Susan Jeanne Altman

Use of X-ray absorption imaging to examine heterogeneous diffusion in fractured crystalline rocks

Heterogeneous diffusion in different regions of a fractured granodiorite from Japan has been observed and measured through the use of X-ray absorption imaging. These regions include gouge-filled fractures, recrystallized fracture-filling material and hydrothermally altered matrix. With the X-ray absorption imaging technique, porosity, relative concentration, and relative mass of an iodine tracer were imaged in two dimensions with a sub-millimeter pixel size. Because portions of the samples analyzed have relatively low porosity values, imaging errors can potentially impact the results. For this reason, efforts were made to better understand and quantify this error. Based on the X-ray data, pore diffusion coefficients (Dp) for the different regions were estimated assuming a single diffusion rate and a lognormal multirate distribution of Dp. Results show Dp for the gouge-filled fractures are over an order of magnitude greater than those of the recrystallized fracture-filling material, which in turn is approximately two times greater than those for the altered matrix. The recrystallized fracture-filling material was found to exhibit the greatest degree of variability. The results of these experiments also provide evidence that diffusion from advective zones in fractures through the gouge-filled fractures and recrystallized fracture-filling material could increase the pore space available for matrix diffusion. This evidence is important for understanding the performance of potential nuclear waste repositories in crystalline rocks as diffusion is thought to be an important retardation mechanism for radionuclides.

Effect of Permeable Biofilm on Micro- And Macro-Scale Flow and Transport in Bioclogged Pores

Simulations of coupled flow around and inside biofilms in pores were conducted to study the effect of porous biofilm on micro- and macro-scale flow and transport. The simulations solved the Navier-Stokes equations coupled with the Brinkman equation representing flow in the pore space and biofilm, respectively, and the advection-diffusion equation. Biofilm structure and distribution were obtained from confocal microscope images. The bulk permeability (k) of bioclogged porous media depends on biofilm permeability (kbr) following a sigmoidal curve on a log-log scale. The upper and lower limits of the curve are the k of biofilm-free media and of bioclogged media with impermeable biofilms, respectively. On the basis of this, a model is developed that predicts k based solely on kbr and biofilm volume ratio. The simulations show that kbr has a significant impact on the shear stress distribution, and thus potentially affects biofilm erosion and detachment. The sensitivity of flow fields to kbr directly translated to effects on the transport fields by affecting the relative distribution of where advection and diffusion dominated. Both kbr and biofilm volume ratio affect the shape of breakthrough curves.

Mineral influence on microbial survival during carbon sequestration

Geologic carbon sequestration involves the injection of supercritical carbon dioxide into deep saline aquifers. Some of the CO2 dissolves into the brines, perturbing water chemistry and water-rock interactions, and impacting microbial habitat and survival. In this study 3 model organisms were tested for their ability to survive high pressures of CO2 exposure in batch cultures: the gram-negative Shewanella oneidensis (SO) strain MR-1, the gram-positive Geobacillus stearothermophilus (GS), and the methanogenic archaeon Methanothermobacter thermoautitrophicus (MT). Results indicate that GS can survive the highest pressures of CO2 for the longest periods of time while SO is the most sensitive to CO2 toxicity. Survival was then evaluated for SO with various minerals and rocks representative of deep saline aquifers to determine if minerals enhanced survival. Cultures were exposed to 25 bar of CO2 for 2 to 8 h and were plated for viable cell counts. Results show that biofilm formation on the mineral surface is important in protecting SO from the harmful effects of CO2 with quartz sandstones providing the best protection. The release of toxic metals like Al or As from minerals such as clays and feldspars, in contrast, may enhance microbial death under CO2 stress.

Systematic analysis of micromixers to minimize biofouling on reverse osmosis membranes.

Micromixers, UV-curable epoxy traces printed on the surface of a reverse osmosis membrane, were tested on a cross-flow system to determine their success at reducing biofouling. Biofouling was quantified by measuring the rate of permeate flux decline and the median bacteria concentration on the surface of the membrane (as determined by fluorescence intensity counts due to nucleic acid stains as measured by hyperspectral imaging). The micromixers do not appear to significantly increase the pressure needed to maintain the same initial permeate flux and salt rejection. Chevrons helped prevent biofouling of the membranes in comparison with blank membranes. The chevron design controlled where the bacteria adhered to the membrane surface. However, blank membranes with spacers had a lower rate of permeate flux decline than the membranes with chevrons despite having greater bacteria concentrations on their surfaces. With better optimization of the micromixer design, the micromixers could be used to control where the bacteria will adhere to the surface and create a more biofouling resistant membrane that will help to drive down the cost of water treatment.

Site characterization methodology for deep borehole disposal.

