title: Use of Nitrogen-15-Enriched Escherichia coli as a Bacterial Tracer in Karst Aquifers.
abstract: Karst aquifers are susceptible to contamination by microorganisms, but relatively few studies have used bacteria as tracers. We demonstrate the utility of Escherichia coli enriched in the stable isotope nitrogen-15 (15N) as a novel bacterial tracer. Nonpathogenic E. coli from two springs in central Kentucky were grown on15N-enriched media. Survival of E. coli and persistence of the isotopic signal were assessed in two sets of laboratory experiments conducted with sterilized spring water in dark microcosms at 14 °C. First, isotopically labeled bacteria survived for 130 d at concentrations within one log unit of the average initial value, and there was no significant difference in δ15N values from Day 1 to Day 130. Second, water samples with E. coli were inoculated with either of two different species of protozoa (Tetrahymena pyriformis or Colpoda steinii). During 7 d, δ15N values increased in T. pyriformis while bacterial populations decreased. In a field test, following a 2.1-cm rainfall,15N-labeled E. coli, solutes (rhodamine WT dye and bromide), and latex microspheres were injected into a sinkhole approximately 530 m upgradient of a spring. Breakthrough of all tracers coincided, but microspheres were remobilized by subsequent storms, unlike other tracers. Enriched E. coli exhibited more tailing than solute tracers during the initial storm-flow recession. These results indicate that15N-enriched E. coli is a viable tracer of bacterial transport in karst aquifers, although predation may attenuate the isotopic signal in systems that are not rapidly flushed.
authors.affiliation: Department of Physics and Geosciences, Angelo State University, 124 Vincent Nursing-Physical Science Building, San Angelo, TX, 76909-0904.
authors.forename: James W
authors.initials: JW
authors.lastname: Ward
authors.affiliation: Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX, 78712-1722.
authors.forename: John G
authors.initials: JG
authors.lastname: Warden
authors.affiliation: Department of Earth and Environmental Sciences, University of Kentucky, 101 Slone Building, Lexington, KY, 40506-0053.
authors.forename: Ashley M
authors.initials: AM
authors.lastname: Bandy
authors.forename: Alan E
authors.initials: AE
authors.lastname: Fryar
authors.affiliation: Department of Civil Engineering, University of Kentucky, 161 Raymond Building, Lexington, KY, 40506-0281.
authors.forename: Gail M
authors.initials: GM
authors.lastname: Brion
authors.affiliation: Department of Environmental Sciences, University of Virginia, Clark Hall, Charlottesville, VA, 22904-4123.
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Macko
authors.affiliation: Department of Earth and Environmental Sciences, Furman University, 3300 Poinsett Highway, Greenville, SC, 29613.
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Romanek
authors.affiliation: Department of Plant and Soil Sciences, University of Kentucky, N122N Agricultural Science Center, Lexington, KY, 40546-0091.
authors.forename: Mark S
authors.initials: MS
authors.lastname: Coyne
mesh_terms: D004926:Escherichia coli; D060587:Groundwater; D007629:Kentucky; D009584:Nitrogen; D014872:Water Movements
chemical_list: D009584:Nitrogen