Clinton S. Willson

Quantification of Grain, Pore, and Fluid Microstructure of Unsaturated Sand from X-Ray Computed Tomography Images

A comprehensive series of three-dimensional x-ray computed tomography (XCT) imaging experiments was conducted to quantitatively assess the multiphase particle- and pore-scale properties of fine Ottawa (F-75) sand. The specimens were prepared to saturations ranging from approximately 5 % to 80 %. Specimens were doped with 10 % CsCl pore fluid solution and imaged using a monochromatic synchrotron x-ray source at energies below and above the Cs x-ray absorption k-edge to allow for high contrast between the solid, liquid, and air phases. Multiphase properties quantified from the XCT images included individual particle sizes and areas, as well as grain size distribution, pore shape and size distribution, water menisci distribution, solid, liquid, and gas surface areas, and particle contact coordination number. At low saturations, pore water is distributed primarily in the form of pendular rings and liquid bridges located between individual grains and in the smallest pore throats and bodies. A highly discontinuous water phase is evident as a large number of separately identifiable water units having very small volume. As the water saturation increases, the number of individual water units decreases; as expected, the average volume of these units increases significantly as the pore water coalesces into larger and larger units. Results obtained using SEM imaging and conventional geotechnical testing methods for particle-size distribution and soil–water retention were compared with those derived from analysis of the XCT images. Results compare very well in each case, typically within a few %. It is shown that the XCT is a reliable and non-destructive method to quantify pore-scale information vital to advance understanding of the hydrologic and mechanical behavior of unsaturated soils at the macroscale.

Mass transfer rate limitation effects on partitioning tracer tests

Abstract Partitioning tracer tests are often used to quantify the amount of non-aqueous phase liquid (NAPL) present in a porous medium. Results from such tests are usually interpreted by using models that assume local equilibrium exists between the NAPL and the aqueous phase or by using a moment method to solve for the fraction of the pore space occupied by the NAPL. We investigated the transport of partitioning alcohol tracers through heterogeneous sand porous medium systems containing a stationary trichloroethylene (TCE) phase for a range of aqueous phase velocities. NAPL saturations were quantified using a non-destructive X-ray absorption methodology. Experimental results were simulated using a numerical solution to an advective–dispersive model in which mass transfer between the NAPL and the aqueous phase was approximated using a dual-resistance approach consisting of a boundary layer mass transfer resistance and a diffusional resistance within the NAPL. The numerical model agreed well with the experimental data, and moment analysis yielded reasonable estimates of actual NAPL saturations.

A Laboratory Study of Sediment and Contaminant Release during Gas Ebullition

Abstract Significant quantities of gas are generated from labile organic matter in contaminated sediments. The implications for the gas generation and subsequent release of contaminants from sediments are unknown but may include enhanced direct transport such as pore water advection and diffusion. The behavior of gas in sediments and the resulting migration of a polyaromatic hydrocarbon, viz phenanthrene, were investigated in an experimental system with methane injection at the base of a sediment column. Hexane above the overlying water layer was used to trap any phenanthrene migrating out of the sediment layer. The rate of suspension of solid particulate matter from the sediment bed into the overlying water layer was also monitored. The experiments indicated that signifi-cant amounts of both solid particulate matter and contaminant can be released from a sediment bed by gas movement with the amount of release related to the volume of gas released. The effective mass transfer coefficient of gas bubble-facilitated contaminant release was estimated under field conditions, being around three orders of magnitude smaller than that of bioturbation. A thin sand-capping layer (2 cm) was found to dramatically reduce the amount of contaminant or particles released with the gas because it could prevent or at least reduce sediment suspension. Based on the experimental observations, gas bubble-facilitated contaminant transport pathways for both uncapped and capped systems were proposed. Sediment cores were sliced to obtain phenanthrene concentration. X-ray computed tomography (CT) was used to investigate the void space distribution in the sediment penetrated by gas bubbles. The results showed that gas bubble migration could redistribute the sediment void spaces and may facilitate pore water circulation in the sediment.

