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

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.

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

“…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 (Oostrom et al, 2009, 2012a,2012b). Modeling studies (Ward et al, 2008; Truex et al, 2011) 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 (Ward et al, 2008; Oostrom et al, 2009, 2012a,2012b; Truex et al, 2011). 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 NaNO 3 (Truex et al, 2011).…”

“…In addition to rewetting from aqueous‐phase movement, rewetting can also occur through movement of vapor‐phase moisture (humid soil gas). Truex et al (2011) demonstrated that vapor‐phase rewetting can increase the moisture content to near the residual moisture content of tested porous media. However, the vapor‐phase rewetting process is also very slow without soil gas advection because of the relatively low moisture content of soil gas and slow diffusion‐driven movement of the humid gas.…”

“…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 toward the water table (Truex et al, 2011). To apply desiccation, a dry gas (relative humidity less than 100% at the in situ temperature) is injected into the subsurface.…”

“…Laboratory and modeling studies (Ward et al, 2008; Oostrom et al, 2009, 2012a,2012b; Truex et al, 2011) suggest that the overall performance of desiccation in limiting water and contaminant flux to the groundwater is 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. For instance, Truex et al (2011) demonstrated through numerical modeling that combinations of a surface infiltration barrier and subsurface desiccation reduced the contaminant flux and maximum resulting groundwater contaminant concentration compared to no‐treatment or surface‐barrier‐only scenarios. The extent of contaminant flux reduction 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.…”

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

“…Similar to the BY Cribs example, lateral spreading diminishes moisture conditions as an attenuation mechanism and an assessment of moisture conditions and porous media properties could be used to assess current fluxes in relation to the fluxes expected under long-term recharge-driven conditions (e.g., to assess whether future fluxes are expected to resemble Figure 1A or Figure 1B). The BC Cribs and Trenches site has been the focus of vadose zone investigations (e.g., Ward et al 2004;Truex et al 2011bTruex et al , 2012. These efforts have included simulation of water and Tc-99 flux to groundwater (Ward et al 2004).…”