At high injection rates,
CO2 displaced water in the low permeability zone with capillary
fingering as the dominant mechanism. CO2 saturation (SCO2) as a function
of injection rate was quantified using fluorescent microscopy. In all
experiments, more than 50% of CO2 resided in the active flowpaths, and
this fraction increased as displacement transitioned from capillary to
viscous fingering. A continuum-scale two-phase flow model with
independently determined fluid and hydraulic parameters was used to
predict liquid CO2 saturation (SCO2) in the dual-permeability field.
Agreement with the micromodel experiments was obtained for low injection
rates. However, the numerical model does not account for the unstable
viscous fingering processes observed experimentally at higher rates and
hence overestimated SCO2.
[Show abstract][Hide abstract] ABSTRACT: Wettability is a critical parameter influencing immiscible fluid
displacements relevant to geological carbon sequestration. Fully
water-wet clean silica surfaces can be modified with silanes to alter
the wettability, with the majority of such efforts to date related to
conversions of water-wet to oil-wet systems. While a sizable literature
exist on contact angles obtained on silanized surfaces, these are by and
large air-water contact angle data, not the oil-water contact angles
needed. We have investigated a large range of silanes to modify silica
surfaces over a range of wettabilities, measuring both air-water and
oil-water contact angles. We have identified surface modifications to
produce intermediate wet surfaces. We have found a linear correlation
between air-water contact angles and oil-water contact angles, enabling
literature data on air-water contact angles to be interpreted in terms
of likely oil-water contact angles. In addition, we have found that
while glass and silica surfaces modified by the same chemistry give the
same contact angles in terms of air water contact angles, the surfaces
are not as similar in terms of oil-water contact angles. These studies
are being carried out in conjunction with immiscible displacements of
water by liquid and supercritical CO2 in microfabricated pore network
micromodels in silicon with oxidized silica surfaces and glass cover
[Show abstract][Hide abstract] ABSTRACT: Unstable immiscible fluid displacement in porous media affects geological carbon sequestration, enhanced oil recovery, and groundwater contamination by nonaqueous phase liquids. Characterization of immiscible displacement processes at the pore-scale is important to better understand macroscopic processes at the continuum-scale. A series of displacement experiments was conducted to investigate the impacts of viscous and capillary forces on displacement stability and fluid saturation distributions in a homogeneous water-wet pore network micromodel with precisely-microfabricated pore structures. Displacements were studied using seven wetting-nonwetting fluid pairs with viscosity ratios M (viscosity of the advancing nonwetting fluid divided by the viscosity of the displaced wetting fluid) ranging four orders of magnitude from logM = -1.95 to 1.88. The micromodel was initially saturated with either polyethylene glycol 200 (PEG200) or water as a wetting fluid, which was then displaced by a nonwetting alkane fluid under different flow rates. Capillary numbers (Ca) ranged over four orders of magnitude for the reported experiments, from logCa = -5.88 to -1.02. Fluorescent microscopy was used to visualize displacement and measure nonwetting fluid saturation distributions. These experiments extend the classical work by Lenormand et al. by using water-wet micromodels, high-precision fabrication, and enhanced image analysis of the saturation distributions. In the micromodel experiments initially saturated with PEG200, a viscous wetting fluid, unstable displacement occurred by viscous fingering over the whole range of imposed capillary numbers. For the experiments initially saturated with water, unstable displacement occurred by capillary fingering at low capillary numbers. When the viscous forces were increased by increasing the injection rate, crossover into stable displacement was observed for the fluid pairs with M > 0. For unstable displacement experiments applying the same capillary number for the various fluid pairs, nonwetting fluid saturations were higher when capillary fingering was the dominant fingering process compared to viscous fingering. Our saturation distributions are consistent with other published experimental work and confirm the numerical results obtained by Lenormand et al.
