No abstract

643T techniques may not be applicable at sites where inorganic or radionuclide contaminants in the unsaturated zone are located deeply across a large area. Under these conditions, mobile contaminants might reach the water table at unacceptable concentrations. Desiccation applied to the deep vadose zone has the potential of minimizing downward contaminant migration toward the water table. Th e basic idea of desiccation technology is to inject dry air and to withdraw an equal volume of wet air in an array of wells. In combination with surface fl ux control, the technique can be used to immobilize contamination by reducing aqueous-phase transport.Currently, a soil desiccation pilot test is being prepared at the U.S. Department of Energy's Hanford Site in Washington. The objective of the test is to determine if the mobility of contaminants such as 99 Tc and NO 3 can be signifi cantly lessened by removing sediment pore water, resulting in a reduced threat to the groundwater. Th e test will be conducted in the BC Cribs and Trenches waste site area where the largest vadose zone inventory of mobile 99 Tc contamination on the Hanford Site has been identifi ed.A technical panel, convened at the Hanford Site (Fluor Hanford, Inc., 2006), identifi ed a number of implementation issues to be addressed before desiccation can be deployed. Th ese issues included (i) energy limitations on the volume of water that can be removed, (ii) osmotic eff ects during soil drying, and (iii) potential remobilization of contaminants after cessation of desiccation.Th e fi rst issue is related to the relatively large amount of energy that is needed to evaporate water (~2500 J g −1 ). Th is energy has to be provided by the injected air or the subsurface conducting the air. Th e amount of water that can potentially be removed per unit volume of air, at a given temperature, is the diff erence between the saturated water-vapor density and the water-vapor density of the injected dry air. Since energy has to be provided for water evaporation, however, it is more likely that water-vapor saturation will take place at the lower wet-bulb temperature, yielding a lower saturated water-vapor density. For example, the saturated water-vapor density of 15°C air is 13 g m −3 ; however, the density at the wet-bulb temperature of 5.5°C is only 7 g m −3 . Th e considerable diff erence between these two numbers may result in a slower water removal than the theoretical rate.Th e second issue relates to potential osmotic eff ects, because vapor pressures are lower in zones of increased salt concentrations and lower vapor pressures may reduce the effi ciency and A : PVC, polyvinyl chloride.O R Soil desicca on (drying), involving water evapora on induced by air injec on and extrac on, is a poten ally robust vadose zone remedia on process to limit migra on of inorganic or radionuclide contaminants through the vadose zone. Desicca on also has the poten al to improve gas-phase-based treatments by reducing water satura on and therefore increasing sediment gas-phase permeabil...

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.

No abstract

Approximately 190 kg of 2 μm‐diameter zero‐valent iron (ZVI) particles were injected into a test zone in the top 2 m of an unconfined aquifer within a trichloroethene (TCE) source area. A shear‐thinning fluid was used to enhance ZVI delivery in the subsurface to a radial distance of up to 4 m from a single injection well. The ZVI particles were mixed in‐line with the injection water, shear‐thinning fluid, and a low concentration of surfactant. ZVI was observed at each of the seven monitoring wells within the targeted radius of influence during injection. Additionally, all wells within the targeted zone showed low TCE concentrations and primarily dechlorination products present 44 d after injection. These results suggest that ZVI can be directly injected into an aquifer with shear‐thinning fluids to induce dechlorination and extends the applicability of ZVI to situations where other emplacement methods may not be viable.

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.

No abstract

Methods are developed to use data collected during cyclic operation of soil vapor extraction (SVE) systems to help characterize the magnitudes and timescales of mass flux associated with vadose zone contaminant sources. Operational data collected at the Department of Energy’s Hanford site are used to illustrate the use of such data. An analysis was conducted of carbon tetrachloride vapor concentrations collected during and between SVE operations. The objective of the analysis was to evaluate changes in concentrations measured during periods of operation and non-operation of SVE, with a focus on quantifying temporal dynamics of the vadose zone contaminant mass flux, and associated source strength. Three mass-flux terms, representing mass flux during the initial period of a SVE cycle, during the asymptotic period of a cycle, and during the rebound period, were calculated and compared. It was shown that it is possible to use the data to estimate time frames for effective operation of an SVE system if a sufficient set of historical cyclic operational data exists. This information could then be used to help evaluate changes in SVE operations, including system closure. The mass-flux data would also be useful for risk assessments of the impact of vadose-zone sources on groundwater contamination or vapor intrusion.