scispace - formally typeset
Search or ask a question
Report•DOI•

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

Michael J. Truex, Martinus Oostrom, Vicky L. Freedman, Christopher E. Strickland, Thomas W. Wietsma, Guzel D. Tartakovsky, Anderson L. Ward 
23 Feb 2011-
TL;DR: 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 as mentioned in this paper.
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.

Summary (2 min read)

Jump to: [Introduction] – [2.1 Effect of Evaporative Cooling and Simple Heterogeneities on Desiccation] – [2.2 Solute Transport] – [2.2.1 Description of Experiments] – [2.2.2 Results of Experiments with 40/50-Mesh Sand] – [2.2.3 Experiments with 70-Mesh Sand] – [2.2.4 Experiments with Hanford Site Sand] and [2.2.5 Conclusions]

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

Figures (169)

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
Report•DOI•
Advanced Monitoring Systems for Deep Vadose Zone Applications Interim Status Report

[...]

Christopher E. Strickland1, Robert D. Mackley1, Vincent R. Vermeul1, Ray E. Clayton1, Jonathan N. Thomle1, Benjamin Q. Roberts1 •
Pacific Northwest National Laboratory1
30 Sep 2018

2 citations

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

  • ...…the same as those presented and described in detail in the U.S. Environmental Agency’s technical protocol for monitored natural attenuation of inorganic contaminants in groundwater (EPA 2007a,b, 2010; ITRC 2010) and described with respect to conceptual site models by Truex et al. (2011)....

    [...]

Technical Basis for Gas-Phase Vadose Zone Remediation Technologies at Hanford: A Review - 12186

[...]

M. Oostrom
01 Jan 2012
TL;DR: In situ vadose zone remediation approaches are being evaluated as potential options to mitigate the transport of inorganic and radionuclide contaminants from the vaderose zone to the groundwater as mentioned in this paper.
Abstract: In situ vadose zone remediation approaches are being evaluated as potential options to mitigate the transport of inorganic and radionuclide contaminants from the vadose zone to the groundwater. Some of the candidate approaches are based on changing the contaminant or subsurface conditions in a way that slows downward migration of the contaminants through the vadose zone using amendments delivered in the gas-phase. Two promising approaches that have undergone testing at Hanford include soil desiccation to address technetium-99 contamination and ammonia-induced sequestration of uranium. For soil desiccation, a dry gas is injected to desiccate a targeted portion of the subsurface and thereby decrease contaminant movement by removing moisture and decreasing the hydraulic conductivity of the desiccated zone. Ammonia-induced sequestration of uranium relies on changing the pore water chemistry, primarily through pH changes, to induce dissolution and precipitation processes that decrease the amount of mobile uranium in the vadose zone.
Advanced Simulation Capability for Environmental Management: Development and Demonstrations - 12532

[...]

Mark D. Freshley, Vicky L. Freedman, Ian Gorton, Susan S. Hubbard, J. David Moulton, Paul Dixon 
01 Jul 2012
TL;DR: The ASCEM project continues to make significant progress in development of capabilities, which are organized into Platform and Integrated Tool-sets and a High-Performance Computing Multi-process Simulator.
Abstract: The U.S. Department of Energy Office of Environmental Management (EM), Technology Innovation and Development is supporting development of the Advanced Simulation Capability for Environmental Management (ASCEM). ASCEM is a state-of-the-art scientific tool and approach for understanding and predicting contaminant fate and transport in natural and engineered systems. The modular and open source high-performance computing tool facilitates integrated approaches to modeling and site characterization that enable robust and standardized assessments of performance and risk for EM cleanup and closure activities. The ASCEM project continues to make significant progress in development of capabilities, which are organized into Platform and Integrated Tool-sets and a High-Performance Computing Multi-process Simulator. The Platform capabilities target a level of functionality to allow end-to-end model development, starting with definition of the conceptual model and management of data for model input. The High-Performance Computing capabilities target increased functionality of process model representations, tool-sets for interaction with Platform, and verification and model confidence testing. The new capabilities are demonstrated through working groups, including one focused on the Hanford Site Deep Vadose Zone. The ASCEM program focused on planning during the first year and executing a prototype tool-set for an early demonstration of individual components. Subsequently, ASCEM has focused on developingmore » and demonstrating an integrated set of capabilities, making progress toward a version of the capabilities that can be used to engage end users. Demonstration of capabilities continues to be implemented through working groups. Three different working groups, one focused on EM problems in the deep vadose zone, another investigating attenuation mechanisms for metals and radionuclides, and a third focusing on waste tank performance assessment, continue to make progress. The project experience has been that the working group mechanism continues to be useful for helping guide development and prepare the capabilities for end users. (authors)« less

Additional excerpts

  • ...The technology currently under evaluation is soil desiccation, an approach that minimizes Tc-99 movement in the vadose zone by removing pore water via the injection of dry air and extraction of water vapor [8]....

    [...]

Remediation Technologies Screening Report for the Deep Vadose Zone, Hanford's Central Plateau - 12414

[...]

