LNAPLs do not Always Float: An Example Case of a Viscous LNAPL under Variable Water Table Conditions

An intermediate-scale experiment was conducted to investigate the behavior of a viscous LNAPL under variable water table conditions. Two LNAPL volumes were released from a small source zone on top of the flow cell into a partly saturated, homogeneously packed porous medium. Following a 30-day redistribution period, the water table was increased 0.5 m in 50 minutes. After the water table rise, LNAPL behavior was monitored for an additional 45 days. Fluid saturation scans were obtained periodically with a fully automated dual-energy gamma radiation system. Results show that both spills follow similar paths downwards. LNAPL drainage from the unsaturated zone was relatively slow and a considerable residual LNAPL saturation was observed after 30 days of drainage. Most of the mobile LNAPL moved into the capillary fringe during this period. After the water table rise, LNAPL moved up in a delayed fashion. After 45 days, the LNAPL has moved up only approximately 0.2 m. Since the LNAPL has only moved up a limited amount, nonwetting fluid entrapment was also limited. The experiment was simulated using the STOMP multifluid flow simulator. A comparison indicates that the simulator is able to predict the observed phenomena well, including residual saturation formation in the vadose zone, and limited upward LNAPL movement after the water table rise. The results of this experiment show that viscous mobile LNAPL, subject to variable water table conditions, does not necessarily float on the water table and may not appear in an observation well

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Two-phase immiscible displacement in porous media is controlled by capillary and viscous forces when gravitational effects are negligible. The relative importance of these forces is quantified through the dimensionless capillary number Ca and the viscosity ratio M between fluid phases. When the displacing fluid is Newtonian, the effects of Ca and M on the displacement patterns can be evaluated independently. However, when the injecting fluids exhibit shear-thinning viscosity behaviour the values of M and Ca are interdependent. Under these conditions, the effects on phase entrapment and the general displacement dynamics cannot be dissociated. In the particular case of shear-thinning aqueous polymer solutions, the degree of interdependence between M and Ca is determined by the polymer concentration. In this work, two-phase immiscible displacement experiments were performed in micromodels, using shear-thinning aqueous polymer solutions as displacing fluids, to investigate the effect of polymer concentration on the relationship between Ca and M, the recovery efficiency, and the size distribution of the trapped non-wetting fluid. Our results show that the differences in terms of magnitude and distribution of the trapped phase are related to the polymer concentration which influences the values of Ca and M

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

Apart 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 fill 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

Experimental Plan: 300 Area Treatability Test: In Situ Treatment of the Vadose Zone and Smear Zone Uranium Contamination by Polyphosphate Infiltration

The overall objectives of the treatability test is to evaluate and optimize polyphosphate remediation technology for infiltration either from ground surface, or some depth of excavation, providing direct stabilization of uranium within the deep vadose and capillary fringe above the 300 Area aquifer. Expected result from this experimental plan is a data package that includes: 1) quantification of the retardation of polyphosphate, 2) the rate of degradation and the retardation of degradation products as a function of water content, 3) an understanding of the mechanism of autunite formation via the reaction of solid phase calcite-bound uranium and aqueous polyphosphate remediation technology, 4) an understanding of the transformation mechanism, identity of secondary phases, and the kinetics of the reaction between uranyl-carbonate and –silicate minerals with the polyphosphate remedy under solubility-limiting conditions, 5) quantification of the extent and rate of uranium released and immobilized based on the infiltration rate of the polyphosphate remedy and the effect of and periodic wet-dry cycling on the efficacy of polyphosphate remediation for uranium in the vadose zone and capillary fringe, and 6) quantification of reliable equilibrium solubility values for autunite under hydraulically unsaturated conditions allowing accurate prediction of the long-term stability of autunite. Moreover, results of intermediate scale testing will quantify the transport of polyphosphate and degradation products, and yield degradation rates, at a scale that is bridging the gap between the small-scale UFA studies and the field scale. These results will be used to test and verify a site-specific, variable saturation, reactive transport model and to aid in the design of a pilot-scale field test of this technology. In particular, the infiltration approach and monitoring strategy of the pilot test would be primarily based on results from intermediate-scale testing. Results from this experimental plan will be documented in a PNNL report