Deep Borehole Disposal (DBD) for radioactive waste has many clear advantages over mined repositories: 1) the possibility of incremental construction and loading at multiple locations, 2) the enhanced natural barriers in the deep continental crystalline basement, and 3) reduced site characterization. This report identifies characterization methods relevant to DBD of spent nuclear fuel or vitrified high-level waste (HLW). A systematic process based on performance assessment methodology and in particular an analysis of features, events, and processes (FEPs) is used to focus the selection of characterization methods. Exclusion criteria for a DBD site include 1) upward vertical gradient, 2) economically exploitable natural resources, 3) an interconnected zone of high permeability from the waste disposal zone to the surface or shallow subsurface, and 4) the occurrence of Quaternary-age volcanic rocks or igneous intrusions as an indication of a potentially significant probability of future volcanic activity. Based on these criteria, site characterization activitities should be focused on characterizing 1) faults and fractures, 2) stratigraphy, 3) physical, chemical, and transport properties and lithological information, 4) fluid chemistry, 5) well and seal integrity, 6) likelihood of human intrusion, and 7) structural stability. M ethods that can be used for characterizing each of these features or processes are presented and described in detail in appendices. Methods are divided into surface based and borehole based. Surface geological mapping will be the first activity to screen potential DBD sites. After there is confidence that exclusion conditions are not present, surface-based characterization would be the next step in site characterization. If

Nanofiltration treatment options for thermoelectric power plant water treatment demands.

.......................................................................................................................................3 ACKNOWLEDGEMENTS ......................................................................................................................5 TABLE OF CONTENTS ........................................................................................................................7 LIST OF TABLES ................................................................................................................................9 List of Figures ................................................................................................................................11

Dual-Permeability Modeling of Capillary Diversion and Drift Shadow Effects in Unsaturated Fractured Rock

The drift-shadow effect describes capillary diversion of water flow around a drift or cavity in porous or fractured rock, resulting in lower water flux directly beneath the cavity. This paper presents computational simulations of drift-shadow experiments using dual-permeability models, similar to the models used for performance assessment analyses of flow and seepage in unsaturated fractured tuff at Yucca Mountain. Comparisons were made between the simulations and experimental data from small-scale drift-shadow tests. Results showed that the dual-permeability models captured the salient trends and behavior observed in the experiments, but constitutive relations (e.g., fracture capillary-pressure curves) can significantly affect the simulated results. Lower water flux beneath the drift was observed in both the simulations and tests, andfingerlike flow patterns were seen to exist with lower simulated capillary pressures. The dual-permeability models used in this analysis were capable of simulating these processes. However, features such as irregularities along the top of the drift (e.g., from roof collapse) and heterogeneities in the fracture network may reduce the impact of capillary diversion and drift shadow. An evaluation of different meshes showed that at the grid refinement used, a comparison between orthogonal and unstructured meshes did not result in large differences.

pH modification for silica control

ABSTRACT Lowering solution pH slows the polymerization of silica and formation of silica scale. In batch systems, lowering the pH of approximately 200 ppm silica solutions prevents scale formation for over 300 h. Silica scale forms most quickly near pH 8. Solutions with pH 3.6–3.7 can maintain silica levels of 1,000–3,000 ppm for roughly 90 h. Bench-scale membrane testing showed that silica scale formation lag times of approximately 72 h were achievable after lowering the pH to 4.5–4.7, which might allow flushing of silica-laden solutions through, for example, flow reversal, before scale formation occurs during water treatment.

Water recovery using waste heat from coal fired power plants.

The potential to treat non-traditional water sources using power plant waste heat in conjunction with membrane distillation is assessed. Researchers and power plant designers continue to search for ways to use that waste heat from Rankine cycle power plants to recover water thereby reducing water net water consumption. Unfortunately, waste heat from a power plant is of poor quality. Membrane distillation (MD) systems may be a technology that can use the low temperature waste heat (<100 F) to treat water. By their nature, they operate at low temperature and usually low pressure. This study investigates the use of MD to recover water from typical power plants. It looks at recovery from three heat producing locations (boiler blow down, steam diverted from bleed streams, and the cooling water system) within a power plant, providing process sketches, heat and material balances and equipment sizing for recovery schemes using MD for each of these locations. It also provides insight into life cycle cost tradeoffs between power production and incremental capital costs.

Analysis of micromixers and biocidal coatings on water-treatment membranes to minimize biofouling.

Biofouling, the unwanted growth of biofilms on a surface, of water-treatment membranes negatively impacts in desalination and water treatment. With biofouling there is a decrease in permeate production, degradation of permeate water quality, and an increase in energy expenditure due to increased cross-flow pressure needed. To date, a universal successful and cost-effect method for controlling biofouling has not been implemented. The overall goal of the work described in this report was to use high-performance computing to direct polymer, material, and biological research to create the next generation of water-treatment membranes. Both physical (micromixers - UV-curable epoxy traces printed on the surface of a water-treatment membrane that promote chaotic mixing) and chemical (quaternary ammonium groups) modifications of the membranes for the purpose of increasing resistance to biofouling were evaluated. Creation of low-cost, efficient water-treatment membranes helps assure the availability of fresh water for human use, a growing need in both the U. S. and the world.