Transport of pathogens through saturated porous rocks with variable flow and salinity

The transport of pathogens through porous rocks is often regarded as a negligible process unless fractures are present within the medium. However, sedimentary porous rocks may have porosity that is capable of allowing the migration of pathogens through them even if there are no fractures. In this work, we investigated the transport behavior of pathogens (namely Escherichia coli and Enterococcus faecalis) through a sedimentary porous rock containing calcite (CaCO₃). Core-flooding experiments were performed under saturated conditions and variable head; the flow was continuously monitored, together with pH, salinity, and pathogen concentration. After an initial stabilization of the core, a saline suspension containing a known concentration of pathogens was superimposed onto the sample and allowed to drain through it. Upon complete suspension drainage, several cycles with either bacteria-free saline solution (NaCl 0.9 vol.%) or distilled water were carried out until the pathogen concentration at the outlet became negligible. A one-dimensional (1D) reactive transport model through saturated porous media was developed. The model couples conservation laws for flow and transport under variable head conditions with constitutive equations of attachment/detachment and straining. The experiments show a much more important retention of Enterococcus faecalis within the core than of Escherichia coli. The retained bacteria are, however, mobilized rapidly as salinity is decreased by draining distilled water through the core. This behavior is well captured by the model, which predicts that pathogens can migrate through rocks, but the magnitude of this migration changes with the type of microorganism. Overall, our results show that porous rocks can allow the transport, the accumulations, and the release of bacteria as chemical conditions vary, suggesting that porous rock deposits cannot be assumed as protective barriers for underground water resources. Rather, they may even exacerbate contamination of underlying aquifers if intermittent conditions of accumulation and release are established.