Quantification of Bacterial Transport Processes in Subsurface Environments

原位生物修复依赖于细菌种群将化学污染物（如氯化碳氢化合物）生物转化为毒性较小的物质的能力。成功实施该技术的一个关键因素是确保细菌和污染物之间有足够的接触，以允许降解进行。因此，能够预测受污染含水层内细菌种群动态分布的数学模型对于确定由于细菌降解而引起的污染物水平变化是必要的。许多降解化学污染物的土壤细菌是可移动的，能够根据化学浓度梯度（趋化性）指导它们的迁移。这项工作的目标是根据可以在简单的实验室实验中测量的基本特性来评估细菌运输系数。这些系数的可靠值需要作为描述细菌在地下环境中迁移的平流-分散模型的输入。目前评估这些系数的方法是高度经验性的，通常不能推广到各种地下条件。实现这一目标的进展将以下列具体目标为指导：建立并实验证实了在无平流条件下细菌有效随机运动系数的理论表达式。2. 基于细菌游动行为、多孔基质和流体流动的微观特征，建立并实验证实了细菌分散系数的理论表达式。3. 定量评估运动对不可逆附着在多孔基质上的影响及其与胶体过滤理论定义的收集器效率的关系。4. 将目前用于描述细菌迁移的溶质迁移、色谱和胶体过滤的理论关系统一为一个基于微观表征的一致理论。将采用数学模型、实验室规模实验和计算机模拟相结合的方法来研究大肠杆菌、恶臭杆菌和土壤分离株（来自萨凡纳河深层地下收集的A0500、来自弗吉尼亚州Oyster的P. fluorescens Pf0-15粘附突变体和E1B2）的行为。利用跟踪显微镜对单个细菌的游泳行为进行速度、游长和转角分布分析。细菌种群的随机运动系数将在停止流动扩散室（SFDC）中测量，并与从单细胞特性确定的理论预测进行比较。更复杂的具有平流和多孔介质的实验系统将在砂柱和具有明确孔隙模式的微模型中进行研究，以获得有效的随机运动、分散和不可逆吸附系数。这些系数的理论关系基于溶质扩散和输运、胶体过滤理论和细胞动力学计算机模拟的类比，将根据细菌特性、流动特性和多孔介质几何来评估，以努力统一各种方法。这些宏观输运系数的值在平流-分散模型中需要用来描述地下环境中的细菌分布。跟踪显微镜、SFDC分析和细胞动力学算法，以及PI最先进的实验和计算实验室所独有的能力，为预测大规模迁移的基于机械的模型的发展提供了理想的环境。未来的研究将采用同样的数学建模、实验室实验和计算机模拟相结合的方法来确定细菌运输模型中使用的表观速度和可逆吸附/解吸系数。

9524544 Ford In situ bioremediation relies on the ability of bacterial populations to biologically transform chemical contaminants such as chlorinated hydrocarbons into less toxic substances. A critical factor in successfully implementing this technology is assuring sufficient contact between the bacteria and contaminant to allow degradation to proceed. Therefore, mathematical models capable of predicting the dynamic distribution of bacterial populations within contaminated aquifers are necessary for determining the changing contaminant levels due to bacterial degradation. Many soil-inhabiting bacteria which degrade chemical contaminants are motile and capable of directing their migration in response to chemical concentration gradients (chemotaxis) The goal of this work is to evaluate bacterial transport coefficients in terms of fundamental properties which can be measured in simple laboratory experiments. Reliable values for these coefficients are required as input for advection-dispersion models which describe bacterial migration in subsurface environments. Current methods for evaluating these coefficients are highly empirical and typically not generalizable to a variety of subsurface conditions. Progress toward achieving this goal will be guided by the following specific objectives: 1. To develop and experimentally confirm a theoretical expression for calculating the effective random motility coefficient for bacteria in the absence of advective flow. 2. To develop and experimentally confirm a theoretical expression for the bacterial dispersion coefficient based on a microscopic-level characterization of the swimming behavior of bacteria, the porous matrix and the fluid flow. 3. To quantitatively assess the impact of motility on irreversible attachments to the porous matrix and its relationship to collector efficiency as defined by colloid filtration theory. 4. To unify theoretical relationships from solute transport, chromatography and colloid filtration currently u sed to describe bacterial transport into one consistent theory based on microscopic-level characterization. A combination of mathematical modeling, laboratory-scale experiments and computer simulation will be used to study the behavior of E. coli, P. putida, and soil isolates (A0500 from the Savannah River Deep Subsurface Collection, P. fluorescens Pf0-15 adhesion mutant and E1B2 from a field site in Oyster, VA). The swimming behavior of individual bacteria will be analyzed in terms of speed, run length and turn angle distribution with a tracking microscope. Random motility coefficients for the bacterial population will be measured in the stopped-flow diffusion chamber (SFDC) and compared to theoretical predictions determined from the single cell properties. More complex experimental systems with advection and porous media will be studied in sand columns and micromodels with well-defined porous patterns to obtain effective random motility, dispersion and irreversible adsorption coefficients. Theoretical relationships for these coefficients based on analogies to solute diffusion and transport, colloid filtration theory, and cellular dynamics computer simulations will be evaluated in terms of bacterial properties, flow characteristics and porous media geometry in an effort to unify the various approaches. Values for these macroscopic transport coefficients are required in advection-dispersion models to describe bacterial distributions in subsurface environments. The tracking microscope, SFDC assay and cellular dynamics algorithms, capabilities unique to the PI's state-of-the-art experimental and computational laboratories, provide an ideal environment for the proposed development of mechanistic-based models for predicting large-scale migration. Future studies will apply the same combination of mathematical modeling, laboratory experiments, and computer simulation to determine apparent velocities and reversible adsorption/desorption coefficients used in models for bacterial transport.