H23H-01 INVITED 13:40h
Colloid Transport under Partially Saturated Transient and Preferential Flow Conditions
This study evaluates the impacts of natural flow regimes on colloid transport. While experiments that impose steady-state flow conditions are critical for developing fundamental understanding of colloid-media interactions, flow in natural subsurface environments is dominated by transient flow involving temporary increases in water content and flow rate during rain events, followed by decreasing water content during drainage. There is a strong relationship between water content and colloid transport efficiency, making it important to evaluate how transient flow conditions affect the rate and extent of colloid transport. In addition, water flow is only distributed uniformly near the surface even such uniform wetting throughout the columns is often assumed in models of water and colloid transport under partial saturation. Rather, infiltrating water and colloids are transported via preferential flow paths under near saturated conditions and with higher velocities than that would occur under uniform wetting conditions. To our knowledge, infiltration, remobilization and percolation of colloids in porous media under such conditions has not been previously documented, but is critical to evaluating the potential for colloid transport in natural subsurface systems. Colloid transport and mobilization in one- and two-dimensional experimental systems are being examined using both model porous media (Accusand) and natural Hanford sediments, and includes the transport of both model colloids (montmorillonite), as well as colloids derived from Hanford sediments. Because Hanford sedimentary deposits exist as sorted layers of coarse and fine sediments, the effects of such layering on the distribution of water flow through these layers and the subsequent impacts on preferential flow and colloid transport are also being evaluated. The upper and lower boundary conditions are a constant flux less than saturated hydraulic conductivity and a constant matric potential, respectively. Transient changes in water content are measured at several locations along the flow paths via gamma-ray attenuation. These results demonstrate the importance of the combined effects of preferential flow and transient water flow on colloid transport.
H23H-02 14:00h
Transport of colloids in unsaturated porous media: Explaining large scale behavior based on pore scale mechanisms
We conducted pore and column scale experiments to study the transport of colloids (latex particles and Bacteriophage MS2) under water unsaturated conditions. The objective was to draw connections between observations at the pore scale and the results obtained from column scale experiments. The same system had been previously operated under saturated conditions to determine colloid collision efficiency. Breakthrough of colloids was first evaluated under unsaturated but steady water content conditions, with constant trickling flow. After monitoring the steady breakthrough of the colloids, the column was flushed with water at higher flow rate to increase the water content up to a saturated condition. Colloid breakthrough was monitored during the entire experiment, as water content increased. Colloid removal increases significantly with decreasing initial water saturation, reflecting retention at the air-water interface and straining in thin-water films. Colloid breakthrough occurs earlier than a conservative tracer even under unsaturated conditions, although the colloid concentrations are much lower than the tracer. After flushing at similar flowrates, there is increased colloid retention under unsaturated conditions even as the system approaches water saturation, indicating that additional removal is occurring, possibly due to the formation of colloidal clusters. These results can be explained to a great extent by pore scale observations of retention and remobilization mechanisms.
H23H-03 INVITED 14:15h
Visualization of Colloid Deposition and Mobilization During Unsteady and Steady Porewater Flow Through Unsaturated Porous Media.
Mineral colloids that are mobilized from near-surface soils during infiltration events may carry sorptive contaminants through the vadose zone and into drinking-water aquifers. The vadose-zone flux of colloid-associated contaminants depends, in part, on the difference between colloid mobilization rates and deposition rates. Our research is aimed at improving current understanding of colloid effects on subsurface-contaminant transport by identifying the mechanisms that govern colloid mobilization and deposition in unsaturated porous media. We present pore-scale observations of the transport of fluorescent microspheres through transparent flow cells packed with a thin layer of partially saturated sand. These visualization experiments were conducted under steady-flow and transient-flow conditions. In experiments in which the air phase was discontinuous and occurred as insular air bubbles, the negatively charged microspheres accumulated at the air-bubble surface and moved freely about this air-water interface. A fraction of these colloids eventually migrated from the air-water interface to the air-water-solid interface, whereupon their motion stopped. Destruction of the air bubbles during imbibition led to the release of colloids retained previously by the air-water interface, but not to the release of colloids held at the air-water-solid interface. Colloids were also trapped upon entry into dead-end water conduits that split from the primary flow channels. The exchange of colloids and water between a dead zone and primary flow channel was slow under steady flow; however, the reconnection of dead-end zones as moisture content increased during imbibition resulted in the mobilization of large concentrations of colloids. Our findings show that multiple mechanisms govern the deposition and mobilization of colloids in unsaturated porous media and provide direction for refining mathematical models for colloid and colloid-facilitated contaminant transport within the vadose zone.
H23H-04 14:35h
Simultaneous in-situ measurements of colloid concentration and water saturation using synchrotron x-rays
Simultaneous in-situ measurements of colloid concentration and water saturation are necessary to elucidate the transport properties of colloidal matter in unsaturated flow. Using a synchrotron x-ray source and cadmium tagged colloids we have developed a method to obtain rapid measurements of colloid concentration and water saturation during a rapid flow event. The x-ray source was tuned to an energy of 31.5 KeV which caused the cadmium attached to the colloids to fluoresce at 23.1 KeV. This fluorescence was recorded using a Ge-solid state detector, and detection limits of a few ppm were achieved. Water saturation was measured using x-ray attenuation. Time-lapsed measurements were taken with a sand porous medium undergoing a rapid flow event where the water content increases and then decreases in time. Results show that the cadmium transport was retarded with respect to the water when the cadmium was in solution, but moved with the water phase when it was attached to colloidal particles. Additionally no significant retention of the colloidal particles was found with increasing air/water interfaces (decreasing water saturation) during a flow event.
