Jiamin Wan

Water film flow along fracture surfaces of porous rock

This study shows that hydraulic properties of individual fracture surfaces can be meaningfully defined and measured, and that water film flow is a mechanism contributing to fast, unsaturated flow in fractures. The hydraulic conductivity of an unconfined block of Bishop Tuff was measured over a range of near-zero matric potentials, where differences between hydraulic conductivities obtained without and with wax sealing of its lateral sides allowed isolation of film flow effects. Tensiometer and flux measurements showed that surface film flow in this system was significant for matric potentials greater (more positive) than about −250 Pa. In this range the average film thickness was shown to be potential dependent and proportional to the observed enhanced hydraulic conductivity. Measured average surface film thicknesses ranged from 2 to 70 μm, with average film velocities in the range of 2 to 40 m d−1 (about 103 times faster than that of the pore water under unit gradient saturated flow). Our experiments demonstrate that hydraulic properties of macroscopic surfaces of porous media are quantifiable, related to surface roughness, and potentially important in the flow of water in vadose environments. This study further shows that contrary to existing conceptual models, unsaturated flow in fractures cannot generally be predicted solely on the basis of aperture distribution information. The high velocities of these surface films suggest that film flow can be an important mechanism contributing to fast flow in unsaturated fractures and macropores, especially in media characterized by low-permeability matrix and along regions of convergent flow in partially saturated fractures.

Dewetting of Silica Surfaces upon Reactions with Supercritical CO2 and Brine: Pore-Scale Studies in Micromodels

Wettability of reservoir minerals and rocks is a critical factor controlling CO(2) mobility, residual trapping, and safe-storage in geologic carbon sequestration, and currently is the factor imparting the greatest uncertainty in predicting capillary behavior in porous media. Very little information on wettability in supercritical CO(2) (scCO(2))-mineral-brine systems is available. We studied pore-scale wettability and wettability alteration in scCO(2)-silica-brine systems using engineered micromodels (transparent pore networks), at 8.5 MPa and 45 °C, over a wide range of NaCl concentrations up to 5.0 M. Dewetting of silica surfaces upon reactions with scCO(2) was observed through water film thinning, water droplet formation, and contact angle increases within single pores. The brine contact angles increased from initial values near 0° up to 80° with larger increases under higher ionic strength conditions. Given the abundance of silica surfaces in reservoirs and caprocks, these results indicate that CO(2) induced dewetting may have important consequences on CO(2) sequestration including reducing capillary entry pressure, and altering quantities of CO(2) residual trapping, relative permeability, and caprock integrity.

Transient film flow on rough fracture surfaces

Transient film flow in unsaturated fractures was investigated conceptually and experimentally. By considering films on fracture surfaces as analogs to water in partially saturated porous media, the film hydraulic diffusivity and equation for transient film flow are obtained from their porous medium counterparts, the hydraulic diffusivity and the Richards equation. Experiments on roughened glass show that the average film thickness dependence on matric potential can be approximated as a power function. It is also shown that the film hydraulic diffusivity increases with increased film thickness (and with increased matric potential). Fast film flow (average velocities greater than 3×10−7 m s−1 under unit gradient conditions) was observed for average film thicknesses greater than 2μm and matric potentials greater than −1 kPa.

Transport and deposition of functionalized CdTe nanoparticles in saturated porous media

Comprehensive understanding of the transport and deposition of engineered nanoparticles (NPs) in subsurface is required to assess their potential negative impact on the environment. We studied the deposition behavior of functionalized quantum dot (QD) NPs (CdTe) in different types of sands (Accusand, ultrapure quartz, and iron-coated sand) at various solution ionic strengths (IS). The observed transport behavior in ultrapure quartz and iron-coated sand was consistent with conventional colloid deposition theories. However, our results from the Accusand column showed that deposition was minimal at the lowest IS (1mM) and increased significantly as the IS increased. The effluent breakthrough occurred with a delay, followed by a rapid rise to the maximum normalized concentration of unity. Negligible deposition in the column packed with ultrapure quartz sand (100mM) and Accusand (1mM) rules out the effect of straining and suggests the importance of surface charge heterogeneity in QD deposition in Accusand at higher IS. Data analyses further show that only a small fraction of sand surface area contributed in QD deposition even at the highest IS (100mM) tested. The observed delay in breakthrough curves of QDs was attributed to the fast diffusive mass transfer rate of QDs from bulk solution to the sand surface and QD mass transfer on the solid phase. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were used to examine the morphology and elemental composition of sand grains. It was observed that there were regions on the sand covered with layers of clay particles. EDX spectra collected from these regions revealed that Si and Al were the major elements suggesting that the clay particles were kaolinite. Additional batch experiments using gold NPs and SEM analysis were performed and it was observed that the gold NPs were only deposited on clay particles originally on the Accusand surface. After removing the clays from the sand surface, we observed negligible QD deposition even at 100mM IS. We proposed that nanoscale charge heterogeneities on clay particles on Accusand surface played a key role in QD deposition. It was shown that the value of solution IS determined the extent to which the local heterogeneities participated in particle deposition.

