Measurement of Bacterial Transport and Immobilization in Heterogeneous Geologic Materials Using Novel Radiolabeling Methods

Understanding the mechanisms of transport and immobilization of pathogenic microorganisms, sourced from human and animal wastes, under hydrogeologic conditions is important to prevent public health crises and address groundwater contamination. Discrepancies between bacteria measurements in drinking water wells and the expected near-surface immobilization from laboratory experiments in homogeneous geologic materials indicate an incomplete understanding of dynamic processes of bacteria transport and immobilization in realistic geologic systems. Our approach involves radiolabeling E. coli bacteria with positron-emitting radioisotopes and using positron emission tomography (PET) imaging to quantify in-situ bacterial transport and attachment in heterogeneous column experiments. The attachment coefficients (kf) were calculated directly from the attached bacteria fraction based on PET data. In layered sand columns, kf in similar sand layers were different despite similar bacteria attachment. Thus, kf for the same geologic media is found spatially influenced by heterogeneity, which indicates the breakdown of colloid filtration theory in studying bacterial colloid transport in heterogeneous columns. These direct measurements of the attachment coefficients and coefficient distributions in bacterial transport experiments provide new understanding of bacterial transport mechanisms, improved model parameterization, and more accurate predictions of how local geologic conditions influence bacterial fate and transport in groundwater.