H23H-05 14:50h
Colloid Formation and Transport at Waste Plume Fronts
We have recently identified the phenomenon of massive colloid formation and transport within moving waste plume fronts during infiltration of high-salinity waste solutions into sediments. Colloid formation and transport was thousands of times higher within a narrow zone at the moving plume front than in the plume body and the leaching stage. The newly formed plume front colloids were primarily in the size range from tens of nm to a few micron meters. The underlying process begins with rapid and completed cation exchange of Na+ from the infiltrating waste solution replacing Ca2+ and Mg2+ from the sediments, coupled with flow of the infiltrating waste solution enriched these divalent cations within the moving plume front. Subsequent precipitation of colloids containing these divalent cations released protons and reduced the plume front pH to as low as neutral. This substantially reduced pH in turn promoted precipitation of other pH sensitive minerals and amorphous phases as colloids. This plume front colloid generation phenomenon can occur under a wide range of conditions including sediment type, waste solution pH, temperature, and chemical composition, with the only necessary condition being that of a high ionic strength waste solution. Because of the large mass of suspended colloids generated, this phenomenon could significantly affect the fate and transport of the contaminant trace elements contained within the waste plumes. Depending on the chemical properties of the individual radionuclide or toxic metal, it can co-precipitate with or adsorb onto the plume front colloids. As an example, uranium was predicted and found preferentially precipitated as a mobile colloid phase within a uranium waste plume front. Funding of this research was provided by the Geosciences Research Program of Basic Energy Science, U.S. Department of Energy.
H23H-06 INVITED 15:05h
Colloid Facilitated Transport of Plutonium in Fractured Volcanic Tuff
The transport of low-solubility radionuclides in a colloidal- or colloidal bound state is frequently suspected or observed. Groundwater contaminated with radionuclides associated with underground nuclear tests was collected from several different well locations at the Nevada Test Site (NTS). In each case, the low-levels of plutonium detected in the groundwater were overwhelmingly (>95percent) associated with the colloidal and not the dissolved fraction of the groundwater. The colloidal fractions consisted of secondary minerals such as clays and zeolites. To better understand the mechanisms controlling the potential colloidal transport of plutonium, colloid-facilitated fracture flow laboratory experiments are being conducted. Pseudocolloids consisting of Pu(IV) sorbed to clinoptilolite were combined with a radionuclide solution cocktail consisting of Np, U, Cs, Sr, Sm and 3H and Re (analog to Tc) tracers in NTS-type synthetic groundwater (4.5mM NaHCO3-). The cocktail was injected into a smooth fracture in a volcanic tuff rock core from the NTS and the effluent analyzed. Autoradiography and secondary ion mass spectrometry will be used to understand the mineral -colloid-radionuclide interactions in the fracture volcanic tuff.
H23H-07 15:25h
Colloid-Facilitated Transport of Plutonium, Pu(+V), in Saturated Alluvium
Natural groundwater colloids can facilitate the subsurface transport of strongly-sorbing radionuclides, such as plutonium (Pu). To evaluate this mechanism, packed-bed column experiments were conducted, comparing the simultaneous transport of dissolved plutonium (Pu-239) of an initial oxidation state (+V), Pu sorbed onto natural colloids, 190-nm and 500-nm diameter fluorescent Carboxylate Modified Latex (CML) microspheres, and tritium, as a conservative tracer, in saturated alluvium. The experiments were conducted in two columns having slightly different porosities at two flow rates, resulting in average linear velocities, v, of 0.6 to 3.65 cm/hr in one column and 0.57 to 2.85 cm/hr in the other. In all experiments, Pu associated with natural colloids transported through alluvium essentially unretarded, while dissolved Pu was entirely retained. These results were consistent with the strong sorption of Pu to alluvium and the negligible desorption from natural colloids, observed in separate batch experiments, over time scales exceeding those of the column experiments. The breakthrough of natural colloids preceded that of tritium in all experiments, indicating a slightly smaller effective pore volume for the colloids. The enhancement of colloids' transport over tritium decreased with v, implying ~40% enhancement at v = 0. The 500-nm CML microspheres were significantly attenuated in the column experiments compared to the 190-nm microspheres, which exhibited slightly more attenuation than natural colloids.
http://pearl1.lanl.gov/external/INC/geochemistry/geochemistry.htm
H23H-08 15:40h
Stochastic Simulations of Colloid-Facilitated Transport for Long Time and Space Scales
Although it is widely recognized that naturally occurring inorganic colloids can potentially enhance the transport of radionuclides in the subsurface, comparatively few analyses have considered the long times and large travel distances associated with potential nuclear waste repositories. One-dimensional transient simulations in a stochastic Lagrangian framework are used to explore model and parameter sensitivities for colloid-facilitated transport at large scales. The model accounts for (i) advection and dispersion of radionuclides and colloids, (ii) radionuclide decay, (iii) exchange of radionuclides among colloid-bound, dissolved, and fixed substrate phases, and (iv) attachment and detachment of colloids to the fixed substrate. Kinetics of the exchanges between dissolved and colloid-bound states are addressed using linear and non-linear models. Generic sensitivity studies addressing both fractured and granular aquifers are considered, as is an example based on the groundwater transport pathway for the potential repository at Yucca Mountain, Nevada. In the absence of mitigating factors such as permanent filtration of colloids, transport may be enhanced over the situation without colloids, but only for strongly sorbing radionuclides. Mass transfer between solution and immobilized colloids makes colloid retardation relatively ineffective at reducing facilitated transport except when the retardation factor is large. Results are particularly sensitive to the rate of desorption from colloids, a parameter that is difficult to measure with short-duration experiments. This paper is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the U.S. Nuclear Regulatory Commission.