H53F-1473
Visualization and Quantification of Fingering Flow Using Light Transmission Method
With the aim of studying the physical process concerning the unstable fingering phenomena in two dimensions, experiments of vertical infiltration through layered sand were carried out in the laboratory using Hele-Shaw cells. We developed a light transmission method to measure the dynamics of water saturation within flow fingers in great detail with high spatial and temporal resolution. The method was calibrated using X-ray absorption. We improved the measured light transmission with correction for scattering effects through deconvolution with a point spread function which allows us to obtain quantitative high spatial resolution measurements. After fingers had fully developed, we added a dye tracer in order to distinguish mobile and immobile water fractions. Fully developed fingers consist of a tip, a core with mobile water, and a hull with immobile water. We analyzed the dynamics of water saturation within the finger tip, along the finger core behind the tip, and within the fringe of the fingers during radial growth. Our results confirm previous findings of saturation overshoot in the finger tips and revealed a saturation minimum behind the tip as a new feature. The finger development was characterized by a gradual increase in water content within the core of the finger behind this minimum and a gradual widening of the fingers to a quasi-stable state which evolves at time scales that are orders of magnitude longer than those of fingers' evolution. In this state, a sharp separation into a core with fast convective flow and a fringe with exceedingly slow flow was detected. All observed phenomena, with the exception of saturation overshoot, could be consistently explained based on the hysteretic behavior of the soil-water characteristic. http://www.iup.uni-heidelberg.de
H53F-1474
Numerical study of the airflow in the unsaturated zone induced by sea tides
Air pressure in coastal unsaturated zones fluctuates in response to tidal fluctuations, as does the ground water level in the aquifer. Air-water two-phase flow induced by sea tides in a coastal two-layered subsurface system is investigated through numerical simulations. The system consists of upper and lower layers with permeabilities of kU and kL, respectively. Water table exists in the lower layer. The tide-induced water level fluctuation may result in significant air pressure fluctuations above the water table in air-confined aquifers, i.e., the permeability of the upper layer is about two orders of magnitude lower than that of the lower layer. The air pressure fluctuations attenuate upward in the vertical direction, while the air pressure amplitudes tend to increase until reaching a maximum and then decrease in the horizontal direction. The simulation results show that the local hydraulic head, rather than the sea tide, is the direct driving force of the air pressure fluctuations in the unsaturated zones. When kU is reasonably great, the air pressure depends mainly on the changing rate of the local hydraulic head. When kU is low, however, the air pressure depends mainly on the amplitude of the local hydraulic head. The air flux fluctuations across the ground surface attenuate landward gradually. An interesting phenomenon is that the air flux amplitude tends to increase with kU at areas close to the coast, but to decrease with kU at areas far away from the coast.
H53F-1475
Gas Diffusivity And Air Permeability In Sandy Soils: Effect Of Particle Size, Compaction And Sample Scale
The transport and fate of gases in soils is mainly governed by gas diffusion and advection. The gas diffusivity (Dp/D0) is the transport parameter for the gas diffusion due to gas concentration gradient, while the air permeability (ka) is the transport parameter for advective gas transport due to soil-air pressure gradient. Hence, those gas transport parameters play a crucial role in simulating transport of gaseous contaminants such as volatile organic chemicals and in quantifying emission and exchange of greenhouse gases from/at the soil- atmosphere interface. In this study, we measured Dp/D0 and ka for total of six sandy soils and examined the effects of soil physical properties such as particle size, soil compaction, and sample scale on the gas transport parameters. Toyoura sand (0.106-0.50 mm) and Narita sands with three different particle size fractions (0.106-0.25, 0.25- 0.425, 0.425-0.85 mm) were used as experimental materials for the measurements of Dp/D0 and ka. The sand materials were repacked with given bulk densities into small-scale cores of 100 cm3 (for all materials) and large-scale cores of 2120 cm3 (only for Toyoura sand) at given water contents. In addition to the measurements, Dp/D0 and ka of Oso Flaco fine sand and Oakley sand from literature were also analyzed in this study. For all sand materials, we observed the threshold soil-air content (εth) below which Dp/D0 and ka are negligible, and measured Dp/D0 and ka increased linearly with increasing air-filled porosity (ε) from εth to soil total porosity (Φ). At high ε, sand materials with larger average particle diameter (APD) gave higher Dp/D0 and ka than those with smaller APD sands at a given ε due to the existence of rapid air flow through the highly continuous large pores. At low ε near the εth however, the measured Dp/D0 for large APD sands were lower than those for small APD sands. The measurements for Narita sand fractions with different bulk densities indicated that soil compaction gave the small effects of water blocking for gas diffusion (high Dp/D0) and the reduction of highly continuous large pores (low ka). The measured Dp/D0 and ka for Toyoura sand with two different soil cores exhibited that no significant scale effect was seen in Dp/D0, while the measured ka for large scale were lower than those for small scale cores. Based on the measurements of Dp/D0 and ka, we developed new predictive Dp(ε)/D0 and ka(ε) models considering εth and pore connectivity factor as functions of APD and bulk density for sandy materials. The new models agreed well with the measured Dp/D0 and ka and were useful for examining the effects of particle size and soil compaction in sandy soils.
