SOLUTE AND BACTERIAL TRANSPORT TO A KARST SPRING, INNER BLUEGRASS REGION, KENTUCKY

Although techniques for conducting solute tracer tests in karst watersheds are well established, tracing the movement of bacterial pathogens is more challenging. From June 2006 to April 2007, we conducted three tracer tests under various flow conditions in a karst conduit system in Woodford County, Kentucky. Blue Hole Spring drains the city of Versailles (~ 8,000 population) and outlying farmland. Tracers were injected as slugs into a swallet ~ 500 m upgradient of the spring. Tracers consisted of solutes (rhodamine WT (RWT) fluorescent dye and bromide (Br^-)), particles (1-µm fluorescent latex microspheres) and microorganisms (wild E. coli originally isolated from the spring and enriched in ¹⁵N).

The first trace occurred on June 2, 2006, under low-flow conditions and consisted of only solutes. Discharge averaged ~ 0.079 m³/s. RWT and Br^- arrived at the spring ~ 6.16 hours and ~ 6.5 hours post-injection, respectively. The second trace occurred on July 11, 2006, under storm-flow conditions and consisted of solutes and particles. Discharge averaged 0.165 m³/s (maximum 0.262 m³/s) for this trace. Breakthrough began ~ 2.33 hours post-injection for the solutes and ~ 2.5 hours for the microspheres. Solute concentrations at the spring peaked ~ 2.67 hours after injection. Microspheres were detected at the spring until 164 hours after injection. The third trace transpired on April 1, 2007, during storm flow, and consisted of solutes, particles and mass growths of ¹⁵N-enriched E. coli. Discharge averaged 0.100 m³/s, with a maximum of 0.357 m³/s. Breakthrough began ~ 0.75 hours post-injection for the solutes and ~ 1.08 hours post- injection for the microspheres. For all three tests, breakthrough curves for the solutes were smooth, yet the microsphere breakthrough curves were jagged. Subsequent storm-flow pulses resuspended microspheres that had settled out following initial injections. Enriched E. coli breakthrough results are pending.