H51F-0842
Effects of Ethanol on the Behaviour of Gasoline at the Water Table: Numerical Simulation of Laboratory Experiments
A numerical model has been used to reproduce a series of laboratory experiments designed to examine the effects of ethanol on gasoline movement in the vadose zone, particularly in the capillary fringe. The multi-phase multi-component model, CompFlow Bio was modified to incorporate two significant effects of ethanol dissolved in the aqueous phase on the fate and transport of gasoline in the subsurface: the enhanced dissolution of gasoline in the aqueous phase due to cosolvency effects, and the reduction in surface/interfacial tension (and hence capillary pressure curve) between the total liquid and gas phases as well as the aqueous and non- aqueous phases. To simulate these processes, a log-linear relationship was added to the model to represent the cosolvency effect, while empirical relationships were used to scale the surface/interfacial tensions inherent in the capillary pressure curves. A series of 2D laboratory experiments were performed in a plexiglass box packed with 390 micrometer glass beads to visualize the impact of ethanol on the fate and transport of gasoline both above and below the water table. These experiments consisted of injecting gasoline into the plexiglass box, allowing it redistribute itself above, within, and below the capillary fringe, and then injecting ethanol above the gasoline source zone. The experiments were used to establish a qualitative comparison with the numerical model. Comparison between the numerical model and the experiments indicate that the model is capable of capturing the general behaviour of the system; specifically, the collapse of the capillary fringe followed by the remobilization and subsequent collection of non-aqueous phase gasoline in at the bottom of collapsed capillary fringe region. Surprisingly, the remobilized non-aqueous phase gasoline that collected at the bottom of the collapsed capillary fringe exhibited higher saturations than those observed prior to the injection of ethanol. The empirical relationships added to CompFlow Bio used to simulate cosolvency effects as well as scaling of the capillary pressure curves were essential to capture the general behaviour of the gasoline source zone following the injection of ethanol.
H51F-0843
Effective Hydraulic Properties for Transient Flow in Unsaturated Zone
Unsaturated zone plays important role in many hydrological and environmental processes. Deriving average/effective parameters is essential for many large scale hydrological, environmental or climate studies. Upscaling is a process of using local-scale hydraulic properties to derive effective properties representing a large scale. In this study, we investigate: 1) how the hydrologic processes impact on the effective hydraulic properties; 2) how the effective hydraulic properties schemes are sensitive to the initial conditions; and 3) how the effective hydraulic properties are related to boundary conditions for heterogeneous soils. The main idea is whether the interested process behavior in heterogeneous soils can be captured by a process that assumes only one set of soil parameters, such that the heterogeneous system is replaced by an equivalent homogeneous system. By using field and re-generated hydraulic data sets, this study derives effective hydraulic properties under transient flow conditions. Precipitation, evaporation, and root water uptake and their effects on the effective properties are also considered during the simulations. An inverse procedure is used to determine the effective hydraulic parameters various flow scenarios. Results show that effective hydraulic parameters change with time scale initially, and then approach nearly constant values, indicating an equivalent homogeneous medium is a more viable alternative for large spatial and temporal scale transient infiltration process. Keywords: Upscaling, Transient Flow, Effective Parameters, Heterogeneous Soil
H51F-0844
Flow and transport at the hillslope scale: New experimental evidence from whole-hillslope irrigation and excavations
Lateral subsurface flowpaths and flow velocities are poorly quantified. The controls on subsurface flow characteristics are needed for better modeling of nutrient and contaminant transport and flood prediction. We present results from a whole-hillslope investigation designed to characterize unsaturated zone subsurface flow paths and velocities. Specifically, we address the questions: What are flow velocities of lateral subsurface flow at the soil bedrock interface? What controls flowpath location and extent? We performed two irrigation experiments on a 6m (parallel to stream) by 8m (upslope) section of hillslope at the Woods and Rowe Maimai experimental hillslope on the South Island of New Zealand. Lateral flow on this hillslope is monitored at an exposed trench face. In the first experiment water with a dye was pumped into a pit excavated to bedrock 8 meters upslope the hillslope base until steady state conditions were reached at the trench face. Flowpath locations were then identified by excavating soil in 0.2-0.3 m slices upslope from the trench. A Bromide solution was injected every 0.4-0.6 m to determine flow velocities. The second experiment was identical except that water was applied on the soil surface as a line 4 m upslope of the trench. Once the hillslope was cleared of soil, the irrigation experiment was repeated on the exposed bedrock. A very high resolution DEM (grid spacing = 5mm) of bedrock topography was constructed using a Trimble laser scanner. The dominant subsurface flow paths were continuous and predominantly at the soil bedrock interface, less than 0.1 m above the bedrock in both experiments. In the pit experiment unbroken flow paths along the entire 8m excavated hillslope were observed. In the second experiment water applied to the soil surface moved via subvertical cracks through the soil profile for <1.5m and then traveled laterally along the bedrock interface. The bedrock topography proved to be the dominant control on flowpath locations. Topographic analysis of the bedrock suggests that while the correlation length scale is on the order of 2m, small scale (<0.1 m) features significantly affect flow routing, and had a large impact on final flow path location. The tracer experiments indicate that subsurface flow velocities at the soil bedrock interface were on the order of 0.25 cm/s - more than an order of magnitude faster than predicted in the Darcy flow regime (with laminar flow assumptions) but more than 2 orders of magnitude slower than predicted in the Hagen Poiseuille flow regime (with pipe flow or open channel flow assumptions). Tracer velocities were similar for both the surface and direct bedrock injections. These results suggest that new models of lateral subsurface flow paths and velocities are needed. Our measured flow velocities poorly fit standard models used to define water delivery. Our experiments show that measuring bedrock topography may improve our ability to identify subsurface flowpaths. These observations are used to develop a new conceptual model of lateral subsurface flow.
