Bacterial Sedimentation Through a Porous Medium

Abstract

Numerous previous studies of bacterial transport in groundwaters and to deep aquifers and sediments have either neglected, or regarded as insignificant, the potential contribution of bacterial sedimentation. This study examines the potential significance of sedimentation as a mechanism for bacterial transport. A simple model is developed to predict the behavior of particles (bacterial or inorganic colloids) sedimenting through granular porous media under hydrostatic conditions. The model indicates that tortuosity‐limited sedimentation velocities through porous media consisting of large, well‐rounded grains can proceed at velocities close to (≈90% that of) free sedimentation in water columns when particle‐grain interactions involve only tortuosity. The assumption of neutral buoyancy of bacteria was demonstrated to be invalid through buoyant density measurements on 25 subsurface bacterial strains (using Percoll density gradient centrifugation). An average buoyant density of 1.088 Mg m−3 was obtained (range from 1.040 to 1.121 Mg m−3). The two nonmotile bacterial strains selected for sedimentation experiments were Arthrobacter globiformis B672 (isolated from the Middendorf aquifer, 259‐m depth), and OYS3, a streptomycin‐resistant strain isolated from shallow groundwaters at Oyster, Virginia. All experiments were carried out under nongrowth conditions. Stokes' law sedimentation velocities for the two bacterial strains calculated from measurements of buoyant densities and characteristic sizes were 5.8 and 40 mm d−1, respectively. Direct measurements of free sedimentation of Arthrobacter B672 and OYS3 through water columns (21°C) yielded median velocities of 7.1 and 42 mm d−1, respectively, in good agreement with Stokes' law calculations. The Arthrobacter B672 and OYS3 strains sedimented through saturated sand columns (quartz sand, 300–420 μm diameter) under hydrostatic conditions at median velocities of 7 and 17 mm d−1. Thus the sedimentation model is consistent with sand column observations on Arthrobacter B672 and too simplistic in the case of OYS3. Bacterial breakthrough by sedimentation exhibited trends consistent with first‐order attenuation with distance. Bacterial deposition coefficients for this first‐order model were in the range of 0.008–0.012 mm−1. Surface physical‐chemical interactions, grain and pore size distributions, and grain surface microtopography can be very important in controlling the effectiveness of bacterial sedimentation as a transport mechanism. This research suggested that if timescales are sufficiently long, spanning many generations, sedimentation can become a significant mechanism for bacterial transport.