2003 Journal Articles
Simulation of carbon tetrachloride migration from the 216-Z-9 Trench at Hanford
M.L. Rockhold1, M. Oostrom1, M.D. White1, P.D. Thorne1, G.V. Last1, M.J. Truex1, V.J. Rohay2
1Pacific Northwest National Laboratory, Richland, WA
2Fluor Hanford, Richland, WA
Abstracts with Programs - Geological Society of America 35(6):449 (2003).
Abstract
From July 1955 through June 1962, approximately 4,090,000 L of waste water, including 316,000 L of a mixture of dense, non-aqueous phase liquids (DNAPL), was discharged into the vadose zone of the Hanford Site through the 216-Z-9 Trench. Approximately 74% (by volume) of this DNAPL was carbon tetrachloride (CCl (sub 4) ). Three-dimensional numerical simulations were conducted of the three-phase (aqueous, gas, NAPL) migration of the disposed liquids from the trench using the STOMP code. The model domain is 440-m (E-W) by 540-m (N-S) by 201-m (ground surface to top of basalt) and consists of 12 hydrostratigraphic units constructed from geologic interpretations of borehole data (including drillers' logs, geologists' logs, particle size data, calcium carbonate content, moisture content, and geophysical logs). EarthVision (super R) software was used to interpolate the hydrostratigraphic units between boreholes. Simulations were conducted from 1955 through present to evaluate the evolution and current status of the plume. In addition to the base case, 20 sensitivity cases were simulated to analyze the sensitivity of the model results to uncertainties in the total volume of DNAPL released, DNAPL fluid properties, source characteristics (area and infiltration rates), soil hydraulic properties (porosity, permeability, anisotropy, fluid entry pressures, pore geometry parameters), and maximum residual DNAPL saturation. Simulation results are compared with observed field data and are evaluated using spatial moment analyses and integrated fluxes of DNAPL, dissolved aqueous-phase CCl (sub 4) , and dissolved gas-phase CCl (sub 4) crossing the model boundaries.
Integration of multi-scale physical and chemical heterogeneities using high-resolution digital images
A.L. Ward, G.W. Glendon, C.J. Murray, Z.F. Zhang
Pacific Northwest National Laboratory, Richland, WA
Abstracts with Programs - Geological Society of America 35(6):531 (2003).
Abstract
High-resolution data sets are needed to improve our understanding of the interaction between subsurface advective, dispersive, and exchange processes and the impact of multi-scale heterogeneity. However, development of these data sets has been hampered by disparities in the scale at which these processes occur and typical scale of characterization. We demonstrate an outcrop analog concept in which high-resolution digital images are used to integrate physical and chemical heterogeneities across multiple spatial scales. High-resolution visible and infrared images of a dike dig face on the Hanford Site were compiled into a mosaic spanning heterogeneities from the millimeter scale to tens of meters. Measurements with in situ characterization tools (water and air permeameters) were used to develop a coarse-scale hydrofacies map. This map was supplemented with sedimentological (grain size distribution) hydraulic (water retention, permeability) and hydrogeochemical (distribution coefficient, cation exchange capacity) properties derived from sediment samples. These properties were regressed on grain size and sorting parameters to obtain predictive relationships for the measurement scale. Hydrogeological and hydrogeochemical properties showed strong correlations with texture, as represented by a mean grain size and sorting index. The resulting relationships were used to transform the digital images into high-resolution lithofacies, hydrofacies and chemofacies distributions for input into predictive flow and reactive transport models. This approach is applicable to the generation of multi-dimensional, multi-parameter data sets for input into high-resolution numerical models. The impact of multi-scale heterogeneities on subsurface flow in the dike outcrop is demonstrated in a series of simulations with the STOMP simulator. A companion paper by Yabusaki and Ward explores the impact of heterogeneous sorption parameters derived by this method on the transport (super 90) Sr. This work was funded through The Hanford Ground Water Protection Project by the U.S. Department of Energy. The Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC06-76RL01830.
Estimating soil hydraulic parameters of a field drainage experiment using inverse techniques
Z.F. Zhang, A.L. Ward, G.W. Gee
Pacific Northwest National Laboratory, Richland, WA
Vadose Zone Journal 2(2):201-211 (2003).