H53F-1476
Gas Transport Parameters for Peaty Soil: Effect of Peat Shrinkage Induced by Successive Drainage
Accurate prediction of gas transport parameters (soil-gas diffusion coefficient, Dp, and air permeability, ka) is important when investigating the fate and transport of gaseous phase contaminants and in quantifying the emission of methane from peat (wetland) soil. However, only limited measurements and knowledge of Dp and ka, especially for peat soils, are available. In this study, Dp and ka were measured on undisturbed 100-cm3 peat soil samples (triplicate) taken from the Bibai wetland in Hokkaido, Japan at 30-, 60-, and 90-cm depths. The undisturbed cores collected at 60-cm depth were sampled in both horizontal and vertical directions. Each soil sample was drained to different soil-water matric potentials of pF (= log (-ψ, cm H2O)) 1.0, 1.5, 1.8, 2.0, 3.0, 4.1, and air-dry condition before measurements of Dp and ka. The peat soil samples showed remarkable shrinkage during successive drainage to the different pF values. Sudden decrease in soil sample volume was seen at the transition from pF 1.5 to 1.8. At pF > 1.8, the soil sample volume continuously decreased to around 30 percent of the original volume at air-dry condition. Shrinkage of the soil sample affected the soil pore structure, and, consequently, markedly influenced the magnitudes of Dp and ka. As a result, Dp and ka did not exhibit an expected monotonically increase with soil-air content, ε (m3 m-3). At pF 2 to pF 4.1, there was no significant increase in Dp and ka. These variations of Dp and ka with ε could be explained from the changes in the pore structure of a peat soil which is classified as macropores (pores in between structures) and matrix pore (structural pores). We evaluated the pore connectivity shape factor X (defined as X = log(Dp/Do)/log(ε)) and equivalent pore diameter dg defined as dg = (8ka/(Dp/Do))1/2) as indices of changes in pore structure. The variation of the X shape factor with the increase in ε involves a three\- fold process. First, at pF<1.5, the large almost highly connected straight macropores were drained resulting to X values less than 2. More macropores become connected increasing the tortuous pathways for gas diffusion thereby increasing the magnitude of X. At pF 1.5, the macropores are assumed to be completely drained, in agreement with the observed high equivalent pore diameter dg. Also at pF 1.5, the soil sample will likely result in a sharp decrease in sample volume due to the closure of macropore apertures because of the absence of soil- water in the macropores. Second, after the sudden sample shrinkage at pF 1.5 to 1.8, the X value followed an almost linear increase with ε within the range of 0.2 to 0.4 m3 m-3. This occurs at the condition where the matrix pores are starting to drain. Both the decrease in soil-water and the further decrease in soil volume resulted in the gentle increase in Dp/Do and ka at pF 1.5 to pF 4.1. Third, at pF > 4.1, the additional matrix pore spaces become highly tortuous because of the observed remarkable shrinkage, at this dry condition. Thus, pore structure changes due to soil shrinkage must be taken into consideration in order to characterize accurately the gas transport parameters.
H53F-1477
Improving Heat Pulse Probe Sensitivity without Changing its Geometry
The heat pulse probe (HPP) has recently received increased attention as it allows in-situ, simultaneous, and automated measurements of soil hydraulic and thermal properties, as well as soil water fluxes. Although the currently-used design allows many applications, changes in HPP design and analysis are needed to increase its sensitivity to smaller water fluxes. In our previous study, we showed that significantly different temperature responses are obtained depending on the axial location of thermistors and that only temperature measurements near the middle of the 33-mm long heater fulfill the assumption of an infinite line heat source. It was also demonstrated that larger heater needle diameters allow larger heat pulses, leading to larger temperature differences between upstream and downstream thermistor needles and thus a higher sensitivity to water flux measurements. Both approaches, however, require changing the physical geometry of HPP, which is not favored by practitioners. In this study, we numerically evaluate the impact of alternating heat pulse durations, rather than changing the HPP geometry, on improving sensitivity of HPP under different conditions. For example, for a standard 1-mm diameter heater needle, five times greater sensitivity was achieved by using eight times longer heat pulse duration, while increasing the maximum temperature at the heater by only 8 degree C. If a standard heat pulse duration of 8 seconds is used, five times greater sensitivity can be achieved only by applying eight times greater heat pulse, which leads to the maximum temperature at the heater exceeding 100 degree C. Longer heat pulse durations are beneficial for the estimation of smaller liquid fluxes. All numerical experiments were conducted using the HYDRUS-2D code.
H53F-1478
Evaluation of Alternative Heat Pulse Probe (HPP) Designs
Water flux and content measurements in variably saturated porous media can be achieved utilizing the heat pulse probe (HPP). However, with current HPP design, the lowest possible measured water flux is approximately 5 cm/day, whereas vadose zone fluxes are generally below 1 cm/day. Furthermore, the current HPP design has relatively low durability in field applications. Numerical analysis results suggest that larger heater needle diameters might be required to make lower unsaturated water flux measurements possible. In this study, we evaluate different HPP designs, using observed data and analytical and numerical model simulation results. To enhance HPP durability, we present experimental and numerical data, demonstrating the capability of shorter HPP needles. For the lowering of the measured water flux threshold, a large-diameter needle HPP wad developed, of which model solutions and experimental data are presented. Moreover, a sensitivity analysis was conducted, evaluating the effects of heater needle diameters, temperature sensing locations, heat intensities and heat patterns on the water flux estimations.
H53F-1479
MONITORING OF WATER AND THERMIC TRANSFERS IN THE VADOSE ZONE OF A GEOLOGICAL CARBONATE FORMATION : EXAMPLE OF AND UNDERGROUND QUARRY, GIRONDE, FRANCE
The aim of this study is the monitoring of water and thermic transfers in vadose zone of a geological carbonate formation during three hydrological cycles (August 2001– November 2004). The application of the Time Domain Reflectometry (TDR) and Self-Potential (SP) methods to determine the water content of porous rock has been widely investigated. More than 285 studied point measurements of rock water content observed during three hydrological cycles and distributed among an abandoned underground quarry in Gironde, France, show a permanently undersaturated limestone (between 35 and 50 percents). We also investigated the unsaturated zone in a borehole between 0 and 20 m depth until the water table. 14 TDR and SP electrodes investigate the vadose zone. For the understanding of the streaming potential and electric behaviour from the SP method of a vadose zone we performed an experimental device which allows us to quantify the measurements of electrokinetic coupling coefficient at various saturation conditions. The results show that the vadose zone is characterized by three different sub-zones which are different water dynamics. The shallow zone down to a depth of seven meters corresponds to a zone with a significant variation of water saturation related to evapotranspiration dynamic water. The second zone (so-called transition zone) between seven to sixteen meters displays a high stability. The third zone (zone of capillary fringe) between sixteen to twenty meter shows a high and constant water saturation. Experimental results show three periods of maximum water content corresponding to three occurring effective precipitations. The dephasing and the amplitude attenuation of the hydraulic and thermic waves with the depth can be modelled and explained by the physical properties of the porous medium in an unsaturated zone such as the diffusivity, the water relative permeability, the capillarity pressure versus water saturation and the effective porosity.
