H33B-0459 1340h
A preferential flow model based on flow variability in macropores
Simulating infiltration in soils containing macropores still provides unsatisfactory results, as existing models seem not to capture all relevant processes. Recent studies of macropore flow initiation in natural soils containing earthworm channels revealed a distinct flow rate variability in the macropores depending on the initiation process (Weiler & Naef, 2003, J of Hydrology, 273: 139-154). When macropore flow was initiated at the soil surface, most of the macropores received very little water while a few macropores received a large proportion of the total inflow. In contrast, when macropore flow was initiated from a saturated or nearly saturated soil layer, macropore flow rate variation was much lower. The objective of this study was to develop and test a model, which combines the macropore flow variability with several established approaches to model dual permeability soils. We then evaluate the INfiltration-INitiation-INteraction Model (IN$^{3}$M) as a tool to explore the influence of macropore flow variability on infiltration behavior by performing a sensitivity analysis and applying IN$^{3}$M to sprinkling and dye tracer experiments at various field sites with different macropore and soil matrix properties. The sensitivity analysis showed that the flow variability in macropores reduces interaction between the macropores and the surrounding soil matrix and thus increases bypass flow, especially for surface initiation of macropore flow and at higher rainfall intensities. The model application shows reasonable agreement between IN$^{3}$M simulations and field data in terms of water balance, water content change, and dye patterns. The influence of macropore flow variability on the hydrological response of the soil was considerable and especially pronounced for soils where initiation occurs at the soil surface.
http://faculty.forestry.ubc.ca/weiler/research.html
H33B-0460 1340h
How Lateral Preferential Flow Influences the Infiltration Behaviour
Fast subsurface flow (SSF) in the vadose zone can play a key role in draining a hillslope. Even relatively small amounts of lateral preferential flow occurring continually can prevent total saturation of the soil profile and the generation of overland flow. In the presented project, the factors controlling this preferential flow were studied to define the soil parameters that control the rate of SSF on a hillslope. To this purpose, flows were closely monitored during sprinkling experiments and natural rainfall events on a hill slope where SSF occurs. Highly resolved measurements of soil moisture and surface and subsurface runoff in different depths were made. Event and pre-event water fractions as well as flow velocities in the different flow components were determined using artificial tracers. In addition, the naturally occurring 222Rn was measured continuously in a new approach to determine flow velocities and residence times. Together with detailed soil investigations and geophysical measurements it was possible to identify and quantify the relevant parameters and soil structures for the occurrence of SSF and for the infiltration behaviour at the test site. The results enabled us to assess the rate of lateral preferential flow, the soil water storage volumes and the infiltration capability between different soil horizons. The well-defined boundary conditions in this relatively simple system allowed insight into the mechanisms of pre-event water mobilization and the dependence of subsurface flow on the rainfall intensity.
H33B-0461 1340h
Forward and inverse dual-permeability model simulation of water flow in a soil column containing a preferential flow path
It is not clear if hydraulic parameters of dual-permeability models (DPM) can be properly identified by inverse analysis of preferential water flow data. In this study, we applied a DPM based on two coupled Richards' equations to compare inverse and forward simulations of preferential water flow observations obtained in well-controlled laboratory column experiments. Infiltration and drainage experiments were conducted using a repacked loam soil column (80 cm long, 24 cm diameter) containing a cylindrical sand region (2.4 cm diameter) as preferential flow path (PFP) along its central axis. The forward DPM simulations relied on fixed hydraulic parameters for both the soil matrix and the PFP, as determined by means of separate infiltration and drainage experiments performed on loam columns and sand columns, respectively. The parameters of the first-order water exchange term were derived based on the column geometry. Two different types of data were utilized for the inverse parameter identification. The first inverse approach relied on observations of accumulated infiltration and outflow, along with water contents and pressure heads in the loam matrix. The second inverse approach was identical except for including region-specific outflow out of matrix and PFP. The results showed that individual outflow out of matrix and PFP could not be properly described when fitting the DPM to bulk-soil related flow data, as opposed to the inverse DPM approach based on region-specific outflow data which reproduced those data well. On the other hand, bulk soil related infiltration and outflow was matched by both inverse approaches. The forward approach in most cases produced satisfactory results for both bulk soil related and region-specific outflow. For natural soils where region-specific flow data are not available, the soil hydraulic DPM parameters can not be inversely identified to correctly describe region-specific flow. This prohibits the subsequent use of the DPM for solute transport analysis. Future work may address the question if DPM hydraulic parameters can be identified when simultaneously using hydraulic and solute transport observations.
