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

C E Strickland

G B Chronister

J E Szecsody

M J Truex

M Oostrom

M W Benecke

CiteSeerX

WM2012 Conference, February 26 – March 1, 2012, Phoenix, Arizona, USA
1
Technical Basis for Gas-Phase Vadose Zone Remediation Technologies 
at Hanford: A Review – 12186
M.J. Truex*, M. Oostrom*, J.E. Szecsody*, C.E. Strickland*, 
G.B. Chronister**, and M.W. Benecke**
*Pacific Northwest National Laboratory, Richland, Washington
**CH2M Hill Plateau Remediation Company, Richland, Washington
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.
INTRODUCTION
Inorganic and radionuclide contaminants in the deep vadose zone are at depths below the limit 
of direct exposure pathways, but may need to be remediated to protect groundwater [1].  The 
groundwater contaminant concentrations that result from vadose zone contamination are a 
function of the rate of contaminant movement through the vadose zone.  For remediation, the 
magnitude of contaminant discharge from the vadose zone to the groundwater must be 
maintained low enough to achieve groundwater protection goals.  
The Department of Energy Hanford Site contains a significant amount of contamination that 
resides in a 60-to-100-m-thick vadose zone due to past discharges associated with plutonium 
production operations.  Much of this contamination is deep in the vadose zone where 
remediation options are limited by the physical and hydrogeologic properties of the vadose 
zone.  There are several distinct categories of deep vadose zone problems at Hanford.  The two 
principal deep vadose zone contaminants of concern are technetium-99 and uranium [2].  Other 
contaminants such as iodine-129 and nitrate are also prevalent in the deep vadose zone and 
groundwater.  The BC cribs and trenches waste site at Hanford is an example of vadose zone 
contamination issues.  This waste site contains 26 cribs and trenches that received about 
110 million liters of liquid waste, primarily in the mid 1950s.  The waste contained about 410 
curies of technetium-99 [3].  There is no evidence that the contamination has reached 
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groundwater, located about 100 m below ground surface (bgs) in this area.  Initial 
characterization efforts indicate that the technetium-99 inventory is located mostly at a depth in 
the vadose zone of between about 30 and 50 m bgs.  Transport model predictions, however, 
indicate the potential for this contamination to adversely impact groundwater in the future [4].
In 2008, the Department of Energy initiated a treatability test program to evaluate potential deep 
vadose zone remedies for protection of groundwater [2].  As part of this effort, 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 transport 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.  
SUMMARY OF SOIL DESICCATION
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 
[5].  To apply desiccation, a dry gas (relative humidity less than 100% at the in situ temperature) 
is injected into the subsurface.  The dry gas evaporates water from the porous medium until the 
gas reaches 100% relative humidity.  The evaporation process can remove pore water and 
result in very low moisture content in the desiccated zone [5, 6, 7].  The desiccation process 
removes previously disposed water from the vadose zone and significantly decreases the 
aqueous-phase permeability of the desiccated zone.  Through these mechanisms, the future 
rate of movement of moisture and contaminants through the desiccated zone is decreased.  
Laboratory and modeling studies have been conducted to study desiccation and provide a 
technical basis for its use as a potential remedy [5, 6, 7].  In these studies, the overall 
performance of desiccation in limiting water and contaminant flux to the groundwater was shown 
to be a function of the final moisture content, contaminant concentration, sediment properties, 
size of the desiccated zone, the hydraulic properties and conditions in surrounding subsurface 
zones, and the net surface recharge rate.  Desiccation was shown to be capable of reducing the 
moisture content to below the residual moisture content of the porous medium [5, 6, 7].  Under 
these conditions, the relative aqueous-phase permeability is near zero and subsequent moisture 
movement is significantly hindered [8].  Truex et al. [5] demonstrated through numerical 
modeling that combinations of a surface infiltration barrier and subsurface desiccation enhanced 
protection of groundwater compared to no-treatment or surface-barrier-only scenarios.  The 
effectiveness of desiccation was related to the thickness and vertical location of the imposed 
