The effect of salinity and particle size on bacteria transport in porous media

Abstract:

Fecal bacteria can be discharged accidentally into surface water bodies or shallow aquifers, and pose a risk to human health. Once they are in surface water, they can be transported downstream with the flow as well as settle towards the bottom and potentially migrate within the sediments of the river bed. The fate and the transport of bacteria in porous media is mainly governed by attachment/detachment and straining processes at the solid-liquid interface. The salinity and the grain size are among the most important factors controlling these phenomena. Few studies quantitatively analyze the effect of the salinity and particle size on the transport behavior of bacteria in porous media linking observation to mathematical equations of interaction. These studies do not account for the combination of bacteria-salinity-grain size, which is evident from experimental observations. They miss therefore the possibility to describe the effect of the continuous variations of salinity which can be important in estuarine sediments and shallow aquifers.

Here, a study is presented which combines laboratory tests and modeling to describe the transport of bacteria as a function of salinity and grain size. Column-flood experiments were conducted to investigate the transport of bacteria through sand and they were continuously monitored with UV-Vis spectrometer and conductivity, pressure and pH probes. One dimensional transport model was developed and consists of two mass conservation equations of the bacteria and salt concentration coupled through the constitutive equations of attachment/detachment and straining processes. The results show that under salinity reduction, a peak of bacteria concentration forms, whose magnitude is controlled by the attachment kinetics and grain size. Retardation of bacteria increased with salinity and decreasing grain size. Nearly constant pressure gradient was measured through the column suggesting that retardation was controlled by attachment. As the salinity was reversed, a detachment peak of up to 20 times the injected concentration formed without clogging. Overall, this work can help to gain an insight into the role of salinity and grain size on bacteria transport through sediment in brackish water and determine the retardation and the enhanced detachment under tidal conditions.