H33B-0462 1340h
Non-Equilibrium and Two-Dimensional Flow Field Effects on Bromide Transport at a Tile-Drained Field Site
Systematically tile drained field sites have been recognized as one major source for surface water contamination with agrochemicals. To study the effects of tile drainage and physical non-equilibrium on solute transport in structured soil, bromide (Br-) transport experiments were carried out on three plots (N2, N4, and N8) with different tile drain spacings (16, 18, and 12 m, respectively) and depths (128, 101, and 96 cm) at the Infeld experimental field site (North-West Germany). Tile drain outflow along with Br- concentrations were monitored over a half-year period. For all three plots, the observed Br- concentrations fluctuated around low levels below 8 mg/l during the experiment without showing a distinct concentration maximum. Experimental observations of the N4 plot were analyzed using one- and two-dimensional (1D and 2D, respectively) single- and dual-porosity (SPM and MIM, respectively) model approaches. All SPM and MIM parameters were obtained from independent measurements, except for the calibrated MIM water and solute transfer coefficients. Water flow and Br- transport were then predicted for the N2 and N8 plots using the N4 model parameters. Measured low-level Br- concentrations could only be consistently calibrated (N4) and predicted (N2 and N8) using the 2D-MIM approach, while the 1D-MIM, 2D-SPM, and 1D-SPM approaches (in this order) increasingly deviated from the experimental data. SPM approaches yielded unrealistically high Br- peaks. MIM simulations suggested that solute transfer into the immobile region represented more than 60 % of the surface applied Br-, thus effectively reducing Br- peak concentrations in the drain effluent. Model simulations further suggested that the two-dimensional flow field induced by tile drains caused dispersion and dilution of the Br- observed in the drainage effluent. This study showed that both the 2D flow field and physical non-equilibrium transport should be explicitly accounted for in physically based model simulations of solute transport in tile-drained structured field soils.
H33B-0463 1340h
Hydraulic Properties of Deformable Structured Soils
We examine the hydraulic and mechanical behavior of structures such as aggregates separated by interaggregate pores. Structured soil is susceptible to deformation under hydraulic and mechanical loading, leading to soil compaction. To describe water flow in structured soils, dual-permeability models consider the soil as two separate but interacting porous domains: a macro-pore system and a less permeable matrix pore system. Most of the existing models assume rigidity of the soil. On the other side most of the models describing deformable soils assume that the soils are homogeneous and their hydraulic properties time invariant (i.e. no change during compaction). The goals of our research is (i) to test whether Richards equation properly describes the flow in the micro-porous system of aggregates; (ii) to identify their hydraulic properties and (iii) to predict the change of the material functions in the course of compaction of the aggregated medium. We use neutron radiography and tomography to quantify the water exchange between aggregates. The aggregates were assembled in 2 mm thick slab forming quasi a 2-dimensional bedding. The larger aggregates were wetted with water and were embedded in smaller aggregates wetted a lower water content with heavy water. The water/heavy water exchange was imaged and simulated by solving the Richard's equation. The trend of the experimental and the simulated water exchange (both proportional to the square root of time) was a first test of the validity of Richard's equation in an aggregated soil. By matching the observed flow field and solving an inverse problem, the hydraulic properties of the sample (i.e. the properties of the individual aggregates) will be identified. A second goal was to find effective constitutive relations for the aggregated medium. This experimental system will be tested by dynamic tomography in a triaxial-load-cell.
H33B-0464 1340h
Correlated Moisture Content, Pressure and Temperature Data for Development of Hysteretic Moisture Retention Curves.
This study was performed as the first part of an effort to collect high quality, repeatable, hydrologic data in a laboratory environment to provide a sound basis for future macropore model development, validation, and calibration. The objective of the experiment was to study the hysteretic function (drainage/wetting/scanning) of soils and the role of hysteresis in fluid transport. Our hysteretic dataset provides detailed information of coupled fluid transport behavior that includes temperature, moisture, and concentration. Hysteretic data collected has been further correlated with soil matric potential and temperature to define moisture-retention curves in replicates of a coarse-sandy, mixed, mesic Fluventic Hapludoll, loam soil (Eudora loam of the Kansas River Valley). Large (28 L-soil core) volume minimized statistical uncertainty that customarily accompany small cores and provided robust results in determining scanning curves (wetting and drying). Integration of a bromide tracer applied to the large cores at saturation indicated that macropore transport occurred within the range of 0-20 kPa thereafter, transport was via soil matrix flow. The tracer also provided further information about the physical and chemical properties of the soil, which can be utilized in the future development and calibration of robust, versatile computer models. Continuing research of these same cores will focus on macropore transport as it correlates to our existing research.
