Leuconostoc mesenteroides is a model bacterium capable of producing a viscous polysaccharide, dextran, when supplied with sucrose. When L. mesenteroides bacteria are grown in soil, the accumulation of dextran has profound effects on the physical and transport properties of soils. This study examines the feasibility of using P-wave and S-wave signatures to monitor the accumulation of dextran in sand and its consequential effects on bioclogging in sands. To achieve this goal, a column experiment stimulating the production of dextran by L. mesenteroides was performed along with monitoring of changes in permeability, responses of P-and S-wave transmission and electrical resistivity. After 41 d, scanning electron microscopy images of the tested sand showed a notable amount of dextran accumulated in the pore space as well as coating the sand grains. The results show that permeability was reduced by more than one order of magnitude, the attenuation factor characteristics of the soils were increased and electrical resistivity increased. However, minimal changes in the P-and S-wave velocities were observed, indicating that dextran production has little effect on soil stiffness. NotationA cross-sectional area of the soil sample (mm 2 ) C ratio of geometrical factors f wave frequency k permeability of the soil sample (m 2 ) L distance between two permeability measured points (mm) Q quality factor Q −1 attenuation coefficient in spectral ratio analysisvolumetric flow rate V p P-wave velocity (m/s) V s S-wave velocity (m/s) z wave travel distance DP pressure difference between two measured points (kPa) m fluid viscosity of the growth media (Pa s) r electrical resistivity
This study investigates changes in low-frequency attenuation responses of sands during microbial formation of soft viscous biofilms, or extracellular polymeric substances (EPS). The resonant column experiments were conducted with two model bacteria Shewanella oneidensis MR1 and Leuconostoc mesenteroides, while monitoring changes in the wave velocities and damping ratios associated with EPS formation in sands. The results show that the accumulation of soft, viscous EPS hardly changes the wave velocities, both the shear and flexural modes. By contrast, the low-frequency attenuations, both torsional and flexural damping ratios, show significant increases with the accumulation of highly viscous EPS. It is found that the contribution of EPS to seismic responses of water-saturated sands is mainly limited to the pore fluid component, causing additional energy dissipation during wave propagation, but with no or minimal impact on skeletal stiffness or no involvement in seismic stress transfer. With these unique and unprecedented low-frequency seismic data of biofilm-associated sands, the results suggest that the formation and accumulation of soft, viscous EPS or biofilms by bacterial activities can be detected by monitoring seismic attenuation and can also alter the seismic attenuation responses of sands, such as the case under earthquake loading or blast-induced compaction.
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