Energy & Fuels 08/2011; 25(8):3493-3505. DOI:10.1021/ef101732k · 2.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Permeability contrasts exist in multilayer geological formations under consideration for carbon sequestration. To improve our understanding of heterogeneous pore-scale displacements, liquid CO(2) (LCO(2))-water displacement was evaluated in a pore network micromodel with two distinct permeability zones. Due to the low viscosity ratio (logM = -1.1), unstable displacement occurred at all injection rates over 2 orders of magnitude. LCO(2) displaced water only in the high permeability zone at low injection rates with the mechanism shifting from capillary fingering to viscous fingering with increasing flow rate. At high injection rates, LCO(2) displaced water in the low permeability zone with capillary fingering as the dominant mechanism. LCO(2) saturation (S(LCO2)) as a function of injection rate was quantified using fluorescent microscopy. In all experiments, more than 50% of LCO(2) resided in the active flowpaths, and this fraction increased as displacement transitioned from capillary to viscous fingering. A continuum-scale two-phase flow model with independently determined fluid and hydraulic parameters was used to predict S(LCO2) in the dual-permeability field. Agreement with the micromodel experiments was obtained for low injection rates. However, the numerical model does not account for the unstable viscous fingering processes observed experimentally at higher rates and hence overestimated S(LCO2).
[Show abstract][Hide abstract] ABSTRACT: The effectiveness of in situ treatment using zero-valent iron (ZVI) for nonaqueous phase or significant sediment-associated contaminant mass can be limited by relatively low rates of mass transfer to bring contaminants in contact with the reactive media. For a field test in a trichloroethene (TCE) source area, combining moderate-temperature subsurface electrical resistance heating with in situ ZVI treatment was shown to accelerate TCE treatment by a factor of about 4 based on organic daughter products and a factor about 8 based on chloride concentrations. A mass-discharge-based analysis was used to evaluate reaction, dissolution, and volatilization processes at ambient groundwater temperature (~10 °C) and as temperature was increased up to about 50 °C. Increased reaction and contaminant dissolution were observed with increased temperature, but vapor- or aqueous-phase migration of TCE out of the treatment zone was minimal during the test because reactions maintained low aqueous-phase TCE concentrations.
[Show abstract][Hide abstract] ABSTRACT: Soil desiccation (drying), involving water evaporation induced by dry air injection and extraction, is a potentially robust remediation process to slow migration of inorganic or radionuclide contaminants through the vadose zone. The application of gas-phase partitioning tracer tests has been proposed as a means to estimate initial water volumes and to monitor the progress of the desiccation process at pilot-test and field sites. In this paper, tracer tests have been conducted in porous medium columns with various water saturations using sulfur hexafluoride as the conservative tracer and tricholorofluoromethane and difluoromethane as the water-partitioning tracers. For porous media with minimal silt and/or organic matter fractions, tracer tests provided reasonable saturation estimates for saturations close to zero. However, for sediments with significant silt and/or organic matter fractions, tracer tests only provided satisfactory results when the water saturation was at least 0.1 - 0.2. For dryer conditions, the apparent tracer retardation increases due to air soil sorption, which is not included in traditional retardation coefficients derived from advection-dispersion equations accounting only for air water partitioning and water soil sorption. Based on these results, gas-phase partitioning tracer tests may be used to determine initial water volumes in sediments, provided the initial water saturations are sufficiently large. However, tracer tests are not suitable for quantifying moisture content in desiccated sediments.
Vadose Zone Journal 05/2011; 10(2):634-641. DOI:10.2136/vzj2010.0101 · 1.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Deep Vadose Zone Treatability Test Plan for the Hanford Central Plateau includes testing of the desiccation technology as a potential technology to be used in conjunction with surface infiltration control to limit the flux of technetium and other contaminants in the vadose zone to the groundwater. Laboratory and modeling efforts were conducted to investigate technical uncertainties related to the desiccation process and its impact on contaminant transport. This information is intended to support planning, operation, and interpretation of a field test for desiccation in the Hanford Central Plateau.