Abstract
Accurate assessment of water flow and contaminant transport in unsaturated porous media at the field scale is often hindered by difficulties associated with obtaining reliable estimates of soil hydraulic properties. The unsteady drainage-flux method is one of the commonly used methods to measure in situ unsaturated hydraulic properties of soils. However, the properties obtained by this method using instantaneous profile data analysis may not be the best estimation of actual values of hydraulic properties. We present an improved analysis of the data from drainage experiments using inverse modeling, which uses nonlinear regression methods to estimate hydraulic parameters. Parameter identifiability is evaluated through sensitivity and uniqueness analyses. We used the combination of the inverse modeling program, UCODE, with the flow simulator, STOMP, for inverse modeling. Applying the inverse method to a field drainage experiment in sandy soil showed that all the van Genuchten (1980) hydraulic parameters could be estimated uniquely when both water content and pressure head (h) data were used. The parameter estimates by inverse technique using both and h data simulated the flow better than the parameter values obtained by the conventional instantaneous-profile analysis method. After the spatial and temporal sensitivities were analyzed, a more rational experimental design was recommended.
Carbon Tetrachloride Flow Behavior in Unsaturated Hanford Caliche Material An Investigation of Residual Nonaqueous Phase Liquids
M. Oostrom, R. J. Lenhard
Pacific Northwest National Laboratory, Richland, WA
Vadose Zone Journal 2: 25-33 (2003).
Abstract
At many contaminated sites, nonaqueous phase liquids (NAPLs) persist in the vadose zone for long periods of time. This occurs because the permeability of the NAPL becomes negligible at some saturation and downward movement ceases, resulting in residual NAPL. To obtain data that can be used to study the development of a residual NAPL saturation and to test corresponding models, a detailed transient experiment was conducted in a 170 cm long by 90 cm high by 5.5 cm wide flow cell. Fluid saturation measurements were obtained with a dual-energy gamma radiation system. The experimental conditions reflected those at the Hanford Site in Washington State, where an estimated 363 to 580 m3 of carbon tetrachloride (CCl4) was disposed to the subsurface. A key subsurface feature at the Hanford Site is a sloped Plio-Pleistocene caliche layer, which was reproduced in the experiment as a sloped lens in a medium-grained, uniform, sand matrix. The caliche contains considerable amounts of CaCO3 and may have fluid wettability properties other than strongly water wet. A total of 800 mL of CCl4 was injected into the experimental domain at a rate of 0.5 mL min-1 from a small source area located at the surface. After apparent static conditions were obtained with respect to CCl4 redistribution, saturation measurements indicated that all of the dense nonaqueous phase liquids (DNAPL) that had initially moved into the caliche remained in this layer. Water was subsequently applied to the surface at a constant rate over the full length of the caliche layer to study CCl4 displacement as a result of changing water saturations. Water saturation in the caliche layer rose to as high as 0.91 during water infiltration. Results show that 25% of the DNAPL present in the caliche migrated from this layer as a consequence of water infiltration, while 75% remained in the caliche layer. The experimental results could not be reproduced with numerical multifluid flow simulations based on common constitutive theory. This indicates that improvements in constitutive theory may be needed to accurately model air-DNAPL-water flow behavior.
Flow behavior and residual saturation formation of liquid carbon tetrachloride in unsaturated heterogeneous porous media
M. Oostrom
Pacific Northwest National Laboratory, Richland, WA
C. Hofstee
Netherlands Institute of Applied Geoscience, Utrecht, The Netherlands
R. J. Lenhard
Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID
T. W. Wietsma
Pacific Northwest National Laboratory, Richland, WA
Journal of Contaminant Hydrology 64: 93-112 (2003).
Abstract
The formation of residual, discontinuous nonaqueous phase liquids (NAPLs) in the vadose zone is a process that is not well understood. To obtain data that can be used to study the development of a residual NAPL saturation in the vadose zone and to test current corresponding models, detailed transient experiments were conducted in intermediate-scale columns and flow cell. The column experiments were conducted to determine residual carbon tetrachloride (CCl4) saturations of two sands and to evaluate the effect of CCl4 vapors on the water distribution. In the intermediate-scale flow cell experiment, a rectangular zone of the fine-grained sand was packed in an otherwise medium-grained matrix. A limited amount of CCl4 was injected from a small source and allowed to redistribute until a pseudo steady state situation had developed. A dual-energy gamma radiation system was used to determine fluid saturations at numerous locations. The experiments clearly demonstrated the formation of residual CCl4 saturations in both sands. Simulations with an established multifluid flow simulator show the shortcomings of current relative permeability-saturation-capillary pressure (k-S-P) models. The results indicate that nonspreading behavior of NAPLs should be implemented in simulators to account for the formation of residual saturations.
