Sensor and Numerical Simulator Evaluation for Porous Medium Desiccation and Rewetting at the Intermediate Laboratory Scale

Abstract

Soil desiccation, in conjunction with surface infiltration control, is considered at the Hanford Site as a potential technology to limit the flux of technetium and other contaminants in the vadose zone to the groundwater. An intermediate‐scale experiment was conducted to test the response of a series of instruments to desiccation and subsequent rewetting of porous media. The instruments include thermistors, thermocouple psychrometers, dual‐probe heat pulse sensors, heat dissipation units, and humidity probes. The experiment was simulated with the multifluid flow simulator STOMP, using independently obtained hydraulic and thermal porous medium properties. All instrument types used for this experiment were able to indicate when the desiccation front passed a certain location. In most cases the changes were sharp, indicating rapid changes in moisture content, water potential, or humidity. However, a response to the changing conditions was recorded only when the drying front was very close to a sensor. Of the tested instruments, only the heat dissipation unit and humidity probes were able to detect rewetting. The numerical simulation results reasonably match the experimental data, indicating that the simulator captures the pertinent gas flow and transport processes related to desiccation and rewetting and may be useful in the design and analysis of field tests.

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

“…Thus, while the leading edge of desiccation is following preferential flow pathways, the desiccated zone broadens over time and includes initially wetter regions closer to the injection well. This type of pattern of desiccation for adjacent coarse‐ and fine‐grained layers has also been observed in laboratory flow cell tests (Oostrom et al, 2009, 2012a, 2012b). At locations L1 through L4, all within about 3 m of the injection well, moisture content was also reduced at depths below the well screened interval (15.2 to 16.8‐m bgs) in a zone with an initial moisture content similar to the zone shown with arrow A (Fig.…”

“…Extensions of the advection‐based moisture content–matric potential relations, to include the region below the residual water contents where water vapor transport dominates, have been discussed by, e.g., Fayer and Simmons (1995), Morel‐Seytoux and Nimmo (1999), and Webb (2000). The relatively simple extension theory by Webb (2000) has been used successfully by Oostrom et al (2012a, 2012b) to simulate desiccation in intermediate‐scale flow cells.…”

“…Relative permeability (Mualem, 1976) as a function of volumetric moisture content, for a porous medium with a van Genuchten (1980) n value of 3.64 and residual moisture contents of 0, 0.03, 0.042, and 0.06 m 3 m −3 . The van Genuchten n value of 3.64 and residual moisture content of 0.042 m 3 m −3 are representative of the Hanford lysimeter sand (Oostrom et al 2012a). …”

“…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.…”

“…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).…”