Because of limited filtration and rapid transport of groundwater and particulate matter, karst aquifers are susceptible to bacterial contamination. Water-quality standards are commonly tied to fecal indicator bacteria such as E. coli, but few studies have used E. coli as part of a suite of tracers to assess contaminant transport in karst environments. Conventional solute and particulate tracers, such as fluorescent dyes and microspheres, may not accurately represent processes that attenuate bacterial transport (e.g., sedimentation, straining, adhesion, and predation).
We used non-pathogenic E. coli isolates, together with rhodamine WT dye and 1-micron fluorescent latex microspheres, as tracers at three sites in Kentucky (USA). These included karst-conduit aquifers in Ordovician limestone of the Inner Bluegrass region (Blue Hole and Royal Spring basins) and epikarst in Lower Carboniferous limestone of the Mississippian Plateaus region (Crumps Cave site). In the Blue Hole basin, we injected wild-type E. coli labeled with the stable isotope nitrogen-15 into a swallet during a storm and monitored tracer arrival at the spring (~ 530 m downgradient) for 24 d. In the Royal Spring basin, we injected two E. coli isolates (kps [low attachment] and iha [high attachment]) into a sinkhole during baseflow conditions, and monitored tracer arrival at a well and the spring (~ 750 m and ~ 6.25 km downgradient, respectively) for 50 d. We injected kps and iha isolates at the top of epikarst above Crumps Cave during a storm and monitored tracer arrival at a cave waterfall ~ 30 m below for 109 d. E. coli were analyzed by isotope-ratio mass spectrometry for the Blue Hole trace and by quantitative polymerase chain reaction for the other traces.
Differences in the relative timing of tracer arrivals and peak concentrations at each site reflect differences in tracer characteristics and flow-path complexity. At Blue Hole spring, breakthrough of all tracers coincided, but E. coli exhibited more tailing than dye during the initial storm-flow recession, and only microspheres were remobilized by subsequent storms. In the Royal Spring basin, microspheres arrived at monitoring sites before E. coli, while dye arrived after the particulate tracers. Dye peaked prior to particulate tracers at the well but between microspheres and bacteria at the spring. In the epikarst trace, dye arrived at the waterfall prior to particulates, and its concentration peaked after kps but prior to microspheres and iha. In both the Royal Spring and Crumps Cave tests, all tracers were remobilized from storage during subsequent storms. We conclude that transport behaviors can vary not only between bacteria and abiotic tracers, but also between different E. coli isolates. Furthermore, tracer remobilization and recovery depend upon antecedent moisture conditions in the vadose zone and the extent to which flow occurs along multiple pathways.