H33B-0465 1340h
Models to simulate preferential/nonequilibrium flow and transport in the vadose zone.
We present a new version of the HYDRUS code that includes various approaches for modeling preferential and nonequilibrium flow and transport in the vadose zone. Existing approaches differ in terms of their underlying assumptions and complexity. They range from relatively simplistic models to more complex physically based dual-porosity and dual-permeability type models. A relatively simple dual-porosity flow model results when the Richards equation is combined with composite equations for the hydraulic properties to account for both soil textural and soil structural effects on flow. The simplest nonequilibrium flow model, a single-porosity model, which distinguishes between actual and equilibrium water contents, is based on a formulation by Ross and Smettem (2000) that requires only one additional parameter to account for nonequilibrium. A more complex dual-porosity, mobile-immobile water flow model results when the Richards or kinematic wave equations are used for flow in the fractures, and immobile water is assumed to exist in the matrix. We also discuss various dual-permeability models, including the formulation of Gerke and van Genuchten (1993a) and the kinematic wave approach as used in the MACRO model of Jarvis (1994). These models differ mainly in the description of the flow in the macropores. Several examples and comparisons of equilibrium and various nonequilibrium flow and transport models are also provided.
H33B-0466 1340h
Sub-Surface Flow Processes at a Juniper Covered Plot in a Karst Region of the Edwards Plateau.
The impacts of juniper trees on the water cycle at the Edwards plateau with karst geology have been the focus of active research for several years. The objective of this paper is to gain a comprehensive insight about the sub-surface flow processes occurring within a 7m X 14 m plot at the Edwards plateau (Honey Creek) containing juniper trees. A 2.3 m deep trench was excavated at the downslope end of the plot and TDR probes were installed at various locations within the trench to measure volumetric water contents. A rainfall simulator consisting of 6 individual (15 m high) telescopic masts was set up to provide artificial rainfall on the plot. Eight rainfall simulations (with different intensities and durations) and two dye-tracer tests were conducted on the plot during a 7-month period. Sub-surface interflow was visually inspected at various locations on the trench face and monitored by TDR probes. The total volume of sub-surface flow was also recorded after each simulation. The results demonstrated that sub-surface flow occurred in a tri-modal manner, consisting of flow in karst conduits, planar fractures in the limestone, and soil matrix. Conduit and fracture flow response time decreased exponentially with increasing rainfall intensity and comparable responses for similar boundary conditions were observed. Matrix flow response time decreased linearly with increasing rainfall intensity and total amount of rainfall. The increase in matrix water content was inversely proportional to the antecedent moisture content. During large rainfall events, water exchange was observed between the fractures and matrix which was absent during smaller rainfall events. The dye studies indicated that fractures and juniper roots are primary pathways for preferential flow occurring within the plot.
H33B-0467 1340h
Intermittent Filtration of Bacteria and Colloids at Pore and Column Scales
Intermittent sand filters used for water and wastewater treatment can achieve high disinfection performance if properly designed and operated. Soil filtration can also play a significant role in pathogen removal. In order to help predict removal of pathogenic bacteria in sand filters and natural porous media, the effects of cyclic infiltration and draining events (transient unsaturated flow) on microorganism fate were investigated. We visualized bacterial transport in unsaturated porous media at the pore scale using micromodels. Column experiments provided quantitative measurements of the phenomena observed at the pore scale. Escherichia coli and a conservative tracer (NaI) were applied once in a pulse to a 1.5 m sand column. Outflow concentrations during subsequent tracer-free pulses were monitored for 4 days. The reproducibility of the breakthrough curve was established in five repeated experiments. We observed earlier breakthrough of bacteria compared to the dissolved tracer, as predicted from pore scale studies. Transport of bacteria and tracer was influenced by the temporal variations in pore water velocity and moisture content. Advancement of the wetting front remobilized bacteria either attached to the air-water interface (AWI) or entrapped in stagnant pore water between gas bubbles. Remobilization leads to successive concentration peaks of bacteria and tracer in the effluent. Overall microbial retention rate was high, 99.972 %. Observations at the pore-scale showed that bacteria retention was due to reversible bacteria entrapment in stagnant regions and sorption onto the AWI and essentially irreversible attachment onto solid-water interface (SWI). Bacterial detachment from the AWI was only observed during complete gas bubble dissolution or if bubble interface stress occurred during the dissolution process.
