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2004 Journal Articles

Transport of carbon-14 in a large unsaturated soil column

MA Plummer, LC Hull, DT Fox
Idaho National Engineering and Environmental Laboratory, Idaho Fall, ID

Vadose Zone Journal 3 (1): 109-121 FEB (2004)

Abstract

Wastes buried at the Radioactive Waste Management Complex (RWMC) of the Idaho National Engineering and Environmental Laboratory (INEEL) include activated metals that release radioactive C-14 as they corrode. To test and refine transport predictions that describe releases to the environment with time, we conducted a series of transport experiments with nonreactive gas- and aqueous-phase tracers and inorganic C-14 species in a large unsaturated soil column filled with sediment representative of that at the RWMC. The tracer tests, hydraulic measurements, and chemical monitoring provided constraints on physical transport parameters, water content, and aqueous gas partitioning behavior. With those constraints, we estimated a solid - aqueous distribution coefficient for the sediment through in verse modeling of the C-14 transport data, using both a simple gas-diffusion model and a multiphase flow and transport simulator ( STOMP). Results indicate that C-14 transport in this system is well described by a reactive gas diffusion model, with a pH-dependent factor. Fitting transport simulations to the early-time transport data yielded K-d approximate to 0.5 +/- 0.1 mL g(-1), while soil samples removed approximately 1 yr later yielded K-d values of 0.8 to 2.4 mL g(-1). These values are consistent with those derived from smaller-scale experiments, demonstrating that laboratory-based measurements pro vide a valid means of estimating transport behavior at much larger spatial and temporal scales. Assuming that (CO2)-C-14 migration in the RWMC is dominated by gas transport, our results suggest that most C-14 released from the RWMC would discharge to the atmosphere rather than to the underlying Snake River Plain aquifer.

Infiltration and redistribution of LNAPL into unsaturated layered porous media

EL Wipfler, M Ness, GD Breedveld, A Marsman, SEATM Van Der Zee
University of Wageningen and Research Center, Wageningen, Netherlands

Journal of Contaminant Hydrology 71 (1-4): 47-66 JUL (2004)

Abstract

Enhanced understanding of light non-aqueous phase liquid (LNAPL) infiltration into heterogeneous porous media is important for the effective design of remediation strategies. We used a 2-D experimental facility that allows for visual observation of LNAPL contours in order to study LNAPL redistribution in a layered porous medium. The layers are situated in the unsaturated zone near the watertable and they are inclined to be able to observe the effect of discontinuities in capillary forces and relative permeabilities. Two experiments were performed. The first experiment consisted of LNAPL infiltration into a fine sand matrix with a coarse sand layer, and the second experiment consisted of a coarse sand matrix and a fine sand layer. The numerical multi-phase flow model STOMP was validated with regard to the experimental results. This model is able to adequately reproduce the experimental LNAPL contours. Numerical sensitivity analysis was also performed. The capillarity contrast between sands was found to be the main controlling factor determining the final LNAPL distribution.

A numerical study of micro-heterogeneity effects on upscaled properties of two-phase flow in porous media

DB Das1, SM Hassanizadeh2, BE Rotter3, B Ataie-Ashtiani4
1University of Oxford, Oxford, England
2Delft University of Technology, Delft, Netherlands
3University of Edinburgh, Midlothian, Scotland
4Sharif University of Technology, Tehran, Iran

Transport in Porous Media 56 (3): 329-350 SEP (2004)

Abstract

Commonly, capillary pressure - saturation - relative permeability (P-c - S - K-r) relationships are obtained by means of laboratory experiments carried out on soil samples that are up to 10 - 12 cm long. In obtaining these relationships, it is implicitly assumed that the soil sample is homogeneous. However, it is well known that even at such scales, some micro-heterogeneities may exist. These heterogeneous regions will have distinct multiphase flow properties and will affect saturation and distribution of wetting and non-wetting phases within the soil sample. This, in turn, may affect the measured two-phase flow relationships. In the present work, numerical simulations have been carried out to investigate how the variations in nature, amount, and distribution of sub-sample scale heterogeneities affect P-c - S - K-r relationships for dense non-aqueous phase liquid (DNAPL) and water flow. Fourteen combinations of sand types and heterogeneous patterns have been defined. These include binary combinations of coarse sand imbedded in fine sand and vice versa. The domains size is chosen so that it represents typical laboratory samples used in the measurements of P-c - S - K-r curves. Upscaled drainage and imbibition P-c - S - K-r relationships for various heterogeneity patterns have been obtained and compared in order to determine the relative significance of the heterogeneity patterns. Our results show that for micro-heterogeneities of the type shown here, the upscaled P-c - S curve mainly follows the corresponding curve for the background sand. Only irreducible water saturation ( in drainage) and residual DNAPL saturation ( in imbibition) are affected by the presence and intensity of heterogeneities.

A parameter scaling concept for estimating field-scale hydraulic functions of layered soils

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

Journal of Hydraulic Research 42:93-103 (2004)

Abstract

Predicting flow and transport in unsaturated porous media is often hampered by limited data and the uncertainties in constitutive property information at the appropriate spatial scales. Some studies have used inverse flow modeling for parameter estimation to overcome these limitations. However, determination of the soil hydraulic parameters of layered soils remains a challenge since inverting for too many parameters can lead to the non-uniqueness of parameter values. Here we propose a parameter scaling method that reduces the number of parameters to be estimated. First, parameter scaling factors are determined using local-scale parameter values. After assigning scaling factors to the corresponding soil textures in the field, the reference hydraulic parameter values at the field scale can be estimated through inverse modeling of well-designed field experiments. Finally, parameters for individual textures are obtained through inverse scaling of the reference values. The number of unknown variables is reduced by a factor equal to the number of textures (M) and the simulation time is reduced by the square of the number of textures (M (super 2) ). The proposed method was tested using two infiltration-drainage experiments in layered soils. The STOMP numerical simulator was combined with the inverse modeling program, UCODE, to estimate the hydraulic parameters. Simulation errors were significantly reduced after applying parameter scaling and inverse modeling. When compared to the use of local-scale parameters, parameter scaling reduced the sum of squared weighted residual by 93-96%.

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