Numerical modeling to assess DNAPL movement and removal at the Scenic Site Operable Unit near Baton Rouge, Louisiana: A case study.
M. Oostrom1,2, P.D. Thorne1, M.D. White1, M.J. Truex1, and T.W. Wietsma3
1Environmental Technology Division; Battelle, Pacific Northwest Division; P.O. Box 999, Richland, WA 99352
2Corresponding author; tel. (509) 372-6044; fax. (509) 372-6089; mart.oostrom@pnl.gov
3Environmental Molecular Sciences Laboratory; Battelle, Pacific Northwest Division; P.O. Box 999, Richland, WA 99352
Journal of Soil & Sediment Contamination in press (2003).
Abstract
Detailed three-dimensional multifluid flow modeling was conducted to assess movement and removal of dense nonaqueous phase liquid (DNAPL) movement at a waste site in Louisiana. The site's subsurface consists of several permeable zones separated by (semi) confining clays. In the upper subsurface, the two major permeable zones are, starting with the uppermost zone, the +40- and +20-MSL (mean sea level) zones. At the site, a total of 23,000 m3 of DNAPL was emplaced in an open waste pit between 1962 and 1974. In this period, considerable amounts of DNAPL moved into the subsurface. By 1974 a portion of the DNAPL was removed and the waste site was filled with low-permeability materials and closed. During this process, some of the DNAPL was mixed with the fill material and remained at the site. Between 1974 and 2000, no additional DNAPL recovery activities were implemented. In an effort to reduce the DNAPL source, organic liquid has been pumped through a timed-pumping scheme from a total of 7 wells starting in calendar year 2000. The recovery wells are screened in the lower part of the waste fill material. In site investigations, DNAPL has been encountered in the +40-MSL but not in the +20-MSL zone. The following questions are addressed: (1) Where has the DNAPL migrated vertically and laterally? (2) How much further is DNAPL expected to move in the next century? (3) How effective is the current DNAPL pumping in reducing the DNAPL source? The computational domains for the simulations were derived from 3-D interpolations of borehole logs using a geologic interpretation software (EarthvisionTM ). The simulation results show that DNAPL primarily entered the subsurface in the period 1962 - 1974, when the waste site was operational. After 1974, the infiltration rates dropped dramatically as a result of the infilling of the waste pit. The simulation results indicate that DNAPL moved from the pit into the underlying +40-MSL zone through two contact zones at the west side of the pit. Lateral movement of the DNAPL body has been relatively slow as a result of the high viscosity and the rapidly decreasing driving force after the waste pit was filled in. For all simulations, lateral movement of DNAPL in the period 1962 - 2001 is predicted to be less than 60 m from the two contact areas, while additional movement in the next century is expected to be less than 30 m. No DNAPL is predicted to enter the +20-MSL zone, which agrees with site information. The simulations also clearly demonstrate the minimal effect of the current pumping scheme on source reduction and DNAPL movement.
A practical model for nonvolatile NAPL-aqueous flow in variably saturated porous media, distinguishing mobile, residual, and entrapped NAPL
M.D. White1, M. Oostrom1,*, and R.J. Lenhard2
1 Hydrology Group, Pacific Northwest National Laboratory, P.O. Box 99 MS K9-33, Richland, WA 99352, USA
2Subsurface Science Initiative, Idaho National Environmental and Engineering Laboratory, P.O. Box 1625, Idaho Falls, ID 93415-2025, USA
*Corresponding author; tel. (509) 372-6044; fax. (509) 372-6089; mart.oostrom@pnl.gov
3Environmental Molecular Sciences Laboratory; Battelle, Pacific Northwest Division; P.O. Box 999, Richland, WA 99352
Groundwater submitted (2003).