H53F-1480
Geochemical signature and transfers in the Vadose zone of a geological carbonate formation : example of an underground quarry, Gironde, France
The aim of this study is to understand physico-chemical weathering processes in the vadose zone. The study combines hydrogeological and hydrogeochemical data on pore water with information about solid phase composition of the unsaturated zone. A monitoring station in the vadose zone is established in the Oligocene Aquitain limestone : the soil (0.40 meter thick) is lies on the Oligocene limestone (30 meters thick). The water table is 19.80 m average. A borehole with a diameter of 0.8 m is instrumented with "Teflon- quartz" cells (for the sampling of the pore water), with TDR probes (Time Domain Reflectometry) and SP (Self-Potential) electrodes (to measure the water content of the porous rock) and that all along the unsaturated zone. Chemically, the pH, alkalinity and the content of cations and anions are measured. The contents of cations and anions show variations with the depth. Results of chemical water analysis show that the unsaturated zone can be devided in three zones where the equilibrum of the calco-carbonic system can be moved with the precipitation or dissolution of dominating phenomena. The shallow zone down to a depth of seven meters corresponds to a zone with a significant decrease of mineralization (calcium, magnesium, sulfate) and evapotranspiration dynamic water. The second zone between seven to sixteen meters displays a high stability. The third zone (zone of capillary fringe) between sixteen to twenty meter shows a rise of certain ionic concentrations. The saturation index for the calcite calculated from PHREEQC program as a function of depth gives both dissolution/precipitation processes in the carbonate rock.
H53F-1481
Potential for Recharge in Agricultural Soils of the Mississippi Delta
Ground water models predict that 5 percent or less of precipitation in the Mississippi Delta region recharges the heavily-used alluvial aquifer; however the presence of agricultural chemicals in ground water suggests more substantial recharge. In a preliminary assessment of the potential for aerial recharge through the agricultural soils of the Bogue Phalia basin in the Mississippi Delta, we applied a method for rapidly measuring field- saturated hydraulic conductivity (Kfs) in 26 locations in cotton and soybean fields. The technique makes use of a portable falling-head, small-diameter, single-ring infiltrometer and an analytical formula for Kfs that compensates both for falling head and for subsurface radial spreading. Soil samples were also collected at the surface and at about 6 cm depth at each location for particle size analysis. Kfs values are generally higher than anticipated and vary over more than three orders of magnitude from 1x10-2 to 5x10-6 cm/s. There is also a correlation between Kfs and mean particle size which may prove useful in generalizing recharge rates over larger areas. A 2-m ring infiltration test is planned that will include the use of tracers and subsurface instruments for measuring water content and matric potential from the near surface to about 5 m to evaluate flow and transport below the root zone.
H53F-1482
Transport and Straining of Colloid-Sized Materials in Saturated Sand
The fact that colloids facilitate transport of contaminants such as heavy metals, pesticides, radionuclides etc. through porous media has gained widespread acceptance in the last few decades. Colloid attachment and straining have been identified as key mechanisms on colloid retention/filtration in porous media. Much research attention was focused on colloid straining during recent years since the attachment-based classical filtration theory could not accurately predict colloid deposition, especially under unfavorable attachment conditions. Recent studies further revealed that transport and filtration of colloids largely depend on initial colloid concentration and flow rate. Despite growing attention, however, the knowledge of colloid filtration, especially for natural soil colloids, is still limited. This study investigated attachment and straining of colloid-sized glass beads with the diameter of 1-10μm and soil colloids with the diameter of less than 1 μm extracted from a volcanic ash soil in saturated sand (Toyoura sand) by means of a series of column experiments at different colloid concentrations and flow rates (Darcian flux 0.16-1.0cm/min). The height and internal diameter of the sand column were 10cm and 4.91cm, respectively. Bromide was also added to colloidal solution as a conservative tracer. In each column experiment, 3 pore volumes of artificial rainwater was initially applied downward at a steady flow rate, then shifted to 10 pore volumes of colloidal solution, followed by another 5 pore volumes of artificial rainwater. By measuring the colloid concentration of effluents, colloid breakthrough and breakdown curves were observed. Glass bead colloids exhibited essentially no breakthrough on both high and low flow rates, suggesting all colloids deposited in the soil column. Since colloid attachment is unlikely due to mutual repulsion of negatively charged colloids and sand grains, we presume all colloids were physically strained in porous media interstices. Soil colloids, on the other hand, exhibited complete deposition at low flow rate, but only 31 percent deposition at high flow rate. Particle size distribution measurements of effluent colloids of the latter revealed that both smaller and larger size regions of input colloids preferentially deposited in sand column. Deposition of large soil colloids can be attributed to straining while smaller colloids are presumably attached to sand grains. Following completion of each column experiment, sand columns were disjointed in order to examine the colloid retention profile. Colloid retention profile of glass bead colloids was non monotonic with multiple deposition peaks. The highest peak occurred near the column inlet while two relatively smaller secondary peaks occurred at dimensional depths of 0.35 and 0.75 respectively. Particle size distribution measurements of deposited colloids revealed that larger size colloids were retained near the column inlet while relatively smaller size colloids were captured by deeper layers. Furthermore, HYDRUS-1D code will be used to estimate the straining parameters of colloids.
