BREAKING DOWN A TRACER IN A SATURATED BUFFER ZONE: COMPARING THE USE OF RHODAMINE WT TO SODIUM CHLORIDE AND SODIUM BROMIDE

Tracer tests are a common means to discern hydrologic and transport properties of an aquifer. Conservative ion tracers, such as dissolved salts, are common choices for use as groundwater tracers, but the introduction of large quantities of salts can elevate the concentrations to acute levels for organisms and can create density driven flows. Rhodamine WT dye is widely used in karst systems but not in Darcian systems. We explored the questions “Can Rhodamine WT dye be used as a groundwater tracer is a Darcian system comprised of weathered diamicton?”, and “How well does Rhodamine transport compare to transport of ion tracers?” Six (6) Kg of sodium chloride (NaCl), 4 Kg of sodium bromide (NaBr), and 1 Kg of Rhodamine WT dye (25000 ppm) were injected into a saturated buffer zone (SBZ) via diversion box and drainage tiles. Following the injection, water samples from wells installed in the SBZ were collected and analyzed for dye, Cl, and Br concentrations. Breakthrough curves for the dye were similar to those of Cl and Br, confirming similar transport dynamics. Inspection of the breakthrough curves for travel times indicated that on average 12 days after injection each well received its largest concentration of the dye (Peak), and two days after peak arrival dye concentrations for all wells had lowered. Peak concentrations for Cl and Br arrived on average 16 and 14 days after injection, respectively. Identification of the time each ion tracer had left the system was difficult to discern, a storm event occurring 31 days after injection will serve as the upper limit. Breakthrough curves modeled utilizing TRAC simulated travel velocities based upon known hydrologic parameters for the system. Differences among the travel velocities exist among the tracers; however, the differences are minimal. Overall, the similar arrival times and breakthrough curves of Rhodamine dye, Cl, and Br suggest that Rhodamine dye can be used as a groundwater tracer in weathered diamicton. Future research and replication must be conducted in order to further explore and refine this technique.