BACTERIAL SEDIMENTATION THROUGH A POROUS-MEDIUM

Numerous previous studies of bacterial transport in groundwaters and t o deep aquifers and sediments have either neglected, or regarded as in significant, the potential contribution of bacterial sedimentation. Th is study examines the potential significance of sedimentation as a mec hanism for bacterial transport. A simple model is developed to predict the behavior of particles (bacterial or inorganic colloids) sedimenti ng through granular porous media under hydrostatic conditions. The mod el indicates that tortuosity-limited sedimentation velocities through porous media consisting of large, well-rounded grains can proceed at v elocities close to (approximate 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 b e invalid through buoyant density measurements on 25 subsurface bacter ial strains (using Percoll density gradient centrifugation). An averag e 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 sedi mentation experiments were Arthrobacter globiformis B672 (isolated fro m the Middendorf aquifer, 259-m depth), and OYS3, a streptomycin-resis tant strain isolated from shallow groundwaters at Oyster, Virginia. Al l experiments were carried out under nongrowth conditions. Stokes' law sedimentation velocities for the two bacterial strains calculated fro m measurements of buoyant densities and characteristic sizes were 5.8 and 40 mm d(-1), respectively. Direct measurements of free sedimentati on of Arthrobacter B672 and OYS3 through water columns (21 degrees C) yielded median velocities of 7.1 and 42 mm d(-1) respectively, in good agreement with Stokes' law calculations. The Arthrobacter B672 and OY S3 strains sedimented through saturated sand columns (quartz sand, 300 -420 mu m 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 th e case of OYS3. Bacterial breakthrough by sedimentation exhibited tren ds consistent with first-order attenuation with distance. Bacterial de position 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 be come a significant mechanism for bacterial transport.