Influences of biological and physical heterogeneities on microbial transport through porous media
Lawson, Jessica A.
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Although enhanced virus transport has been observed in anoxic aquifers, little is known about the effects of biological heterogeneity – including microbially-induced zonation of terminal electron-acceptor processes – on microbial transport in groundwater. An improved understanding of the influence of heterogeneities of physical and biological origin on microbial transport would benefit water supply and water reuse applications, including riverbank filtration (RBF). Laboratory studies of planktonic S. oneidensis MR-1 cultures confirmed the influence of metabolic state, represented by electron acceptor conditions and growth phase, on transport-relevant surface properties of the organism. Discernible differences in zeta potential and apparent hydrophobicity (as measured by the MATH test) were detected between aerobic and anaerobic cultures. Zeta potentials were generally in the range of -4 to -10 mV. Results of EPS analysis were in qualitative agreement with the electrokinetic findings that nitrate-reducing cultures had lower net surface charge than aerobic cultures at log phase. However, similar qualitative agreement between the results of cell surface characterization by MATH and electrokinetic analyses was not observed. Our results confirm previous reports that charge, non-polar interactions, and steric factors contribute to adhesion and attachment behavior in complex ways, and further demonstrate that redox conditions can affect transport-relevant properties. Stochastic modeling studies in one and three dimensions explored the influences of physical and biological heterogeneity on microbial transport. Both models showed the potential for heterogeneity to adversely impact system performance. The 1D model demonstrated that correlations between biological and physical heterogeneities can influence virus breakthrough in complex, varied, and sometimes counterintuitive ways. The 3D study, based on a novel dimensionless framework to describe an RBF flow field, separated the contribution of the pumping-induced distribution of flow path lengths from the overall filtration behavior of the system. While a less linear flow field improves removals and apparent filtration efficiency, heterogeneity in hydraulic conductivity hurts filtration performance on average; physical and flow heterogeneities thus counteract each other. Our results further underscored how a failure to fully account for correlations between physical/flow heterogeneities and attachment processes can produce artificial scale dependency in macroscale estimates of attachment parameters.