USE OF STABLE ISOTOPE-LABELED ESCHERICHIA COLI AS A TRACER IN KARST AQUIFERS

Contamination of karst aquifers is a large concern across the globe, yet bacterial transport in them is not currently well understood. Groundwater tracers typically used in karst systems include fluorescent dyes and latex microspheres. Not only can these tracers be cost-prohibitive, depending on the system being studied, but they may not accurately mimic the transport behaviors of bacteria and other potential pathogens, and thus may not be good proxies for risk assessment involving microorganisms. This study examines the movement and attenuation of two serotypes of Escherichia coli (E. coli) with differing attachment efficiencies compared to traditional tracers (Rhodamine WT dye and 1-μm diameter fluorescent microspheres). Study sites include epikarst above Cave Springs Cavern near Bowling Green, KY, and a karst aquifer that emerges at Royal Spring in Georgetown, KY.

E. coli is enriched with either ¹³C or ¹⁵N and is injected into the area of interest (epikarst or aquifer conduit) along with dye and microspheres. Quantification of E. coli will be performed through molecular methods (qPCR) and dual isotope analysis. Preliminary findings suggest that the two methods may be complementary, having limitations across the range of isotopically enriched bacteria or number of gene copies they can detect.

Based on prior research, it was hypothesized that the three tracers will exhibit differential transport times in the Cane Run/Royal Spring basin under base flow conditions, with microorganisms arriving at the spring prior to microspheres or conservative dyes. For the epikarst trace, the E. coli isolate that exhibits higher attachment efficiency and contains the iha gene is expected to have greater attenuation and emerge from the epikarst after the isolate that contains the kps gene. These two isolates of E. coli will have transport times differing from microspheres or dyes, with the potential for multiple storm events occurring before bacteria are flushed through the epikarst, and dye emerging from the epikarst prior to particulate tracers.