Tracers and Modelling in Hydrogeology (Proceedings of the TraM'2000 Conference held at Liege, Belgium, May 2000). IAHS Publ. no. 262, 2000. 201 Rhodamine WT as a reactive tracer: laboratory study and field consequences I). J. SUTTON, Z. J. KABALA Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina 27708, USA e-mail: kabala@copemicus.egr.duke.edu D. VASUDEVAN Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, USA Abstract 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. INTRODUCTION Tracer tests have been and continue to be used by the subsurface hydrology community for aquifer characterization (Sutton et al., 2000a; Pang & Close, 1999; Datta-Gupta et al, 1995; Welty & Gelhar, 1994; and others). Rhodamine WT (RWT), developed in 1966 specifically for use as a general tracer (Smart & Laidlaw, 1977), is easily measured with a fluorometer, is favoured by the hydrology community, and has been used extensively in ground water studies (Pang & Close, 1999; Derouane & Dassargues, 1998; Ptak & Schmid, 1996; and others). However, numerous studies have shown that RWT sorbs during field scale groundwater tracer tests (Ptak & Schmid, 1996) and laboratory column and batch experiments with a variety of subsurface materials (Kasnavia et al, 1999; Everts & Kanwar, 1994; Shiau et al, 1993; Sabatini & Austin, 1991; Smart & Laidlaw, 1977). In addition, Sabatini & Austin (1991) noted that RWT breakthrough curves are inconsistent with equilibrium sorption of a single solute, and Shiau et al (1993) reported isolating, from the commercially available RWT mixture, two RWT isomers with significantly different sorption properties. Despite the overwhelming evidence that RWT consists of two isomers with different soiption properties, it continues to be treated as a single solute and by some
202 D. J. Sutton et al. as a conservative tracer (Pang & Close, 1999; Derouane & Dassargues, 1998). This paper describes some of the complications and errors that result from using commercially available RWT and a fluorometer as a groundwater tracing system. MATERIALS AND METHODS The subsurface material selected for this study is a sample of the medium used in the sand packs around the monitoring wells L-35, L-36, and L-39 located at the Lizzie Field Site near Greenville, North Carolina (Sutton et al, 2000a). Its porosity, 9, is 0.40 and its bulk density, p^, is 1.40 g cm" 3. The RWT solution was obtained from Turner Designs. Chemical constituents of the solution, including the two fluorescing isomers of RWT and a smaller, non-fluorescing component, were separated using HPLC with UV/Diode Array Detection. The emission spectra of the two isomers were obtained for an excitation wavelength of 555 nm. Batch studies were conducted with concentrations ranging from 1 ppb to 12 ppm (active ingredient RWT) with an equilibration time of 24 h. Additional batch studies with 6 ppm of RWT were conducted on time scales from five minutes to 24 h to analyse the kinetics of the sorption. For these studies, the RWT isomers were separated with HPLC and the concentrations were determined with UV absorbance at 256 nm or fluorescence at 555 nm excitation and 580 nm emission. LABORATORY RESULTS Sutton et al. (2000b) provides detailed results of the RWT chemical characterization, batch, column, and modelling studies. As shown in Fig. 1, the two isomers of RWT have relatively similar emission spectra at an excitation wavelength of 555 nm. However they possess distinct emission maxima: 585 nm for isomer 1, and 588 nm for isomer 2. The emission intensity at their respective maxima are similar. As summarized in Sutton et al. (2000b), the batch studies show that both the time to equilibrium sorption and the partitioning coefficient of isomer 1 are an order of magnitude lower than those of isomer 2. While isomer 1 sorption was accurately described by a linear isotherm, isomer 2 sorption followed a nonlinear Freundlich isotherm. Although Shiau et al. (1993) reported that the tracer grade RWT contains by weight 40% isomer 1 and 60% isomer 2, they used a fluorometer for this estimate and therefore did not account for the distinct emission spectra of the two isomers. IMPLICATIONS The RWT chromatographic-separation errors The Turner Designs Model 10-AU fluorometer measures the concentration of RWT between 0.1 ppb and 500 ppb by exciting the sample solution with 550 nm wavelength light, measuring the emitted light at wavelengths above 570 nm, and converting the
