scispace - formally typeset
SciSpace - Your AI assistant to discover and understand research papers | Product Hunt

ReportDOI

Laboratory and Modeling Evaluations in Support of Field Testing for Desiccation at the Hanford Site

23 Feb 2011-

AbstractThe 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.

Topics: Hanford Site (57%), Vadose zone (55%)

Summary (2 min read)

Introduction

  • Laboratory and modeling efforts were conducted to investigate technical uncertainties related to the desiccation process and its impact on contaminant transport.
  • The experimental results also suggest that for slowly moving desiccation fronts and high solute concentrations (>100 g/L), some redistribution of solute may occur in the soil moisture and in the direction of the solute concentration gradient.
  • This report documents the modeling and laboratory results pertinent to these elements conducted in support of evaluating desiccation and planning for a field test.

2.1 Effect of Evaporative Cooling and Simple Heterogeneities on Desiccation

  • Soil desiccation , involving water evaporation induced by air injection and extraction, is a potentially robust vadose zone remediation process to limit migration of inorganic or radionuclide contaminants through the vadose zone.
  • A series of detailed, intermediate-scale laboratory experiments, using unsaturated homogeneous and heterogeneous systems, were conducted to improve understanding of energy balance issues related to soil desiccation.
  • The fine-grained sand embedded in the medium-grained sand of the heterogeneous system showed two local temperature minima associated with the cooling.
  • Results of the laboratory tests were simulated accurately only if the thermal properties of the flow cell walls and insulation material were taken into account, indicating that the appropriate physics were incorporated into the simulator.
  • Details of these laboratory experiments are reported in Oostrom et al. (2009).

2.2 Solute Transport

  • Experiments were conducted to examine the impact of solute concentration on the desiccation process.
  • Results suggest that desiccation rate is not a function of solute concentration.
  • The experimental results also suggest that for slowly moving desiccation fronts and high solute concentrations (>100 g/L), some redistribution of solute may occur in the soil moisture and in the direction of the solute concentration gradient.
  • Because the sediment is relatively dry behind the desiccation front, solute migration will occur in the direction of the desiccation front movement or laterally at the edges of the desiccated area.
  • Maximum concentration factors of about 120% of the initial concentration were observed in the onedimensional column experiments.

2.2.1 Description of Experiments

  • A series of one-dimensional column experiments were conducted to evaluate the movement of NaNO3 salt during desiccation.
  • All experiments were conducted in the vertical direction.
  • To verify that assumption, two experiments were conducted for each porous material in which fluids in the packed column were allowed to redistribute for 2 weeks.
  • The average desiccation rate for each column was computed by dividing the distance from the upper to lower humidity probe (90 cm) divided by the difference in arrival time of the drying fronts at these locations.

2.2.2 Results of Experiments with 40/50-Mesh Sand

  • Results shown in Figures 2.1 and 2.2 demonstrate that water and salt do not migrate during a 14-day redistribution period.
  • The results show that for the experiments with initial salt concentrations of 1 and 10 g/L, no preferential salt movement could be observed.
  • A clear trend in the concentrations was obvious for the experiments conducted with 100 and 500 g/L salt.
  • For the 100 g/L experiment, the dimensionless concentration ranged from 0.95 at the inlet to 1.02 at the outlet.

2.2.3 Experiments with 70-Mesh Sand

  • Results shown in Figures 2.6 and 2.7 demonstrate that water and salt do not migrate during a 14-day redistribution period for this particular sand, although the added volume per Kg of sand is 50 mL. 2.8 A comparison of the desiccation experiments with a rate of 1 L/min are shown in Figure 2.8.
  • An increase in the salt concentrations with distance from the inlet is observed for the experiments conducted with 100 and 500 g/L salt.
  • For both experiments, the range is about the same as for the experiments in the 40/50 sand.
  • As for the 40/50 sand, the salt concentration ranges were smaller for the higher rate than for the lower rate .