H51F-0845
Incorporating Soil Hydraulic Parameter Statistics in Developing Pedo-transfer Functions
In this study, we develop artificial neural network (ANN) based pedotransfer functions (PTFs) to predict soil hydraulic properties. The PTF approach is an efficient way of translating less costly available data, such as particle-size distributions, soil textures and other geophysical measurements, to soil hydraulic parameters required for numerical simulations and other applications. The ANN PTFs need to be trained before being used to transfer indirect measurements to soil hydraulic parameters. The traditional training process, in general, is to adjust ANN's coefficients to solely minimize the difference between the estimated and measured soil hydraulic parameters. The training process, however, did not consider the distributions of soil hydraulic parameters and the trained neural networks may yield improper distributions, which may severely affect probabilistic predictions. We incorporate the distributions of the soil hydraulic parameters into the ANN PTF development. In addition, it has been observed that PTFs can introduce unrealistic correlations between the output parameters. The unwanted artificial correlations need to be penalized during the training process, since it is well known that parameter correlations have significant effect on predictions. We achieve these two goals by adding two regularization terms to the ANN objective functions. A suite of new neural network models are developed to estimate soil hydraulic parameters. These neural network models have the same input and output variables, but different objective functions, which incorporate sequentially the site soil hydraulic parameter measurements, parameter probability distributions, and parameter correlations.
H51F-0846
Hydraulic Similitude for Variably-Saturated Media
We introduced a dimensionless formulation for water flow in variably-saturated media that provides a basis for similitude between domains of various sizes, anisotropy ratios, hydraulic conductivities, and capillary properties. The formulation indicated, for example, that short coarse-textured domains behave similarly to tall fine-textured domains. It also indicated that the hydraulics in a high conductivity beach subjected to a fast rising tide is similar to that in a low conductivity beach subjected to a slow rising tide. Measurements of water level and solutes (salt, nutrients) at two beaches in the Prince William Sound, Alaska, were modeled using the MARUN model, a 2D finite element code for flow in variably-saturated media, and were then generalized using the dimensionless formulation.
H51F-0847
Hydraulic Conductivity Anisotropy of Unsaturated Soils
The effects of saturation degree (or tension) on hydraulic conductivity anisotropy in unsaturated soils have been recognized for long time, but they have not been fully described conceptually. Previous models include quantifying saturation-dependent anisotropy of soil formations that consist of many thin layers each with its own hydraulic properties characterized by a uniform density distribution of saturated hydraulic conductivity or soil bulk density. Some other approaches have also been developed to study the soil anisotropy behavior in dealing with flow and transport problems in saturated and unsaturated soils, such as tensorial connectivity-tortuosity concept. In this study, we investigate soil unsaturated hydraulic conductivity anisotropy that mainly arises from a combination of both wide range of soil texture variations and within narrow range of texture units in conjunction with pedotransfer functions (PTFs) of soil hydraulic properties. PTFs are often used to estimate soil hydraulic properties because direct measurements are too expensive and difficult. PTFs transform basic soil properties such as texture, bulk density into water retention and saturated or unsaturated hydraulic conductivity. We develop a new approach to combine the neural network based PTF results with the thin layer approach to explore saturation-dependent anisotropy behavior for a wide range of texture and bulk density conditions. Anisotropy models are developed that quantify saturated and unsaturated hydraulic conductivities for soils composed of many thin layers distinguished by texture, organic carbon content and bulk density.
