RECURRING, MULTICOMPONENT GROUND-WATER TRACING EXPERIMENTS AT A WELL-CHARACTERIZED BASIN IN MANTLED KARST--LESSONS FROM THE SAVOY EXPERIMENTAL WATERSHED

More than 20 ground-water tracer tests in Basin 1 of the Savoy Experimental Watershed (SEW) have been conducted during the last 3 years, many at the same injection point, but under varying hydrologic conditions. Interpretation of these tests are providing understanding not only about the detailed hydrogeology of this basin, but also are providing insight into the effectiveness of different tracers under true field conditions in a mantled karst setting typical of the southern Ozarks. The SEW is a long-term field research facility of 1250 hectares owned by the University of Arkansas, and shared with state and federal researchers. . Basin 1 is well characterized, and typifies mantled karst including important hydrologic features such as springs, seeps, a losing stream, a regolith mantle of varying thickness, and a losing stream. In place in basin 1 is an extensive infrastructure of weirs, flumes, wells, and a weather station. To date, the following tracers have been used in Basin 1: fluorescein, rhodamine WT, chloride, bromide, lanthanum-labeled clay, indigenous E. coli labeled with europium, temperature, and specific conductance. Studies to date reveal that fast-flow components of the shallow karst aquifer at SEW transmits conservative tracers with point-to-point velocities that range from 15 to more than 800 meters per hour (m/hr). Computations of dilution factors for low-flow conditions all are within the range 2 - 9 x 10-7. Conservative tracer transport times through the vadose zone (1-2 meters) typically are several orders of magnitude longer than flow in the carbonate aquifer, reflective not only of a lower conductivity in the soil and regolith, but also of temporary storage along permeability contrasts and in response to recharge pulses. In the rapid flow part of the saturated aquifer, suspended sediments and bacteria reside in the aquifer longer than dissolved tracers (weeks and months versus days). Velocity calculations for suspended tracers are misleading, however, because the particles don’t move uniformly or continuously, but rather in a series of pulses related to turbulence along the flowpath. Coupled with other ongoing, field-intensive research at SEW, tracing constrains our conceptual model, and significantly enhances understanding of a complex karst flow system.