Microbial and solute transport through intact vadose zone cores of heterogeneous alluvial gravel under variably saturated conditions

The movement of bacterial and viral pathogens through soil and vadose zone and subsequently into groundwater is a major public health concern. There are relatively few studies on the transport and fate of microbes through variably saturated vadose zone media compared with their transport in the soil and saturated groundwater zones. In this study, we investigated the transport of Escherichia coli, F-RNA bacteriophage MS2, and a conservative solute tracer bromide through three intact vadose zone cores, under saturated (discharge rate similar to 100 mm h(-1)) and unsaturated (discharge rate 10 and 0.5 mm h(-1)) flow conditions. The vadose zone media were sandy gravel overlying a sand lens in core 1, a heterogeneous SG mix in core 2, and SG with an open framework gravel lens through the middle of the core in core 3. The three flow regimes resulted in different transport characteristics through each of the cores. As expected, microbial transport through all cores was higher under saturated conditions, compared with unsaturated conditions. Overall, E. coli removal was consistently greater than that of MS2 phage irrespective of core media or flow conditions. There were relatively minor removals (factors of 1-2.5) of both microbes under saturated conditions, reductions of 2-3 orders of magnitude under the high flow unsaturated conditions, and almost complete removal (4 to >5 orders of magnitude) under the low flow unsaturated conditions. The much greater removal of microbes under unsaturated conditions has significant implications and potential benefits for land management decisions.