H53F-1483
Quantification of Microbial Activities in Near-Surface Soils
Microbial processes in near-surface soils play an important role in carbon and nutrient cycling, and specifically in the turnover of greenhouse gases such as CO2 and CH4. We modified a recently developed technique, the gas push-pull test (GPPT), to allow for the in-situ quantification of microbial activities in near-surface soils. A GPPT consists of the controlled injection of a gas mixture containing reactive gases (e.g., CH4, O2, CO2) and nonreactive tracer gases (e.g., Ar, Ne) into the soil, followed by the extraction of the gas mixture/soil-air blend from the same location. Rates of microbial activities are computed from the gases" breakthrough curves obtained during the GPPT's extraction phase. For a GPPT to be applied successfully, it is important that sufficient mass of the injected gases can be recovered during the test, even after prolonged incubation in soil. But this may be difficult to achieve during GPPTs performed in near- surface soils, where gas loss to the atmosphere can be substantial. Our modification consisted of performing GPPTs within a steel cylinder (8.4-cm radius), which was previously driven into the soil to a depth of 50 cm. During the GPPTs, the cylinder was temporarily closed with a removable lid to minimize gas loss to the atmosphere. We performed a series of numerical simulations as well as laboratory experiments to test the usefulness of this modification. Numerical simulations confirmed that without use of the cylinder, typical near- surface GPPTs (e.g., injection/extraction depth 20 cm below soil surface) are subject to extensive gas loss to the atmosphere (mass recovery < 20% for most gases), whereas mass recovery of injected gases increased dramatically when the cylinder was employed (mass recovery > 90% for most gases). Results from laboratory experiments confirmed this observation. We will also present results of a first field application, in which a near- surface GPPT was successfully conducted in a sandy soil to quantify in-situ rates of CH4 oxidation.
H53F-1484
Natural Colloid Mobilization in Unsaturated Hanford Coarse Sand Under Transient Flow and Transient Chemical Conditions
Colloid-sized clay, carbonate, and metal oxide particles are ubiquitous in the vadose zone and strongly adsorb dissolved contaminants such as metals and radionuclides. Under certain conditions, colloid particles are readily mobilized (released) into pore water and travel in a nearly conservative fashion and thus can facilitate the transport of contaminants. Although much progress has been made toward identifying and modeling colloid mobilization and transport processes in ideal, homogeneous systems, our understanding of the phenomenon in non-ideal, heterogeneous systems is still limited. We investigated natural colloid mobilization and transport in laboratory columns packed with Hanford Coarse Sand, a heterogeneous natural sediment. Our major focus was the role of transient flow and transient chemical conditions on colloid release and transport in unsaturated media. We found that a moving air-water interface had the greatest effects on the mobilization of colloid, and up to ~1000 mg/L of colloid was mobilized during column drainage at an ionic strength of 2 mM. An increase in flow rate or decrease in ionic strength also mobilized colloids. A model that accounts for transient pore water flow, colloid transport, and mass transfer in unsaturated media was developed to describe colloid mobilization in our column experiments. Both our experimental and modeling results showed the important role of moving air-water interfaces, changes in moisture content, and changes in ionic strength in mobilizing natural colloids in heterogeneous natural sediments. This work has contributed to our knowledge of colloid and colloid-associated contaminant mobilization in real vadose-zone environments under transient flow and transient chemical conditions.
H53F-1485
Drainage Experiments in Heterogeneous Sand Columns With Different Geometric Structures
This poster presents results of multi-step drainage experiments, carried out with two sand columns (10x10x20
cm
H53F-1486
Direct laboratory quantification of dynamic coefficient of a field soil for drainage and wetting cycles
The constitutive relationships between capillary pressure and wetting fluid saturation (retention curve) are needed in the modeling of unsaturated and multi-phase flow in porous media. These relationships are usually measured under equilibrium conditions of the two phases (wetting and non-wetting). The question is whether such curves adequately describe the relationships between capillary pressure and saturation in drainage or imbibition events with time scales on the order of hours. Hassanizadeh et al. [2002] suggest that differences in capillary pressures measured under equilibrium and non-equilibrium conditions is due to a phenomenon that is referred to as dynamic effects in capillary pressure. Typically, the difference between dynamic and static capillary pressures is defined as a product of a dynamic coefficient (tau) and rate of change in wetting phase saturation (- dSw/dt). In this study, we quantify tau in a direct, systematic, and consistent manner through careful laboratory experiments. Using a fine field sand filled in a 10 cm-tall cell, we performed drainage and wetting experiments under identical packing conditions, but differing imposed boundary conditions (i.e., static and dynamic with various drainage/wetting rates). The dynamic coefficient was estimated for primary drainage, main wetting, and main drainage cycles. In the primary drainage cycles tau values increased with decreasing wetting phase saturation. In the case of the main wetting cycle, performed under the same dynamic conditions, tau values were somewhat smaller and increased slightly with increasing wetting phase saturation. These findings imply that tau may be hysteretic. Finally, tau values for the main drainage cycle were similar to those of the primary drainage cycle (tau increases with decreasing wetting phase saturation). Overall, the estimated tau values varied over a range of 2e5 to 1e7 kg/m/s. This is consistent with the range of tau that Hassanizadeh et al., [2002] estimated based on published studies that exhibited dynamic effect in capillary pressure. A numerical simulator, modified to incorporate dynamic effects in capillary pressure, was used to model the outflow experiments and yielded good agreement between observed and modeled outflow.
H53F-1487
Geotechnical Centrifuge Studies of Unsaturated Transport
Improved understanding of contaminant migration in heterogeneous, variably saturated porous media is required to better define the long-term stewardship requirements for U.S. Department of Energy (DOE) lands and to assist in the design of effective vadose-zone barriers to contaminant migrations. A geotechnical centrifuge provides an experimental approach to explore vadose zone transport over a wide range of relevant conditions in time frames not possible for conventional bench-top experiments. Our research to date resulted in the design, construction, and testing of in-flight experimental apparatus allowing the replication of traditional bench top unsaturated transport experiments using the 2-meter radius geotechnical centrifuge capabilities at the Idaho National Laboratory. Additionally we conducted a series of unsaturated 1-dimenstional column experiments using conservative tracers to evaluate the effects of increased centrifugal acceleration on derived transport properties and assessing the scaling relationships for these properties. Our experimental results indicated that breakthrough times for a conservative tracer decreased significantly and systematically as a function of increased centrifugal acceleration. Differences between these experimental results and estimates based on predictive scaling rules are due to slight moisture content differences between experiments at different centrifugal accelerations. In contrast, dispersion coefficients varied systemically with centrifugal acceleration in accordance with predictive scaling rules. The results we obtained in this study indicate that the centrifuge technique is a viable experimental method for the study of subsurface processes where gravitational acceleration is important. The geotechnical centrifuge allows experiments to be completed more quickly than tests conducted at 1-gravity and can be used to experimentally address important scaling issues, and permits experiments under a range of conditions that would be difficult or impossible using conventional approaches.