2.2.4 Experiments with Hanford Site Sand

  • Results shown in Figures 2.10 and 2.11 demonstrate that water and salt do not migrate during a 14-day redistribution period for the Hanford Site sand.
  • The water saturations for both experiments after 14 days are near the initial 0.28 .
  • As for the experiments with the 40/50 and 70 laboratory sands, results show for the initial salt concentrations of 1 and 10 g/L, no preferential salt movement occurred.
  • An obvious increase in salt 2.10 concentrations with distance from the inlet is observed for the experiments conducted with 100 and 500 g/L salt.
  • The data in Table 2.3 show that for the Hanford Site sand experiments, the desiccation rate is not affected by the initial salt concentration.

2.2.5 Conclusions

  • Experiments reported herein examined the impact of salt concentration on the desiccation process.
  • Because the sediment is relatively dry behind the desiccation front, solute migration will occur in the direction of the desiccation front movement or laterally at the edges of the desiccated area.
  • Maximum concentration factors of about 120% of the initial concentration were observed in the one-dimensional column experiments.
  • This moderate concentration increase does not affect the desiccation process because the desiccation rate is independent of the salt concentration.
  • The impact of the solute concentration front on rewetting and over larger distances in the subsurface still needs to be investigated.

Did you find this useful? Give us your feedback

...read more

Content maybe subject to copyright    Report

PNNL-20146
Prepared for the U.S. Department of Energy
under Contract DE-AC05-76RL01830
Laboratory and Modeling
Evaluations in Support of Field
Testing for Desiccation at the
Hanford Site
MJ Truex
M Oostrom
VL Freedman
CE Strickland
TW Wietsma
GD Tartakovsky
AL Ward
February 2011


PNNL-20146
Laboratory and Modeling
Evaluations in Support of Field
Testing for Desiccation at the
Hanford Site
MJ Truex
M Oostrom
VL Freedman
CE Strickland
TW Wietsma
GD Tartakovsky
AL Ward
February 2011
Prepared for
the U.S. Department of Energy
under Contract DE-AC05-76RL01830
Pacific Northwest National Laboratory
Richland, Washington 99352

iii
Abstract
The Deep Vadose Zone Treatability Test Plan for the Hanford Central Plateau
1
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.
1
U.S. Department of Energy. 2008. Deep Vadose Zone Treatability Test Plan for the Hanford Central Plateau.
DOE/RL-2007-56, Rev. 0, U.S. Department of Energy, Richland Operations Office, Richland, Washington.

v
Summary
The Deep Vadose Zone Treatability Test Plan for the Hanford Central Plateau (DOE/RL 2008)
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.
A vadose zone technical panel was convened in 2005 to evaluate potential vadose zone technologies,
including desiccation (FHI 2006). In their evaluation, panel members provided guidance on the type of
uncertainties that need to be resolved before applying desiccation as part of a remedy. This guidance,
additional external technical review comments, and subsequent development of data quality objectives for
the desiccation field test were used to develop a scope for modeling and laboratory efforts in support of
the desiccation treatability test.
Described below are the primary conclusions of the laboratory and modeling efforts as related to the
elements of the project scope in support of applying desiccation for the Hanford Central Plateau vadose
zone.
Impact of evaporative cooling on desiccation rate. Evaporative cooling occurs during desiccation
at and adjacent to desiccation fronts to an extent that can be accurately quantified based on known
processes. The impact of locally decreased temperatures on the overall desiccation rate is relatively small
because the soil gas is warmed as it moves away from the desiccation front. For estimation purposes, the
moisture capacity and volumetric rate of the injected gas at the in situ temperature is reasonable to use in
estimating the desiccation rate.
Impact of solutes on desiccation and the fate of solutes during desiccation. Experiments demon-
strated the desiccation rate is not a function of salt concentration. As such, inclusion of salt concen-
trations in estimates of desiccation rate is not necessary. The experimental results also suggest that for
slowly moving desiccation fronts and high solute concentrations (>100 g/L), some redistribution of solute
may occur in the soil moisture and in the direction of the solute concentration gradient. Because the
sediment is relatively dry behind the desiccation front, solute migration will occur in the direction of the
desiccation front movement or laterally at the edges of the desiccated area. Maximum concentration
factors of about 120% of the initial concentration were observed in the one-dimensional column
experiments. This moderate concentration increase does not affect the desiccation process because the
desiccation rate is independent of the salt concentration.
Impact of porous media heterogeneity on desiccation. Desiccation rate is a function of soil gas
flow rate. Thus, where layers of contrasting permeability are present, desiccation occurs to the greatest
extent in higher permeable layers.
Evaluation of rewetting phenomena after desiccation. Vapor-phase rewetting increases moisture
content to less than the irreducible water saturation value, but not further. Thus, the desiccated zone
relative aqueous phase permeability may be assumed to be negligible, and therefore short-term advective
water movement induced by vapor-phase rewetting can be ignored. Advective rewetting of a desiccated
zone occurs based on standard unsaturated water flow processes. For the field test, humidity will be the