Rhodamine WT as a reactive tracer: laboratory study and field consequences 203 400 570 nrn isomer 1 isomer 2 0 i <: l v< i. _, 1 500 600 700 800 wavelength (nm) Fig. 1 Emission spectra for isomers 1 and 2 at an excitation of 555 nm. The emission maximum for isomer 1 is at 585 nm and that for isomer 2 is at 588 nm. The Turner Designs Model 10-AU Field Fluorometer measures the light at a wavelengths longer than 570 nm. intensity of the emitted light to concentration based on a linear calibration curve obtained with standard solutions of the commercial grade dye (Turner Designs, 1998). However, Fig. 1, shows that the emission spectra of RWT isomers 1 and 2 are different at wavelengths above 570 nm when excited by light with a similar wavelength, 555 nm. Because the isomers emit differently, the relationship of fluorescence to concentration for each isomer is also different. The average intensity, I, of light emitted by an isomer between 570 nm and 800 mn can be calculated by: 800 (1) where I(k) is the intensity of emitted light from a given isomer as a function of wavelength. The average intensities for isomers 1 and 2 between 570 nm and 800 nm are 55.8 and 65.2, respectively. Therefore, for equal concentrations of the two isomers in the sample chamber of the Turner Designs Model 10-AU fluorometer, 46.1 % of the light intensity is due to isomer 1 and 53.9% is due to isomer 2. Because of the significantly different sorption properties of isomers 1 and 2, the transport of RWT through porous media should produce a chromatographic effect, separating the two isomers and thus changing their relative concentrations in solution. Given sufficient travel distance (or time) the two isomers will fully separate and the arriving RWT becomes nearly 100% isomer 1 and 0% isomer 2. Its light intensity is then only 92.2% of what it would be had the two isomers not separated. Because of the linear relationship between emission intensity and RWT concentration (Turner Designs, 1998), one may thus expect to underestimate the RWT concentration by up to 7.8%. Ambiguity in interpretation of field tracer tests A concentration breakthrough curve with two peaks obtained from a tracer test with RWT and a fluorometer may be ambiguous because the peaks may result from a heterogeneity in the hydraulic conductivity or the isomers arriving separately.
204 D. J. Sutton et al. 0.001 0 50 100 150 200 250 300 350 lime (minutes) Fig. 2 Data from a dipole-flow test with a tracer (DFTT) conducted with RWT (Sutton et al, 2000a) showing heterogeneities and possibly the partially overlapping signals of isomers 1 and 2. Arrows on the time axis denote the arrival times of the peaks and shoulders. The ambiguity that arises from using commercially available RWT in small-scale tracer tests is illustrated in Fig. 2 by the breakthrough curve from a small-scale, singleborehole, tracer test called the dipole-flow test with a tracer (DFTT), conducted on the length scale of approximately one metre in well L-32 at the Lizzie Field Site near Greenville, North Carolina (Sutton et al, 2000a). The first peak and shoulder that arrive in approximately 15 and 40 min are likely due to heterogeneities in the hydraulic conductivity of the aquifer, while the peak and shoulder at approximately 70 and 120 min are likely due to isomers 1 and 2 arriving at different times. Yet, without knowing independently the RWT sorption properties of the porous medium one cannot be sure. They could also be the result of a heterogeneity in the hydraulic conductivity. Single-isomer RWT as an ideal groundwater tracer Based on the above, we find that RWT isomer 1 would be an excellent groundwater tracer that sorbs to a relatively small extent and can be modelled relatively easily. In contrast, we find that RWT isomer 2 is a poor groundwater tracer that sorbs to a relatively large extent and follows a nonlinear isotherm. The effectiveness of currently available tracer grade RWT is hampered by the presence of both isomers. We therefore suggest that efforts be made to separate the two isomers and provide RWT solutions that contain isomer 1 as the only active ingredient. Acknowledgments This research was partially supported by the US Geological Survey, USGS Agreement # 1434-HQ-96-GR-02689, North Carolina Water Resources Research Institute, WRRI Project # 70165, and National Science Foundation under grant DMS-9873275.
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