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...

Review: Technical and policy challenges in deep vadose zone remediation of metals and radionuclides.

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.

Monitoring Vadose Zone Desiccation with Geophysical Methods

Integrated hydrogeophysical modelling and data assimilation for geoelectrical leak detection.

Evaluation of deep vadose zone contaminant flux into groundwater: Approach and case study.

The U.S. Department of Energy (DOE) recently invested in developing a numerical modeling toolset called ASCEM (Advanced Simulation Capability for Environmental Management) to support modeling analyses at legacy waste sites. This investment includes the development of an open-source user environment called Akuna that manages subsurface simulation workflows. Core toolsets accessible through the Akuna user interface include model setup, grid generation, sensitivity analysis, model calibration, and uncertainty quantification. Additional toolsets are used to manage simulation data and visualize results. This new workflow technology is demonstrated by streamlining model setup, calibration, and uncertainty analysis using high performance computation for the BC Cribs Site, a legacy waste area at the Hanford Site in Washington State. For technetium-99 transport, the uncertainty assessment for potential remedial actions (e.g., surface infiltration covers) demonstrates that using multiple realizations of the geologic conceptual model results in greater variation in concentration predictions than when a single model is used. Akuna provides integrated toolset needed for subsurface modeling workflow.Akuna streamlines process of executing multiple simulations in HPC environment.Akuna provides visualization tools for spatial and temporal data.Example application demonstrates risk with remediation impacting infiltration rates.

A high-performance workflow system for subsurface simulation

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.

Field-Scale Assessment of Desiccation Implementation for Deep Vadose Zone Contaminants

Super-absorbent polymers (SAPs) have the potential to remove water and associated contaminants from unsaturated sediments in the field. Column and flow cell experiment were conducted to test the ability of four types of SAPs to remove water from unsaturated porous media. Column experiments, with emplacement of a layer of polymer on top of unsaturated porous media, showed the ability of the SAPs to extract up to 80% of the initially emplaced water against gravity into the sorbent over periods up to four weeks. In column experiments where the sorbent was emplaced between layers of unsaturated porous media, gel formation was observed at both the sorbent-porous medium interfaces. The extraction percentages over four weeks of contact time were similar for both column configurations and no obvious differences were observed for the four tested SAPs. Two different flow cells were used to test the wicking behavior of SAPs in two dimensions using three configurations. The largest removal percentages occurred for the horizontal sorbent layer configuration which has the largest sorbent-porous medium interfacial area. In a larger flow cell, a woven nylon “sock” was packed with sorbent and subsequently placed between perforated metal plates, mimicking a well configuration. After one week of contactmore » time the sock was removed and replaced by a fresh sock. The results of this experiment showed that the sorbent was able to continuously extract water from the porous media, although the rate decreased over time. The declining yield during both periods is associated with the sharp reduction in water saturation and relative permeability near the sorbent. It was also observed that the capillary pressure continued to increase over the total contact time of 14 days, indicating that the sorbent remained active over that period. This work has demonstrated the potential of soil moisture wicking using SAPs at the proof-of-principle level.« less

Vadose Zone Soil Moisture Wicking Using Super-Absorbent Polymers

Perched-water conditions have been observed in the vadose zone above a fine-grained zone that is located a few meters above the water table within the B, BX, and BY Tank Farms area. The perched water contains elevated concentrations of uranium and technetium-99. This perched-water zone is important to consider in evaluating the future flux of contaminated water into the groundwater. The study described in this report was conducted to examine the perched-water conditions and quantitatively evaluate 1) factors that control perching behavior, 2) contaminant flux toward groundwater, and 3) associated groundwater impact.

/pdf/perched-water-evaluation-for-the-deep-vadose-zone-beneath-547uphkutp.pdf

Perched-Water Evaluation for the Deep Vadose Zone Beneath the B, BX, and BY Tank Farms Area of the Hanford Site

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.

/pdf/deep-vadose-zone-treatability-test-for-the-hanford-central-1n7zbjwjs8.pdf

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

/pdf/deep-vadose-zone-treatability-test-of-soil-desiccation-for-4hnc8em47h.pdf

Glen B. Chronister

Papers

Field-Scale Assessment of Desiccation Implementation for Deep Vadose Zone Contaminants

Vadose Zone Soil Moisture Wicking Using Super-Absorbent Polymers

Perched-Water Evaluation for the Deep Vadose Zone Beneath the B, BX, and BY Tank Farms Area of the Hanford Site

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

Deep Vadose Zone Treatability Test of Soil Desiccation for the Hanford Central Plateau: Final Report