Can varying velocity conditions be one possible explanation for differences between laboratory and field observations of bacterial transport in porous media?

• Cannot model micro-particle transport using theory developed for constant flow conditions.
• Models developed for constant flow underestimate particle BTCs when velocity increases.
• The influence of velocity history on particle transport has implications for modeling field-scale transport.
• Fluctuating velocity conditions might explain widespread fecal contamination of aquifers.

Laboratory column experimental results are frequently used to estimate field-scale, fecal bacterial transport distances. However, it is not uncommon for fecal bacteria to be observed at greater distances than predicted by up-scaling laboratory results. Fluctuating or varying velocity conditions is one complex in-situ condition that might account for such inaccurate prediction, yet it is often neglected in laboratory column experiments. In this study, one-dimensional, laboratory column experiments were performed under both constant and varying velocity conditions using 2 µm microspheres and 100 µm glass beads to simulate bacterial transport in saturated porous media. Particle breakthrough curves and particle concentrations retained in the column at the end of an experiment were obtained for five constant and three varying velocity conditions. The range of constant velocities investigated was between 3.17 m/day and 27.65 m/day. For varying velocity conditions, the velocity was steadily increased and/or decreased over the period of the experiment within the same range. Results from the constant velocity experiments were successfully modeled using first order, irreversible particle attachment kinetics. The irreversible attachment coefficients obtained from the constant velocity experiments were used to derive a power function relationship between a dimensionless irreversible attachment coefficient, Ki* and velocity, v.   This relationship was then used to model the varying velocity experiments, with limited success (NRMSE > 10% for all model fits). A comparison of Ki* values obtained from direct fitting of the varying velocity tests, with the Ki* values derived from the results of the constant velocity experiments, revealed a potential dependence of Ki* on the rate of change of velocity. Observed particle breakthrough curves (BTCs) for the varying velocity experiments were also modeled using a constant value of Ki* based on the average velocity of each experiment. The results of this modeling under-estimated observed maximum breakthrough concentrations for the column experiments where velocity increased, and especially under conditions where velocity increased then decreased. Overall, the results of this study point to the need for better understanding of how varying velocity conditions impact bacterial transport in the field.