The impact of immobile zones on the transport and retention of nanoparticles in porous media: IMMOBILE ZONES AND NANOPARTICLE TRANSPORT AND RETENTION

Nanoparticle transport and retention within porous media is treated by conceptualizing the porous media as a series of independent collectors (e.g., Colloid Filtration Theory). This conceptualization assumes that flow phenomena near grain-grain contacts, such as immobile zones (areas of low flow), exert a negligible influence on nanoparticle transport and assumes that retention and release of particles depends only on surface chemistry. This study investigated the impact of immobile zones on nanoparticle transport and retention by employing Synchrotron X-ray Computed Microtomography (SXCMT) to examine pore-scale silver nanoparticle distributions during transport through three sand columns: uniform iron oxide, uniform quartz and well graded quartz. Extended tailing was observed during the elution phase of all experiments suggesting that hydraulic retention in immobile zones, not detachment from grains, was the source of tailing. A numerical simulation of fluid flow through an SXCMT dataset predicted the presence of immobile zones near grain-grain contacts. SXCMT-determined silver nanoparticle concentrations observed that significantly lower nanoparticle concentrations existed near grain-grain contacts throughout the duration of all experiments. In addition, the SXCMT-determined pore-scale concentration gradients were found to be independent of surface chemistry and grain size distribution, suggesting that immobile zones limit the diffusive transport of nanoparticles towards the collectors. These results suggest that the well-known overprediction of nanoparticle retention by traditional CFT may be due to ignoring the influences of grain-grain contacts and immobile zones. As such, accurate prediction of nanoparticle transport requires consideration of immobile zones and their influence on both hydraulic and surface retention. This article is protected by copyright. All rights reserved.

Adapting to Change in the Lowermost Mississippi River: Implications for Navigation, Flood Control and Restoration of the Delta Ecosystem

The Lowermost Mississippi River (LMMR), from the Gulf of Mexico to 520 km above Head of Passes, remains critical for flood conveyance and transport of agricultural and industrial bulk products from the central United States. The US Army Corps of Engineers (USACE) has managed it with little change for 80 years using the levees and spillways constructed under the Mississippi River and Tributaries project (MR&T). At the same time, public demand for reconnection of the Mississippi to the deteriorating delta ecosystem has grown. Significant sediment diversion projects have been authorized downstream of New Orleans to restart deltaic wetland building to conserve fish and wildlife resources and reduce hurricane flood risk to delta communities. Recent research and observations from the back-to-back record 2011 high-, and 2012 low-discharge events indicate that LMMR hydraulics have changed significantly, and that sea level rise, subsidence and a reduction in sand transport through the Plaquemines-Balize birdsfoot delta (PBD) now favor formation of new, unregulated outlets upstream. During the peak of the 2011 flood, only 27 % of the 65,000 m3-s−1 discharge entering the LMMR reached the Gulf via Head of Passes, compared to 36 % passing through the two outlets of the shorter Atchafalaya distributary. About 20 % of the water lost from the LMMR occurred through unregulated flow overbank and through small, but growing, distributaries between New Orleans and Head of Passes. Adding delta restoration to existing USACE missions will require adjusting the MR&T but has potential to lower flood flow lines and reduce navigation dredging costs sufficiently to allow LMMR ports to accommodate larger, Post-Panamax ships.

Method for obtaining silver nanoparticle concentrations within a porous medium via synchrotron X-ray computed microtomography.

Attempts at understanding nanoparticle fate and transport in the subsurface environment are currently hindered by an inability to quantify nanoparticle behavior at the pore scale (within and between pores) within realistic pore networks. This paper is the first to present a method for high resolution quantification of silver nanoparticle (nAg) concentrations within porous media under controlled experimental conditions. This method makes it possible to extract silver nanoparticle concentrations within individual pores in static and quasi-dynamic (i.e., transport) systems. Quantification is achieved by employing absorption-edge synchrotron X-ray computed microtomography (SXCMT) and an extension of the Beer-Lambert law. Three-dimensional maps of X-ray mass linear attenuation are converted to SXCMT-determined nAg concentration and are found to closely match the concentrations determined by ICP analysis. In addition, factors affecting the quality of the SXCMT-determined results are investigated: 1) The acquisition of an additional above-edge data set reduced the standard deviation of SXCMT-determined concentrations; 2) X-ray refraction at the grain/water interface artificially depresses the SXCMT-determined concentrations within 18.1 μm of a grain surface; 3) By treating the approximately 20 × 10(6) voxels within each data set statistically (i.e., averaging), a high level of confidence in the SXCMT-determined mean concentrations can be obtained. This novel method provides the means to examine a wide range of properties related to nanoparticle transport in controlled laboratory porous medium experiments.

Using What We Have: Optimizing Sediment Management in Mississippi River Delta Restoration to Improve the Economic Viability of the Nation

Management practices on the Mississippi River have reduced the amount of sediment in the river by approximately half. Some have questioned whether the current sediment load in the river is sufficient for restoration of the delta. The Mississippi River does not now, nor has it ever supplied enough sediment to continuously sustain the entire Mississippi Delta coastline. Nevertheless, the available sediment supply is still huge, and so we must use this valuable resource efficiently and effectively.