Uranium(VI) Adsorption and Surface Complexation Modeling onto Background Sediments from the F-Area Savannah River Site

The mobility of an acidic uranium waste plume in the F-Area of Savannah River Site is of great concern. In order to understand and predict uranium mobility, U(VI) adsorption experiments were performed as a function of pH using background F-Area aquifer sediments and reference goethite and kaolinite (major reactive phases of F-Area sediments), and a component-additivity (CA) based surface complexation model (SCM) was developed. Our experimental results indicate that the fine fractions (≤45 μm) in sediments control U(VI) adsorption due to their large surface area, although the quartz sands show a stronger adsorption ability per unit surface area than the fine fractions at pH < 5.0. Kaolinite is a more important sorbent for U(VI) at pH < 4.0, while goethite plays a major role at pH > 4.0. Our CA model combines an existing U(VI) SCM for goethite and a modified U(VI) SCM for kaolinite along with estimated relative surface area abundances of these component minerals. The modeling approach successfully predicts U(VI) adsorption behavior by the background F-Area sediments. The model suggests that exchange sites on kaolinite dominate U(VI) adsorption at pH < 4.0, goethite and kaolinite edge sites cocontribute to U(VI) adsorption at pH 4.0-6.0, and goethite dominates U(VI) adsorption at pH > 6.0.

Impacts of bridging complexation on the transport of surface-modified nanoparticles in saturated sand

The transport of polyacrylic acid capped cadmium telluride (CdTe) quantum dots (QDs), carboxylate-modified latex (CML), and bare silica nanoparticles (NPs) was studied in packed columns at various electrolyte concentrations and cation types. The breakthrough curves (BTCs) of QDs and CML particles in acid-treated Accusand showed significant amounts of increasing deposition with 0.5, 1, and 2 mM Ca(2+), but only minute deposition at 50 and 100 mM Na(+). Negligible QD and CML deposition occurred at 2mM Ca(2+) in columns packed with ultrapure quartz sand that was similar in size to the Accusand. These observations are not consistent with interpretations based on Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations of interaction energies. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis demonstrated that there were regions on the acid-treated Accusand covered with small amounts of clay that were absent on the ultrapure quartz sand. A salt cleaning method was therefore used to remove the clay from the acid-treated Accusand. The BTCs of QDs and CML in this acid+salt treated Accusand exhibited much less deposition at any given Ca(2+) concentration compared to those obtained from the acid-treated sand. SEM images showed that most of the QD deposited in acid-treated Accusand occurred on clay surfaces. Unlike our results with QDs and CML, negligible deposition of bare silica NPs occurred at 5 and 10 mM Ca(2+) in acid-treated Accusand. The high deposition of QDs and CML particles was therefore attributed to bridging complexation in which Ca(2+) serves as a bridge between the cation exchange locations on the clay and carboxyl functional groups on the QD and CML particles, which were absent on the bare silica NPs. Our results suggest that the transport of carboxylic ligand-modified NPs may be limited in subsurface environments because of the ubiquitous presence of clay and divalent cations.

Moisture characteristics of Hanford gravels: Bulk, grain-surface, and intragranular components

Although gravels comprise large portions of some vadose zones, their unsaturated hydraulic properties have received relatively little attention. This study examines moisture retention relations in the 2- and 6-mm size fractions of gravels from the Hanford formation vadose zone (Washington State). Understanding flow and transport within this formation is important because parts of it have become contaminated by leakage of radioactive wastes at the Hanford Site. Moisture retention relations were obtained for a very wide energy range, with attention to water retained in intragranular pores and along grain surfaces. External surfaces of these gravels have root mean-squared roughnesses (rmsr) in the micrometer range, with sparsely distributed deep (hundreds of micrometers) pits. Water films on these external surfaces are volumetrically insignificant at matric potentials less than about −2 kPa. Residual water in these gravels occurs in intragranular pores, accounts for about 10% of the total porosity, and is effectively hydraulically immobile. The intragranular domain in Hanford gravels also has a large specific surface area of about 11 m 2 g −1 . Thus, exchanges of solutes (including contaminants) between the intragranular domain of Hanford gravels and their immediate surrounding are significant and diffusion limited.