Martin Doornbos, John Morse
01 Jul 2012
TL;DR: Deep Vadose Zone contamination is defined as the sediment below the limit of typical surface-based remedies (such as, excavation or caps), but above the water table as mentioned in this paper.
Abstract: Deep Vadose Zone contamination is a significant issue because it represents a potential source for continued release of contamination to the groundwater and associated receptors. This contamination, which is the result of past waste disposal practices on the Hanford Site Central Plateau, occurs deep in the subsurface and is not easily remediated by typical surface remedies. The Deep Vadose Zone is defined as the sediment below the limit of typical surface-based remedies (such as, excavation or caps), but above the water table. The Central Plateau Deep Vadose Zone begins at a depth of approximately 15 m (50 ft) below ground surface and extends to a depth of approximately 76 m (250 ft) below ground surface. Cleanup of the Deep Vadose Zone is challenging because contamination is difficult to access and expensive to characterize; contaminants occur at different depths and soil types; conventional, surface-based remedies have limited effectiveness; and remedy performance is difficult to predict, test, and monitor. Typically, remedial technologies for Deep Vadose Zone contamination are less developed than for the shallow soil contamination or saturated groundwater contaminants. In addition, few remediation technologies have been tested in the field, and fewer still have been successfully implemented as full remedial actions. These challenges, along with the limited number of potentially applicable remediation technologies, complicate the decision-making process for evaluating and selecting Deep Vadose Zone remedial alternatives.
Report•DOI•
Deep Vadose Zone Treatability Test for the Hanford Central Plateau: Interim Post-Desiccation Monitoring Results, Fiscal Year 2014

[...]

Michael J. Truex, Christopher E. Strickland, Christian D. Johnson, Timothy C. Johnson, Ray E. Clayton, Glen B. Chronister 
01 Sep 2014
TL;DR: A field test of desiccation is being conducted as an element of the DOE test plan published in more than March 2008 to meet Milestone M 015 50 as mentioned in this paper, which directed DOE to submit a treatability test plan for remediation of technetium-99 and uranium in the deep vadose zone.
Abstract: Over decades of operation, the U.S. Department of Energy (DOE) and its predecessors have released nearly 2 trillion L (450 billion gal.) of liquid into the vadose zone at the Hanford Site. Much of this discharge of liquid waste into the vadose zone occurred in the Central Plateau, a 200 km2 (75 mi2) area that includes approximately 800 waste sites. Some of the inorganic and radionuclide contaminants in the deep vadose zone at the Hanford Site are at depths below the limit of direct exposure pathways, but may need to be remediated to protect groundwater. The Tri-Party Agencies (DOE, U.S. Environmental Protection Agency, and Washington State Department of Ecology) established Milestone M 015 50, which directed DOE to submit a treatability test plan for remediation of technetium-99 (Tc-99) and uranium in the deep vadose zone. These contaminants are mobile in the subsurface environment and have been detected at high concentrations deep in the vadose zone, and at some locations have reached groundwater. Testing technologies for remediating Tc-99 and uranium will also provide information relevant for remediating other contaminants in the vadose zone. A field test of desiccation is being conducted as an element of the DOE test plan published in more » March 2008 to meet Milestone M 015 50. The active desiccation portion of the test has been completed. Monitoring data have been collected at the field test site during the post-desiccation period and are reported herein. This is an interim data summary report that includes about 3 years of post-desiccation monitoring data. The DOE field test plan proscribes a total of 5 years of post-desiccation monitoring. « less

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

  • ...Desiccation of a portion of the vadose zone, in conjunction with a surface infiltration barrier, has the potential to minimize migration of deep vadose zone contaminants towards the water table (Truex et al. 2011)....

    [...]

  • ...In the laboratory, desiccation was shown to be capable of reducing the moisture content to below the residual moisture content of the porous medium (Truex et al. 2011; Ward et al. 2008; Oostrom et al. 2009)....

    [...]

  • ...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 slowly by diffusion of water vapor and increases the moisture content of desiccated porous medium to…...

    [...]

  • ...Laboratory and modeling studies have been conducted to study desiccation and provide a technical basis for its use as a potential remedy (Truex et al. 2011; Ward et al. 2008; Oostrom et al. 2009, 2011, 2012a and b)....

    [...]

  • ...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 and b; Truex et al. 2011, 2012a and b, 2013a and b, 2014)....

    [...]

Related Papers (5)
Desiccation of Unsaturated Porous Media: Intermediate-Scale Experiments and Numerical Simulation

[...]

01 Aug 2009-Vadose Zone Journal
Martinus Oostrom, Thomas W. Wietsma, Jacob H. Dane, Michael J. Truex, Anderson L. Ward 
Field-Scale Assessment of Desiccation Implementation for Deep Vadose Zone Contaminants

[...]

01 Nov 2012-Vadose Zone Journal
Michael J. Truex, Martinus Oostrom, Christopher E. Strickland, Glen B. Chronister, Mark W. Benecke, Christian D. Johnson 
Electrical Resistivity Correlation to Vadose Zone Sediment and Pore-Water Composition for the BC Cribs and Trenches Area

[...]

01 Jun 2009
R. Jeffrey Serne, Anderson L. Ward, Wooyong Um, Bruce N. Bjornstad, Dale F. Rucker, David C. Lanigan, Mark W. Benecke 
Vadose Zone Contaminant Fate and Transport Analysis for the 216-B-26 Trench

[...]

14 Oct 2004
Anderson L. Ward, Glendon W. Gee, Zheng Zhang, Jason M. Keller 
Review: Technical and policy challenges in deep vadose zone remediation of metals and radionuclides.

[...]

11 Mar 2011-Environmental Science & Technology
P. Evan Dresel, Dawn M. Wellman, Kirk J. Cantrell, Michael J. Truex 
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.