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

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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.