desiccated zone in relation to the location of the elevated moisture and contaminant conditions.  
While the concentration of solutes increased in the desiccated zone in these simulations, this 
effect did not lead to a significant high-concentration pulse to the groundwater.  
After a targeted portion of the vadose zone is desiccated, rewetting of this zone can occur by 
vapor- and aqueous-phase moisture transport.  The timescale of rewetting is related to the 
overall performance of desiccation in minimizing contaminant flux to the groundwater.  Truex et 
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al. [5] 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 a limited extent, nominally to near the residual moisture content for the porous 
medium.  The aqueous-phase rewetting rate was found to be a function of the relative aqueous-
phase permeability of the porous medium. 
Key factors that impact applying desiccation are the initial moisture content, permeability 
contrasts between adjacent sediment layers, and temperature and relative humidity of the 
injected gas.  Laboratory studies [5, 6, 7] and field testing [8] have shown that the rate of 
desiccation is directly related to the water-holding capacity of the injected dry gas, the initial 
moisture content, and the number of pore volumes of dry gas transported through the porous 
medium.  Because the transport of dry gas is directly related to the permeability of the porous 
medium, higher permeability zones in soil columns and flow cells packed with heterogeneous 
media dried more quickly than lower permeability zones [7].  Modeling studies [5, 6]
demonstrated that the desiccation rate is increased with higher temperature and lower relative 
humidity of the injected dry gas, consistent with laboratory studies where the thermodynamic 
factors controlling the water-holding capacity of the injected dry gas were correlated with the 
desiccation rate [5, 6, 7].  Laboratory studies have also demonstrated that the concentration of 
solutes in the pore water does not significantly affect the desiccation rate for solute 
concentration ranging up to 5.8M of sodium nitrate [5].  Field test data were consistent with 
expectations based on these laboratory and modeling efforts [8].  
SUMMARY OF AMMONIA TREATMENT FOR URANIUM CONTAMINATION
The subsurface geochemistry can be altered to sequester uranium by injecting vapor-phase 
ammonia. 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.  Figure 1 depicts the three primary 
elements of uranium treatment by ammonia vapor.  When a mixed gas containing ammonia 
vapor is injected into an unsaturated porous medium, a large percentage of the ammonia 
partitions into the pore water due to the low Henry’s Law constant (a dimensionless value of 
about 6.5E10-4) of ammonia (Step 1, Figure 1).  For instance, a 5% by volume ammonia vapor 
produces an equilibrium pore water concentration of about 3 M ammonia.  Through dissociation, 
this ammonia concentration results in the pore water, starting at around pH 8, rising to about pH 
11.5 [9, 10, 11].  Ion exchange and mineral dissolution (including silicate dissolution) is caused 
by the caustic pH (Step 2, Figure 1) [9, 10, 11].  With high total dissolved solids, precipitates 
start to form, especially as the pH is buffered back toward neutral.  The precipitates may 
incorporate uranium (e.g., sodium boltwoodite) or may be compounds such as quartz,
chrysotile, calcite, diaspore, hematite that could coat uranium already precipitated or adsorbed 
on the sediment surface (Step 3, Figure 1) [9, 10, 11].  The goal of the dissolution and re-
precipitation process is to create uranium precipitates or coatings that render uranium less 
mobile than before treatment.  Note that chemical reduction of the uranium is not a component 
of these processes; thus, the oxidation state of the vadose zone does not directly impact the 
longevity of precipitates that incorporate or coat uranium.
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Figure 1. Overview of the processes that occur with injection of ammonia vapor into an 
unsaturated contaminated Hanford sediment.
The ammonia process targets uranium contaminants in the vadose zone that were the result of 
past disposal processes.  Vadose zone contamination at the Hanford Site results from past 
uranium and plutonium enrichment activities and the intended or unintended release of 
202,703 kg of uranium to the ground surface [12] in a variety of aqueous solutions (i.e., acidic, 
basic, with organic complexants [citrate, ethylenediaminetetraacetic acid]) and inorganic ligands 
(CO3, PO4).  Uranium contamination exists in multiple forms in the subsurface as a result of 