H33B-0468 1340h
Properties of Flow Zones in Fractured Rock
Observations over the last 25 years from various field studies suggest that preferential flow is common in soils and rocks. Despite this realization, very little is known about the large-scale properties (e.g., structure, distribution, continuity) of such flow regimes. This information is important for predictive models, but it remains elusive, mainly because of the difficulties involved in characterizing flow that has substantial spatial (both vertical and horizontal) and temporal variability. To better understand preferential flow in fractured rock, we carried out an in situ field experiment in the Topopah Spring tuff found in Exploratory Studies Facility at Yucca Mountain, Nevada. This experiment involved the release of ~22 m3 of ponded water (at a pressure head of ~0.04 m) over a period of 7 months, directly onto a 12 m2 infiltration plot. As water was released, changes in moisture content were monitored along horizontal boreholes located in the formation ~19-22 m below. Distinct flow zones, with significant differences in flow velocity, size, and extent of lateral movement, intercepted the 6-9 m long monitoring boreholes. Further, in some flow zones saturation levels persisted for the time period in which water was released, while in others there were periodic fluctuations. There was also evidence of water being diverted above the ceiling of a cavity in the immediate vicinity of the monitoring boreholes. Observations from this field experiment suggested that inconsistencies exist in present conceptual models of flow in fractured rock. Particularly, these observations suggest that isolated conduits within the fractured rock formation encompass a large number of fractures to form preferential flow paths that persist if there is a continuous supply of water. It appears that in fractured welded tuffs, the propensity for vertical dispersion and fracture-matrix interactions may be significantly greater than suggested by existing conceptual models. These observations indicate that refinements in the understanding of flow and transport in fractured, welded rock may be realized through additional field investigations conducted at spatial scales of tens of meters. This work was supported by the Director, Office of Civilian Radioactive Waste Management, U.S. Department of Energy, through Memorandum Purchase Order EA9013MC5X between Bechtel SAIC Company, LLC, and the Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab). The support is provided to Berkeley Lab through the U.S. Department of Energy Contract No. DE-AC03-76SF00098.
H33B-0469 1340h
Modeling Conditional Stability of Gravity-Driven Unsaturated Infiltrating Flows Using a Non-equilibrium Capillary Pressure-Saturation Relation
Gravity-driven unstable flow during infiltration in unsaturated porous media is considered to be an important preferential flow process. The conventional equation for modeling unsaturated flows is the Richards equation, but this equation cannot be used to model unstable flows as it has been shown (A.G. Egorov, R.Z. Dautov, J.L. Nieber, and A.Y. Sheshukov, 2003. Stability analysis of gravity-assisted infiltrating flow, Water Resour. Res., 39:,1266, doi:10.1029/2002WR001886) that solutions to the Richards equations are unconditionally stable. Therefore, modeling these unstable flows requires the use of new forms of the mass balance equation which contain features that might account for the cause of instabilities. One postulated cause for instabilities is the existence of non-equilibrium in the capillary pressure-saturation relation. In this presentation we will examine the conditional stability that results from the use of a non-equilibrium relation of the form $\tau(S,p){\partial S}/{\partial t}=p-P(S)$, where $\tau$ is the relaxation coefficient, $S$ is the saturation, $p$ is the dynamic pressure, and $P(S)$ is the equilibrium pressure. The stability analysis is based on the perturbation of the traveling wave form of the governing mass balance equation and the associated relaxation equation. The numerical determination of the eigenvalues of the resulting perturbation equations yields the critical growth factor as related to perturbation frequency. The derived critical growth factor is used in evaluating the conditional stability as to its sensitivity to the flux and magnitude of the relaxation coefficient. It is interesting to see that as $\tau(S,p)\rightarrow 0$ the flow becomes unconditionally stable, the result being that found for the Richards equation. The perturbation frequency associated with the critical growth factor is related to finger width and this is confirmed with simulations in the two-dimensional vertical plane.
