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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.

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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
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Report•DOI•
Remedy Evaluation Framework for Inorganic, Non-Volatile Contaminants in the Vadose Zone

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Michael J. Truex, Kenneth C. Carroll
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Abstract: Contaminants in the vadose zone may act as a potential long-term source of groundwater contamination and need to be considered in remedy evaluations. In many cases, remediation decisions for the vadose zone will need to be made all or in part based on projected impacts to groundwater. Because there are significant natural attenuation processes inherent in vadose zone contaminant transport, remediation in the vadose zone to protect groundwater is functionally a combination of natural attenuation and use of other remediation techniques, as needed, to mitigate contaminant flux to groundwater. Attenuation processes include both hydrobiogeochemical processes that serve to retain contaminants within porous media and physical processes that mitigate the rate of water flux. In particular, the physical processes controlling fluid flow in the vadose zone are quite different and generally have a more significant attenuation impact on contaminant transport relative to those within the groundwater system. A remedy evaluation framework is presented herein that uses an adaptation of the established EPA Monitored Natural Attenuation (MNA) evaluation approach and a conceptual model based approach focused on identifying and quantifying features and processes that control contaminant flux through the vadose zone. A key concept for this framework is to recognize that MNAmore » will comprise some portion of all remedies in the vadose zone. Thus, structuring evaluation of vadose zone waste sites to use an MNA-based approach provides information necessary to either select MNA as the remedy, if appropriate, or to quantify how much additional attenuation would need to be induced by a remedial action (e.g., technologies considered in a feasibility study) to augment the natural attenuation processes and meet groundwater protection goals.« less

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  • ...The BC Cribs and Trenches site has been the focus of vadose zone investigations (e.g., Ward et al. 2004; Truex et al. 2011b, 2012)....

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  • ...This approach supports consideration of active remedies to augment natural attenuation within the context of the recognized “enhanced attenuation” approach (ITRC 2010; Truex et al. 2011a)....

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  • ...These processes are essentially the same as those presented and described in detail in the EPA technical protocol for MNA of inorganic contaminants in groundwater (EPA 2007a, 2007b, 2010; ITRC 2010) and described with respect to conceptual site models by Truex et al. (2011a)....

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Journal Article•DOI•
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  • ...After desiccation, the flux rate of water and contaminants toward groundwater is reduced, as shown in laboratory and field studies (Oostrom et al., 2009; Truex et al., 2011, 2012, 2018)....

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  • ...Soil desiccation has also been demonstrated to be effective at creating dry conditions at the USDOE Hanford Site in southeastern Washington State, where moisture content and contaminant flux were lowered to near zero (Oostrom et al., 2009; Truex et al., 2011, 2012, 2013, 2018; Zhang, 2016)....

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Book Chapter•DOI•
Near-Surface Geophysics at the Hanford Nuclear Site, the United States

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Timothy C. Johnson1, Dale F. Rucker, Danney R. Glaser•
Pacific Northwest National Laboratory1
01 May 2015
TL;DR: In this paper, the authors discuss the Hanford Site operations and waste management practices that resulted in vadose zone and groundwater radionuclide contamination, and present several examples of how near-surface geophysical methods are being used to inform site operators of the properties and processes governing contaminant distribution and behavior.
Abstract: Operations associated with nuclear weapons development resulted in subsurface heavy metal and radionuclide contamination at many sites within the former US nuclear weapons development complex. Of these, the Hanford Site, which housed research and plutonium production facilities, faces some of the most complex and challenging subsurface remediation problems. In this chapter, we briefly discuss the Hanford Site operations and waste management practices that resulted in vadose zone and groundwater radionuclide contamination. We then present several examples of how near-surface geophysical methods are being used to inform site operators of the properties and processes governing contaminant distribution and behavior.

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Report•DOI•
Deep Vadose Zone Treatability Test for the Hanford Central Plateau: Interim Post-Desiccation Monitoring Results

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Michael J. Truex, Martinus Oostrom, Christopher E. Strickland, Timothy C. Johnson, Christian D. Johnson, Ray E. Clayton, Glen B. Chronister 
01 Sep 2013
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Abstract: A field test of desiccation is being conducted as an element of the deep vadose zone treatability test program. Desiccation technology relies on removal of water from a portion of the subsurface such that the resultant low moisture conditions inhibit downward movement of water and dissolved contaminants. Previously, a field test report (Truex et al. 2012a) was prepared describing the active desiccation portion of the test and initial post-desiccation monitoring data. Additional monitoring data have been collected at the field test site during the post-desiccation period and is reported herein along with interpretation with respect to desiccation performance. This is an interim report including about 2 years of post-desiccation monitoring data.

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Wave-Propagation Modeling and Inversion Using Frequency-Domain Integral Equation Methods

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  • ...2011), modeling studies (Ward et al. 2008; Truex et al. 2011), and field testing (Truex et al....

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  • ...Soil desiccation was investigated as a potential vadose zone remediation technology, including laboratory studies (Ward et al. 2008; Oostrom et al. 2009 2012a,b; Truex et al. 2011), modeling studies (Ward et al....

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

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  • ...Evaporation can remove pore water and result in very low moisture content in the desiccated zone (Ward et al. 2008; Oostrom et al. 2009; Truex et al. 2011)....

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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.