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

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 does the transport of Rhodamine compare to transport of chloride?” Six (6) Kg of sodium chloride (NaCl) 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 the dye and Clconcentrations. Breakthrough curves for the dye were similar to those of Cl, confirming similar transport dynamics. Inspection of the breakthrough curves indicated the average travel time for the peak dye concentration was 12 days following injection and two days after the peak dye concentrations within all wells had lowered. Peak concentrations for Cl arrived on average 16 days after injection. Identification of the time each 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 and longitudinal dispersitives of the tested constituents. Average velocities for both Cl and dye were relatively equal, however, dye displayed high variance. Modeled Cl dispersitives are two orders of magnitude larger than the dye. Overall, the similar arrival times and breakthrough curves of the constituents 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