Spontaneous Detachment and Retardation of Bacteria: Physical and Chemical Controls on these Processes and Their Impact on Bacterial Transport in Groundwater

0087522W。约翰森低浓度细菌在地下水中的长期迁移对于理解保护地下水资源的目的以及解释在地下观察到的细菌的广泛分布是重要的。相比之下，现有的大部分细菌运输知识侧重于高浓度细菌的短期运输，并相应地关注细菌在沉积物上的附着速度。然而，对于长期的运输行为，细菌运输的其他方面变得重要，例如观察到在没有任何物理或化学扰动的情况下从沉积物中分离的缓慢速度(自发脱离)，以及观察到的低浓度细菌在沉积物中的滞留。自发性脱落和滞留对细菌在地下水中可能运输的距离的影响尚不清楚。本文提出的工作将有助于确定细菌在运输过程中自发脱离和滞留的机制，并将确定它们对地下水中低浓度细菌长期运输的潜在影响。实验室实验将使用重新填充的沉淀柱来检查描述细菌突破、滞留和延长尾矿(自发脱离)的动力学常数的变化，以及描述细菌与沉积物颗粒之间相互作用力的势能分布的变化。预计动力学常数的变化将揭示控制延迟的附着机制与控制稳态突破的附着机制的不同。柱状实验和平行板室中的直接视觉分析将检验这样一种假设，即淋洗过程中观察到的细菌和其他胶体从沉积物中的自发分离可能是由沉积物颗粒表面的流体动力剪切引起的。对于不同胶体尺寸的等效实验，动力学常数随孔隙水速度的变化将表明流体动力剪切在自发剥离中是否重要(如较大尺寸胶体相对于较小胶体优先剥离所示)。实验还将检验响应于颗粒表面的流体动力剪切而产生的自发剥离是否涉及到阴影效应必然产生的“侵蚀”效应。实验还将检验这样一种假设，即随着细菌在颗粒表面停留时间的增加，观察到的脱落概率的下降并不一定与细菌的代谢活动有关。为了观察附着的种群质心的潜在差异，并进一步约束数值模型，将检查长时间和短时间洗脱时间后沉积物上细菌的浓度。在不同的物理和化学条件下，通过实验确定的动力学常数，可以用一维模型来模拟细菌在长时间内的自发性脱落和滞留对细菌传输的潜在影响。

0087522W. JohnsonThe long-term transport of low concentrations of bacteria in groundwater is important to understand the purpose of protecting groundwater resources, and for interpreting the wide-ranging distribution of bacteria that is observed in the subsurface. In contrast, the bulk of the existing body of bacterial transport knowledge focuses upon the short-term transport of high concentrations of bacteria, and correspondingly on the rate of bacterial attachment to sediment. However, for long-term transport behavior, other aspects of bacterial transport become important, such as the observed slow rate of detachment from sediment that occurs in the absence of any physical or chemical perturbations (spontaneous detachment), and the observed retardation of low concentrations of bacteria in sediment. The impact of spontaneous detachment and retardation on the distance over which bacteria may be transported in groundwater is unknown. The work proposed herein will serve to determine the mechanisms governing spontaneous detachment and retardation of bacteria during transport, and will determine their potential impact on the long-term transport of low concentrations of bacteria in groundwater.Laboratory experiments will be performed using repacked sediment columns to examine changes in kinetic constants that describe bacterial breakthrough, retardation, and extended tailing (spontaneous detachment) with variations in the potential energy profile describing interaction forces between the bacteria and the sediment grain. It is expected that variation in the kinetic constants will illuminate differences in the mechanisms of attachment that control retardation, as opposed to the mechanisms of attachment that control steady state breakthrough.Column experiments and direct visual analyses in parallel plate chambers will examine the hypothesis that spontaneous detachment of bacteria and other colloids from sediment that is observed during elution may be caused by hydrodynamic shear operating at the surface of the sediment grains. The variation in kinetic constants with pore water velocity for equivalent experiments performed with different colloid sizes will indicate whether hydrodynamic shear is important in spontaneous detachment (as indicated by preferential detachment of larger-sized colloids relative to smaller-sized colloids). Experiments will also examine whether spontaneous detachment in response to hydrodynamic shear at the grain surface involves an "erosion" effect that is corollary to the shadow effect. Experiments will also examine the hypothesis that the observed decrease in probability of detachment with increased bacterial residence time on a grain surface is not necessarily related to bacterial metabolic activity.Concentration of bacteria on the sediment following long and short elution times will be examined in order to observe potential differences in the centers of mass of the attached populations, and to further constrain numerical models. Potential impacts of spontaneous detachment and retardation to bacterial transport over long time periods under different physical and chemical conditions will be simulated with a one-dimensional model using experimentally determined kinetic constants.