Enhanced transport of bacteria in porous media by sediment-phase and aqueous-phase natural organic matter

Aqueous-phase dissolved natural organic matter (DOM) and sediment organic matter (SOM) were shown in laboratory mini-column experiments to affect the transport of bacteria within porous media. Attachment efficiencies of bacteria were estimated from their retention on quartz, iron oxide coated quartz (Fe-quartz), and Fe-quartz coated with SOM (SOM-Fe-quartz). Suwannee River Humic Acid (SRHA) and Soil Humic Acid (SHA) were used to represent organic matter (SOM and DOM), and were added to radiolabeled bacterial suspensions (10⁶ cells/ml, pH = 7.7) prior to transport. Coating quartz with iron oxide increased bacterial retention 160% relative to uncoated quartz. Coating Fe-quartz with SOM lowered bacterial retention, resulting in a fraction retained only 33% greater than retained on uncoated quartz. Compared to these effects, the effect of DOM on bacterial retention was secondary, and reflected the extent of DOM adsorption to the porous media. When DOM did not interact with the porous media, as in the case of quartz, bacterial retention in the presence of DOM was reduced by 20%. However, when DOM adsorption to the porous media was increased by coating the quartz with iron oxide, bacterial retention on the Fe-quartz increased by 10%. When Fe-quartz surfaces were loaded with DOM to equilibrium conditions to produce SOM-Fe-quartz, the presence of DOM in the applied solution also increased bacterial retention by 10%. The effects of DOM were the same for both types of humic acids (SHA or SRHA). These results suggest that SOM and DOM affect bacterial transport by increasing the negative surface charge of the Fe-quartz and bacteria, respectively. The largest decrease in bacterial retention (60%) was associated with coating of Fe-quartz by SOM in the absence of DOM.