H53F-1488
Measurement of Relative Permeability Using Single-Step Transient Flow Centrifuge Experiments
Steady-state flow centrifuge methods have been widely used to determine the relative permeability function, krw, of porous media. The centripetal acceleration greatly accelerates the speed at which equilibrium conditions are found in drainage or imbibition experiments. While equilibrium conditions are required by steady- state flow centrifuge methods, transient flow centrifuge methods utilize the time variant adaptation of state variables (e.g. fluid saturation, pressure or flux) to sudden changes in boundary conditions. Transient flow centrifuge experiments were conducted using a commercial rock core ultracentrifuge to displace wetting fluids from an initially fully-saturated sample by non-wetting fluids. The cumulative production of wetting fluid leaving the sample was recorded using an automated digital camera recording system. The time series of cumulative wetting fluid production forms the input for the objective function of inverse numerical modeling efforts and for the independent estimation of krw using an analytical solution for the transient flow of fluids through a porous medium in a centripetal field (the Hagoort method). In this study, single-step transient outflow experiments were run on Berea sandstone cores. The angular velocity was 942 sec-1. Independent laboratory techniques were used to measure the saturated water content, θs, and the intrinsic permeability, ksat. Besides these measurements, synthetic production data were generated with forward simulations of Hydrus-1D using the standard van Genuchten parameters for 'Sand' and 'Silt' soils. The models simulated acceleration of the centrifuge rotor during the first ~95 seconds of the experiment by increasing the angular velocity stepwise in 2.5 second intervals to 733 sec-1. The data were interpreted using a modified version of the Hagoort method in which numerical derivatives are used to compute the slope of the production curve in order to calculate relative saturation and the corresponding krw function. We also used Hydrus-1D to inversely estimate the parameters of the van Genuchten relative permeability model, θr, a, n, and l. Systematic variation of the initial parameters values used in the inverse simulations showed that some of the parameter estimates obtained with inverse simulation were non-unique. For both the synthetic simulations and the measurement data, the difference between the original krw curve and the krw curve obtained with inverse simulations was most sensitive to the initial estimate of the shape parameter, n. Estimation of initial parameter values could be augmented by the good correspondence between the krw curves obtained with the Hagoort method and those used and obtained in numerical (inverse) simulations. Transient flow centrifuge experiments and their interpretation are expected to have applications in predicting infiltration and remediation of environmental contamination within the vadose zone.
H53F-1489
A Comparison of Soil-Water Sampling Techniques
The representativeness of soil pore water extracted by suction lysimeters in ground-water monitoring studies is a problem that often confounds interpretation of measured data. Current soil water sampling techniques cannot identify the soil volume from which a pore water sample is extracted, neither macroscopic, microscopic, or preferential flowpath. This research was undertaken to compare values of extracted suction lysimeters samples from intact soil cores with samples obtained by the direct extraction methods to determine what portion of soil pore water is sampled by each method. Intact soil cores (30 centimeter (cm) diameter by 40 cm height) were extracted from two different sites - a sandy soil near Altamonte Springs, Florida and a clayey soil near Centralia in Boone County, Missouri. Isotopically labeled water (O18 - analyzed by mass spectrometry) and bromide concentrations (KBr- - measured using ion chromatography) from water samples taken by suction lysimeters was compared with samples obtained by direct extraction methods of centrifugation and azeotropic distillation. Water samples collected by direct extraction were about 0.25 ‰ more negative (depleted) than that collected by suction lysimeter values from a sandy soil and about 2-7 ‰ more negative from a well structured clayey soil. Results indicate that the majority of soil water in well-structured soil is strongly bound to soil grain surfaces and is not easily sampled by suction lysimeters. In cases where a sufficient volume of water has passed through the soil profile and displaced previous pore water, suction lysimeters will collect a representative sample of soil pore water from the sampled depth interval. It is suggested that for stable isotope studies monitoring precipitation and soil water, suction lysimeter should be installed at shallow depths (10 cm). Samples should also be coordinated with precipitation events. The data also indicate that each extraction method be use to sample a different component of soil-pore water. Centrifugation can be used with success, particularly for efficient sampling of large areas. Azeotropic distillation is more appropriate when strict qualitative and quantitative data on sorption/desorption and various types of kinetic studies may be needed.
H53F-1490
Solute Diffusivity of Repacked Volcanic Ash Soil: Effect of Changes in Pore Size Distribution due to Soil Compaction
Diffusion is the dominant spreading mechanism of contaminants dissolved in soil-water in the absence of soil- water flow. Solute diffusion coefficient, Ds, is a key parameter in investigating the fate and transport of contaminants from a polluted soil site. However, only a few studies on quantifying Ds as a function of soil- water content were done, especially for aggregated soils with a dual pore system such as volcanic ash soils (Andisols). In this study, we investigated the effect of bulk density on pore size distribution, and, consequently, on solute diffusivity (Ds/Do, where Do is the solute diffusion coefficient in pure water) in repacked volcanic ash soil taken at 5-10 cm depth at a pasture site in Nishi-Tokyo, Japan. Measurements of Ds were done on sieved and repacked soil at three bulk densities (0.62 g cm-3 , 0.7 g cm-3, and 0.8 g cm-3 ) and at three soil moisture conditions at pF (= log (-ψ; soil-water matric potential in cm H2O)) 1.8, 2, and 3 for each bulk density. Half-cell method was used to measure Ds where the source and sink half cells (each cell of 10-cm length and 4.9 cm in diameter) were joined together and the concentration profile was analyzed after a substantial time to determine Ds. Results showed that at a particular bulk density, Ds decreased with decreasing degree of saturation. This is expected since as the soil becomes drier, water films become disconnected resulting in a decrease in Ds. On the other hand, at a particular degree of saturation, the magnitude of Ds considerably decreases with increasing dry bulk density. As soil is compacted (and thus the increase in bulk density), the observed pore size distribution obtained from soil-water retention curve changes where the mainly inter-aggregate large pores become smaller and soil particles become closer to each other. This reduction in inter-aggregate pore size likely increases the liquid-phase tortuosity resulting in the decrease in Ds/Do at soil-water content at pF < 3. The soil-water retention point at pF 3 was observed to be the separation between the inter- and intra-aggregate pore space regions, where the inter- aggregate pore space was completely drained. Thus at pF close to 3, the difference of Ds/Do among three bulk densities becomes smaller, probably due to high possibility to ensure continuous water pathways among intra-aggregate pores caused by inter-connection of aggregate. Although volcanic ash soils are distributed across around 0.84% of the earth's land surface, only a limited number of studies about solute diffusion for Andisols are available as compared to numerous studies on water permeability (liquid-phase convection parameter). Therefore, this study contributes to a valuable data set of solute diffusion coefficient for volcanic ash soils.