Abstract
Flow of nonvolatile NAPL and aqueous phases that accounts for mobile, entrapped (water-occluded), and residual NAPL in variably saturated porous media is numerically modeled and compared against results from detailed laboratory-scale experiments. Residual saturation formation in the vadose zone is a process that is often ignored in multifluid flow simulators. Mobile NAPL is defined as being continuous in the pore space and flows under a pressure gradient or gravitational body force. Entrapped NAPL is defined as being occluded by the aqueous phase, occurring as immobile ganglia surrounded by aqueous phase in the pore space and formed when NAPL is replaced by thee aqueous phase. Residual NAPL is defined as immobile, non-water entrapped NAPL that does not drain from the pore spaces and is conceptualized as being either continuous or discontinuous. The numerical model is formulated on mass conservation equations for oil and water, transported via NAPL and aqueous phases through variably saturated porous media. To account for phase transitions, a primary variable switching scheme is implemented for the oil-mass conservation equation over three phase conditions: 1) aqueous or aqueous-gas with dissolved oil, 2) aqueous or aqueous-gas with entrapped NAPL, and 3) aqueous-free NAPL or aqueous-free NAPL-gas. An algorithm has been included that computes contact angles for nonspreading NAPLs. Two laboratory-scale column experiments are modeled to verify the numerical model. Comparisons between the numerical simulations and experiments demonstrate the necessity to include the residual NAPL formation process in multifluid flow simulators.
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 in press (2003).
Abstract
Predicting flow and transport in unsaturated porous media is often hampered by limited data and the uncertainties in constitutive property information at appropriate the 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 (M2). 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%.
A tensorial connectivity-tortuosity (TCT) concept to describe the unsaturated hydraulic properties of anisotropic soils
Z.F. Zhang, A.L. Ward, G.W. Gee
Pacific Northwest National Laboratory, Richland, WA
Vadose Zone Journal in press (2003).
Abstract
The anisotropy in unsaturated hydraulic conductivity is saturation-dependent. Yet, there are few options for modeling this phenomenon in natural soils. A tensorial connectivity-tortuosity (TCT) concept is proposed to describe the unsaturated soil hydraulic conductivity. The TCT concept assumes that soil pore connectivity and/or tortuosity are anisotropic and can be described using a tensor. Saturation dependent anisotropy can be easily invoked into common models of relative permeability by incorporating the connectivity tensor. Synthetic Miller-similar soils having hypothetical anisotropy are defined by allowing the saturated hydraulic conductivity to have different correlation range for different directions of flow. The TCT concept was tested using the synthetic soils with four levels of heterogeneity and four levels of anisotropy. The results show that the soil water retention curves were independent of flow direction but dependent on soil heterogeneity, while the connectivity-tortuosity coefficient is a function of both soil heterogeneity and anisotropy. The TCT model can accurately describe the unsaturated hydraulic functions of anisotropic soils and can be easily combined into commonly used relative permeability functions for use in numerical solutions of the flow equation.
Inversely estimating soil hydraulic parameters through field drainage experiment
Z.F. Zhang, A.L. Ward, G.W. Gee
Pacific Northwest National Laboratory, Richland, WA
Vadose Zone Journal 2: 201-211, (2003).
Abstract
Accurate assessment of water flow and contaminant transport in unsaturated porous media at the field scale is often hindered by difficulties associated with obtaining reliable estimates of soil hydraulic properties. The unsteady drainage-flux method is one of the commonly used methods to measure in situ unsaturated hydraulic properties of soils. However, the properties obtained by this method using instantaneous profile data analysis may not be the best estimation of actual values of hydraulic properties. We present an improved analysis of the data from drainage experiments using inverse modeling, which uses nonlinear regression methods to estimate hydraulic parameters. Parameter identifiability is evaluated through sensitivity and uniqueness analyses. We used the combination of the inverse modeling program, UCODE, with the flow simulator, STOMP, for inverse modeling. Applying the inverse method to a field drainage experiment in sandy soil shows that all the van Genuchten (1980) hydraulic parameters could be estimated uniquely when both water content (Θ) and pressure head (h) data were used. The parameter estimates by inverse technique using both and h data simulate the flow better than the parameter values obtained by the conventional instantaneous-profile analysis method. After the spatial and temporal sensitivities were analyzed, a more rational experimental design was recommended.