Bacterial transport in variably saturated porous media.

Abstract

The transport of Pseudomonas fluorescens strain P17 through saturated and unsaturated porous media was investigated. Continuous-flow column experiments examined the effects of ionic strength and pH on P17 transport in saturated porous media. Bacterial penetration was measured and filtration theory was used to calculate bacterial collision efficiencies (ɑ). A decrease in ionic strength from 10⁻¹ to 10⁻⁵ M produced an approximately 90% decrease in bacterial ɑ's (from 0.12 to 0.015). This change in a is consistent with double-layer theory, but suggests that very large changes in ionic strength are necessary to influence transport. Cell transport was unaffected by pH in the range of 5.5 < pH < 7.0. Column results were compared to a screening technique using large-pore filters. Filter ɑ's followed similar trends, but were about 1.5 times that of column ɑ's, likely due to the use of an idealized model to describe different porous media geometries and system hydraulics. Column results also indicate that uncertainty in measurements of culturable bacteria can preclude reliable estimation of low ɑ values. For ɑ < 0.01, a rapid and more reliable mini-column method is suggested for measuring biocolloid attenuation in saturated porous media. Laboratory experiments also determined P17 transport as a function of water content and the influence of the gas-liquid and solid-liquid interfaces in limiting microbial transport. Cells were suspended in artificial groundwater and injected into saturated and unsaturated quartz sand columns. Total P17 retention (R(t)) was inversely proportional to water content with approximately twice the cell retention at 46% water saturation (R(t) = 0.95) compared to saturated experiments (R(t) = 0.50). Retained cells were divided into fractions attached at the gas-liquid (R(g)) and solid-liquid (R(s)) interfaces. The ratio of R(g)/R(t), increased with decreasing water content suggesting increased bacterial removal was due to cell attachment at the gas-liquid interface and was in proportion to gas-liquid interfacial area. Bacterial transport under unsaturated conditions was enhanced by decreasing the ionic strength of the carrying solution and adding non ionic surfactant. Decreased cell attachment at both the gas-liquid and solid-liquid interfaces was likely due to changes in electrostatic interactions between the interfaces and cell surfaces brought on by water chemistry modifications.