Citations
More filters

Journal ArticleDOI
TL;DR: The major processes for deep vadose zone metal and radionuclide remediation are reviewed, finding efforts to remove contaminants have generally been unsuccessful although partial removal may reduce downward flux.
Abstract: Contamination in deep vadose zone environments is isolated from exposure so direct contact is not a factor in its risk to human health and the environment. Instead, movement of contamination to the groundwater creates the potential for exposure and risk to receptors. Limiting flux from contaminated vadose zone is key for protection of groundwater resources, thus the deep vadose zone is not necessarily considered a resource requiring restoration. Contaminant discharge to the groundwater must be maintained low enough by natural attenuation (e.g., adsorption processes or radioactive decay) or through remedial actions (e.g., contaminant mass reduction or mobility reduction) to meet the groundwater concentration goals. This paper reviews the major processes for deep vadose zone metal and radionuclide remediation that form the practical constraints on remedial actions. Remediation of metal and radionuclide contamination in the deep vadose zone is complicated by heterogeneous contaminant distribution and the sat...

58 citations


Journal ArticleDOI
Abstract: Soil desiccation was recently field tested as a potential vadose zone remediation technology. Desiccation removes water from the vadose zone and significantly decreases the aqueous-phase permeability of the desiccated zone, thereby decreasing movement of moisture and contaminants. The two- and three-dimensional distribution of moisture content reduction with time provides valuable information for desiccation operations and for determining when treatment goals have been reached. This type of information can be obtained through the use of geophysical methods. Neutron moisture logging, cross-hole electrical resistivity tomography, and cross-hole ground-penetrating radar approaches were evaluated with respect to their ability to provide effective spatial and temporal monitoring of desiccation during a treatability study conducted in the vadose zone of the USDOE Hanford site in the state of Washington.

25 citations


Cites background from "Laboratory and Modeling Evaluations..."

  • ...…of movement of moisture and contaminants through this zone is decreased. h e performance of desiccation in terms of mitigating future moisture movement is related to the extent of moisture content reduction and the location and thickness of the desiccated zone (Truex et al., 2011, 2012a, 2012b)....

    [...]

  • ...Evaporation can remove pore water and result in a very low moisture content in the desiccated zone (Ward et al., 2008; Oostrom et al., 2009; Truex et al., 2011). h e desiccation process removes previously disposed and native water from the vadose zone and signii cantly decreases the water relative…...

    [...]

  • ...Desiccation of a portion of the vadose zone, in conjunction with a surface ini ltration barrier, has the potential for minimizing the migration of deep vadose zone contaminants toward the water table (Truex et al., 2011)....

    [...]

  • ...…investigated as a potential vadose zone remediation technology, including laboratory studies (Ward et al., 2008; Oostrom et al., 2009, 2012a, 2012b; Truex et al., 2011), modeling studies (Ward et al., 2008; Truex et al., 2011), and i eld testing (Truex et al., 2012a, 2012b) conducted as part of…...

    [...]

  • ...…including laboratory studies (Ward et al., 2008; Oostrom et al., 2009, 2012a, 2012b; Truex et al., 2011), modeling studies (Ward et al., 2008; Truex et al., 2011), and i eld testing (Truex et al., 2012a, 2012b) conducted as part of the treatability test ef orts at the USDOE Hanford site....

    [...]


