Remobilization of Fecal Indicator Bacteria in Dune Sands During Transients in Water Flow
Summary
The processes controlling remobilization of bacteria in the unsaturated zone under the influence of transients in water flow have not often been studied and are poorly understood. Fecal indicator bacteria, Enterococci, have been observed in water collected after infiltration and soil passage in sand dunes, leading to questions about the capacity of dune sands in general and the unsaturated zone in particular, to filter and retain bacteria from the infiltrating water. This study measured the release of two fecal indicator bacteria, E. coli and E. moraviensis, in dune sands as a result of transients in flow. The release was measured in column experiments with either heavy rainfall events or fluctuations in the groundwater level as the forcing mechanisms. Subsequently, microbial release was modeled with HYDRUS-1D, using either a one- or two-site kinetic attachment/detachment model to account for retention and release of the bacteria in the subsurface. The model was fitted to the experimental data. Experimental results reveal that imbibition dominates the release of E. coli, where drainage is the controlling process for the release of E. moraviensis. Furthermore, E. moraviensis are more easily attached and less easily detached from soil grains than E. coli. Hysteresis and air entrapment occur during the unsaturated experiments and are thought to have a large effect on microbial transport. Agreement between measured and modeled concentrations of bacteria under influence of rainfall is generally good and the attachment and detachment parameters that control the kinetics of bacteria (re)mobilization vary in a recognizable manner with changes in bacteria species and transient flow scenario. The one- and two-kinetic site model simulations of groundwater level fluctuations fitted poorly with the measured effluent concentrations. More than 99% of the bacteria applied to the sand column during inoculation is retained in the sand. However, the large number of bacteria contained in animal feces as well as accumulation of bacteria in the subsurface over time, can results in significant contamination of the groundwater. The results of this study indicate that under certain unfavorable conditions, additional steps need to be taken in drinking water production after dune filtration in order to ensure the required drinking water quality. Further research is needed to fully distinguish the factors controlling remobilization of bacteria in the unsaturated zone in dune sands.