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Saturated Hydraulic Conductivity Reduction Caused by Aerobic Bacteria in Sand Columns

Dep. of Soil, Crop, and Atmospheric Sciences, Bradfield Hall, Cornell Univ., Ithaca, NY 14853

ABSTRACT

Bacterial reductions of the saturated hydraulic conductivity, K_s, of natural porous media have been traditionally associated with the development of anaerobic conditions and the production of large amounts of extracellular polymers by the bacteria. Various researchers have also reported that these reductions occur predominantly at or very near the surfaces of injection of nutrients within the porous media. Attempts to describe mathematically the resulting clogging process have, in the past, been based on the assumption that bacterial cells form impermeable biofilms uniformly covering pore walls. A series of percolation experiments was carried out to determine the extent to which an obligately aerobic bacterial strain, Arthrobacter sp., is able to clog permeameters filled with fine sand. A second objective was to elucidate the mechanism(s) responsible for this process. The experimental results indicate that strictly aerobic bacteria are able to reduce K_s by up to four orders of magnitude. Rapid reductions in K_s are associated with the formation of a bacterial mat at the inlet boundary of the sand columns. When the colonization of the inlet is prevented, clogging proceeds within the bulk of the sand at a noticeably slower rate. Under O₂- or glucose-limited growth conditions, this decrease in K_s within the sand does not appear on scanning electron micrographs to be caused by exopolymers, which are entirely absent, but rather seems to be due to the presence of large aggregates of cells that form local plugs within the pores. Under conditions of severe N limitation, the same mechanism seems to be largely responsible for the observed clogging, in spite of the production by the cells of extracellular substances, visible under light microscopy and on scanning electron micrographs. In all cases, the coverage of the solid surfaces by the bacterial cells is sparse and heterogeneous, contrary to the basic tenets of the biofilm model.

^* Corresponding author.

Received for publication September 19, 1990.