H53F-1491
A Preliminary Description of the Moisture Moment Method to Describe Unsaturated Soil Hydraulic Properties
We will present a new experimental procedure to elicit unsaturated soil hydraulic properties from a bench-scale test. A slightly wetted horizontal soil column is hung from two load cells and water is slowly injected into one end. A data logger records the cumulative change of force acting on each load cell due to redistribution of water. A tensiometer present at the inlet measures soil tension throughout the test. Unlike previous horizontal infiltration tests, the proposed technique does not necessitate maintaining a constant water content at the inlet/outlet of the soil column (i.e. no Boltzmann transformation). By analyzing the change in forces on the two load cells, one can describe water retention curve and unsaturated hydraulic conductivity curve. In the future, we plan to test the procedure on a range of soil textures from sand to clay. We will validate our water retention curve predictions by measuring the actual water retention within the column using a computer controlled gamma-ray attenuation system. The new method requires a few hours to more than a day.
H53F-1492
Complex Unsaturated Zone Flow and Thermohydrologic Processes in a Regulatory Environment: A Perspective on Uncertainty
An important part of a risk-informed, performance-based regulatory review of a potential license application for disposal of high-level radioactive waste at Yucca Mountain, Nevada, is the consideration of alternative interpretations and models of risk significant physical processes. The Nuclear Regulatory Commission (NRC) expects that simplified models will be abstracted from complex process-level models to conduct total-system performance assessments. There are several phases or steps to developing an abstracted model and its supporting basis from more detailed and complicated models for each area of the total system. For complex ambient and thermally perturbed flow in fractured tuffs of the unsaturated zone at Yucca Mountain, these steps c,an be summarized as (i) site characterization and observation, (ii) field and laboratory tests, (iii) conceptual model development, (iv) process-level numerical modeling, and (v) abstraction development. Each step is affected by uncertainty in (i) assessing parameters for models and (ii) conceptualization and understanding of governing processes. Because of the complexity and uncertainty, alternative interpretations and models become important aspects in the regulatory environment. NRC staff gain confidence in performance assessment model results through understanding the uncertainty in the various models. An example of a complex process in the unsaturated zone is seepage into drifts, which leads to liquid water potentially contacting waste packages. Seepage is a risk-important process for the unsaturated zone at Yucca Mountain because of its potential effect on waste package integrity and trainsport of potentially released radionuclides. Complexities for seepage include (i) characterization of fractures that carry flow, (ii) effect of small to intermediate scale structural features on flow, (iii) consideration of the diverse flow regimes (rivulets, film flow, capillarity) in fractures, (iv) effect of vapor transport associated with convection along drifts and air flow through fractures, (v) effect of the thermal perturbation caused by radioactive waste, and (vi) consideration of drift stability and climate change because of the long time periods of interest-I 0,000 to a million years. This poster tracks the available information starting from site characterization and continuing on to abstractions used in performance assessment models. At each step, multiple interpretations or alternatives can be identified based on fundamental observations and measurements. From a regulatory perspective, confidence is gained when alternative models or abstractions lead to similar results from a performance assessment; otherwise uncertainty from alternative explanations should be considered in terms of a defendable review. This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory positions of the NRC. The NRC staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed or of the acceptability of a license application for a geologic repository at Yucca Mountain.
H53F-1493
Land Retirement as a Habitat Restoration Tool
Use of intensive irrigation in arid and semi-arid areas usually leads to gradual salination of the soil leading to crop yield decline. The salination problem is mitigated by applying irrigation in excess of crop requirements, which leaches the excess salt load to the groundwater. Insufficient natural or man made drainage to dispose off this saline recharge to the groundwater leads to a gradual rise in the water table and eventual encroachment upon the root zone. This may ultimately make the land unfit for any economically productive activity. The abandoned land may even lead to desertification with adverse environmental consequences. In drainage basins with no surface outflow (sometimes called closed basins), land retirement has been proposed as a management tool to address this problem. Land retirement essentially entails intentionally discontinuing irrigation of selected farmlands with the expectation that the shallow water table beneath those lands should drop and the root zone salinity level should decrease. In the San Joaquin Valley of California, intensive irrigation in conjunction with a shallow underlying layer of clay, known as the Corcoran clay layer and absence of a drainage system caused the root zone to become highly saline and the shallow water table to rise. Land retirement would remove from production those farmlands contributing the poorest quality subsurface drain water. Based on numerical models results, it was expected that with land retirement of substantial irrigated lands with poor drainage characteristics, beneath which lies shallow groundwater with high salt load, the shallow water table beneath those lands should drop. A part of the retired lands could also be used for wildlife habitat. A potential negative side of the land retirement option that has to be considered is that in certain enabling evapotranspiration, soil and water table conditions, water will be drawn upwards and evaporated, leaving a deposit of salts on the surface and in the root zone. Salt on the surface may then be wind blown to adjacent areas creating a potential environmental hazard. Using field results from the U.S. Department of the Interior Land Retirement Demonstration Project at the Tranquillity site located in western Fresno County, principles of mass balance in a fixed control volume, the HYDRUS-1D Software Package for Simulating the One-Dimensional Movement of Water, Heat, and Multiple Solutes in Variably-Saturated Media, and PEST, a model-independent parameter optimizer, we have investigated the processes of soil water and salinity movement in the root zone and the deep vadose zone. Various combinations of evapotranspiration, soil water retention properties, water table condition and top and bottom boundary condition were tested. We show that certain Land Retirement scenarios decrease shallow water table and soil water salinity and enhance development of native plants as a means to facilitate habitat restoration for certain combination of soil and bottom boundary condition. Other combinations are not sustainable.
