APPLICATION OF MULTIPLE TRACERS TO ELUCIDATE COMPLEX TRANSPORT PHENOMENA IN A KARST SPRING SYSTEM

Multiple tracers were injected into a losing stream on the recessional limb of a storm hydrograph to characterize structural and stratigraphic controls on groundwater flow, and to elucidate the fate and transport of sediment and Escherichia. coli in the mantled karst aquifer at the Savoy Experimental Watershed, Northwest Arkansas. Rhodamine WT, fluorescein, lanthanum-labeled clay and europium-labeled E. coli were injected under natural gradient into the losing stream approximately 490 m and 453 m upgradient from two primary spring discharge points. Discrete time-series of samples were collected to characterize tracer breakthrough at each spring under a steadily recessing hydrograph for the first 10-days following injection, and then over several storm-induced transient flows for the next 35-days.

Tracer breakthrough data indicated the arrival of suspended solids and E. coli approximately 10.7- and 5.9-hours, respectively, ahead of the conservative-dye tracers at the more distant spring (490 m downgradient) but later than the arrival of the conservative tracers at the proximal spring. The early arrival of sediment and E. coli at the distal spring is hypothesized to result from differential gravitational settling in pools along the flow path and preferential flow into conduits connected to these pools, coupled with the effect of pore size exclusion. It is proposed that the observed sequence of tracer arrival at the proximal spring is also a function of tracer density. The more dense sediment and E. coli settle into the pools along the flow path and thus are retarded relative to the more conservative dyes. During storm-induced recharge events, all tracers were observed to arrive simultaneously at both springs. The difference in the arrival time of all tracers under storm-induced transient flow was only an hour, arriving first at the overflow spring and second at the underflow spring. This pattern of movement suggests that the tracers are being stored in pools in the subsurface during periods of low-flow and are flushed by the onset of more turbulent flow associated with storm-induced recharge, moving to the primary spring discharge points as a pulse on the rising limb of the hydrograph. This pulse arrives first at the closer overflow spring because of higher transmissivity along that specific flow path.