Physical Pore Properties and Grain Interactions of SAX04 Sands

During the 2004 Sediment Acoustic eXperiment (SAX04), values of sediment pore properties in a littoral sand deposit were determined from diver-collected cores using traditional methods and image analysis on X-ray microfocus computed tomography (XMCT) images. Geoacoustically relevant pore-space properties of sediment porosity, permeability, and tortuosity were evaluated at scales ranging from the pore scale to the core scale from “mud-free” sediments collected within the 0.07-km² study area. Porosity was determined from water-weight-loss measurements to range from 0.367 to 0.369, from 2-D image analysis to range from 0.392 to 0.436 and from 3-D image analysis to range from 0.386 to 0.427. The range of permeability from all measurements was 2.8 × 10^-11 m² to 19.0 × 10^-11 m², however the range of permeability within each technique was much narrower. Permeability was determined using a constant head (CH) apparatus (<i>k</i>_range = 2.88 to 3.74 × 10^-11 m²), from a variant of the Kozeny-Carman (KC) equation (<i>k</i>_range = 12.4 to 19.0 × 10^-11 m²), from an effective medium theory technique (<i>k</i>_range = 5.60 to 13.3 × 10^-11 m²) and from a network model (<i>k</i>_range = 8.49 to 19.0 × 10^-11 m² ). Permeability was determined to be slightly higher in the horizontal than in the vertical direction from the network model. Tortuosity ranged from 1.33 to 1.34. Based upon the small coefficients of variation for the conventionally determined pore-space properties, the sand sediment within these core samples was deemed homogeneous at all of the SAX04 sites. Additionally, grain interactions, specifically grain coordination number and grain contact areas, were determined from XMCT images. Grain contacts ranged in size from small point contacts of 136 μm² to large-area contacts the size of grain faces ( >4500 × μm²). The mean coordination number was similar to that of a cubic packing (six), but sometimes exceeded 12, which is the coordination number for a hexagonal close packing of spheres.

Imaging tissue structures: assessment of absorption and phase-contrast x-ray tomography imaging at 2nd and 3rd generation synchrotrons

Several methods have been proposed for imaging biological tissue structures at the near micron scale and with user-control of contrast mechanisms that differentiate among the tissue structures. On the one hand, treatment with high-Z contrast agents (Ba, Cs, I, etc.) by injection or soaking and absorption edge imaging distinguishes soft tissue from cornified or bony tissue. This experiment is most compatible with high-bandpass monochromators (ΔE/E between 0.01 - 0.03), such as recently installed at the LSU synchrotron (CAMD). On the other hand, phase contrast imaging does not require any pre-treatment except preservation in formalin, but places more demands upon the X-ray source. This experiment is more compatible with beam lines, such as 13 BM-D at APS, which operates with a narrow bandpass monochromator (ΔE/E ≈ 10-4). Here, we compare imaging results of soft, cornified and bony tissues across the 2x2 matrix of absorption edge versus phase contrast, and high versus narrow bandpass monochromators. In addition, we comment on new data acquisition strategies adapted to the fragile character of biological tissues: (a) a 100 % humidity chamber, and (b) a data acquisition strategy, based on the Greek golden ratio, that more quickly leads to image convergence. The latter incurs the minor cost of reprogramming, or relabeling, images with order and angle. Subsequently, tomography data sets can be acquired based on synchrotron performance and sample fragility.

A screening model for simulating DNAPL flow and transport in porous media: theoretical development

In the last two decades there has been an increased awareness of the contamination of groundwater due to the presence of denser-than-water nonaqueous phase liquids (DNAPLs). Numerous theoretical, experimental and numerical investigations have been conducted to study the various processes that impact aquifer contamination. These studies have provided us with greater insight into the individual processes and the complex nature of the problem. In spite of this progress, there still exists a need within the environmental community for a simple tool that will allow us to analyze a DNAPL contamination scenario from free-product release to transport of soluble constituents to downgradient receptor wells. Such a model may be useful in source term characterization for DNAPL releases to groundwater. The objective of this manuscript is to present the conceptual model and formulate the equations and modules which are utilized in this screening model. Three hypothetical releases are simulated and the results discussed to demonstrate the application and usefulness of this model. Due to its simplicity and ease of use, this screening model will be useful to industry, regulatory agencies and educators for estimating the impact of a DNAPL release on an aquifer.