Extended tailing of low bacterial concentrations following breakthrough at the Narrow Channel focus area was observed for 4 months. Bacterial attachment and detachment kinetics associated with breakthrough and extended tailing were determined by fitting a one-dimensional transport model to the field breakthrough-tailing data. Spatial variations in attachment rate coefficient (kf) were observed under forced gradient conditions (i.e., kf decreased as travel distance increased), possibly because of decreased bacterial adhesion with increased transport distance. When pore water velocity decreased by an order of magnitude at 9 days following injection, apparent bacterial attachment rate coefficients did not decrease with velocity as expected from filtration theory, but, instead, increased greatly for most of the wells. The coincidence of the increase in apparent attachment rate coefficient with the occurrence of protist blooms suggested that the loss of bacteria from the aqueous phase during the protist blooms was not governed by filtration but rather was governed by predation. Simulations were performed to examine the transport distances achieved with and without detachment, using attachment and detachment rate coefficients similar to those obtained in this field study. Simulations that included detachment showed that transport distances of bacteria may significantly increase because of detachment under the conditions examined. ¿ 2001 American Geophysical Union