H33B-0470 1340h
2D Resistivity in Discontinuous Permafrost, Fort Wainwright, Alaska
2D resistivity profiles were collected in an area containing discontinuous permafrost, fractured bedrock, and groundwater contamination on Fort Wainwright, Alaska in June 2004. The stratigraphy consists of the Chena Alluvium over weathered and fractured Birch Hill Schist. Contaminant migration pathways are complex due to numerous fractures and frozen zones. Our purpose was to supplement existing permafrost data and to identify other features that might influence ground water flow and contaminant transport. This study used an Iris Instruments Syscal Pro-ten, 10-channel switch resistivity meter and a 96-electrode cable with 5-meter electrode spacing. Three resistivity array configurations were used during this study: dipole-dipole, Wenner and Schlumberger. Resistivity models were generated with RES2DINV software and were compared along identical profiles for the three array types. Modeled resistivity values of over 2000 ohm-m were interpreted as permafrost in most cases based on previous resistivity studies at the site. Borehole logs and known frozen zones from monitoring wells were used to verify the permafrost interpretations. Dynamic Graphics software EarthVision was used to generate a 3-D permafrost model of the area. This model was updated at the completion of the 2D resistivity survey.
H33B-0471 1340h
Influence of Air Entry Pressure and Degree of Sorting on Air Entrapment below the Water Table: Simulation Results
Our previous experimental studies have shown that air can be entrapped below the water table during periods of a rising or fluctuating water table. This is particularly evident when the aquifer material consists of heterogeneous sediments containing discontinuous lenses of coarse material in an otherwise relatively uniform medium. In this study, TOUGH2 (a finite difference model) is applied to, and verified against, imbibition results from our prior laboratory experiments. The model is then used to extend these experimental results to the investigation of the impact of air entry pressure and degree of sorting (of the coarse sediment lenses) on the air entrapment that is likely to occur below the water table. Within TOUGH2, the van Genuchten expression for the saturation-capillary pressure relationship is utilized, as is the van Genuchten - Mualem expression for relative permeability. The numerical portion of this study is based on variation of two parameters within the pressure-saturation expression, Po and lambda. Po is related to the air entry pressure for the sediments (with increasing Po associated with higher air entry pressure). The second parameter, lambda, relates to the degree of sorting within the sediments (with higher lambda associated with greater sorting - more uniformity in grain size of the sediments). Based on the numerical results, it is observed that the volume of air entrapped below the water table increases as the difference in air entry pressure between the coarse lens and the surrounding sediment matrix increases. Further, the volume of entrapped air increases with an increase in the uniformity of grain size of the coarse sediment (i.e., the sediment in the coarse lens is increasingly well sorted). These results provide substantial insight into the factors that control the degree of air entrapment below the water table.
H33B-0472 1340h
Assessment of Long-term Nitrate Transport to Groundwater in a Deep Alluvial Unsaturated Zone
As part of a comprehensive assessment of the fate and transport of applied nitrate in the vadose zone under an orchard, detailed numerical modeling was conducted to simulate transient, two-dimensional flow and nitrate transport over a seven year period to estimate potential for N leaching to groundwater. Simulations reflect two alternative N management practices with respective fertilizer application rates of 110 and 365 kg N/ha/yr, with realistic, temporally varying site boundary conditions for flow and nitrate transport utilizing data obtained from a 12-year fertilizer experiment (1982-1995) conducted at Kearney Research Site in Fresno, California. Subsurface sampling shows the presence of eight non-horizontal lithofacies based on soil color, texture, and cementation. Each lithofacies is conceptualized as a homogenous unit with soil hydraulic parameters that are defined in the form of van Genuchten model. Nitrate is treated as a tracer in the deep vadose zone based on the data interpretation of the measured biochemical properties. The results indicate that despite of the presence of the deep vadose zone, there is a profound effect of atmospheric boundary conditions on water flux to groundwater. Temporal changes in the mean pressure head both in the root zone and in the deep vadose zone are primarily controlled by irrigation wetting regimes. The results further suggest that irrigation management practices are not well integrated with the site climate conditions and crop water requirements. A substantial amount of nitrate is carried through the root zone primarily by irrigation water applications that took place after the fall fertilizer application and to a lesser extent by winter precipitation. The results indicate the need for a rigorous examination of current fertilizer application strategies, especially in light of worldwide vulnerability of shallow groundwater resources to nitrate contamination.