Limitations and potential of commercially available rhodamine WT as a groundwater tracer

Abstract

Abstract. We conducted chemical characterization, batch, column, and modeling studies to elucidate the sorption and transport of rhodamine WT (RWT) in the subsurface. The sand-pack material from the Lizzie field site near Greenville, North Carolina, served as our porous media. Our study confirms earlier results that RWT consists of two isomers with different sorption properties. It also shows that the two isomers have distinct emission spectra and are equally distributed in the RWT solution. The presence of the two isomers with different sorption properties and distinct emission spectra introduces an error in measuring the RWT concentration with fluorometers during porous media tracer studies. The two isomers become chromatographically separated during transport and thus arrive in a different concentration ratio than that of the RWT solutions used for fluorometer calibration and test injection. We found that this groundwater tracer chromatographic error could be as high as 7.8%. We fit six different reactive-solute transport models of varying complexity to our four column experiments. A two-solute, two-site sorption transport model that accounts for nonequilibrium sorption accurately describes the breakthrough curves of the shorter-timescale column experiments. However, possibly due to the groundwater tracer chromatographic error we discovered, this model, or a similar one that accounts for a Freundlich isotherm for one of the solutes, fails to describe the RWT transport in the longer-timescale column experiments. The presence of the two RWT isomers may complicate the interpretation of field tracer tests because a shoulder, or any two peaks in a breakthrough curve, could result from either aquifer heterogeneity or the different arrival times of the two isomers. In cases where isomer 2 sorbs to such an extent that its breakthrough is not recorded during a test, only isomer 1 is measured, and therefore only 50% of the injected mass is recorded. Isomer 1 of RWT can be accurately modeled with a one-solute, two-site, nonequilibrium sorption model. This conclusion and the results from our batch studies suggest that RWT isomer 1 is an effective groundwater tracer but that the presence of isomer 2 hampers its effectiveness.

Nitrogen transformations in a through‐flow wetland revealed using whole‐ecosystem pulsed 15N additions

“…We used RWT as a conservative tracer in this study in spite of evidence that under some circumstances sorption and photo-degradation may cause RWT to act nonconservatively, resulting in poor tracer recovery (Sutton et al 2001). However, in wetlands with limited sediment-water exchange and short residence times (less than a week), recoveries are sufficient for RWT to be useful in studies of hydrologic residence (Lin et al 2003).…”

Modification of Rhodamine WT tracer tests procedure in activated sludge reactors

“…Thorough laboratory research on the RWT sorption characteristics has shown that the extent of sorption is dependent on different factors such as sediment type, organic matter concentration, initial RWT concentration, pH, ionic strength and the ionic composition of the solution [4,[9][10][11][12]. In addition, RWT is made up of two isomers [4,9,10] with different sorption properties and different emission spectra, which are equally distributed in RWT solution.…”

“…In addition, RWT is made up of two isomers [4,9,10] with different sorption properties and different emission spectra, which are equally distributed in RWT solution. Isomer 1 (para) is an almost conservative tracer, while isomer 2 (meta) is a nonconservative tracer due to its sorption to sediments [4,9,10].…”

Adsorption and transport of cadmium and rhodamine WT in pumice sand columns

“…However, RWT has been found to adsorb to many subsurface media (Sabatini & Austin 1991;Shiau et al 1993) because one of the two major isomers in commercially available rhodamine WT, isomer 2, can be significantly and irreversibly adsorbed (Shiau et al 1993). A few studies have evaluated the usefulness of RWT as a groundwater tracer in porous media (Sabatini & Austin 1991;Difazio & Vurro 1994;Sutton et al 2001;Harden et al 2003). However, no evaluation has been done for pumice sand aquifer media.…”