H53F-1494
Coupling Inverse Parameter Estimation Software with a 1D Soil Heat and Water Transport Model
Inverse parameter estimation is becoming an increasingly popular tool for calibrating and evaluating hydrologic models. In this study, PEST (Paramater ESTimation) software was used in conjunction with the one-dimensional soil transport model SHAW (Simultaneous Heat and Water Transport). The overall goal of the research project was to evaluate the impacts of winter climate variations on spring snowmelt and infiltration in south-central Wisconsin. In this study, SHAW was used with known parameter values to create a synthetic drainage and soil moisture data set. Various combinations of the synthetic observation data and methods for dealing with parameter correlation were used to test the ability of PEST to reproduce the known values for air entry potential, hydraulic conductivity, and pore size distribution index in the SHAW model. Results are presented for a synthetic short-term fall drainage experiment, and for soil subject to natural winter seasonal weather conditions.
H53F-1495
Application of Fluorescent Tracers on Observation of Preferential Flow
Around 60 % of the soils in the Czech Republic is composed of Cambisols formed on weathered crystalline bedrock. These soils are texturally heterogeneous and exhibit fast preferential flow. The goal of our study was to assess the preferential flow in relation with numerical modelling. The methodology was based on infiltration experiments and dye tracers. Connectivity, volumetric ratio and spatial development of preferential pathways were evaluated as the necessary information for numerical simulations with dual-permeability approach. Inflow- outflow ponded infiltration experiments with the use of fluorescent dye tracer Rhodamine 6G and Brilliant Blue tracer were performed on undisturbed core samples taken from two experimental sites in the Czech Republic. The outflow concentrations of Rhodamine 6G were continuously measured with fluorometer. Immediately after the experiments the saturated samples were horizontally sliced and each layer was photographed. The images were digitally treated to visualize and quantify geometry of the preferential flow domain. Results were used as an input into the S1D_DUAL dual-permeability numerical model to solve the infiltration-outflow experiment inversely. Breakthrough curves of Rhodamine 6G and cumulative outflows served as the calibration data for the model. The research has been carried out within the internal CTU grant 0702811 and research projects MSMT 1K05024 and VZ 04 CEZ MSM 6840770005.
H53F-1496
Evaluation of the Transport of Natural Radioactive Materials in Large Lysimeters Using Hydrus-1D
The mining industry in Brazil often uses raw materials that contain relatively high concentrations of naturally occurring radioactive materials (referred to as NORM). Ores of relatively low grade typically are used to produce refined metals of high purity (e.g., Nb, Ta, Sn, and Au) using pyrometallurgic processes. The final waste is a slag rich in natural radioactive contaminants (the U and Th decay series), which are then usually deposited in industrial landfills. To study the long-term fate and transport of radionuclides leached from the NORM wastes, several large (3 m deep) lysimeters were constructed at the Pocos de Caldas Laboratory of the Brazilian Nuclear Energy Commision (CNEN). The lysimeters were packed with surface soils and slags from one of the mining sites in South East Brazil. Main purpose of our lysimeter experiments was to follow the dissolution and transport of radionuclides from the slags under natural climatic conditions. Leaching rates and radionuclide concentrations of the effluent were observed during a three-year time period. A variety of physical and chemical properties of the soils and slags (including laboratory batch equilibrium sorption values) were also determined. The data were analyzed using several computer software packages, including the STANMOD code for analytical modeling of decay chain transport during steady flow, the HYDRUS-1D code for variably-saturated flow and the transport of multiple solutes, and the HP1 code for a more comprehensive analysis of the geochemistry involved. In this presentation we describe the experimental setup and provide preliminary results of the theoretical analyses, especially those using HYDRUS-1D.
H53F-1497
Development and Validation of a Numerical Model for Non-isothermal Multiphase Flow in Partially Frozen Porous Media
A general numerical model for the simultaneous transport of water, air, and heat in geological media under transient freezing conditions will be presented. The code solves conservation equations for air (as gas and dissolved in liquid water) and water (ice, liquid and vapor phases) using an integrated finite difference method with fully implicit time stepping. Key physical processes represented include advection as gas or liquid, vapor and aqueous phase diffusion, and heat conduction. Key relationships among liquid and ice saturations, interfacial pressures, and temperatures are based on empirical unfrozen soil moisture retention curves combined with thermodynamic relationships between ice/liquid capillary pressure and temperature. Saturated, partially saturated and partially frozen, and fully dry conditions are accommodated; phase disappearance/reappearance is handled with a variable substitution approach. Simulations of freezing-induced moisture redistribution agree well with published laboratory data on freezing-induced moisture redistribution (cryosuction). However, the comparison is sensitive to the assumed relationship between thermal conductivity and ice content, which is uncertain. Acknowledgment: This work was supported by SwRI(TM) Southwest Initiative for Mars (SwIM) under project number R9313 and by NASA Mars Fundamental Research Program award NNX06AB19G. Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the author and do not necessarily reflect the views of the National Aeronautics and Space Administration.
H53F-1498
Development of a Long-Column Method to Test Constitutive Relations for LNAPL Movement in Two-Phase Systems
Multifluid relative permeability – saturation - capillary pressure (k-S-P) empirical constitutive models are components of numerical simulators that are used to predict fluid distributions following a nonaqueous phase liquid (NAPL) contamination event or during remediation. The S-P parameter values for these empirical models are either obtained from the literature or determined experimentally by fitting the models to measured data. Most of the experimental emphasis so far has been on testing the S-P component of the k-S-P constitutive relations. Due to the difficulties in obtaining quality relative permeability laboratory data for multiphase systems, testing of the k-S models that are used in multifluid flow simulators has been virtually non-existent. A new tool, the Multiple Location Saturation Pressure Apparatus (MLSPA), located in PNNL's EMSL Subsurface Flow and Transport Laboratory, has been developed to obtain data sets that can be used to test both S-P and k-S relationships for two-phase NAPL-water systems. The MLSPA is a long column (~1 m) equipped with several hydrophilic and hydrophobic pressure transducers. Fluid saturations are determined along the length of a column using a dual-energy gamma radiation system. Although the MLSPA is limited to porous media with a relatively small entry pressure and fairly homogeneous pore-size distributions, it offers the distinct advantage of obtaining S-P data at multiple locations. Besides for static determinations of S-P relations, the MLSPA offers the benefit that it can be used for more dynamic experiments where fluid pressures are changed more rapidly. The data sets produced by the dynamic experiments can be used in relative permeability models. Results of several experiments with crude-oil brine systems will be presented.
