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Abstracts
Plateau, where reprocessing operations occurred. The vadose zone at this location is comprised of about 75 meters of
water-unsaturated, unconsolidated, stratified sediments above groundwater that discharges to the Columbia River.
Contaminants in this zone originated from intentional discharges to cribs, retention basins, and trenches, and from
unintended tank waste releases in the tank farms. The “deep vadose zone” is defined as the region below the practical
depth of surface remedy influence (e.g., excavation or surface barrier). At the Hanford Site, this region poses unique
challenges for characterization and remediation.
In 2008, the Department of Energy initiated a deep vadose zone treatability test to seek remedies for technetium-99
and uranium contamination. The treatability approach includes laboratory, modeling, and field tests building on previous
work at the Hanford Site. Initial laboratory and field tests for technetium-99 have focused on use of desiccation which
could be used in combination with an infiltration barrier to slow the transport of technetium-99 in the subsurface. For
uranium contamination, reactive gas technologies are being tested as one component of the overall treatability test
approach.
More recently, in recognition of the need for a broader strategy, the Department of Energy initiated an integrated
study of the deep vadose zone to establish a technical basis for addressing risk-driving contaminants and supporting
selection of remediation approaches for the deep vadose zone that are protective of groundwater. This is a cooperative
effort that combines the resources and contributions of research scientists, technology developers, and remediation
contractors.
The objectives of the deep vadose zone strategy are to:
• Develop sufficient understanding of the nature and extent of deep vadose zone contamination and processes that
affect fate and transport;
• Develop predictive capabilities for describing contaminant fate and transport as well as flux from the vadose
zone to the groundwater;
• Develop, test and deploy effective methods for remediating contaminated areas;
• Develop and deploy effective monitoring methods for assessing the performance of remedies and for
determining the long-term threat of contaminants to the groundwater.
5) 40235 – Advanced Remedial Methods for Metals and Radionuclides in Deep Vadose Zone Environments
Dawn Wellman, Shas Mattigod, Ann Miracle, Lirong Zhong, Danielle Jansik, PNNL (USA);
Susan Hubbard, Yuxin Wu, LBNL (USA); Martin Foote, MSE (USA)
Deep vadose zone environments can be a primary source and pathway for contaminant migration to groundwater.
These environments present unique characterization and remediation challenges that necessitate scrutiny and research.
The thickness, depth, and intricacies of the deep vadose zone, combined with a lack of understanding of the key
subsurface processes (e.g., biogeochemical and hydrologic) affecting contaminant migration, make it difficult to create
validated conceptual and predictive models of subsurface flow dynamics and contaminant behavior across multiple scales.
These factors also make it difficult to design and deploy sustainable remedial approaches and monitor long-term
contaminant behavior after remedial actions.
Functionally, the methods for addressing contamination must remove and/or reduce transport of contaminants. This
problem is particularly challenging in the arid western USA where the vadose zone is hundreds of feet thick, rendering
transitional excavation methods exceedingly costly and ineffective. Delivery of remedial amendments is one of the most
challenging and critical aspects for all remedy-based approaches. The conventional approach for delivery is through
injection of aqueous remedial solutions. However, heterogeneous deep vadose zone environments present hydrologic and
geochemical challenges which limit the effectiveness. Because the flow of solution infiltration is dominantly controlled
by gravity and suction, injected liquid preferentially percolates through highly permeable pathways, by-passing
low-permeability zones which frequently contain the majority of contamination. Moreover, the wetting front can readily
mobilize and enhance contaminant transport to the underlying aquifer prior to stabilization. Development of innovative,
in-situ technologies may be the only way to meet remedial action objectives and long-term stewardship goals.
Shear-thinning fluids (i.e., surfactants) can be used to lower the liquid surface tension and create stabile foams,
which readily penetrate low permeability zones. Although surfactant foams have been utilized for subsurface
mobilization efforts in the oil and gas industry, so far, the concept of using foams as a delivery mechanism for
transporting remedial amendments into deep vadose zone environments to stabilize metal and long-lived radionuclide
contaminants has not been explored. Foam flow can be directed by pressure gradients, rather than being dominated by
gravity; and, foam delivery mechanisms limit the volume of water (< 5% vol.) required for remedy delivery and
emplacement, thus mitigating contaminant mobilization. We will present the results of a numerical modeling and
integrated laboratory- / intermediate-scale investigation to simulate, develop, demonstrate, and monitor (i.e. advanced
geophysical techniques and advanced predictive biomarkers) foam-based delivery of remedial amendments to remediate
metals and radionuclides in vadose zone environments.
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