Journal ArticleDOI
Abstract: Desiccation of the vadose zone has the potential to reduce the flux of contaminants to underlying groundwater by removing moisture and decreasing the aqueous-phase permeability of the desiccated zone. However, data to evaluate implementation of desiccation are needed to enable consideration of desiccation as a potential remedy. Implementation of desiccation was field tested by injecting dry nitrogen gas to a target treatment zone and monitoring the spatial and temporal progress of the drying process. Aqueous waste discharges to disposal cribs approximately 50 years ago distributed water and contaminants, including primarily technetium-99 and nitrate, within the 100-m deep vadose zone at the test site. A field test location was selected adjacent to one of the former disposal cribs. The test was conducted in a contaminated portion of the vadose zone dominated by fine sands with lenses of silt material. Desiccation reduced volumetric moisture content to as low as 0.01. The lateral and vertical distribution of drying from the injection well was influenced by the subsurface heterogeneity. However, over time, desiccation occurred in the initially wetter, lower permeability lenses.

21 citations


Cites background from "Laboratory and Modeling Evaluations..."

  • ...Evaporation can remove pore water and may result in very low moisture contents and decreased water relative permeability in the desiccated zone (Ward et al., 2008; Oostrom et al., 2009, 2012a,2012b; Truex et al., 2011)....

    [...]

  • ...Laboratory and modeling studies (Ward et al., 2008; Oostrom et al., 2009, 2012a,2012b; Truex et al., 2011) suggest that the overall performance of desiccation in limiting water and contaminant l ux to the groundwater is a function of the i nal moisture content, contaminant concentration, sediment…...

    [...]

  • ...Truex et al. (2011) examined rewetting of desiccated zones in the laboratory and found that vapor-phase rewetting from adjacent humid soil gas, in the absence of advective soil gas movement, occurs by dif usion of water vapor and increases the moisture content of desiccated porous medium to a…...

    [...]

  • ...Truex et al. (2011) demonstrated that vapor-phase rewetting can increase the moisture content to near the residual moisture content of tested porous media....

    [...]

  • ...Desiccation of a portion of the vadose zone, in conjunction with a surface ini ltration barrier, has the potential of minimizing migration of deep vadose zone contaminants toward the water table (Truex et al., 2011)....

    [...]


Journal ArticleDOI
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.

12 citations


Cites background or methods from "Laboratory and Modeling Evaluations..."

  • ...…zone, in conjunction with a surface ini ltration barrier, has the potential of minimizing downward contaminant migration toward the water table (Truex et al., 2011). h e basic idea of desiccation technology is to inject dry air to desiccate a portion of the vadose zone and slow downward water…...

    [...]

  • ...2). h e distance from the 1 to 10% water content contour is only a few centimeters, indicating sharp desiccation fronts that were also observed in l ow cell experiments described by Oostrom et al. (2009) and by Truex et al. (2011)....

    [...]

  • ...Details of the l ow cell are reported in Truex et al. (2011). h e instruments were emplaced in i ve bundles (Fig....

    [...]


Journal ArticleDOI
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 agreementsmore » 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.« less

9 citations


Cites background from "Laboratory and Modeling Evaluations..."

  • ...…removes pore water and results in lower moisture content and aqueousphase relative permeability in the desiccated zone (Oostrom et al., 2009; Truex et al., 2011, 2012). h rough this mechanism, the future rate of moisture movement and contaminant transport through this zone may be decreased....

    [...]

  • ...Several of these concerns have been studied in considerable detail through numerical modeling and intermediate-scale experimentation (Oostrom et al., 2009, 2011; Truex et al., 2011), with the exception of the ef ects of permeability contrasts on desiccation ef ectiveness....

    [...]

  • ...…dry air has recently been proposed as a means to reduce water contents in subsurface layers (Fluor Hanford, Inc., 2006; Ward et al., 2008; Truex et al., 2011). h e evaporation process resulting from dry air injection removes pore water and results in lower moisture content and…...

    [...]

  • ...Recent i eld-scale modeling studies (Ward et al., 2008; Truex et al., 2011) have indicated that the overall performance of desiccation in limiting water and contaminant l ux to the groundwater is a function of the i nal moisture content (at er desiccation), contaminant concentration, extent of the…...

    [...]


Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "Laboratory and modeling evaluations in support of field testing for desiccation at the hanford site" ?

In this paper, the authors evaluated the desiccation properties of different types of sensors, such as thermistors, TCPs, DPHP sensors, HDUs, and humidity probes.