Experimental and Numerical Investigations of Soil Desiccation for Vadose Zone Remediation: Report for Fiscal Year 2007

Abstract

SummaryApart from source excavation, the options available for the remediation of vadose zone metal and radionuclide contaminants beyond the practical excavation depth (0 to 15 m) are quite limited. Of the available technologies, very few are applicable to the deep vadose zone with the top-ranked candidate being soil desiccation. An expert panel review of the work on infiltration control and supplemental technologies identified a number of knowledge gaps that would need to be overcome before soil desiccation could be deployed. The report documents some of the research conducted in the last year to address some of these knowledge gaps. This work included 1) performing intermediate-scale laboratory flow cell experiments to demonstrate the desiccation process, 2) implementing a scalable version of Subsurface Transport Over Multiple Phases-Water-Air-Energy (STOMP-WAE), and 3) performing numerical experiments to identify the factors controlling the performance of a desiccation system.Four laboratory experiments were conducted in an intermediate-scale flow cell to demonstrate the desiccation process and assess the impact of the energy balance on the desiccation process. One of the experiments was performed in homogeneous medium-grained sand, while the rest of the simulations were completed in a heterogeneous system with an inclusion of a fine-grained sand zone and varying levels of external insulation. Evaporative cooling was observed in the experiments and generally decreased with increased distance from the gas inlet chamber. Observations of temperature in fine-grained sands in the heterogeneous systems show two local minima associated with the cooling because of evaporation in the adjacent medium-grained sand whereas the second is attributed to evaporative cooling in the fine-grained sand itself. The occurrence of these temperature minima coincided with passage of the desiccation front. Results of the laboratory tests are consistent with the theory showing that the injection of air leads to significant temperature and moisture changes in the porous medium. In all the experiments, the injection of dry air proved to be an effective means for removing essentially all moisture from the test media. The results of this study provide a unique data set that will be used to calibrate the STOMP code for use in the designs of field-scale desiccation systems.To aid in the design and evaluation of desiccation technologies, STOMP-WAE-B has been enhanced with the addition of gas injection/withdrawal wells. The upgraded version of STOMP-WAE-B is now available for scalable execution on multiple processor (i.e., parallel) computers. The parallel version of the simulator is written in pure FORTRAN 90 with embedded directives that are interpreted by a FORTRAN preprocessor. Without the preprocessor, the scalable version of the simulator can be executed sequentially on a single processor computer. The scalable version of the simulator is designated as STOMP-WAE-B-Sc and is expected to find applications in the design and optimization of ...

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

“…Thus, the impact of non‐uniform temperature is to spatially spread out the evaporation process. In laboratory flow cell tests, very sharp transitions between the zone of desiccation and nondesiccated zones were observed when temperature was relatively constant due to fast heat transfer from the flow cell walls that minimized evaporative cooling impact on temperature (Ward et al, 2008; Oostrom et al, 2009). For field applications, evaporative cooling can decrease temperatures over a large extent.…”

“…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.…”

“…Evaporation can remove pore water and may result in very low moisture contents and decreased water relative permeability in the desiccated zone (Ward et al, 2008;Oostrom et al, 2009Oostrom et al, , 2012aOostrom et al, ,2012bTruex et al, 2011). Due to these desiccation-induced changes, the future rate of movement of moisture and contaminants through this zone is decreased.Laboratory and modeling studies (Ward et al, 2008;Oostrom et al, 2009Oostrom et al, , 2012aOostrom et al, ,2012bTruex et al, 2011) suggest that the overall performance of desiccation in limiting water and contaminant l ux to the groundwater is a function of the i nal moisture content, contaminant concentration, sediment properties, size of the desiccated zone, the hydraulic properties A procedure for desicca ng vadose zones by injec on of dry gas was fi eld tested in a contaminated vadose zone at the DOE Hanford site. The results show the poten al for reducing the fl ux of contaminants to underlying groundwater by removing moisture and decreasing the water relative permeability.…”

Effects of Porous Medium Heterogeneity on Vadose Zone Desiccation: Intermediate‐Scale Laboratory Experiments and Simulations

“…All these applications are passive in nature. In combination with surface flux control, active subsurface desiccation through injection of relatively dry air has recently been proposed as a means to reduce water contents in subsurface layers (Fluor Hanford, Inc., 2006; Ward et al, 2008; Truex et al, 2011). The evaporation process resulting from dry air injection removes pore water and results in lower moisture content and aqueous‐phase relative permeability in the desiccated zone (Oostrom et al, 2009; Truex et al, 2011, 2012).…”

“…Recent field‐scale modeling studies (Ward et al, 2008; Truex et al, 2011) have indicated that the overall performance of desiccation in limiting water and contaminant flux to the groundwater is a function of the final moisture content (after desiccation), contaminant concentration, extent of the desiccated zone, the hydraulic properties and conditions in surrounding subsurface zones, and the net surface recharge rate. These modeling studies demonstrated that the desiccation rate is increased with higher temperature and lower relative humidity of the injected dry gas.…”