FY2010 - Oak Ridge National Laboratory

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Director’s R&D Fund— Neutron Sciences 05551 Neutron Imaging of Fluids within Plant-Soil-Groundwater Systems Hassina Z. Bilheux Project Description This project develops a collaborative science program to investigate and model the phase structure and flow dynamics of fluids (water, brines, air, CO 2 ) within plants, soils, and rocks using noninvasive, nondestructive neutron imaging techniques. The theoretical treatment of fluids in porous media has improved substantially over the last several decades; however, model validation using time-resolved (seconds to minutes), high-resolution (tens of microns) measurements of fluid distributions in heterogeneous natural systems has been a major obstacle. Neutron imaging provides high sensitivity to light elements in fluids (e.g., hydrogen) and deep penetration into plants and earth materials. The scientific objectives of this project are to (1) develop quantitative imaging techniques to accurately measure 3D phase structures and 2D fluid flow in porous media, (2) test and refine imaging/modeling capabilities using homogenous model systems, and (3) apply imaging/modeling capabilities to identify fluid pathways, rates of flow, and interactions between porous media, fluids, and plants under dynamic and complex environmental drivers. Mission Relevance Utilizing the High Flux Isotope Reactor (HFIR) R&D Cold Guide 1 (CG-1) and the <strong>National</strong> Institute of Standards and Technology (NIST) BT-2 beamlines, we have developed in situ measurement to investigate soil-plant-atmosphere water exchange dynamics, water retention, unsaturated flow and solute transport in the vadose zone, and multi-phase flow and transport in groundwater systems. Results and Accomplishments To address several key questions regarding the distributions and dynamic flow of fluids (air, CO 2 , water, brines) within plant–soil–groundwater systems, using 2D and 3D neutron imaging techniques, we have identified three main tasks for our project: Measuring and Quantifying the Distribution of Fluids in Porous Media Evaluating Water Transport Limitations in Soil-Plant Systems Assessment of Analytical and Numerical Models for Predicting Fluid Flow We have made significant progress on many aspects of our project over the past 9–10 months and met our deliverables for the firstyear as summarized here. Measuring and quantifying the distribution of fluids in porous media. Typical 2D neutron images contain over 4 million pixels (2048 × 2048). A 3D reconstructed data set, in turn, usually consists of hundreds of 2D images, depending on the rotational increment selected (usually 0.5° or better). The boundary of a sample within a container is located in the image, and the thickness of water is calculated on a pixel-bypixel basis using the Beer-Lambert equation: I = I 0 exp( −µ ⋅ ∆x) , where I is the transmitted beam intensity, I 0 is the incident beam intensity, µ is the attenuation coefficient, and ∆x is the thickness of the sample. Two sets of very similar experiments were conducted at the imaging facility and HFIR CG-1D development beamline for imaging and quantifying the amount and distribution of water in a soil column as a function of water potential. The NIST BT-2 is at the only world-class neutron imaging facility in the United States, whereas the newly commissioned HFIR CG-1D instrument is a development beamline that is not designed for imaging measurements. Nevertheless, CG-1D has been successfully used for this 60
Director’s R&D Fund— Neutron Sciences project. Comparative experiments at the two imaging facilities have been performed. Flint #13 sand and Hanford soil were packed inside an aluminum cylinder with a 2.7 cm OD and initially saturated with water. Various water potentials (ψ) were applied to the bottom of the column using the hanging water column, and radiography (2D) images were taken at each equilibrium state during drying and wetting processes with a 60 s exposure time. Three-dimensional images with a rotational increment of 0.25° were also acquired at about half way in the drying and wetting stages. The 2D images of the soil column using NIST BT-2 illustrate water movement into the soil during the wetting process. The average water content of Flint #13 sand (4 cm height) at each equilibrium state during the drying and wetting cycles was measured by two methods: (1) volumetric measurements from the hanging water column and (2) integration of pixel-based water contents using neutron imaging data. The two methods yielded very similar average water retention curves. Similar measurements were performed at HFIR CG-1, using two different samples, Hanford and Flint, respectively. Evaluating water transport limitations in soil–plant systems. Switchgrass seeds were grown in pure silica sand within aluminum cylinders and subjected to low moisture conditions. Water was applied to the young seedlings through an injection port at the base of roots. A series of neutron images were taken every 2 min at HFIR CG-1D (exposure time was 120 s). After 18 min, there was little detectable change in the water content of the roots, albeit the plants were exposed to low light conditions which likely limited their uptake and transport of the injected water. Further work has utilized both D 2 O and H 2 O to enhance image contrast, addition of illumination treatments to enhance water flux, thin rectangular containers to improve root display, and longer intervals to track water flux. Maize seeds were germinated in silica sand and watered with D 2 O. After several weeks, 9 mL of water was injected into the bottom of the container and water distribution was tracked through time. The analysis reveals areas of increased moisture (large deep root, root tips) and decreased moisture (darker shallow roots) that establish the utility of this technique to determine water transport limitations in situ, specifically dynamics of water content surrounding root tissue. The relative soil–root hydration surrounding a growing root can reveal rhizosphere hydration at 80 m resolution through time, which will be assessed during periods of drying to determine loss of conductivity. Our 2D/3D neutron imaging data for water in roots and soil clearly demonstrate the potential of the technique for tracing the flow of water to roots and quantifying the spatial distribution of water in partially saturated soil. Our future research will focus on investigating (1) differences in point water retention curves estimated from conventional hanging water column experiments and those measured directly by neutron imaging; (2) comparison of forward numerical simulations with direct neutron-based measurements of water distribution; (3) reduction in hydraulic conductance of rhizosphere as soil dries under undisturbed conditions; and (4) fine-scale, 3D geometry of air–water interfaces using neutron tomography. Assessment of analytical and numerical models for predicting fluid flow. We have extracted point water retention functions for Flint #13 sand by constructing the drying curve on a pixel-by-pixel basis and then averaging row-by-row across the cylinder. The range in point functions obtained from different heights in the column is quite small as expected for a relatively homogenous material like Flint sand. We are now in the process of modeling the average curves using TRUECELL to analytically predict the point function, which can be compared with the observed point functions obtained by neutron imaging. Inverse numerical modeling of the 2D water distributions using HYDRUS 2D is also under way. The degree to which the observed and predicted functions agree with each other can provide an objective evaluation of the assumptions inherent within the different models. 61
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FY2010 - Oak Ridge National Laboratory

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