Release of Quantum Dot Nanoparticles in Porous Media: Role of Cation Exchange and Aging Time

Understanding the fate and transport of engineered nanoparticles (ENPs) in subsurface environments is required for developing the best strategy for waste management and disposal of these materials. In this study, the deposition and release of quantum dot (QD) nanoparticles were studied in saturated sand columns. The QDs were first deposited in columns using 100 mM NaCl or 2 mM CaC12 solutions. Deposited QDs were then contacted with deionized (DI) water and/or varying Na(+) concentrations to induce release. QDs deposited in 100 mM Na(+) were easily reversible when the column was rinsed with DI water. Conversely, QDs deposited in the presence of Ca(2+) exhibited resistance to release with DI water. However, significant release occurred when the columns were flushed with NaCl solutions. This release behavior was explained by cation exchange (Ca(2+) in exchange sites were replaced by Na(+)) which resulted in the breakdown of calcium bridging. We also studied the effect of aging time on the QD release. As the aging time increased, smaller amounts of QDs were released following cation exchange. However, deposited QDs were subsequently released when the column was flushed with DI water. The release behavior was modeled using a single first-order kinetic release process and changes in the maximum solid phase concentration of deposited QDs with transition in solution chemistry. The results of this study demonstrate that the presence of carboxyl groups on ENPs and divalent ions in the solution plays a key role in controlling ENP mobility in the subsurface environment.

Surface‐zone flow along unsaturated rock fractures

Although fractures in rock are well recognized as pathways for fast percolation of water, processes which permit fast flow along unsaturated fracture pathways remain to be identified and understood. Earlier aperture-based models of flow in partially saturated fractures permit fast flow only through a continuous network of locally saturated segments. Film flow was recently identified as a mechanism capable of sustaining fast flow along truly unsaturated fractures when the matric potential is very close to zero. Another mechanism for fast flow along unsaturated fractures is introduced in this study, “surface-zone flow,” which can be important when the permeability of the rock along fractures (fracture skin) is significantly greater than that of the bulk rock matrix. In such systems the fracture surface zone provides low resistance pathways through which fast flow (relative to the bulk matrix rock) can occur, even when the fractures are at very low water saturation. Initial experimental tests of surface-zone fast flow were performed. Surface-zone fast imbibition of water was measured on a welded tuff and a rhyolite. However, because (1) imbibition rates are also strongly influenced by rock wettability, (2) these initially air-dry rocks exhibited finite contact angles upon exposure to water, and (3) we lack methods to reliably measure permeabilities of thin regions on rock surfaces, quantification of permeability contrasts was not possible in these initial tests.

Geochemical evolution of highly alkaline and saline tank waste plumes during seepage through vadose zone sediments

Abstract Leakage of highly saline and alkaline radioactive waste from storage tanks into underlying sediments is a serious environmental problem at the Hanford Site in Washington State. This study focuses on geochemical evolution of tank waste plumes resulting from interactions between the waste solution and sediment. A synthetic tank waste solution was infused into unsaturated Hanford sediment columns (0.2, 0.6, and 2 m) maintained at 70°C to simulate the field contamination process. Spatially and temporally resolved geochemical profiles of the waste plume were obtained. Thorough OH − neutralization (from an initial pH 14 down to 6.3) was observed. Three broad zones of pore solutions were identified to categorize the dominant geochemical reactions: the silicate dissolution zone (pH > 10), pH-neutralized zone (pH 10 to 6.5), and displaced native sediment pore water (pH 6.5 to 8). Elevated concentrations of Si, Fe, and K in plume fluids and their depleted concentrations in plume sediments reflected dissolution of primary minerals within the silicate dissolution zone. The very high Na concentrations in the waste solution resulted in rapid and complete cation exchange, reflected in high concentrations of Ca and Mg at the plume front. The plume-sediment profiles also showed deposition of hydrated solids and carbonates. Fair correspondence was obtained between these results and analyses of field borehole samples from a waste plume at the Hanford Site. Results of this study provide a well-defined framework for understanding waste plumes in the more complex field setting and for understanding geochemical factors controlling transport of contaminant species carried in waste solutions that leaked from single-shell storage tanks in the past.