Transport of pathogens in saturated porous rocks under variable flow and salinity

The transport of pathogens through rocks is often regarded as negligible unless there are fractures in the medium. However, sedimentary rocks may have a porosity that allows the migration of pathogens even when they are unfractured.
In this work, we investigated the transport behavior of several pathogens (namely Escherichia coli and Enterococci faecalis) through a sedimentary porous rock made of 98.5 wt.% of calcite (CaCO3), hydraulic conductivity 6·10−6 m/s, and porosity 0.43. Core flooding experiments were performed under variable head conditions, ensuring full saturation of the samples. During the experiments, the flow and pathogen concentration were monitored. After an initial stabilization of the core, a suspension containing a known concentration of pathogens was superimposed onto the sample and allowed to drain through. Upon complete suspension drainage, several cycles of sterile saline solution (0.9 vol.%) were performed until the pathogen concentration at the outlet became negligible. A reactive transport model through saturated porous media was developed and implemented to describe the tests. The model couples conservation laws for flow and transport under variable head conditions with constitutive equations of straining and attachment/detachment. The data show significant retention of pathogens within the core during suspension drainage and rapid mobilization during distilled water infiltration. This behavior is well captured by the model and shows that rocks can act as bioreactors for pathogens that favor accumulation and growth during loading and mobilization during flooding with low-salinity water. This may suggest that porous rock deposits may exacerbate contamination of the underlying aquifers under intermittent conditions of accumulation/growth and release rather than protecting underground water resources, as generally assumed.
This topic is the objective of DY.MI.CR.ON. project “Predictive dynamics of microbiological contamination of groundwater in the earth critical zone and impact on human health (DY.MI.CR.ON Project)” funded by the European Union – Next Generation EU, mission 4 component 1, CUP I53D23000500001.