H53F-1499
Experimental and Numerical Investigation of Subsurface Desiccation
Traditional remediation techniques are not applicable at some locations at the Hanford site where contaminants in the unsaturated zone are located too deep and spread over a large area. Non-reactive or mobile contaminants might reach the water table at unacceptable concentrations. Desiccation, in addition to surface infiltration control, is considered to have considerable benefits for deep vadose zone application and is currently viewed as a potentially viable deep vadose zone technology for Hanford. Several technical issues need to be addressed as part of developing and applying soil desiccation as a viable technique for deep vadose zone treatment. Wedge-shaped, two dimensional experiments have been conducted in an attempt to provide insight into key issues regarding 1) energy limitations on the volume of water that can be removed, 2) osmotic effects during soil drying, and, 3) potential remobilization of contaminants after cessation of desiccation. The flow cell experiments have been conducted in homogeneous and heterogeneous systems for various imposed boundary conditions. A dual-energy gamma system was used to determine water saturations for some experiments, while temperature and humidity have been measured with internal probes. Experimental variables are temperature and rate of the injected air, boundary conditions (zero flux and constant temperature), and porous media properties. The experimental results are compared with simulations conducted with the STOMP simulator.
H53F-1500
Characterization of Potential Preferential Pathways in Soil Columns by X-ray Tomography and Infiltration Experiments
Disturbances of soil structure, which originate from both biological and physical processes in soil often represent pathways of the preferential flow. Larger structures can be well identified by X-ray computed tomography (CT) of the soil. In this study the regions of high porosity, which can represent preferential pathways, were identified by thresholding of the CT images followed by modified morphological operation of erosion and dilation. Five undisturbed soil columns from Cambisol and Oxisol series from different locations have been studied. Volumes of high porosity regions were calculated for each threshold value and semivariograms were calculated for original images as well as for high porosity regions only. Data from CT were analyzed in connection with the flow dynamics as monitored on the same soil columns by experiments in laboratory. Experiments consisted of series of infiltration events done using modified tension infiltration disc placed on the top of the soil column. The bottom surface of soil column was supported by the perforated plate and was exposed to the atmospheric pressure. Cumulative outflow and the total mass of the soil core were recorded as well as pressure heads measured by tensiometers. Experiments revealed occurrence of the preferential flow for the soil from Cambisol series. For soil core from Oxisol series the preferential flow was less pronounced. These observations correspond with the results of CT images analysis, where soils from Cambisol series showed distinct networks of high porosity regions. This research has been supported by MSMT 1K05024.
H53F-1501
Variably-saturated Flow Through Mine Waste Rock in a Permafrost Environment
Mining in northern Canada creates waste rock piles with the potential of generating acid rock drainage (ARD). Test piles fifteen meters high and smaller-scale collection lysimeters have been constructed to investigate infiltration and variably-saturated water flow through heterogeneous material in a region of continuous permafrost. Data collection includes time domain and frequency domain reflectrometry, lysimetry, tensiometers, electrical conductivity sensors, and flow gauges. Capillarity-driven flow travels slower than seasonal frost and thaw propagation, resulting in wetting fronts that freeze and are remobilized in the following summer period. In addition, highly permeable zones of waste rock can transmit water at rates as high as meters per hour, under certain conditions. Results from the research may be incorporated into mine closure strategies to minimize contaminant loading in cold climates.
H53F-1502
Effect of Cyclic Precipitation and Radiation on the Fate and Transport of TNT and DNT Near Soil-Atmospheric Surfaces
Many explosive-related compounds (ERCs) are found near the soil-atmospheric surface in sites containing buried explosive devices, such as landmines and unexploded ordnance, detonation-residual solid explosives, and munitions residues from explosive manufacturing facilities. Accurate assessment of the fate and transport processes is essential for predicting their movement to the surface, groundwater, or any other important environmental compartment. The dynamics of ERCs movement is complex, involving multiple, interrelated processes. The transport processes controlling the direction and magnitude of the movement, and chemical, physical, and biological processes controlling the fate of the chemicals vary with environmental conditions. This research addresses the effect of variable rainfall, evaporation, temperature, and solar radiation on fate and transport of 2,4,6-Trinitrotoluene (TNT), 2,4-Dinitrotoluene (DNT), and other related chemicals in partially saturated soil. Fate and transport experiments were conducted in a laboratory-scale geo-environmental system containing a 3D SoilBed packed with a sandy soil, and equipped with rainfall and solar radiation simulators, and temperature control settings. Experiments were conducted by burying a TNT/DNT source under the soil surface and applying cyclic rainfall and radiation over the surface of the soil. Aqueous and gaseous concentrations of tracers, TNT, DNT and other related ERCs were monitored through time at several distances from the source. Measured hydraulic conditions and tracer concentrations during the experiments indicate the existence of preferential flow paths during infiltration events and solute accumulation during evaporation periods. TNT, DNT and an undetermined chemical believed to be a degradation by-product of TNT were detected non-continuously in space and time. Concentrations of TNT and DNT were much lower than their solubility limits, indicating rate- limited mass-transfer, dissolution limitations, and dilution processes. Concentrations of the degradation by- product were measured at all sampling depths, but mostly in the upper segments of the SoilBed, suggesting greater degradation processes resulting from radiation-induced conditions near the soil-atmospheric surface. ERC vapors were mostly detected in the upper soil segments and above the soil surface toward the end of evaporation periods. In general, it is deduced that ERCs follow preferential flow path during infiltration periods but that the overall transport is further influenced by vapor transport, sorption, mass transfer, and degradation processes. Results indicate that the magnitude and direction of their transport were strongly influenced by changes in environmental conditions.