H51F-0848
Validating Thermohydrologic Models Using the Drift Scale Test of the Proposed Repository at Yucca Mountain: Impact of Capillary-Pressure Cap
The MultiScale ThermoHydrologic Model (MSTHM) supports the total system performance assessment (TSPA) for the proposed nuclear-waste repository at Yucca Mountain. The MSTHM uses the Nonisothermal Unsaturated Flow and Transport (NUFT) code to represent thermal-hydrologic (TH) processes occurring at scales from a few tens of centimeters around individual waste packages and emplacement drifts (tunnels) all the way to the kilometer scale for heat flow through the mountain. The TH model involves two-phase (liquid and gas) nonisothermal flow and transport in an unsaturated fractured rock system, using a dual-permeability model of overlapping fracture and matrix continua. The TH model depends on calibrated system parameters, including the van~Genuchten α and m parameters for the capillary pressure versus saturation relationship. Waste- package heat generation can drive liquid saturation to below residual saturation. Extending the van Genuchten capillary-pressure function to below residual saturation involves establishing a physically reasonable capillary- pressure cap. Various extension methods are considered for the van Genuchten capillary-pressure function as applied to a 3-D nested-mesh TH model of the Drift Scale Test (DST), as well as a corresponding 2-D drift-scale TH submodel, which supports the MSTHM. Simulated temperatures and liquid saturations are compared with field measurements from the DST. Compared to past DST model-validation studies, agreement between the simulated results and field measurements is improved, partially due to implementing a capillary-pressure cap. Because the same hydrologic properties and capillary-pressure cap are applied in the TH submodels supporting the MSTHM, this model-validation study builds confidence in the MSTHM as it is applied to Yucca Mountain TSPA. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
H51F-0849
Validating the Multiscale Thermohydrologic Model Using an Alternative Model of the Proposed Repository at Yucca Mountain
The MultiScale ThermoHydrologic Model (MSTHM) is used in the total system performance assessment (TSPA) for the proposed nuclear-waste repository at Yucca Mountain. The MSTHM uses the Nonisothermal Unsaturated Flow and Transport (NUFT) code to represent thermal-hydrologic (TH) processes occurring at scales from a few tens of centimeters around individual waste packages and emplacement drifts (tunnels) all the way to the kilometer scale for heat flow through the mountain. The MSTHM is computationally more efficient than a monolithic 3-D TH model because it breaks the problem into smaller tractable pieces. An MSTHM simulation is constructed with NUFT-submodel calculations of various levels of coupled-process detail and scale, by superposing results of 3-D mountain- and drift-scale thermal submodels onto those of 2-D drift-scale TH submodels. The MSTHM was previously validated against an alternative (monolithic) 3-D TH model for a test problem at a scale smaller than the Yucca Mountain repository. Good agreement with the alternative model was obtained for temperature, relative humidity, and liquid saturation. A parallel-CPU version of NUFT handles much larger and more computationally demanding problems. This version is applied to an alternative (monolithic) 3-D "pillar-scale" TH model to simulate near-field and in-drift TH behavior along a full-scale (~1000-m long) emplacement drift. Results from the alternative model are compared to corresponding MSTHM results. Agreement with the alternative model builds confidence in the MSTHM applied at the scale of the Yucca Mountain repository. This study justifies key assumptions used in the MSTHM, such as that of negligible vapor flow along emplacement drifts. The pillar-scale model is also used to conduct a sensitivity study of parameters that control the magnitude of vapor flow and condensation along emplacement drifts. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
H51F-0850
Multiscale Thermohydrologic Model Supporting the Total System Performance Assessment for the Proposed Repository at Yucca Mountain
The MultiScale ThermoHydrologic Model (MSTHM) is used in the total system performance assessment (TSPA) for the proposed nuclear-waste repository at Yucca Mountain. The MSTHM uses the Nonisothermal Unsaturated Flow and Transport (NUFT) code to represent thermal-hydrologic (TH) processes occurring at scales from a few tens of centimeters around individual waste packages and emplacement drifts (tunnels) all the way to the kilometer scale for heat flow through the mountain. The MSTHM is used to predict the anticipated range of TH conditions within emplacement drifts and adjoining host rock. To be defensible, the range in predicted TH conditions must address the influence of the variability and uncertainty of engineered- and natural-system parameters that significantly influence those conditions. Parameter-sensitivity analyses show that the most important natural-system parameters are host-rock thermal conductivity and percolation flux through the repository. These analyses show that the key engineered-system parameter is the waste-package-to-waste- package variability in heat output. The range in TH conditions is also influenced by the "edge-cooling" effect, where waste packages closer to the repository edge cool more quickly than those closer to the repository center. To account for this effect, the MSTHM represents the geometric details of the repository layout. Improvements have also been made to how the MSTHM incorporates hydrostratigraphic and percolation-flux data from the Unsaturated Zone Flow Model, which supports ambient flow and transport simulations for TSPA. Other improvements allow more parameter sensitivity cases to be investigated. Twelve cases are analyzed, including four percolation-flux scenarios (10-, 30-, 50- and 90-percentile) and three host-rock thermal-conductivities (10- percentile, mean, and 90-percentile). Using the results of stochastic analyses, weighting factors are applied to the twelve cases. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.