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2002 Journal Articles

Fluid flow, heat transfer, and solute transport at nuclear waste storage tanks in the Hanford vadose zone

K. Pruess1, S. Yabusaki2, C. Steefel3, P. Lichtner4

1Lawrence Berkeley National Laboratory, Berkeley, CA
2Pacific Northwest National Laboratory, Richland, WA
3Lawrence Livermore National Laboratory, Livermore, CA
4Los Alamos National Laboratory, Los Alamos, NM

Vadose Zone Journal 1(1):68-88 (2002).

Abstract

At the Hanford Site, highly radioactive and chemically aggressive waste fluids have leaked from underground storage tanks into the vadose zone. This paper addresses hydrogeological issues at the 241-SX tank farm, especially focusing on Tank SX-108, which is one of the highest heat load, supernate density and ionic strength tanks at Hanford and a known leaker. The behavior of contaminants in the unsaturated zone near SX-108 is determined by an interplay of multiphase fluid flow and heat transfer processes with reactive chemical transport in a complex geological setting. Numerical simulation studies were performed to obtain a better understanding of mass and energy transport in the unique hydrogeologic system created by the SX tank farm. Problem parameters are patterned after conditions at Tank SX-108, and measured data were used whenever possible. Borrowing from techniques developed in geothermal and petroleum reservoir engineering, our simulations feature a comprehensive description of multiphase processes, including boiling and condensation phenomena, and precipitation and dissolution of solids. We find that the thermal perturbation from the tank causes large-scale redistribution of moisture and alters water seepage patterns. During periods of high heat load, fluid and heat flow near the tank are dominated by vapor-liquid counterflow (heat pipe), which provides a much more efficient mechanism than heat conduction for dissipating tank heat. The heat pipe mechanism is also very effective in concentrating dissolved solids near the heat source, where salts may precipitate even if they were only present in small concentrations in ambient fluids. Tank leaks that released aqueous fluids of high ionic strength into the vadose zone were also modeled. The heat load causes formation dry-out beneath the tank, which is accompanied by precipitation of solutes. These may become remobilized at a later time when tank temperatures decline and previously dried out regions are rewetted. Simulated temperature and moisture distributions compare well with borehole measurements performed in 2000. The temperature maximum observed beneath Tank SX-108 can be explained from past thermal history of the tank; it is not necessary to invoke heat generation from leaked radioactive contaminants. A novel composite medium model is used to explore effects of moisture tension-dependent anisotropy, which is shown to have important impacts on fluid flow and solute transport in the Hanford sediments.

Effects of heterogeneities on capillary pressure-saturation-relative permeability relationships

B. Ataie-Ashtiani1, S.M. Hassanizadeh2, M.A. Celia3

1Sharif University of Technology, Department of Civil Engineering, Tehran, Iran
2Delft University of Technology, Netherlands
3Princeton University, United States

Journal of Contaminant Hydrology 56(3-4):175-192 (2002).

Abstract

In theories of multiphase flow through porous media, capillary pressure-saturation and relative permeability-saturation curves are assumed to be intrinsic properties of the medium. Moreover, relative permeability is assumed to be a scalar property. However, numerous theoretical and experimental works have shown that these basic assumptions may not be valid. For example, relative permeability is known to be affected by the flow velocity (or pressure gradient) at which the measurements are carried out. In this article, it is suggested that the nonuniqueness of capillary pressure-relative permeability-saturation relationships is due to the presence of microheterogeneities within a laboratory sample. In order to investigate this hypothesis, a large number of "numerical experiments" are carried out. A numerical multiphase flow model is used to simulate the procedures that are commonly used in the laboratory for the measurement of capillary pressure and relative permeability curves. The dimensions of the simulation domain are similar to those of a typical laboratory sample (a few centimeters in each direction). Various combinations of boundary conditions and soil heterogeneity are simulated and average capillary pressure, saturation, and relative permeability for the "soil sample" are obtained. It is found that the irreducible water saturation is a function of the capillary number; the smaller the capillary number, the larger the irreducible water saturation. Both drainage and imbibition capillary pressure curves are found to be strongly affected by heterogeneities and boundary conditions. Relative permeability is also found to be affected by the boundary conditions; this is especially true about the nonaqueous phase permeability. Our results reveal that there is much need for laboratory experiments aimed at investigating the interplay of boundary conditions and microheterogeneities and their effect on capillary pressure and relative permeability.

The influence of hydraulic nonequilibrium on pressure plate data

G.W. Gee, A.L. Ward, Z.F. Zhang, G.S. Campbell, J. Mathison
Pacific Northwest National Laboratory, Richland, WA

Vadose Zone Journal 1: 172-178, (2002).

Abstract

Pressure plates are used routinely to measure water-retention characteristics of soils. Plates of varying porosity are used, depending on the pressure range of interest. For applied pressures up to 1.5 MPa, 15-bar porous ceramic plates with fine porosity are used because of their high bubbling pressure (>1.5 MPa), which limits airflow through the plate. The typical saturated hydraulic conductivity of the 15-bar plate is less than 3 x 10-11 m s-1. Low plate conductance coupled with decreasing soil hydraulic conductivities (e.g., < 1 x 10-11 m s-1) at high pressures strongly influence equilibrium times, which theoretically may extend to months or years. We measured the soil-water pressures (suctions) for three soils, a sand, a silt loam, and a clay, placed on 15-bar pressure plates for 10 days or longer, with and without static loads and with and without using a kaolinite slurry for improved plate contact. Total matric suctions, inferred from peltier psychrometry data, were always less than 1.0 MPa. When sample height was increased from 1.5 cm to 3 cm, the water contents increased and total suctions decreased to 0.15 MPa for sand, 0.3 MPa for silt loam, and 0.55 MPa for clay. These data suggest that alternative methods to pressure plates may be required to measure equilibrium water suctions of soils in reasonable times in the 1.5 MPa (15 bar) pressure range and that loading of the samples and use of kaolinite slurry appear to be ineffective in speeding equilibrium.

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