Micron-Size Zero-Valent Iron Emplacement in Porous Media Using Polymer Additives: Column and Flow Cell Ex-periments

At the Hanford Site, an extensive In Situ Redox Manipulation (ISRM) permeable reactive barrier was installed to prevent chromate from reaching the Columbia River. However, chromium has been detected in several wells, indicating a premature loss of the reductive capacity in the aquifer. Laboratory experiments have been conducted to investigate whether barrier reductive capacity can be enhanced by adding micron-scale zero-valent iron to the high-permeability zones within the aquifer using shear-thinning fluids containing polymers. Porous media were packed in a wedge-shaped flow cell to create either a heterogeneous layered system with a high-permeability zone between two low-permeability zones or a high-permeability channel sur-rounded by low-permeability materials. The injection flow rate, polymer type, polymer concentration, and injected pore volumes were determined based on preliminary short- and long-column experiments. The flow cell experiments indicated that iron concentration enhancements of at least 0.6% (w/w) could be obtained using moderate flow rates and injection of 30 pore volumes. The 0.6% amended Fe0 concentration would provide approximately 20 times the average reductive capacity that is provided by the dithionite-reduced iron in the ISRM barrier. Calculations show that a 1-m-long Fe0 amended zone with an average concentration of 0.6% w/w iron subject to a groundwater velocity of 1 m/day will have an estimated longevity of 7.2 years

Carbon Tetrachloride Flow and Transport in the Subsurface of the 216-Z-18 Crib and 216-Z-1A Tile Field at the Hanford Site: Multifluid Flow Simulations and Conceptual Model Update

Carbon tetrachloride (CT) was discharged to the 216-Z-9, Z-1A, and Z-18 waste sites that are included in the 200-PW-1 Operable Unit in Hanford 200 West Area. Fluor Hanford, Inc. is conducting a Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) remedial investigation/feasibility study (RI/FS) for the 200-PW-1 Operable Unit. As part of this overall effort, Pacific Northwest National Laboratory (PNNL) was contracted to improve the conceptual model of how CT is distributed in the Hanford 200 West Area subsurface through use of numerical flow and transport modeling. This work supports the U.S. Department of Energy's (DOE's) efforts to characterize the nature and distribution of CT in the 200 West Area and subsequently select an appropriate final remedy

Treatability Test Plan for an In Situ Biostimulation Reducing Barrier

This treatability test plan supports a new, integrated strategy to accelerate cleanup of chromium in the 100 Areas at the Hanford Site. This plan includes performing a field-scale treatability test for bioreduction of chromate, nitrate, and dissolved oxygen. In addition to remediating a portion of the plume and demonstrating reduction of electron acceptors in the plume, the data from this test will be valuable for designing a full-scale bioremediation system to apply at this and other chromium plumes at the Hanford Site

Partitioning Tracers for In Situ Detection and Quantification of Dense Nonaqueous Phase Liquids in Groundwater Systems

The overall goal of the project is to explore the use of an innovative in-situ method (partitioning tracer tests) for detection and quantification of dense nonaqueous phase liquids (DNAPLs) in subsurface systems. DNAPLs occur in the subsurface at numerous contaminated sites and can act as long-term sources of groundwater contamination. Both effective risk assessment and remediation of DNAPL-contaminated sites is limited by current site characterization techniques. A major limitation of the current methods is that they provide data at discrete points, such that the probability of sampling a zone of localized DNAPL is quite small. The results of this research will lead to an improved method for characterizing DNAPL-contaminated sites and will enhance our understanding of the distribution of immiscible liquids in the subsurface. The use of this method will help reduce the uncertainty associated with risk assessments and remediation planning