USE OF E. COLI SEROTYPES AS TRACERS IN EPIKARST AND KARST-CONDUIT AQUIFERS (Invited Presentation)

Because of solution-enhanced porosity and permeability and the development of conduit networks, karst aquifers are particularly susceptible to bacterial contamination. However, conventional tracers of groundwater flow and particle movement (e.g., fluorescent dyes and latex microspheres) may not accurately mimic processes affecting bacteria, such as sedimentation, straining, adhesion, and predation. We conducted tracer tests at two sites in Kentucky using rhodamine WT dye, 1-µm fluorescent latex microspheres, and two non-pathogenic serotypes of E. coli: kps (low attachment) and iha (high attachment). Tracers were injected at the top of epikarst above Crumps Cave (Warren County) during a storm in May 2015 and monitored for 109 d at a cave waterfall ~ 30 m below. Tracers were injected into the Royal Spring aquifer (Fayette and Scott counties) during baseflow in October 2015 and monitored for 50 d in a conduit well (~ 750 m downgradient) and the spring (~ 6.25 km downgradient).

The relative timing of tracer arrivals and peak concentrations differed between sites. In the epikarst trace, dye arrived at the waterfall prior to particulates, and its concentration peaked after the kps isolate but prior to microspheres and the iha isolate. In the aquifer trace, dye arrived at the spring after particulates and at the well after microspheres and the kps isolate; dye peaked between microspheres and bacteria at the spring and after microspheres and the iha isolate at the well. Microspheres arrived simultaneously with the kps isolate in the epikarst trace and prior to bacteria in the aquifer trace. In both tests, tracers were remobilized from storage during subsequent storms, but in the epikarst trace, the kps isolate was remobilized during events when the iha isolate was not. Low recovery of tracers in the epikarst trace probably reflects multiple flow paths, which can be activated depending on antecedent moisture conditions. In the aquifer trace, isolates appeared to be remobilized above a threshold flow rate. These results indicate that transport behaviors can vary not only between bacteria and abiotic tracers, but also between different isolates of the same bacterial species.