these waste discharges.  In addition to dissolved uranium and associated adsorbed uranium [9, 
10, 13], the uranium contamination in Hanford sediments from the Central Plateau area have
been found as a uranium-silicate (Na-boltwoodite; Na(UO2)(SiO4)*1.5H2O), uranophane 
[Ca(UO2)2(SiO3OH)2(H2O)5], and as uranium-calcite co-precipitates [14, 15].  
Recent laboratory investigations of the ammonia treatment process used a sequential extraction 
process to operationally define the initial distribution of uranium within different phases in the 
sediment and to then determine the impact of ammonia treatment on this distribution.  
Sequential liquid extractions at a 2:1 solution:sediment ratio have been used as shown in Table 
1 [9, 10]. Uranium content in the extraction solution was determined by kinetic 
phosphorescence analysis [16] with a detection limit of 0.1 µg/L.  
Table I.  Sequential Extraction Solutions
Extraction 
Solution
Hypothesized 
targeted sediment 
components
Interpreted uranium 
mobility of extracted 
fraction
Color 
Code
1. Aqueous: 
uncontaminated 
Hanford 
groundwater
Uranium in pore water 
and a portion of 
sorbed uranium
Mobile phase
2. Ion Exch.: 
1M Mg-nitrate
Readily desorbed 
uranium
Readily mobile 
through equilibrium 
partitioning
3. Acetate 
pH5: 1 hour in pH 
5 sodium acetate 
Uranium associated 
with surface exposed 
carbonate 
Moderately mobile 
through rapid 
dissolution processes
WM2012 Conference, February 26 – March 1, 2012, Phoenix, Arizona, USA
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solution precipitates, including 
uranium carbonates, 
or other readily 
dissolved precipitates
4. Acetate pH 
2.3: 
1 week in pH 2.3 
acetic acid
Dissolution of most 
carbonate 
compounds, including 
uranium carbonates, 
and sodium 
boltwoodite
Slow dissolution 
processes are 
associated with 
uranium release from 
this fraction such that 
uranium mobility is low 
with respect to 
impacting 
groundwater
5. 8M HNO3: 
2 hours in 8M 
nitric acid at 95oC
Dissolution of most 
minerals expected to 
contain uranium, 
considered to 
represent total 
uranium extraction for 
this study1
Very slow dissolution 
processes are 
associated with 
uranium release from 
this fraction such that 
uranium mobility is 
very low with respect 
to impacting 
groundwater
1some sediment digestion techniques may release more uranium from a given sample
The data from extraction solutions are not necessarily representative of a single uranium 
surface phase [10].  For example, a uranium-carbonate sample (no sediment) subjected to the 5
sequential extractions showed that 84% was dissolved by extraction solution #3, 15% by 
extraction solution #4, and 1% in the extraction #5.  In contrast, the Na-boltwoodite sample 
subjected to the 5 sequential extractions showed that 13% was dissolved by extraction solution 
#3 and 84% by extraction solution #4, with the remainder in extraction #5.
Laboratory efforts to evaluate the ammonia treatment processes have used a range of 
contaminated sediments from the Hanford site that contain the above forms of uranium [9, 10].  
Figure 2 shows a set of sediment samples collected from the 241-U tank farm area at Hanford
[10].  For these samples, both the overall concentration and amount of uranium in the different 
extraction solutions changed with depth.  With ammonia treatment, the dissolved uranium and 
associated adsorbed uranium (extraction solutions #1 and #2) decreased in all but two of the 
samples, uranium in extraction solutions #3 and #4 (Table 1) showed a mixture of increases and 
decreases, and the uranium in extraction solution #5 increased in 79% of the samples [10].  
Qualitatively, these results suggest that ammonia treatment shifts the uranium within most 
samples from this Hanford location to phases or physical locations (e.g., under mineral 
coatings) that would be less mobile than prior to treatment.
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Figure 2. Fraction of the total uranium extracted using the solutions defined in Table 1 from 
sediment samples at the 241-U tank farm area of Hanford for a) untreated sediments and b) 
sediments contacted with 10% ammonia vapor for one month (after [10]).  Color coding of the 
extraction solutions is shown in Table 1. 
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6. Ward A.L., M. Oostrom, and D.H. Bacon.  2008.  “Experimental and Numerical Investigations 
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Technical Basis for Gas-Phase Vadose Zone Remediation Technologies at Hanford: A Review -12186

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

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