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Water retention curves are essential for understanding the hydrologic behavior of partially saturated porous media and modeling flow and transport processes within the vadose zone. We directly measured the main drying and wetting branches of the average water retention function obtained using two- dimensional neutron radiography. Flint sand columns were saturated with water and then drained and rewetted under quasi-equilibrium conditions using a hanging water column setup. Digital images (2048 by 2048 pixels) of the transmitted flux of neutrons were acquired at each imposed matric potential (~ 10-15 matric potential values per experiment) at the National Institute of Standards and Technology Center for Neutron Research BT-2 neutron imaging beam line. Volumetric water contents were calculated on a pixel-by-pixel basis using Beer-Lambert's law after taking into account beam hardening and geometric corrections. To account for silica attenuation and remove scattering effects at high water contents, the volumetric water contents were normalized (to give relative saturations) by dividing the drying and wetting sequences of images by the images obtained at saturation and satiation, respectively. The resulting pixel values were then averaged and combined with information on the imposed basal matric potentials to give average water retention curves. The average relative saturations obtained by neutron radiography showed an approximate one-to-one relationship with the average values measured volumetrically using the hanging water column setup. There were no significant differences (P < 0.05) between the parameters of the van Genuchten equation fitted to the average neutron radiography data and those estimated from replicated hanging water column data. Our results indicate that neutron imaging is a very effective tool for quantifying the average water retention curve. [ABSTRACT FROM AUTHOR]

Neutron radiography was used to investigate capillary uptake of water in Berea sandstone, which is widely used as a standard porous medium in flow and transport studies. The sorptivity and unsaturated diffusivity function were estimated from analyses of the neutron radiographs. These parameters have not been previously reported for this material. Neutron radiography is increasingly being used to study the dynamics of water movement in variably saturated porous media. It has been applied to visualize water imbibition in both natural and engineered materials, including soil, rock, brick, concrete, and glass. The sorptivity, S, and unsaturated diffusivity, D(θ), are important parameters for describing water movement under partially saturated conditions. Estimates of S and D(θ) have been obtained using a variety of techniques, including neutron imaging. However, we could find no previous reports of such measurements for the Berea sandstone, regardless of the method employed. Berea sandstone is a widespread, medium- to fine-grained terrestrial sandstone of Mississippian age that is used extensively as a standard porous medium in the geology and petroleum engineering fields. We used the CG-1D neutron imaging facility at the High Flux Isotope Reactor of Oak Ridge National Laboratory to estimate S and D(θ) from radiographs acquired every 26 s. A 25 -1/4m thick LiF/ZnS scintillator was employed in conjunction with a DW936 IkonL ANDOR charge coupled device (CCD) camera system, giving a spatial imaging resolution of -1/475-1/4m. Four replicate cores were investigated. The positions of the observed wetting fronts were linearly regressed against the square root of time. Sorptivity values calculated from the slopes of these relations ranged from 0.89 to 1.46 mm s-1/2. Further analysis yielded D(θ) functions. These functions were very reproducible and showed good agreement with independent D(θ) values calculated from relative permeability and capillary pressure-saturation data for Berea sandstone. To the best of our knowledge, these are the first published estimates of S and D(θ) for Berea sandstone. Our results clearly demonstrate the effectiveness of neutron imaging in providing high quality, quantitative data for the computation of unsaturated flow parameters [ABSTRACT FROM AUTHOR]