Rhodamine WT as a reactive tracer: Laboratory study and field consequences
We separated the two isomers of Rhodamine WT (RWT) from a commercially available tracer grade solution and found that their fluorescence emission spectra are distinct. In addition, with RWT and the sand used to fill the sand packs around the monitoring wells at the Lizzie Field Site near Greenville, North Carolina, we conducted batch studies that confirm that one RWT isomer (isomer 1) has a partitioning coefficient and a time to equilibrium sorption an order of magnitude lower than those of the other isomer (isomer 2). The combination of two isomers, with different sorption properties and distinct emission spectra, introduces errors in measuring RWT concentrations with fluorometers during groundwater tracer studies. The two isomers become chromatographically separated (due to travelling at different velocities) and thus arrive in a different concentration ratio than that of the RWT solution used in injection and fluorometer calibration. Based on the emission spectra of the two isomers these errors could be as high as 7.8%. The presence of isomer 2 in commercially available RWT hampers its effectiveness as a tracer.
